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.
Water alkalinity is the total content of substances in water that, during dissociation or as a result of hydrolysis, cause an increased concentration of ions HE -.
In source water, alkalinity is usually due to the presence of ions. In softened and boiler water, in addition to the listed substances, alkalinity is also determined by ions. Depending on which anion is present in the water, alkalinity is called, respectively, bicarbonate alkalinity, carbonate alkalinity or hydrate alkalinity.
High alkalinity of water is determined by the amount of hydrochloric acid spent on titration of the analyzed water sample (100 ml of sample) in the presence of the indicator phenolphthalein (stage I) and methyl orange (stage II) 0.1 N. The amount of acid (ml) consumed during titration is equal to the alkalinity of the test water at pH = 3...4.
Low alkalinity of water is determined by titrating 100 ml of sample in the presence of phenolphthalein (stage I) and methylrot or a mixed indicator (stage II) 0.01 N. a solution of sulfuric or hydrochloric acid. The alkalinity value is determined by the formula:
where n is the amount of 0.01 n consumed. acid solution, ml.
Analyzes to determine individual forms of alkalinity are based on the fact that when a water sample is titrated with a strong acid, the reactions occurring between the acid and various anions that determine the form of alkalinity end at different pH values of the solution. Titration is carried out in the presence of two indicators, each of which is designed for a specific pH range. One indicator is methyl orange, the color of which changes to yellow in an acidic environment at pH = 3...4, the second is phenolphthalein, the color of which changes to pink in an alkaline environment at pH > 8.4. It should be especially noted that solutions of pure hydrocarbonates (HCO3), which are constantly present in waters, have a pH value of 8.4. When analyzing a water sample, phenolphthalein is used at the first stage, and methyl orange at the second.
The assessment of individual forms of alkalinity is carried out in accordance with the data obtained during titration. The following cases are possible:
1) phenolphthalein does not give a pink color, i.e. F = 0, where F is the consumption of hydrochloric acid used for titration of the sample stained with phenolphthalein, ml. Methyl orange gives a yellow color to the sample, which is then titrated with hydrochloric acid until the color changes. In this case, only bicarbonates (bicarbonate alkalinity) are present in the water, which is calculated according to formula (1), where A = M, and M is the acid consumption for titrating a water sample colored with methyl orange, mEq/l;
2) phenolphthalein gives a pink color, and when titrating the sample it turned out that 2F< М. В этом случае в воде присутствуют как бикарбонаты, так и карбонаты. Расчет Щб производится по формуле (1), где А = М - 2Ф, для расчета Щк следует принять А = 2Ф;
3) phenolphthalein gives a pink color, and 2Ф = M. In this case, only carbonates are present in the water; to calculate Shk, A = 2F should be substituted into formula (1);
4) phenolphthalein gives a pink color, with 2P>M. In this case, carbonates and hydrates are present in the water. To calculate Shk, A = 2(M-F) should be substituted into formula (1), and to calculate Shg - A = (2F - M);
5) phenolphthalein gives a pink color, and M = 0 (i.e., after phenolphthalein is decolorized, a further increase in the volume of methyl orange immediately causes an orange color in the water sample). In this case, only hydrates are present; to calculate Ag, A = F should be substituted into formula (1). At Kn = 0.1 mg-eq/l and V = 100 ml, it is convenient to use a table to determine the shape and numerical value of alkalinity.
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
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Intensity of taste and aftertaste |
The nature of the appearance of taste and aftertaste |
Intensity rating, point |
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Taste and aftertaste are not felt |
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Very weak |
Taste and aftertaste are not perceived by the consumer, but are detected during laboratory testing. |
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Taste and aftertaste are noticed by the consumer if they pay attention to it |
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Noticeable |
Taste and aftertaste are easily noticed and cause disapproval of water |
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Distinct |
Taste and aftertaste attract attention and make you refrain from drinking |
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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 by experts 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
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Odor designation |
Character of the smell |
Approximate type of smell |
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Aromatic |
Cucumber, floral |
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Bolotny |
Muddy, muddy |
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Putrefactive |
Fecal, waste |
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Woody |
The smell of wet wood chips, woody bark |
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Earthy |
Rotten, smell of freshly plowed earth, clayey |
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moldy |
Musty, stagnant |
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Fish oil smell, fishy |
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Hydrogen sulfide |
Rotten egg smell |
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Grassy |
The smell of cut grass and hay |
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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
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Odor intensity |
Character of the odor |
Intensity rating, point |
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The smell is not felt |
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Very weak |
The smell is not perceived by the consumer, but is detected during laboratory testing |
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The smell is noticed by the consumer if you draw his attention to it |
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Noticeable |
The smell is easily noticed and causes disapproval of the water |
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Distinct |
The smell attracts attention and makes you refrain from drinking |
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Very strong |
The smell is so strong that it makes the water unfit for consumption. |
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 depending on 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 of 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?
