How to find the elongation of a spring if the mass is known. The concept of water hardness, calculation formula, units of measurement, norms in Russia

That tap water too harsh, you have to hear it all the time. How is the "hardness" of a liquid measured? How is water hardness calculated?

In the photo: a heating element "damaged" by hardness salts

If we minimize scientific reasoning about the totality of the properties of a liquid, then the total amount of dissolved salts of alkaline earth metals (mainly calcium and magnesium) can be considered water hardness. From a chemical point of view, all divalent cations, to one degree or another, are able to bind with anions, forming salts that can precipitate. However, in practice, the amount of strontium, barium and manganese dissolved in water is minimized, and aluminum and iron pass into salt complexes only at a certain level of acidity of the medium (pH less than 7), which practically does not occur in nature. We already wrote about how to determine stiffness at home.

Table 1. Cations and anions responsible for hardness

Hardness cations	Hardness anions
Calcium (Ca2+)	Bicarbonate (HCO 3 -)
Magnesium (Mg2+)	Sulfate (SO 4 2-)
Strontium (Sr 2-)	Chloride (Cl-)
Iron (Fe2+)	Nitrate (No 3 -)
Manganese (Mn 2+)	Silicate (Sio 3 2-)

Water hardness formula

Carbonate hardness - the quantitative content of hydrocarbonates and carbonates of magnesium and calcium (MgHCO3, CaHCO3) in water. This type of pollutant is easily eliminated by boiling with the formation of carbonic acid and sediment:

Ca 2+ + 2HCO 3 - (when heated) = CaCO 3 ↓ + H 2 O + CO 2

Non-carbonate (permanent) hardness is due to the presence of magnesium and calcium compounds of strong acids (nitric, hydrochloric, sulfuric). Salts of this type do not decompose when boiled.

The formula for calculating the total hardness of water: Htotal \u003d H carb + H non-carb

Units

The units of measurement in Russia are degrees (°F), which can be expressed in volumetric or mass fraction. 1 degree of hardness is numerically equal to 0.5 molar concentration of the alkaline earth element, expressed in mg / cu. dm. (1 ° W = 1 mg-eq / l.)

Units of measurement in the SI system - mol / cubic meter. However, in practice, the most commonly used mg-eq./l. The concentration per unit mass justifies itself in those cases when it is necessary to analyze water in a different state of aggregation (with a changed density).

For those to whom chemistry was not easy in class, you should repeat:

One meq/l corresponds to 20.04 milligrams of ionsCa 2+ or 12.16 milligrams of ionsMg 2+ (ratio atomic mass and valency of the element).

Stiffness standards

According to the hardness value, water can be divided into three categories:

Soft (up to 2 °F),

Medium hardness (2-10 °F),

Very hard (more than 10 °F).

Rigidity standards in Russia do not allow exceeding 7 mg-eq / l. According to EU standards, the MPC of total water hardness cannot be more than 1.2 mg-eq / l. By simple calculations, we can conclude that in Europe the water is almost 6 times softer than in Russia.

Thus, the numerically confirmed need to install purification systems in Russia is no longer in doubt. In addition, according to the standards adopted throughout the world, the water used by our compatriots needs multi-stage softening and deep filtration.

Now let's figure out how to eliminate the excessive salt content in the water:

Used Books:

1. GOST 2874-82
2. GOST R 52029-2003 | NATIONAL STANDARDS
3. Chemical encyclopedia. - M.: Soviet Encyclopedia, 1990. T. 2. S. 145.

14.1. Hardness of water

Water hardness is a combination of chemical and physical properties water associated with the content of dissolved metal salts in it, mainly calcium Ca 2+ and magnesium Mg 2+ (the so-called "hardness salts"). There are temporary (carbonate) hardness due to calcium and magnesium bicarbonates (Ca (HCO 3) 2; Mg (HCO 3) 2), and permanent (non-carbonate) hardness caused by the presence of other salts that are not released during boiling water: mainly sulfates and Ca and Mg chlorides (CaSO 4, CaCl 2, MgSO 4, MgCl 2). The total hardness of water is equal to the sum of temporary (carbonate) and permanent (non-carbonate) hardness.

