Determination of water hardness and alkalinity. Alkalinity of water, reagents used in its determination Methodology for determining the total alkalinity of water
The alkalinity of water is determined by the presence of compounds that react with strong acids. These may be free hydroxides (in industrial wastewater) or salts formed by weak acids and strong bases (for example, hydrocarbonates, carbonates, silicates, sulfides, alkali metal acetates). Alkalinity due to the presence of soluble hydroxides (ions HE -), called hydrate alkalinity.
In natural waters, alkalinity is usually caused by hydrocarbonates HCO 3 - (hydrocarbonate), in alkaline waters it is also caused by carbonates CO 3 2- (carbonate).
Water alkalinity is characterized by the amount of acid required to neutralize 1 liter of water. It is expressed in mEq/l.
There is a distinction between free and total alkalinity of water. If the pH of the water being tested is more than 8.3, then the water is considered to have free alkalinity. Its value is determined by the amount of acid required to neutralize alkalinity components ( OH - , SiO 3 -2 . CO 3 -2 etc.) until the pH value of the test water reaches 4.5 (or by a change in the color of methyl orange). If the pH of the water is less than 4.5, then the alkalinity of the water is assumed to be zero. It is believed that waters with a pH value<8,3, не содержат свободной щелочности.
Determination of water alkalinity is carried out immediately after sampling or no later than 24 hours, provided that the water is stored in a closed container filled with a stopper.
Difficulties in carrying out analysis and obtaining inaccurate results can be caused by the presence of suspended substances, free carbon dioxide, chlorine and hypochlorites, compounds that cause the color of water. The interfering influence of suspended substances is eliminated by filtering the water. Hypochlorites and free chlorine cause discoloration of acid-base indicators, so they are preliminarily reduced with a 0.1 N solution of sodium thiosulfate, taken in an equivalent amount. Sometimes a 3% hydrogen peroxide solution is used to remove hypochlorites. The color of water can be reduced by filtering it through a layer of activated carbon or macroporous anion exchange resin. Free carbon dioxide is removed by blowing air through the sample water. If in water, along with bicarbonates, hydrosilicates, silicates, hydrosulfites, sulfides or other compounds that cause alkalinity of water are present in noticeable quantities, then to calculate carbonate (bicarbonate) alkalinity it is necessary to subtract from the result obtained the data obtained when determining these components (in mEq /l). For waters with low alkalinity, to obtain a more accurate result (less than 0.2 mEq/l), it is necessary to use 0.05 N solutions of acids (hydrochloric or sulfuric).
The determination of water alkalinity can be carried out by the volumetric neutralization method and electrometrically (by pH value).
Ministry of Education and Science of the Russian Federation
Volgograd State Architecture and Construction
university
Department of Water Supply and Sanitation
Water chemistry
Guidelines for laboratory work in the discipline
« Water chemistry»
Volgograd 2013
Introduction
The laboratory workshop is compiled in accordance with the work program for the discipline “Water Chemistry”.
The workshop was compiled taking into account the fact that students have already attended lectures on the course “Chemistry”, “Applied Chemistry” and are sufficiently familiar with work in a chemical laboratory and have certain theoretical ideas about the basic physical and chemical properties of water. Simultaneously with the laboratory practical work, a course of lectures “Water Chemistry” is given, where students become familiar with the main characteristics of natural and waste waters.
1. Goals and objectives of the workshop
The purpose of the laboratory workshop is to give students practical acquaintance with the equipment and utensils that are used to perform water analyzes in industrial and research laboratories. The future engineer, performing research work, acquires practical skills necessary for his future activities.
The objective of the laboratory workshop is the practical mastery of the “Water Chemistry” course, as well as the acquisition of skills in independently conducting laboratory work of a research nature.
The laboratory workshop covers work on studying various properties of water; the study of methods for determining water quality indicators includes 6 works, which include:
Job No. 1. Determination of physical indicators of water quality.
Job No. 2. Determination of water acidity.
Job No. 3. Determination of water alkalinity.
Work No. 4. Determination of water hardness. Determination of the concentration of calcium and magnesium ions in water.
Work No. 5. Determination of various forms of carbon dioxide.
Work No. 6. Determination of permagane oxidation of water.
Before carrying out any laboratory work, students must familiarize themselves with the purpose, methodology for performing the work and the necessary calculations, the design of the device, be able to handle chemical glassware, reagents, and must also familiarize themselves in detail with the basic provisions of the “Safety Instructions for those working in a chemical laboratory.” At the end of the work, students must fill out a report in the laboratory journal, indicating the main calculations, analyze the data obtained, and draw conclusions about the quality of the analyzed water.
2. Safety rules in the laboratory
Conducting laboratory work requires students to strictly adhere to work safety rules. All reagents must be in a closed container clearly labeled with the name and concentration of the reagent. When using solutions of strong acids and alkalis, it is necessary to exclude the possibility of getting them on your hands, clothes, or face. Sampling concentrated liquids should be done using graduated cylinders or pipettes with rubber cartridges. Spilled acids or alkalis must be immediately covered with sand, neutralized, and then cleaned up. When using heating devices, it is necessary to monitor the serviceability of the wiring, the degree of heating of sockets, plugs, and do not leave heating devices unattended. It is recommended to take heated objects and dishes with tongs, flask holders, or a towel rope.
Each student must observe sanitary precautions when performing laboratory work. Necessary:
– avoid direct contact with the analyzed water and sediment;
– carry out tests using rubber gloves;
– used laboratory glassware must be disinfected with a disinfectant solution;
– after completing the tests, clean up the work area and wash your hands thoroughly.
Laboratory work No. 1
Determination of physical indicators of water quality.
Goal of the work: studying methods for determining physical indicators of water quality (temperature, color, smell, taste, density).
The accuracy of water analysis largely depends on proper sample selection. Since many physical and chemical indicators of water change over time, for certain determinations in official publications the maximum shelf life of the sample is indicated. Samples are taken in bottles with rubber or ground stoppers, which are pre-rinsed with the test water. When placing into storage, the sample is preserved with chloroform (2 ml per 1 liter of water). Before analysis, if necessary, preliminary preparation of the sample is carried out: suspended substances are removed (filtration, centrifugation, settling), evaporated in porcelain cups.
