The method is based on the property of Trilon B (the disodium salt of ethylenediaminoacetic acid) to give exceptionally stable complex compounds with divalent metal ions, including calcium and magnesium.
For this method, the absorbed bases from the soil must be displaced by 1.0 N. ammonium acetate solution at pH 6.5 or 1 N. NaCl solution. The trilonometric method is more convenient to carry out in conditions of low salt concentrations. Therefore, after displacing the absorbed bases, ammonium acetate is destroyed by evaporating the solution, then the resulting residue is calcined on a mantle heater or in a muffle at 400-600 °, and calcium and magnesium are obtained in the form of carbonates or oxides. The organic matter burns out. The resulting precipitate is dissolved with 10% hydrochloric acid and, after making sure that it is completely dissolved (no crystals are visible at the bottom of the cup), the hydrochloric acid solution is diluted with hot water, filtered into a 200 ml volumetric flask and made up to the mark with water.
Ammonium acetate displaces a small amount of sesquioxides from the soil, so in many cases they do not have to be isolated from solution. High concentrations of iron interfere with titration with Trilon - the color transition loses its clarity; in addition, you can get some overestimated data. In such cases, an additional dilution of the solution with water is recommended to reduce the concentration of iron or the release of iron if there is a lot of it in relation to calcium and magnesium. It is better to do this before preparing the solution for the final evaporation - isolate the sesquioxide in the usual way with ammonia, and then bring the evaporation to the end and ignite the precipitate.
The harmful effect of manganese is destroyed by the addition of hydroxylamine hydrochloride (1-2 ml of a 5% solution), which prevents the formation of manganese peroxide, which interferes with titration. It is also necessary to eliminate the harmful effects of copper. All reagents for this purpose are prepared with distilled, copper-free water. Distilled water must be obtained on an apparatus that does not have copper parts. The harmful effect of traces of copper is eliminated by adding 1-2 ml of 2% Na2S to the test solution, which converts it into insoluble copper sulfide.
The determination is made by titrating the test sample with a solution of Trilon B in the presence of the black chromogen indicator, and calcium ions are first bound to the complex, and then magnesium ions. Magnesium ions cause a particularly sharp change in the color of the indicator, while calcium ions do not give a clear change in the color of the solution, and therefore calcium can only be determined in the presence of magnesium, that is, the sum of calcium and magnesium can be determined.
Determination of the amount of calcium and magnesium. A certain part of the analyzed solution of absorbed bases (it is convenient to take 50 ml) is placed in a 250 ml conical flask, diluted with water to approximately 100 ml. The solution is heated to 60-70°, 5 ml of an ammonia buffer solution is added to create an alkaline reaction, then 0.5 ml of Na2S and 0.5 ml of hydroxylamine, 10-15 mg of the black chromogen indicator (or blue-black chromium) and titrated with 0, 01 - 0.05 n. Trilon B solution with vigorous stirring until the color of the solution changes from cherry red through violet-blue to pure blue at the equivalence point. When an excess of trilon is added, the color does not change. Therefore, it is recommended to carry out titration by comparing the color of the solution with a "witness" - a deliberately overtitrated sample.
The sum of calcium and magnesium (in mEq per 100 g of soil) is:

Determination of calcium by the trilonometric method in the presence of the indicator murexide (ammonium salt of monobasic purple acid). With calcium ions, the anion of purple acid in an alkaline medium forms a red-colored complex. This complex is less stable than the calcium compound with Trilon, and upon titration there is a sharp change in color from red to purple at the equivalent point. The harmful effects of copper and manganese are destroyed in the same way as when titrating the sum of calcium and magnesium.
Analysis progress. A certain volume of the solution is placed in a 250 ml conical flask, the solution is diluted with water to approximately 100 ml.
To prevent the co-precipitation of calcium with magnesium in the direct determination of calcium with murexide, 2 ml of 0.5 N sodium hydroxide is added to the sample beforehand (before adding NaOH). Na2CO3 solution. In this case, calcium precipitates in the form of CaCO3, forming a separate phase, which dissolves during the subsequent titration. This eliminates the possibility of co-precipitation of calcium with Mg(OH)2 and ensures the completeness of the determination of calcium. Add 2 ml of 2.0 N. NaOH, 0.5 ml Na2S solution and 0.5 ml hydroxylamine solution, then dry murexide on the tip of a knife and titrate with 0.05 or 0.01 N. Trilon B solution with vigorous stirring until the bright purple color of the solution changes to purple.
In the future, from the addition of trilon, the color does not change, therefore it is better to carry out titration in the presence of a "witness" - a deliberately overtitrated sample.