In the SI system, hardness is measured in mol / m 3. In practice, degrees of hardness are used, hardness is expressed in milligram equivalents per liter, as well as in millimoles equivalents per liter. 1°W corresponds to the concentration of the alkaline earth element, numerically equal to 1/2 of its mole per liter (1°W = 1 mg-eq/l = 1/2 mmol-eq/l). According to the magnitude of the total hardness, soft water (up to 2°F), medium hardness (2-10°F) and hard water (more than 10°F) are distinguished.

In this manual, 1 meq / l is taken as the dimension of water hardness (often the equivalent is skipped and water hardness is expressed in mmol / l), which is expressed as the sum of millimoles (mmol) equivalents of Ca 2+ and Mg 2+ ions contained in 1 liter of water. One mmol of hardness corresponds to 20.04 mg/l Ca 2+ and 12.16 mg/l Mg 2+.

The hardness of water can be calculated using the formula given in general view:

where W is water hardness, mmol-eq/l; m i is the mass of cations (or corresponding salts), mg; M ec i is the molar mass of equivalents of cations (or their corresponding salts), mg/mmol; V- volume of water, l.

1. Determination of water hardness by the mass of salts contained in it.

Example 1 Calculate the total hardness of water (mmol / l), if 0.20 l of water contains 32.42 mg of calcium bicarbonate Ca (HCO 3) 2; 1.46 mg magnesium bicarbonate Mg(HCO 3) 2 ; 22.20 mg calcium chloride CaCl 2 and 4.75 mg magnesium chloride MgCl 2 .

Solution. The total hardness of water can be calculated using the formula (1)

The masses of salts and the volume of water are known from the condition of the problem. Find the molar masses of salt equivalents. The molar mass of salt equivalents is equal to the molar mass of salt divided by the equivalent number Z. For all salts Z equals 2.

M eq Ca(HCO 3) 2 = M Ca (HCO 3) 2 / 2 \u003d 162.11 / 2 - 81.05 mg / mmol;

M eq Mg(HCO 3) 2 = M Mg (HCO 3) 2 / 2 \u003d 146.34 / 2 \u003d 73.17 mg / mmol;

M eq CaCl 2 = M CaCl 2/2 = 110.99/2 = 55.49 mg/mmol;

M ec MgCl 2 = M MgCl 2/2 - 95.21/2 = 47.60 mg/mmol.

Substituting in the mass formula, the molar masses of salt equivalents and the volume of water, we calculate the total hardness of water:

2.0 + 0.1 + 2.0 + 0.5 = 4.6 mmol/L.

Example 2 Calculate the temporary hardness of water, knowing that 500 liters of it contains 162.1 g of Ca (HCO 3) 2.

Solution. The temporary hardness of water can be found by formula (1), substituting into it the mass, the molar mass of Ca (HCO 3) 2 equivalents and the volume of water. The molar mass of Ca(HCO3) 2 equivalents is 81.05 mg/mmol (see example 1), the mass of this salt is 162100 mg, the volume of water is 500 liters. Consequently,

2. Determination of salt content in water by water hardness

Example 3. How many grams of CaCl 2 are contained in 100 liters of water if the water hardness caused by this salt is 2.5 mmol / l?

Solution. The mass of CaCl 2 salt can be found by formula (1). Namely:

Substituting the water hardness, the molar mass of CaCl 2 equivalents (see example 1) and the volume of water into the formula, we get:

M CaCl 2 \u003d 2.5 55.49 100 \u003d 13872.5 mg or 13.8725 g.

3.Determination of the temporary (carbonate) hardness of water by the volume of hydrochloric acid used for its titration

by the most simple method determination of water hardness is the titration method. To determine the temporary hardness of water, a standard solution of hydrochloric acid is used. In this case, the following reaction occurs:

Me (HCO 3) 2 + 2 HC1 \u003d MeC1 2 + 2 H 2 O + 2CO 2,

where Me - Ca 2+, Mg 2+, Fe 2+.

According to the law of equivalents, the number of equivalents of all substances involved in a chemical reaction must be the same. From here:

where C ek1 and V 1 are, respectively, the molar concentration of substance equivalents (mol / l) and the volume (l) of the first solution; C ek2 and V 2 - molar concentration of substance equivalents (mol / l) and volume (l) of the second solution.