Assessing water quality primarily takes into account such important physical indicators as temperature, color, smell, taste, transparency, turbidity.
1.1. Determining water temperature
In plumbing and pumping installations, the temperature is determined by immersing a thermometer in a stream of flowing water. The reading is made without removing the thermometer from the water. For individual determinations, the thermometer is placed for 3–5 minutes. into a large vessel with water. The optimal temperature limits for drinking water are 7 – 12 0 C.
1.2. Determination of water color
The color of water is due to the presence of a large number of suspended particles; it is determined after settling, filtering or centrifugation. Color is assessed in degrees dichromat - cobalt scale.
A qualitative assessment of color is made by comparing it with distilled water. To do this, separate test water and distilled water are poured into special test tubes made of colorless glass. Against the background of a white sheet of paper in daylight, water is viewed from above and from the side. Based on this, color is assessed, i.e. indicate the observed color (faint white, brown, etc.). If there is no color, the water is considered colorless. Color is determined quantitatively using the dichromate-cobalt scale.
Progress. Turbid water is pre-filtered. For determination, colorless cylinders with a diameter of 30 mm and a height of 350 mm are used.
1.3. Determining the smell of water
The smell of wastewater is determined qualitatively by opening the sample. First, they give a qualitative description of the smell according to the corresponding characteristics (swampy, earthy, putrefactive, fishy, aromatic, etc.). Intensity is assessed on a five-point scale (Table 1) at temperatures of 20 0 C and 60 0 C.
Progress. Pour the test water (2/3 of the volume) into a flask with a ground stopper and shake vigorously while closed. Then open it and immediately note the nature and intensity of the smell.
Table 1
The nature of smells and tastes according to the degree of their intensity
|
Smell (taste) |
Intensity |
Score in points |
|
Absent |
Not felt |
|
|
Very weak |
Detectable only by an experienced researcher |
|
|
Detected by the consumer if he pays attention |
||
|
Perceptible |
Easily detectable by the consumer |
|
|
Distinct |
Water is undrinkable |
|
|
Very strong |
Water is undrinkable |
1.4. Determining the taste of water
The different taste of water may be due to the presence of chemical compounds (sodium chloride, iron salts, manganese, magnesium, etc.), as well as waste products of aquatic organisms. According to SanPiN 2.1.4.1074-01, four types of taste are distinguished: bitter, sweet, sour, salty. The remaining taste sensations are characterized as flavors. The intensity of taste is quantified on the same scale as smell (see Table 1).
Water that is safe from a sanitary point of view is examined in its raw form, in other cases - after boiling and subsequent cooling to 18 - 20 0 C. Contaminated water cannot be sampled. To determine the nature and intensity of taste, 10–15 cm 3 of the test water is taken into the mouth and held for 10–15 seconds without swallowing. The intensity of the taste of drinking water, according to SanPiN 2.1.4.1074-01, should not exceed 2 points (see Table 1).
1.5. Determination of water density
The density of pure water depends on temperature. At 15 0 C it is equal to 0.99913 g/cm 3, at 20 0 C – 0.99823 g/cm 3. The density of wastewater also depends on dissolved compounds. Typically, the density of water is close to unity.
The density of water can be determined to the third decimal place using a hydrometer. Pour the test water into a 100 ml cylinder. Carefully lower the hydrometer into it. The water level should be within the hydrometer scale. If the hydrometer scale is higher or lower than the water level, the hydrometer should be replaced. The hydrometer scale reading at the water surface level corresponds to its density at a given temperature.
1.6. Determination of water turbidity
Water turbidity is caused by the presence in natural water of undissolved and colloidal substances of inorganic and organic origin. Water turbidity is characterized by the following terms: clear, slightly turbid, cloudy, etc.
The turbidity of water can be determined by the gravimetric method, visual, turbidity meter, photoelectronic tyndalemeter and photoelectric colorimeter.
1.6.1 Gravimetric method
Progress. Filter 500–1000 ml of turbid water through a dense filter (diameter 9–11 cm), pre-dried at 105 0 C for 1.5–2 hours and weighed in a closed bottle on an analytical balance. After filtering, transfer the filter with the precipitate to the same bottle, dry at 105 - 110 0 C for 1.5 - 2 hours, cool in a desiccator and weigh on an analytical balance in a closed bottle. The content of suspended substances in the test water is calculated using the formula:
q 1 – weight of the bottle with a dried filter after filtering the water, g,
q 2 – weight of the bottle with a dried filter before filtering, g,
V – volume of filtered water, ml.
1.6.2 Visualmethod
The transparency of water depends on its turbidity. A measure of transparency is the height of the water column through which a certain type of font can still be read.
The results are indicated in centimeters. Determine the height of the water column through which the typographic font becomes difficult to distinguish.
Progress. A cylinder, under which a well-lit type is placed, is filled with a mixed sample of water to such a height that the letters, viewed from above, become difficult to distinguish. The sample is viewed in diffuse daylight. The definition is repeated several times. Record the height of the water column in centimeters and calculate the average value. According to SanPiN 2.1.4.1074-01, the transparency of drinking water must be at least 30 cm.
Questions for laboratory report No. 1
1) Name the main physical indicators of water quality. The reasons for their presence (briefly).
2) Temperature. What processes occurring in water does it affect? In what units is it measured? How is temperature measured? Optimal value.
3) What impurities of natural waters determine the smell, taste and taste of water?
4) Methods for determining taste and smell. Optimal values.
5) What is deodorization? Methods for eliminating tastes and odors in water.
6) Transparency and turbidity. What impurities in natural waters cause turbidity. Determination methods. Optimal values.
7) What substances present in water bodies determine the color of the water. Determination methods. Optimal value.
Laboratory work No. 2
Determination of water acidity.
Goal of the work: study methods for determining the acidity (free and total) of water.
2.1 Determination of water acidity
The acidity of water can be due to the presence of free acids or salts formed by weak bases and strong acids (for example, FeSO 4 , AlCl 3 , ZnSO 4 and etc.). In surface natural waters and in most underground waters, the acidity of the water is usually caused by the presence of free carbonic acid. In industrial wastewater, acidity can be caused by the presence of free strong ( HCl, H 2 SO 4 , HNO 3 ) and weak acids ( NSN, H 2 S etc.), as well as salts of heavy metals. When hydrolysis of salts formed by a strong acid and a weak base occurs, the pH value decreases to 4.5 and below.