When it comes to the quality of drinking water, we first of all pay attention to the absence of harmful impurities, color, smell, etc. But not everyone knows about the importance of such an indicator as water alkalinity. In this article we will try to figure out why the water alkalinity level is so important for our health, how it is determined and how to achieve optimal water alkalinity levels.
Determination of alkalinity: a little theory
First, let’s try to figure out what “water alkalinity” actually is. Reference literature offers the following definition of alkalinity: this is the total number of hydroxyl ions contained in water, as well as anions of weak acids. The alkalinity of water can be hydrate, carbonate, bicarbonate, depending on the presence of certain substances in it. You should also distinguish between the concept of “water alkalinity” and its pH value. It shows the concentration of free hydrogen ions in water. If the pH is low (< 7), то мы говорим о кислой среде, если высокий (>7) – about alkaline. The relationship between pH and alkalinity is directly proportional: the greater the alkalinity of the water, the higher the pH. Alkalinity is measured in mmol/dm3, and pH is simply a number of units.
According to State Sanitary Standards, the optimal pH for drinking water is from 6.5 to 8.5 units. This coincides with the requirements used to control the quality of drinking water in the United States. EU standards in this matter differ insignificantly (from 6.5 to 9.5). The indicator of distilled water is 7 units. This is conditionally neutral water. The pH of drinking water is strictly controlled in all countries.
But water alkalinity standards are not regulated by Ukrainian State Sanitary Standards. Although they are taken into account at water supply enterprises for the correct calculation of reagents that are used for water treatment.
Regarding the calculation of water alkalinity, it is worth noting that the optimal standards differ slightly depending on the needs for which the water is used.
Water alkalinity value
Let's try to figure out why such great importance is attached to water alkalinity standards? It turns out that the condition of our body directly depends on the alkalinity of water. The optimal level of water alkalinity will help to establish the following body functions:
- metabolic processes
- restoration of intestinal microflora
- activation of brain activity by enriching the brain with oxygen
- strengthening the immune system
This is not surprising, since neutral or slightly alkaline liquids predominate in the human body. Interestingly, the pH of human blood is 7.43 (that is, almost neutral).
Therefore, it is very important to determine the alkalinity of drinking water and control these indicators.
Japanese scientists have found that if a person constantly drinks water with an alkalinity of 6.5 - 7, then life expectancy increases by 20 - 30%. The fact is that an acidic environment (low alkalinity of water) creates ideal conditions for the development of various diseases.
To maintain a normal acid-base balance and good health, it is important not to “acidify” the body. But the problem is that most foods are acidic. This means that to maintain balance, it is important to drink water, as it helps the blood to carry more oxygen. But not all water is useful. First of all, you need to pay attention to its alkalinity. It's better if it's close to neutral. State sanitary standards regulate the alkalinity of drinking bottled water and water from pump rooms at the level of 6.5 mmol/dm3. The alkalinity level of water entering centralized water supply systems is regulated at the water treatment stage. Increased acidity can be observed in heavily polluted natural waters (for example, after the discharge of waste from industrial enterprises, when a large amount of strong acids and their salts enter the water).
By the way, water with high alkalinity is also not very healthy. So, after water procedures in such water the following may appear:
- itching on the skin
- rashes
- irritation on mucous membranes
Calculation of water alkalinity and pH level regulation
In different regions, the pH level of tap water ranges from 5.5 to 10 units. If necessary, this indicator can and should be adjusted. To measure pH at home, you can use special testers. But to determine the alkalinity of water, you will need to submit samples to a specialized laboratory. After you receive the results of the study, UkrKhimAnaliz specialists will give the necessary recommendations. If the water has high alkalinity, then a filter can be an effective way to reduce it. Reverse osmosis systems, in particular, cope well with this problem. They help not only reduce the alkalinity of water, but also normalize its composition as a whole, reduce mineralization and hardness.