Example 4. Determine the temporary hardness of water if 1.44 10-3 liters of 1.15 N HCl solution are used to titrate 5 10-2 l of water containing calcium bicarbonate.

Solution. According to the condition of the problem, the molar concentration of Ca(HCO 3) 2 equivalents is unknown. Let's denote it by x. Substituting the values ​​into formula (2), we get:

x 5 10 -2 = 1.44 10 -3 0.15.

Thus, the molar concentration of Ca(HCO 3) 2 equivalents is 0.004n, which corresponds to a Ca(HCO 3) 2 content of 0.004 mol/l or 4 mmol/l. Therefore, the temporary hardness of water is 4 mmol/l.

Example 5. What is the temporary hardness of water if 5.75 ml of a 0.07 N HCl solution is used to titrate 100 ml of this water containing iron (II) bicarbonate.

Solution. This problem is solved in the same way as shown in example 5, after converting the volumes of solutions into liters, i.e. V 1 \u003d 0.1 l H 2 O; V 2 \u003d 5.75 10 -3 l HCl. Substituting the values ​​into formula (2), we obtain:

x 0.1 \u003d 5.75 10 -3 0.07.

n.

The molar concentration of equivalents of Fe(HCO 3) 2 corresponds to the content of this salt, equal to 4 · 10 –3 mol/l or 4 mmol/l.

Therefore, the hardness of water is 4 mmol/l.

14.2. Water softening methods

To soften water, methods of precipitation and ion exchange are used. Ca 2+ and Mg 2+ cations are converted by precipitation into sparingly soluble precipitated compounds. This can be achieved by boiling or chemically - by introducing into water, for example, Na 2 CO 3 soda, Ca (OH) 2 slaked lime, etc. When boiling, only temporary (carbonate) hardness is eliminated by the reaction:

Ca(HCO 3) 2 CaCO 3 ¯ + CO 2 + H 2 O.

Mg (HCO 3) 2 Mg (OH) 2 ¯ + CO 2.

During the decomposition of Mg (HCO 3) 2, Mg (OH) 2 is formed, and not MgCO 3, since

The chemical softening method is based on adding soda ash Na 2 CO 3 or slaked lime Ca (OH) 2 to the water. In this case, calcium and magnesium salts pass into insoluble compounds and, as a result, precipitate. For example, the addition of slaked lime results in the conversion of calcium salts into insoluble carbonate:

Ca(HCO 3) 2 + Ca(OH) 2 → 2CaCO 3 ↓ + 2H 2 O

The best reagent to eliminate the general hardness of water is sodium orthophosphate Na 3 PO 4:

3Ca(HCO 3) 2 + 2Na 3 PO 4 → Ca 3 (PO 4) 2 ↓ + 6NaHCO 3

3MgSO 4 + 2Na 3 PO 4 → Mg 3 (PO 4) 2 ↓ + 3Na 2 SO 4

Orthophosphates of calcium and magnesium are very poorly soluble in water, so they are easily separated by mechanical filtration.

The ion exchange method is based on the use of an ion-exchange granular resin that absorbs cations of hardness salts (calcium and magnesium, iron and manganese). In return, depending on the ionic form, the resin gives off sodium or hydrogen ions. These methods are respectively called Na-cationization and H-cationization.

4.Determination of the mass of the reagent required to eliminate water hardness.

Example 6 How many grams of soda Na 2 CO 3 must be added to 150 liters of water to eliminate the hardness of 5 mmol / l?

Solution. 150 liters of water contains 150 5 = 750 mmol / l of salts that cause water hardness. According to the law of equivalents, to eliminate this hardness, it is necessary to add the same amount of water softening equivalents, i.e. 750 mmol Na 2 CO 3 .

To find a substance in grams, the amount of substance equivalents is multiplied by the molar mass of the equivalents of that substance. In this case, the Na 2 CO 3 equivalent number is two, and the molar mass of the Na 2 CO 3 equivalents will be

M eq Na 2 CO 3 \u003d M Na 2 CO 3 / 2 \u003d 106/2 \u003d 53 mg / mmol.

Find the mass of Na 2 CO 3:

m Na 2 CO 3 \u003d n eq Na 2 CO 3 M eq Na 2 CO 3 \u003d 750 53 \u003d 39750 mg or 39.75 g.