The acidity of water is determined by the neutralization method, which is based on the reaction between an acid and a base. The total acidity of water is characterized by the amount of a strong base, such as sodium hydroxide, needed to neutralize compounds contained in the water. It is determined by the number of mg-eq/l of a strong base necessary to neutralize the substances contained in 1 liter of water, the pH of which is 8.3. If the pH of the water is above 8.3, then the acidity of the water is assumed to be zero. In cases where the pH of water is below 4.5, it is considered to have free acidity.
When taking a water sample to determine acidity, it is necessary to take measures to reduce the contact of water with carbon dioxide in the air. For this purpose, hermetically sealed samplers are used, and water is taken from the tap using a rubber tube, which is lowered to the bottom of the bottle. The water should displace the air and change in the bottle several times. For titration, use flasks closed with a rubber stopper with a hole for supplying the working solution.
Acidity determination is carried out by indicator and electrometric methods.
The analysis is hampered by the increased carbonate hardness of the water (more than 4 - 5 mEq/l), the presence of heavy metal salts, for the indicator method - the color and turbidity of the water, free chlorine. If the carbonate hardness of the water is significant, during the titration process, calcium and magnesium bicarbonates transform into sparingly soluble carbonates, causing cloudiness of the test solution. This negative effect can be removed by diluting the sample with boiled distilled water. When adding a working solution of alkali, salts of heavy metals form sparingly soluble compounds that make determination difficult. The interfering effects of these compounds can be reduced by adding a small amount (0.8 - 1.2 ml) of a solution of Rochelle salt or by boiling the sample (it should be taken into account that carbon dioxide is practically removed from the water after boiling)
The electrometric method is applicable for the analysis of colored and turbid waters.
2.1.1 Determination of free acidity
a) Indicator method
Reagents
1) 0.1 N solution NaOH;
2) methyl orange (0.03% aqueous solution).
Progress. Measure 100 ml (or other required) volume of water into a conical flask, add 2-3 drops of methyl orange. If the solution turns pink, it has free acidity. Titrate the sample with 0.1 N sodium hydroxide solution until the pink color changes to yellow-pink, and then calculate the free acidity using the formula:
Where X– free acidity of water, mEq/l;
V NAOH – volume of working solution NAOH, used for titration of water sample, ml;
N NaOH – normality of the solution NaOH, g-eq/l ;
Where X - total acidity of water, mEq/l;
V NAOH – volume of working solution NaOH, used for titration of water sample, ml;
N NAOH – normality of the working solution NaOH, g-eq/l;
where x is the free alkalinity of water, mEq/l
V HCl – volume of HCl working solution used for sample titration, ml;
N HCl – normality of the HCl working solution, g-equiv/l;
Where X– total alkalinity of water, mEq/l;
VHCl– volume of HCl working solution used for sample titration, ml;
NNC l – normality of the working solution HCl, g-eq/l;
Where AND O – total water hardness, mEq/l;
V 1 – volume of Trilon B working solution used for titration of the water sample, ml;
N – normality of the Trilon B working solution, mg-eq.l;
V 2 – volume of water sample, ml.
4.3 Determination of calcium and magnesium ions
Reagents:
1) 0.1 N solution of Trilon B;
2) 2 N NaOH solution;
3) dry murexide indicator;
4) 5% Na 2 S solution;
5) 1% hydroxylamine solution;
6) dark blue chrome indicator;
7) ammonium buffer solution.
4.3.1 Determination of calcium ions
Progress. To 100 ml of test water add 5 ml of NaOH solution, a few (6 – 8) drops of Na 2 S and dry murexide on the tip of a spatula. The solution acquires a dark pink color. Slowly titrate with Trilon B solution until the color changes to wine red. Calcium content (x) is calculated using the formula:
Where X– calcium content in the analyzed water, mg/l;
a
Where at– concentration of magnesium ions in the analyzed sample, mg/l;
b – amount of Trilon B solution used for sample titration, ml;
From the equations it follows that the concentration of free carbonic acid is directly dependent on the concentration of hydrogen ions, and the concentration of carbonate ions is inversely dependent. At pH< 4,2 в природных водах содержится только свободная угольная кислота. Появление в воде гидрокарбонат-ионов повышает рН от 4,2 до 8,3; в воде присутствует свободная угольная кислота и ионы NSO 3 - , and with increasing pH the concentration increases NSO 3 -– ions and the concentration of free carbonic acid decreases. At pH = 8.4, almost only hydrocarbonates are present in the water (99.7%). With a further increase in pH in water, along with hydrocarbonate ions, carbonate ions also appear, which at pH > 10 become predominant.
Carbonic acid in the form of carbonates is considered bound; in the form of bicarbonates, it consists of bound and semi-bound carbonate (equally), since when boiling, half of the bicarbonates are converted into free carbonic acid.
2 HCO 3 - = CO 3 2- + CO 2 + H 2 O.
If there is free carbonic acid in the water and VAT 3 - - ions, then the amount of bound acid is equal to the content of semi-bound. At
pH > 8.4 amount of bound acid ( 
) will be more semi-connected ().
With the simultaneous presence of ions in water HCO 3 - And CO 3 2- The determination is carried out in one sample, neutralizing the alkalinity created by these ions with a working solution of hydrochloric acid. The determination is based on changes in the content of various forms of carbon dioxide depending on pH. In the presence of acid, CO 3 2- and HCO 3 - ions are converted into free carbonic acid. Carbonates react with hydrochloric acid in two stages. At the first stage, carbonates turn into hydrocarbonates:
A sample titrated with a working acid solution in the presence of phenolphthalein contains bicarbonates that were previously in water and bicarbonates formed from carbonates. Hydrocarbonates are converted to free carbonic acid by subsequent titration of the sample with acid in the presence of methyl orange.
When calculating the content CO 3 2--ions, the volume of hydrochloric acid consumed for the titration of a water sample with phenolphthalein doubles accordingly. And when calculating the amount of bicarbonates, the volume used for titration with phenolphthalein is subtracted from the volume of acid spent on titrating water in the presence of methyl orange.
Carbonate ions are found in alkaline waters. In this case, only bicarbonates are determined in water by titration with acid in the presence of methyl orange.
5.1 Determination of free carbon dioxide content
Reagents:
1) 0.02 N NaOH solution;
2) 0.1% alcohol solution of phenolphthalein.
Progress. Measure 100 ml of the test water into a conical flask, add 2-3 drops of phenolphthalein and titrate the sample with 0.02 N NaOH solution until a pink color appears, which does not disappear within 3 minutes. Repeat the determination 3 times and take the average value. The content of free carbon dioxide is calculated according to the formula:
– amount of water taken for determination, ml. – amount of water taken for determination, ml.
Questions for laboratory report No. 5
1) What are the main forms of carbon dioxide?
2) What determines the content of one or another form of carbon dioxide?
3) How do the forms of carbonic acid depend on pH?
4) Equations and dissociation constants of carbonic acid.
Laboratory work No. 6
Determination of permanganate oxidation of water
Goal of the work : study methods for determining permanganate oxidation of water.
The presence in water of organic substances and easily oxidized inorganic compounds (Fe 2+, sulfites, nitrites, H 2 S, etc.) characterizes the oxidability of water. Oxidability is one of the indicators of the degree of water contamination with organic impurities.
The oxidability of organic substances dissolved in water is characterized by the amount of oxygen spent on their oxidation under certain conditions.
The oxidability of water is expressed by the number of milligrams of oxygen spent on the oxidation of organic substances contained in 1 liter of water. Potassium permanganate (permanganate oxidability), potassium dichromate (or iodide) (dichromate (or iodate) oxidability or chemical oxygen demand - COD) are usually used as oxidizers of organic substances when determining the oxidability of water. To avoid errors, inorganic reducing agents are first determined in the test water.
This manual discusses the method for determining the permanganate oxidation of water. If the concentration of chlorides in the test water does not exceed 100 mg/l, then organic substances are oxidized with potassium permanganate in an acidic environment (Kubel method). At higher chloride contents, the oxidation reaction with potassium permanganate in an alkaline medium is used (Schultz method).
Organic substances contained in the water being tested are oxidized by potassium permanganate when boiled in the presence of sulfuric acid. A obviously excessive amount of KMnO 4 solution of a certain concentration is added to the water sample. Under these conditions, not all organic substances are oxidized, so oxidability characterizes the content of only easily oxidized impurities. To obtain more accurate results, back titration is used: an excess of oxalic acid is added to the boiled sample, while part of it is oxidized by the remaining potassium permanganate, and the rest H 2 C 2 O 4 titrate with potassium permanganate.
Pre-treatment of glassware before analysis. To avoid errors associated with accidental contamination of the flask with impurities capable of oxidation, before determining oxidability, 100–150 ml of a concentrated solution of potassium permanganate, acidified with sulfuric acid, is poured into the flask and boiled for 3–5 minutes. Then the mixture is poured, and the precipitate formed on the walls of the flask is dissolved in a small amount of concentrated hydrochloric acid. The flasks are rinsed with distilled water and the oxidizing mixture is poured into it again. Boiling is repeated 2–3 times, after which the flask is rinsed with distilled water.
6.1 Kubel method
Reagents:
1) 0.01 N solution of KMnO 4;
3) 0.01 N solution of H 2 C 2 O 4;
3) 4N solution of H2SO4.
Progress. Measure 100 ml of the test water into a pre-prepared 250 ml conical flask, add 5 ml of a 4N acid solution and pour exactly 10 ml of a 0.01 KMnO 4 solution from a burette. Close the flask with a funnel and boil the mixture for 10 minutes. (counting from the moment of boiling). Then remove from heat.
Turbidity is an indicator of water quality, caused by the presence in water of undissolved and colloidal substances of inorganic and organic origin. Turbidity in surface waters is caused by silt, silicic acid, iron and aluminum hydroxides, organic colloids, microorganisms and plankton. In groundwater, turbidity is caused primarily by the presence of undissolved minerals, and when wastewater penetrates into the ground, it is also caused by the presence of organic substances. In Russia, turbidity is determined photometrically by comparing samples of the test water with standard suspensions. The measurement result is expressed in mg/dm3 when using a basic standard suspension of kaolin or in TU/dm3 (turbidity units per dm3) when using a basic standard suspension of formazin. The last unit of measurement is also called Formazine Turbidity Unit (FTU) or in Western terminology FTU (Formazine Turbidity Unit). 1FTU=1EMF=1EM/dm3. Recently, the photometric method for measuring turbidity using formazin has become established as the main method throughout the world, which is reflected in the ISO 7027 standard (Water quality - Determination of turbidity). According to this standard, the unit of measurement for turbidity is FNU (Formazine Nephelometric Unit). The U.S. Environmental Protection Agency (U.S. EPA) and the World Health Organization (WHO) use the Nephelometric Turbidity Unit (NTU). The relationship between the basic units of turbidity is as follows: 1 FTU=1 FNU=1 NTU.
WHO does not standardize turbidity based on health effects, but from an appearance point of view it recommends that turbidity should not exceed 5 NTU (nephelometric turbidity unit), and for disinfection purposes - no more than 1 NTU.
A measure of transparency is the height of the water column at which one can observe a white plate of a certain size lowered into water (Secchi disk) or distinguish a font of a certain size and type on white paper (Snellen font). Results are expressed in centimeters.
Characteristics of water by transparency (turbidity)
Chroma
Color is an indicator of water quality, mainly due to the presence of humic and sulfic acids, as well as iron compounds (Fe3+) in water. The amount of these substances depends on the geological conditions in the aquifers and on the number and size of peatlands in the basin of the river under study. Thus, the surface waters of rivers and lakes located in areas of peat bogs and swampy forests have the highest color, and the lowest color in steppes and steppe zones. In winter, the content of organic substances in natural waters is minimal, while in spring during the period of high water and floods, as well as in summer during the period of mass development of algae - water blooms - it increases. Groundwater, as a rule, has less color than surface water. Thus, high color is an alarming sign indicating trouble in the water. In this case, it is very important to find out the cause of the color, since the methods for removing, for example, iron and organic compounds are different. The presence of organic matter not only worsens the organoleptic properties of water and leads to the appearance of foreign odors, but also causes a sharp decrease in the concentration of oxygen dissolved in water, which can be critical for a number of water treatment processes. Some, in principle, harmless organic compounds, when entering into chemical reactions (for example, with chlorine), are capable of forming compounds that are very harmful and dangerous to human health.