The same problem can be solved using formula (1). To do this, it must be remembered that, according to the law of equivalents, the masses of reactants are proportional to their molar masses of equivalents. Therefore, in formula (1), instead of the molar mass of the equivalents of salts that cause hardness, you can substitute the molar mass of the equivalents of the substances necessary to eliminate this rigidity in order to find their mass. In this case:

m Na 2 CO 3 \u003d F M eq Na 2 CO 3 V.

Substituting the values ​​of F, M eq Na 2 CO 3 and V, we get:

m Na 2 CO 3 \u003d 5 53 150 \u003d 39750 mg or 39.75 g.

CONTROL TASKS

261. What is meant by hardness of water? How is water hardness measured? Water with a volume of 1 liter contains 38.0 mg of calcium bicarbonate and 19.6 mg of magnesium sulfate. What is the total hardness of water? What hardness group does this water belong to?

Answer: 0.8 mmol / l.

262. Why is it necessary to eliminate excess water hardness? Why is this water dangerous? Give examples. What is the hardness of water containing 0.01 mol/l of calcium chloride? What hardness group does this water belong to?

Answer: 20 mmol / l.

263. What causes constant hardness of water? What are chemical methods elimination of permanent hardness of water? Write the equations for the corresponding reactions. What is the constant hardness of water if 2.5 liters of it contains 40 mg of calcium sulfate? What hardness group does this water belong to?

Answer: 0.23 mmol / l.

264. What makes up the general hardness of water? Explain the answer. What is the total hardness of water if 3.4 liters of it contains 57 mg of magnesium bicarbonate and 33 mg of calcium sulfate? What hardness group does this water belong to?

Answer: 0.37 mmol / l.

265. What causes temporary hardness of water? What methods can be used to eliminate excessive temporary hardness of water? Write the necessary equations chemical reactions. When boiling 0.25 liters of water containing only calcium bicarbonate, 4 mg of calcium carbonate precipitates. What is the temporary hardness of water? What hardness group does this water belong to?

Answer: 0.32 mmol / l.

266. What causes constant hardness of water? What are the chemical ways to eliminate permanent hardness of water? Write the equations for the corresponding reactions. How many grams of magnesium sulfate are contained in 150 liters of water if the water hardness caused by this salt is 4.7 mmol / l? What hardness group does this water belong to?

Answer: 42.441

267. What causes the temporary hardness of water? What methods can be used to eliminate excessive temporary hardness of water? Write the necessary equations for chemical reactions. When boiling 0.5 l of water containing only iron (II) bicarbonate. 8 mg of iron carbonate (II) precipitates. What is the temporary hardness of water? What hardness group does this water belong to?

Answer: 0.28 mmol / l.

268. What is titration? What law is the titration method based on? Give the definition of this law. For titration of 25 mg of water, 2.4 ml of a 0.1 N solution of HCl was used. What is the carbonate hardness of water? What hardness group does this water belong to? What hardness group does this water belong to?

Answer: 9.6 mmol / l.

269. What makes up the total hardness of water? Explain the answer. What methods can be used to eliminate excess total hardness of water? Write the corresponding reaction equations. It took 12.72 g of sodium carbonate to soften 200 liters of water. What is the hardness of water? What hardness group does this water belong to?

Answer: 1.2 mmol / l.

270. What causes temporary hardness of water? What methods can be used to eliminate excessive temporary hardness of water? Write the necessary equations for chemical reactions. The temporary hardness of water is 6.64 mmol / l. What mass of calcium hydroxide must be taken to remove the hardness of 10 liters of water.

Answer: 2.459g.

271. What chemical methods exist to eliminate excess water hardness? Write the corresponding equations for the reactions that take place during water softening. How many grams of sodium orthophosphate must be added to 250 liters of water to eliminate its carbonate hardness of 2.5 mmol / l? What hardness group does this water belong to?

Answer: 34.162

272. What is titration? What law is the titration method based on? Give the definition of this law. For titration of 40 ml of water, 5.7 ml of 0.12 n. HC1 solution. What is the carbonate hardness of water? What hardness group does this water belong to?

Answer: 20 mmol / l.

273. What causes constant hardness of water? What are the chemical ways to eliminate permanent hardness of water? Write the equations for the corresponding reactions. 500 liters of water contains 70 g of calcium sulfate. What is the permanent hardness of this water? What hardness group does this water belong to?

Answer: 2.1 mmol / l.

274. What makes up the total hardness of water? Explain the answer. What methods can be used to eliminate excess total hardness of water? Write the corresponding reaction equations. What is the total hardness of water if 300 liters of it contains 32.85 g of calcium bicarbonate and 30.6 g of magnesium sulfate? What hardness group does this water belong to?

Answer: 3.1 mmol / l.

275. What causes the temporary hardness of water? What methods can be used to eliminate excessive temporary hardness of water? Write the necessary equations for chemical reactions. Water containing only calcium bicarbonate has a hardness of 4.2 mmol/l. What hardness group does this water belong to? How many grams of calcium bicarbonate are there in 250 liters of water?

Answer: 85.102

276. Why is it necessary to eliminate excess water hardness? Why is this water dangerous? Give examples. To 280 liters of hard water was added 62.5 g of sodium carbonate. Write the equations of possible reactions in this case. Calculate by how much mmol/l the water hardness has decreased?

Answer: 4 mmol / l.

277. What causes the temporary hardness of water? What methods can be used to eliminate excessive temporary hardness of water? Write the necessary equations for chemical reactions. Water containing only magnesium bicarbonate has a hardness of 7.8 mmol/l. What hardness group does this water belong to? What mass of magnesium bicarbonate is contained in 350 liters of water?

Answer: 221.266

278. What is meant by hardness of water? How is water hardness measured? What mass of calcium hydroxide must be added to 150 liters of water to eliminate the temporary hardness of water, equal to 2.5 mmol/l? What hardness group does this water belong to?

Answer: 13.89

279. What is titration? What law is the titration method based on? Give the definition of this law. Titration of 40 ml of water required 3.85 ml of 0.15 K HCl solution. What is the carbonate hardness of water? What hardness group does this water belong to?

Answer: 14.4 mmol / l.

280. What causes constant hardness of water? What are the chemical ways to eliminate permanent hardness of water? Write the equations for the corresponding reactions. What is the hardness of water, 10 liters of which contains 0.025 moles of magnesium sulfate? What hardness group does this water belong to?

13. RIGIDITY

Water hardness is a property of natural water, which depends on the presence in it of mainly dissolved salts of calcium and magnesium. The total content of these salts in water in mmol / dm 3 is called the total hardness.

The main sources of calcium and magnesium influx into surface waters are the processes of chemical weathering and dissolution of minerals, primarily limestones and dolomites. Significant amounts of calcium and magnesium ions can enter reservoirs with wastewater from silicate, metallurgical, textile, glass, chemical and other industries.

The ionic form of calcium and magnesium is typical for low-mineralized waters. A significant part of them is in the form of neutral or charged forms () ion pairs, as well as associated in complexes with organic matter water.

The total hardness ranges from units to tens, sometimes hundreds of mmol/dm 3 . Hardness due to calcium ions usually predominates (up to 70%).

The hardness of sea water and ocean water is usually higher, and magnesium hardness often exceeds calcium hardness.

General hardness surface water subject to noticeable seasonal fluctuations, usually reaching its highest value at the end of winter and the lowest during the flood period. Rigidity groundwater more permanent.

High hardness, especially due to magnesium salts, worsens the organoleptic properties of water, giving it a bitter taste, and has an effect on the digestive organs. Depending on the pH and alkalinity of the water, hardness above 10 mmol/dm 3 can cause slag in the water distribution system and scale when heated.

13.1. Determination of total water hardness

complexometric method

Method principle. The method is based on the formation of complex compounds of Trilon B with ions of alkaline earth elements. The determination is carried out by titrating the sample with a solution of Trilon B at pH 10 in the presence of the indicator eriochrome black T.

At pH 10, Trilon B is able to form slightly dissociated complexes with magnesium and calcium ions:

Na 2 H 2 Y + Ca 2+ ↔ Na 2 CaY + 2H +;

Na 2 H 2 Y + Mg 2+ ↔ Na 2 MgY + 2H +.