Color is measured in degrees on the platinum-cobalt scale and ranges from units to thousands of degrees - Table 2.
Characteristics of waters by color
Taste and smack
The taste of water is determined by substances of organic and inorganic origin dissolved in it and varies in character and intensity. There are four main types of taste: salty, sour, sweet, bitter. All other types of taste sensations are called tastes (alkaline, metallic, astringent, etc.). The intensity of taste and aftertaste is determined at 20 °C and assessed using a five-point system, according to GOST 3351-74*.The qualitative characteristics of shades of taste sensations - taste - are expressed descriptively: chlorine, fishy, bitter, and so on. The most common salty taste of water is most often caused by sodium chloride dissolved in water, bitter by magnesium sulfate, sour by excess free carbon dioxide, etc. The threshold of taste perception of salty solutions is characterized by the following concentrations (in distilled water), mg/l: NaCl – 165; CaCl2 – 470; MgCl2 – 135; MnCl2 – 1.8; FeCl2 – 0.35; MgSO4 – 250; CaSO4 – 70; MnSO4 – 15.7; FeSO4 – 1.6; NaHCO3 – 450.
According to the strength of their effect on the taste organs, ions of some metals are arranged in the following rows:
O cations: NH4+ > Na+ > K+; Fe2+ > Mn2+ > Mg2+ > Ca2+;
O anions: OH->NO3->Cl->HCO3->SO42-.
Characteristics of waters by taste intensity
|
Intensity of taste and aftertaste |
The nature of the appearance of taste and aftertaste |
Intensity rating, point |
|
Taste and aftertaste are not felt |
||
|
Very weak |
Taste and aftertaste are not perceived by the consumer, but are detected during laboratory testing. |
|
|
Taste and aftertaste are noticed by the consumer if they pay attention to it |
||
|
Noticeable |
Taste and aftertaste are easily noticed and cause disapproval of water |
|
|
Distinct |
Taste and aftertaste attract attention and make you refrain from drinking |
|
|
Very strong |
The taste and aftertaste are so strong that they make the water unfit for consumption. |
Smell
Odor is an indicator of water quality, determined by the organoleptic method using the sense of smell based on the odor strength scale. The smell of water is influenced by the composition of dissolved substances, temperature, pH values and a number of other factors. The intensity of the odor of water is determined expertly at 20 °C and 60 °C and measured in points according to the requirements.The odor group should also be indicated according to the following classification:
By nature, odors are divided into two groups:
- natural origin (organisms living and dying in water, decaying plant debris, etc.)
- artificial origin (impurities of industrial and agricultural wastewater).
Natural scents
|
Odor designation |
Character of the smell |
Approximate type of smell |
|
Aromatic |
Cucumber, floral |
|
|
Bolotny |
Muddy, muddy |
|
|
Putrefactive |
Fecal, waste |
|
|
Woody |
Smell of wet wood chips, woody bark |
|
|
Earthy |
Rotten, smell of freshly plowed earth, clayey |
|
|
moldy |
Musty, stagnant |
|
|
Fish oil smell, fishy |
||
|
Hydrogen sulfide |
Rotten egg smell |
|
|
Grassy |
The smell of cut grass and hay |
|
|
Uncertain |
Odors of natural origin that do not fall under the previous definitions |
The intensity of the odor according to GOST 3351-74* is assessed on a six-point scale - see the next page.
Characteristics of water by odor intensity
|
Odor intensity |
Character of the odor |
Intensity rating, point |
|
The smell is not felt |
||
|
Very weak |
The smell is not perceived by the consumer, but is detected during laboratory testing |
|
|
The smell is noticed by the consumer if you draw his attention to it |
||
|
Noticeable |
The smell is easily noticed and causes disapproval of the water |
|
|
Distinct |
The smell attracts attention and makes you refrain from drinking |
|
|
Very strong |
The smell is so strong that it makes the water unfit for drinking. |
Hydrogen value (pH)
Hydrogen index (pH) - characterizes the concentration of free hydrogen ions in water and expresses the degree of acidity or alkalinity of water (the ratio of H+ and OH- ions in water formed during the dissociation of water) and is quantitatively determined by the concentration of hydrogen ions pH = - IgIf the water has a reduced content of free hydrogen ions (pH>7) compared to OH- ions, then the water will have an alkaline reaction, and with an increased content of H+ ions (pH<7)- кислую. В идеально чистой дистиллированной воде эти ионы будут уравновешивать друг друга. В таких случаях вода нейтральна и рН=7. При растворении в воде различных химических веществ этот баланс может быть нарушен, что приводит к изменению уровня рН.
Determination of pH is carried out using a colorimetric or electrometric method. Water with a low pH reaction is corrosive, while water with a high pH reaction tends to foam.
Depending on the pH level, water can be divided into several groups:
Characteristics of water by pH
Control over the pH level is especially important at all stages of water treatment, since its “change” in one direction or another can not only significantly affect the smell, taste and appearance of water, but also affect the effectiveness of water treatment measures. The optimal pH value required varies for different water treatment systems according to the composition of the water, the nature of the materials used in the distribution system, and the water treatment methods used.
Typically, the pH level is within the range at which it does not directly affect the consumer quality of water. Thus, in river waters the pH is usually in the range of 6.5-8.5, in precipitation 4.6-6.1, in swamps 5.5-6.0, in sea waters 7.9-8.3. Therefore, WHO does not propose any medically recommended value for pH. At the same time, it is known that at low pH water is highly corrosive, and at high levels (pH>11) water acquires a characteristic soapiness, an unpleasant odor, and can cause irritation to the eyes and skin. That is why the optimal pH level for drinking and domestic water is considered to be in the range from 6 to 9.
Acidity
Acidity is the content of substances in water that can react with hydroxide ions (OH-). The acidity of water is determined by the equivalent amount of hydroxide required for the reaction.In ordinary natural waters, acidity in most cases depends only on the content of free carbon dioxide. The natural part of acidity is also created by humic and other weak organic acids and cations of weak bases (ammonium ions, iron, aluminum, organic bases). In these cases, the water pH does not fall below 4.5.
Polluted water bodies may contain large amounts of strong acids or their salts due to the discharge of industrial wastewater. In these cases the pH may be below 4.5. Part of the total acidity that reduces pH to values< 4.5, называется свободной.
Rigidity
General (total) hardness is a property caused by the presence of substances dissolved in water, mainly calcium salts (Ca2+) and magnesium (Mg2+), as well as other cations that appear in much smaller quantities, such as ions: iron, aluminum, manganese (Mn2+) and heavy metals (strontium Sr2+, barium Ba2+).But the total content of calcium and magnesium ions in natural waters is incomparably greater than the content of all other listed ions - and even their sum. Therefore, hardness is understood as the sum of the amounts of calcium and magnesium ions - the total hardness, which consists of the values of carbonate (temporary, eliminated by boiling) and non-carbonate (permanent) hardness. The first is caused by the presence of calcium and magnesium bicarbonates in water, the second by the presence of sulfates, chlorides, silicates, nitrates and phosphates of these metals.
In Russia, water hardness is expressed in mEq/dm3 or mol/l.
Carbonate hardness (temporary) – caused by the presence of calcium and magnesium bicarbonates, carbonates and hydrocarbons dissolved in water. During heating, calcium and magnesium bicarbonates partially precipitate in solution as a result of reversible hydrolysis reactions.
Non-carbonate hardness (constant) - caused by the presence of calcium chlorides, sulfates and silicates dissolved in water (they do not dissolve and do not settle in the solution when the water is heated).
Characteristics of water by total hardness value
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Water group |
Unit of measurement, mmol/l |
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Very soft |
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Medium hardness |
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Very tough |
Alkalinity
Water alkalinity is the total concentration of weak acid anions and hydroxyl ions contained in water (expressed in mmol/l), which react during laboratory tests with hydrochloric or sulfuric acids to form chloride or sulfuric acid salts of alkali and alkaline earth metals.The following forms of water alkalinity are distinguished: bicarbonate (hydrocarbonate), carbonate, hydrate, phosphate, silicate, humate - depending on the anions of weak acids that determine the alkalinity. Alkalinity of natural waters, the pH of which is usually< 8,35, зависит от присутствия в воде бикарбонатов, карбонатов, иногда и гуматов. Щелочность других форм появляется в процессах обработки воды. Так как в природных водах почти всегда щелочность определяется бикарбонатами, то для таких вод общую щелочность принимают равной карбонатной жесткости.
Iron, manganese
Iron, manganese - in natural water appear mainly in the form of hydrocarbons, sulfates, chlorides, humus compounds and sometimes phosphates. The presence of iron and manganese ions is very harmful to most technological processes, especially in the pulp and textile industries, and also worsens the organoleptic properties of water.In addition, the content of iron and manganese in water can cause the development of manganese bacteria and iron bacteria, colonies of which can cause clogging of water supply networks.
Chlorides
Chlorides – The presence of chlorides in water can be caused by the leaching of chloride deposits, or they can appear in the water due to the presence of effluent. Most often, chlorides in surface waters appear in the form of NaCl, CaCl2 and MgCl2, and always in the form of dissolved compounds.Nitrogen compounds
Nitrogen compounds (ammonia, nitrites, nitrates) arise mainly from protein compounds that enter the water along with wastewater. Ammonia present in water can be organic or inorganic. In the case of organic origin, increased oxidation is observed.Nitrites arise mainly due to the oxidation of ammonia in water; they can also penetrate into it along with rainwater due to the reduction of nitrates in the soil.
Nitrates are a product of the biochemical oxidation of ammonia and nitrites, or they can be leached from the soil.
Hydrogen sulfide
O at pH< 5 имеет вид H2S;
O at pH > 7 appears as the HS- ion;
O at pH = 5:7 can be in the form of both H2S and HS-.
Water. They enter the water due to the leaching of sedimentary rocks, leaching of soil and sometimes due to the oxidation of sulfides and sulfur - protein breakdown products from wastewater. A high content of sulfates in water can cause diseases of the digestive tract, and such water can also cause corrosion of concrete and reinforced concrete structures.
Carbon dioxide
Hydrogen sulfide gives water an unpleasant odor, leads to the development of sulfur bacteria and causes corrosion. Hydrogen sulfide, predominantly present in groundwater, can be of mineral, organic or biological origin, and in the form of dissolved gas or sulfides. The form under which hydrogen sulfide appears depends on the pH reaction:
- at pH< 5 имеет вид H2S;
- at pH > 7 it appears as an HS- ion;
- at pH = 5: 7 can be in the form of both H2S and HS-.
Sulfates
Sulfates (SO42-) – along with chlorides, are the most common types of contaminants in water. They enter the water due to the leaching of sedimentary rocks, leaching of soil and sometimes due to the oxidation of sulfides and sulfur - protein breakdown products from wastewater. A high content of sulfates in water can cause diseases of the digestive tract, and such water can also cause corrosion of concrete and reinforced concrete structures.Carbon dioxide
Carbon dioxide (CO2) – depending on the reaction, the pH of water can be in the following forms:- pH< 4,0 – в основном, как газ CO2;
- pH = 8.4 – mainly in the form of bicarbonate ion HCO3-;
- pH > 10.5 – mainly in the form of carbonate ion CO32-.
Dissolved oxygen
Oxygen enters a body of water by dissolving it upon contact with air (absorption), as well as as a result of photosynthesis by aquatic plants. The content of dissolved oxygen depends on temperature, atmospheric pressure, the degree of water turbulization, water salinity, etc. In surface waters, the content of dissolved oxygen can range from 0 to 14 mg/l. There is practically no oxygen in artesian water.The relative content of oxygen in water, expressed as a percentage of its normal content, is called the degree of oxygen saturation. This parameter depends on water temperature, atmospheric pressure and salinity level. Calculated using the formula: M = (ax0.1308x100)/NxP, where
M – degree of water saturation with oxygen, %;
A – oxygen concentration, mg/dm3;
P – atmospheric pressure in a given area, MPa.