Eriochrome black indicator T (special black chromogen ET-100), when added to a water sample, forms a red-violet complex compound with magnesium ions. When titrated, Trilon B combines with calcium ions, and then with magnesium ions and displaces the indicator, which in its free form has a blue color. At the equivalence point, the indicator gives a sharp color transition:

HInd 2- + Mg 2+ ↔ MgInd - + H +

red-violet

MgInd - + Na 2 H 2 Y ↔ Na 2 MgY + HInd 2- + H + .

red-violet

Ca 2+ ions do not give such a clear change in the color of the indicator, and therefore Ca 2+ ions cannot be determined separately in the presence of the eriochrome black T indicator.

To obtain reproducible results, it is necessary that the titrated solution has a pH = 10 ± 0.2 and a sufficient amount of magnesium ions.

If the test sample was acidified for preservation or the sample has an acidic environment, then sodium hydroxide solution with a molar concentration of 0.2 mol / dm 3 to pH 6 - 7 is added to the sample aliquot. acids with a molar concentration of 0.1 mol / dm 3 to pH 6 - 7. pH control is carried out using universal indicator paper or using a pH meter.

Elimination of interfering influences. The presence in water of more than 10 mg / dm 3 of iron ions, more than 0.05 mg / dm 3 of each of the ions of copper, cadmium, cobalt, lead, more than 0.1 mg / dm 3 of each of the ions of manganese (II), aluminum, zinc , nickel, tin, as well as color more than 200º and increased turbidity cause a fuzzy color change at the equivalence point during titration, and leads to an overestimation of the hardness determination results. Orthophosphate and carbonate ions can precipitate calcium under titration conditions at pH 10. Interfering effect of Zn 2+ ions up to 200 mg/dm 3 ; Al 3+ , Cd 2+ , Pb 2+ up to 20 mg/dm 3 ; Fe 3+ up to 5 mg / dm 3; Mn 2+ , Co 2+ , Cu 2+ , Ni 2+ up to 1 mg / dm 3 are eliminated by adding sodium sulfide solution with a mass fraction of 0.05 (5%) to the titrated sample. To reduce the effect of manganese to 1 mg/dm 3 , iron, aluminum to 20 mg/dm 3 , copper to 0.3 mg/dm 3 add 5 to 10 drops of hydroxylamine hydrochloride solution. The turbidity of the sample is eliminated by filtration through membrane filters with a pore diameter of 0.45 µm or paper ash-free blue ribbon filters.

If the interfering influence cannot be eliminated, then the determination of the stiffness is carried out by the methods of atomic spectroscopy.

Selecting the sample volume for analysis. Before performing an analysis of a water sample with an unknown hardness value, an estimated titration is carried out. To do this, take 10 cm 3 of water, add 0.5 cm 3 of a buffer solution, an indicator and titrate with a solution of Trilon B with from(1/2 Na 2 H 2 Y) \u003d 0.05 mol / dm 3 until the color changes to blue. According to the volume of Trilon B solution used for titration, it is selected from the table. 13.1 appropriate volume of water sample.

Definition progress. The required sample volume is measured with a pipette into a conical flask with a capacity of 250 cm 3, if necessary, adjusted to 100 cm 3 with distilled water, 5 cm 3 of a buffer solution, from 0.05 to 0.1 g of the indicator dry mixture are added. The sample is thoroughly mixed and titrated with Trilon B solution. from(1/2 Na 2 H 2 Y) \u003d 0.05 mol / dm 3 until the red-violet color changes to blue. The titration is repeated and, if the discrepancy between parallel titrations does not exceed 0.05 cm 3 with a volume of Trilon B solution of 5 cm 3 or less than 0.1 cm 3 with a volume of more than 5 cm 3, the average value of the volume of Trilon B solution is taken as the result. Otherwise If necessary, repeat the titration until an acceptable discrepancy between the results is obtained.

Payment. General water hardness X, mmol / dm 3, calculated by the formula

where from(1/2 Na 2 H 2 Y) is the molar concentration of Trilon B equivalent, mol/dm 3 ;

V(Na 2 H 2 Y) is the volume of Trilon B solution used for sample titration, cm 3;

V

13.2. Determination of the mass concentration of calcium and magnesium ions by the complexometric method. Calculation of water hardness in degrees of hardness (ºF)

The method is based on the ability of calcium ions to form stable complexes with Trilon B in a highly alkaline medium (рН = 12–13). A similar complex of magnesium ions in this medium is destroyed with the release of magnesium hydroxide. When titrated with a solution of Trilon B, the color change of the indicator (murexide) from lilac to red-crimson indicates the complete binding of calcium ions:

H 2 Ind 3- + Ca 2+ ↔ CaH 2 Ind -;

red crimson

CaH 2 Ind - + Na 2 H 2 Y ↔ Na 2 CaY + H 2 Ind 3- + 2H +.

Titration of calcium ions is possible with the joint presence of heavy metal ions in concentrations not exceeding the values: for copper - 0.2 mg / dm 3; zinc, lead, nickel, manganese, iron, aluminum - 1 mg / dm 3 and magnesium - 3 mg in a determined volume. At higher concentrations of heavy metal ions, sodium sulfide is added to the sample. The interfering effect of magnesium ions is eliminated either by reducing the volume of the sample taken for analysis, or at a high magnesium content (Mg:Ca ratio more than 1), by precipitating magnesium ions with a sodium hydroxide solution having a concentration of 2 mol / dm 3 (pH 12 - 13) in a measured flask with a capacity of 100 cm 3. To do this, 20−40 cm 3 samples are diluted with distilled water to 90 cm 3 and NaOH solution is slowly added dropwise, mixed well, while a small amount of calcium ions co-precipitate with Mg(OH) 2 . The volume of the solution is brought to the mark with distilled water, and after settling the precipitate for 1.5–2 hours, a transparent aliquot is taken for titration. To reduce calcium loss, settling time should not exceed 2 hours.

Definition progress. The required sample volume is measured into a conical flask with a capacity of 250 cm 3, adjusted, if necessary, to 100 cm 3 with distilled water, 2 cm 3 of NaOH solution with mass fraction 0.08 (8%), 0.1 - 0.2 g of the murekide indicator and slowly titrated with a solution of Trilon B with vigorous stirring until the color changes from crimson red to purple. Repeat the titration and, if the discrepancy between parallel titrations does not exceed those given in Table. 13.2, the average value of Trilon B is taken as the result. Otherwise, the titration is repeated until an acceptable discrepancy between the results is obtained.

Payment. The mass concentration and the amount of the substance of the equivalent of calcium ions in the analyzed water sample are found by the formulas:

where m(Ca 2+) - mass concentration of calcium ions in water, mg / dm 3;

n(1/2Ca 2+) - the amount of substance equivalent of calcium ions in water, mmol / dm 3;

c(1/2Na 2 H 2 Y) is the molar concentration of Trilon B equivalent, mol/dm 3 ;

V(Na 2 H 2 Y) is the volume of Trilon B solution used for sample titration, cm 3;

M(1/2Ca 2+) is the molar mass of the equivalent of calcium ions, mg/mmol;

V- volume of water sample taken for determination, cm3.

The mass concentration of magnesium ions in mg / dm 3 in the analyzed water sample is determined by the formula

where m(Mg 2+) - mass concentration of magnesium ions in water, mg / dm 3;

X- total water hardness, mmol / dm 3;

n(1/2Ca 2+) - the amount of substance equivalent of calcium ions in water, mmol / dm 3;

M(1/2Mg 2+) is the molar mass of Mg 2+ equivalent, mg/mmol.

Table 13.2

Permissible discrepancies between parallel titrations depending on the volume of Trilon B solution

In accordance with GOST R 52029-2003, water hardness is expressed in degrees of hardness (ºF).

The degree of hardness corresponds to the concentration of the alkaline earth element, numerically equal to 1/2 of its mole, expressed in mg / dm 3 (g / m 3).

Water hardness W, ºW, with separate quantitative determination of ions of alkaline earth elements, is calculated by the formula

where m(Ca 2+) is the mass of calcium in the water sample, mg/dm 3 ;

m(Mg 2+) - the mass of magnesium in the water sample, mg / dm 3;

M(Ca 2+) is the molar mass of calcium, mg/mol;

M(Mg 2+) is the molar mass of magnesium, mg/mol.