N is the normal oxygen concentration at a given temperature and total pressure of 0.101308 MPa, given in the following table:
Oxygen solubility depending on water temperature
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Water temperature, °C |
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Oxidability
Oxidability is an indicator characterizing the content of organic and mineral substances in water that are oxidized by a strong oxidizing agent. Oxidability is expressed in mgO2 required for the oxidation of these substances contained in 1 dm3 of the tested water.There are several types of water oxidation: permanganate (1 mg KMnO4 corresponds to 0.25 mg O2), dichromate, iodate, cerium. The highest degree of oxidation is achieved by dichromate and iodate methods. In water treatment practice, permanganate oxidation is determined for natural, slightly polluted waters, and in more polluted waters, as a rule, dichromate oxidation (also called COD - chemical oxygen demand). Oxidability is a very convenient complex parameter that allows one to assess the overall contamination of water with organic substances. Organic substances found in water are very diverse in nature and chemical properties. Their composition is formed both under the influence of biochemical processes occurring in the reservoir, and due to the influx of surface and groundwater, atmospheric precipitation, industrial and domestic wastewater. The amount of oxidizability of natural waters can vary widely from fractions of milligrams to tens of milligrams of O2 per liter of water.
Surface waters have a higher oxidizability, which means they contain high concentrations of organic substances compared to underground waters. Thus, mountain rivers and lakes are characterized by oxidability of 2-3 mg O2/dm3, lowland rivers - 5-12 mg O2/dm3, rivers fed by swamps - tens of milligrams per 1 dm3.
Groundwater has an average oxidizability at a level of from hundredths to tenths of a milligram of O2/dm3 (exceptions include water in areas of oil and gas fields, peat bogs, heavily swampy areas, and groundwater in the northern part of the Russian Federation).
Electrical conductivity
Electrical conductivity is a numerical expression of the ability of an aqueous solution to conduct electric current. The electrical conductivity of natural water depends mainly on the degree of mineralization (concentration of dissolved mineral salts) and temperature. Thanks to this dependence, the value of electrical conductivity can be used to judge the mineralization of water with a certain degree of error. This measurement principle is used, in particular, in fairly common instruments for operational measurement of total salt content (so-called TDS meters).The fact is that natural waters are solutions of mixtures of strong and weak electrolytes. The mineral part of the water consists mainly of sodium (Na+), potassium (K+), calcium (Ca2+), chlorine (Cl–), sulfate (SO42–), and hydrogen carbonate (HCO3–) ions.
These ions mainly determine the electrical conductivity of natural waters. The presence of other ions, for example, ferric and divalent iron (Fe3+ and Fe2+), manganese (Mn2+), aluminum (Al3+), nitrate (NO3–), HPO4–, H2PO4–, etc. does not have such a strong effect on electrical conductivity (provided, of course, that these ions are not contained in the water in significant quantities, as, for example, this can be in industrial or domestic wastewater). Measurement errors arise due to the unequal specific electrical conductivity of solutions of various salts, as well as due to an increase in electrical conductivity with increasing temperature. However, the modern level of technology makes it possible to minimize these errors, thanks to pre-calculated and stored dependencies.
Electrical conductivity is not standardized, but a value of 2000 µS/cm approximately corresponds to a total mineralization of 1000 mg/l.
Redox potential (redox potential, Eh)
The oxidation-reduction potential (a measure of chemical activity) Eh, together with pH, temperature and salt content in water, characterizes the state of stability of water. In particular, this potential must be taken into account when determining the stability of iron in water. Eh in natural waters varies mainly from -0.5 to +0.7 V, but in some deep zones of the Earth's crust it can reach values of minus 0.6 V (hydrogen sulfide hot waters) and +1.2 V (superheated waters of modern volcanism ).Groundwater is classified:
- Eh > +(0.1–1.15) V – oxidizing environment; water contains dissolved oxygen, Fe3+, Cu2+, Pb2+, Mo2+, etc.
- Eh – 0.0 to +0.1 V – transitional redox environment, characterized by an unstable geochemical regime and variable content of oxygen and hydrogen sulfide, as well as weak oxidation and weak reduction of various metals;
- Eh< 0,0 – восстановительная среда; в воде присутствуют сероводород и металлы Fe2+, Mn2+, Mo2+ и др.
The essence of the method. The method for determining the total alkalinity of water is based on the principle of the formation of neutral salts during the interaction of acid with hydrates, bicarbonates and carbonates of alkali and alkali metals, as well as the property of various indicators to change their color depending on the pH value.
Taking these properties into account, the water sample under study is titrated with a solution of hydrochloric or sulfuric acid of the required concentration in the presence of the indicators phenolphthalein and methyl orange.
Reagents used:
decinormal (0.1 N) solution of hydrochloric or sulfuric acid;
1% alcohol solution of phenolphthalein for determining hydrate and carbonate alkalinity;
A 0.1% solution of methyl orange, which serves as an indicator in determining carbonate and bicarbonate alkalinity.
Water sample preparation. When titrating water, the acid interacts with both alkalis and substances that may be suspended in the water and that do not determine the alkalinity of the water. To reduce the consumption of acid for unnecessary reactions and ensure the correct determination of alkalinity, the analyzed sample is cooled to 20 ° C, if it was hot, and passed through a paper filter.
Analysis procedure. To 100 ml of a water sample prepared for titration, add 2-3 drops of phenolphthalein.
When staining, the sample is titrated with a solution of hydrochloric or sulfuric acid of the appropriate normality (0.1 N or 0.01 N) until the color disappears. Titration is carried out slowly, thoroughly mixing the water sample.
Quantity 0.1 n. or 0.01 n. solution of hydrochloric or sulfuric acid used for titration with phenolphthalein is recorded with the mark “ff”. If coloring does not occur during the addition of phenolphthalein, it means that there is no hydrate and carbonate alkalinity in the water. In this case, there is no need to titrate water samples with an acid solution, since there is no alkalinity for phenolphthalein.
After this, 2-3 drops of methyl orange are added to the same sample and titrated with 0.1 N. or 0.01 n. acid solution until the color of the sample changes from yellow to orange. The amount of acid solution used for titration with methyl orange is recorded with the mark “MO”.
To calculate the total alkalinity of water, take the total consumption of acid used for titration with phenolphthalein and methyl orange.
Calculation of analysis results. The calculation of the analysis results is based on the fact that every 1 ml of a normal solution of hydrochloric or sulfuric acid is titrated with 1 mEq of alkalinity. Accordingly, 1 ml of decinornal (0.1 N) solution of hydrochloric acid titrates to 0.1 mg×eq. alkalinity, and 1 ml of centinormal (0.01 N.) solution titrates to 0.01 mEq alkalinity.
Therefore, the total alkalinity of water
where A is the total alkalinity of water, mEq/kg;
1000 - recalculation of analysis results per 1 liter of water;
K is the normality coefficient of the acid solution;
B - total acid consumption for titration, ml;
100 - volume of water sample taken for analysis, ml.
When titrating 100 ml of water sample with a decinormal acid solution (0.1 N), the formula is simplified:
Sh = B, mg×eq/kg.
When using saitinormal acid solution (0.01 N):
Sh = 0.1 B, mg×eq/kg.
For condensate water, alkalinity is usually expressed in microgram equivalents per liter (µg×eq/kg). In this case
Ш =Б 0.01 × 1000 × 1000/100
or Sh=100 B µg×eq/kg.
COMPILATION OF A REPORT
To complete the report, you must fill out the table. 3.
Table 3
Calculation results
CONTROL QUESTIONS
1. What is the reason for and in what units is carbonate hardness measured?
2. What is the reason for and in what units is non-carbonate hardness measured?
3. What is overall hardness?
4. How to determine the water hardness class?
5. Why is carbonate hardness removed by boiling? Write what reactions occur in this case.
6. How is water hardness eliminated in industrial conditions?
7. How is carbonate hardness determined?
8. How is non-carbonate hardness determined?
9. How is overall hardness determined?
10. What is the oxidizability of water and what causes it, in what units is it measured?
11. How is the oxidizability of water determined?
12. What is the total alkalinity of water, in what units is it measured?
13. How is water alkalinity determined?
14. What is dry residue, in what units is it measured and how is it determined?
In the tables of SanPiN of the Russian Federation (“Drinking water”), the maximum permissible concentration for alkalinity indicators is not indicated, therefore most sources, when determining the norm of water alkalinity, refer to WHO standards, an EU directive or sanitary rules of countries with a similar regulatory system.
Thus, the value of 30 mg HCO3-/l is established in the EU directive when determining the quality of water intended for human consumption. In the Ukrainian current rules of the State Sanitary and Norms Regulations for tap water, the parameter is not established, but the value is< 6,5 ммоль/м 3 указывается только для фасованной и бюветной воды. Приведённые в российских тематических источниках значения чаще всего варьируются в пределах от 0,5 до тех же 6,5 ммоль/м 3 .
At the same time, there is GOST 31957-2012 - Interstate standard, signed by standardization bodies of 6 countries and modified in relation to other international standards. Russia, along with Armenia, Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan, is among the countries that have signed a document that describes methods for determining alkalinity in a concentration of 0.1-100 mmol/dm 3.
Definition and content of the concept
The alkalinity of water (“U” in formulas) is the sum of the substances it contains – hydroxyl ions/anions of weak acids – that react with strong acids, divided into:
- bicarbonate (Shb),
- carbonate (Sch k),
- hydrate (Shg),.
The unit of measurement is the milligram equivalent of acid, written as mEq/L. Total alkalinity as the sum of weak acid anions - silicates, borates, carbonates, hydrocarbonates, sulfides, hydrosulfides, sulfites, hydrosulfites, phosphates, humic acid anions) is the ability to bind strong acids (their equivalent amount). The concentration of some ions is insignificant, therefore, when they talk about total alkalinity, they mainly mean the carbonate type (determined by carbonic acid ions), where hydrolyzed anions form hydroxide ions:
The alkaline index for surface waters is associated with the presence in them mainly of hydrocarbonates of alkaline earth metals (and alkaline metals to a lesser extent), and for natural waters with pH< 8,3 он определяется концентрацией гидрокарбонатов магния и кальция. При определённой обработке водоресурса и при pH >8.5 the appearance of the hydrate type occurs.
The alkaline parameter is required for:
- determination of carbonate content, as well as the balance of carbonic acid (together with pH),
- dosing of chemicals used in water supply,
- reagent cleaning,
- establishing the suitability of a water resource for irrigation (in case of an excess of alkaline earth metals).
The northern regions of Russia with low alkalinity and pH values for natural water are characterized by increased corrosive aggressiveness, which affects pipelines and structures made of ferrous metals and concrete.
According to Japanese researchers, in areas where they drink more alkaline water (above 6.5, but below 9), life expectancy is 20-30% higher. In general, alkaline values should be sufficient to allow chemical coagulation to take place, but they should not be too high so as not to provoke physiological disorders in water consumers. The minimum alkaline values are +/- 30 mg/l, and the maximum are in the range of 450-500 mg/l.
The opinion that has spread among owners of various modified aerators about their influence on the alkaline properties of the hydroflow is not confirmed. These water-saving aerators (http://water-save.com/) allow you to reduce water consumption, but do not affect the chemical characteristics of the water resource.
Methods for determining carbonate concentration
The interstate standard describes 2 titrimetric methods for calculating water alkalinity:
- Free and total alkalinity. For drinking - packaged (non-carbonated) and from sources of drinking water supply - natural, as well as waste water by titration (gradual mixing) to a pH value of 8.3 and 4.5. The obtained values are used to calculate the concentration of carbonates (in the range of 6-6000 mg/dm 3) and bicarbonates (6.1-6100 mg/dm 3).
- Carbonate alkalinity. For drinking, natural, technical water at different stages of technological processes by titration to pH 5.4 units.
The end point of the titration is determined by changing the value on the pH meter or the color of the indicator:
- The pH transition from pink to colorless at 8.3-8.0 gives the value of the parameter “for phenolphthalein”,
- The pH transition from orange to yellow at 4.4 gives the parameter value “by methyl orange”.
The parameter is taken equal to zero if the pH for the analyzed sample is<4,5.