From the sum of calcium and magnesium per 100 g of soil, the amount of calcium is subtracted and the amount of magnesium is obtained (in mEq per 100 g of soil).
The obtained data on the content of absorbed calcium and magnesium are recalculated per 100 g of dried soil.
Reagents. 1. Trilon B solution. To prepare 0.05 p. solution, dissolve 9.3 g of trilon in 1 liter of distilled water. 0.01 n. the solution is prepared by diluting 0.05 N. solution. The titer of the Trilon solution is determined by magnesium sulphate. Selling chemically pure salt MgSO4 7H20 is recrystallized, dried for a day between sheets of filter paper and kept in a desiccator over a mixture of 5 parts of MgSO4 7H2O and 1 part of water until constant nose. 0.01 n. the solution contains 1.232 g MgSO4 7H2O in 1 liter of water. It is recommended to check the amount of magnesium in the solution prepared for checking the titer of Trilon by weight pyrophosphate method and make the necessary correction.
2. Buffer solution. 20 g of ammonium chloride are dissolved in 500 ml of distilled water, 100 ml of 25% ammonia solution are added and the volume is adjusted to 1 liter.
3. Indicator for titrating the amount of calcium and magnesium. 0.2 g of black chromogen is dissolved in 10 ml of ammonia buffer and diluted with water to 100 ml. The indicator solution is stable for 1 month. It is convenient to prepare this indicator for analysis by rubbing it with NaCl until a uniformly colored state (5 g of the indicator and 95 g of NaCl), store in a jar with a ground stopper in a dark place. When titrating, add 10-15 mg for each determination.
To check the magnesium titer of trilon, 20 ml of the prepared solution of magnesium sulfate is transferred with a pipette into a 250 ml conical flask, 100 ml of distilled water, 5 ml of ammonia buffer, 10-15 ml of black chromogen are added, and a cherry-red solution is titrated with 0.01 N. Trilon solution until the color of the solution changes to blue.
4. Ready commercial murexide is prepared for analysis by rubbing it with NaCl until the color is uniform (5 g of indicator and 95 g of NaCl). Store in a sealed jar in a dark place. In the analysis take 10-15 mg of the resulting salt.
To create the necessary alkaline reaction when titrating with trilon with the indicator murexide, 2 n. sodium hydroxide solution Small amounts of Na2S are added to hydroxylamine in case of displacement of manganese and copper from the soil.
Determination of mobile magnesium in soils is carried out in 1 N. KCl extract. 100 g of soil, sifted through a sieve with holes of 1 mm, is placed in a bottle, pour 250 ml of 1 N. KCl, shaken on a rotator for 1 hour and filtered through a pleated filter.
To determine the sum of Mg, Ca and Mn, 50 ml of the extract is placed in a 150 ml beaker, 5 ml of an ammonia buffer mixture (20 g of chemically pure NH4Cl and 100 ml of a 25% NH4OH solution in 1 l of water, 2 ml of a 1% hydroxylamine hydrochloric acid solution, 50 ml of distilled water and a dry indicator on the tip of a knife - acid chromium dark blue, sour cream with NaCl in a ratio of 1: 99. Then the extract is titrated with a 0.02 N solution of Trilon B until the color of the solution changes from cherry red to When using a photoelectric titrimeter of the FET-UNIIZ type, titration is carried out until the ammeter pointer stops.
To determine the amount of Ca and Mn, take 50 ml of the extract, add 2 ml of a 1% solution of hydrochloric acid hydroxylamine, 10 ml of a borate buffer mixture (6 ml of a 0.05 N solution of borax and 4 ml of a 0.02 N solution of boric acid) , 10 ml of ammonia buffer mixture and dry murexide on the tip of a knife. To the extract is added from the burette 0.02 N. Trilon B solution until the color changes from orange to crimson. Then 2 ml of a 20% NaOH solution is added and the titration is continued until the titrimeter needle stops or the bright purple color changes to purple during visual titration.
Magnesium is determined by the formula:

Direct titration method. The analyzed solution containing cations of the metal to be determined is diluted in a volumetric flask and an aliquot of the solution is taken for titration.

Titration is carried out with a standard EDTA solution in an alkaline medium with eriochrome black T or in an acidic medium with xylene orange.

To do this, the solution to be titrated is preliminarily adjusted to a certain pH value before titration using a buffer solution. Along with the buffer solution, an auxiliary complexing agent (tartrate, citrate, etc.) is sometimes added, which binds some cations and keeps them in a soluble state in order to avoid precipitation of hydroxides in an alkaline solution.

In the process of direct titration, the concentration of the cation being determined first decreases gradually, then drops sharply near the equivalence point. This moment is noticed by the color change of the introduced indicator, which instantly reacts to the change in the concentration of complexing metal cations.

The method of direct complexometric titration determines Cu 2+, Cd 2+, Pb 2+, Ni 2+, Co 2+, Fe 3+, Zn 2+, Th IV, Al 3+, Ba 2+, Sr 2+, Ca 2 + , Mg 2+ and some other cations. The determination is hindered by complexing substances that retain the ions to be determined in the form of complex ions that are not destroyed by complexing agents.

back titration method. In those cases when, for one reason or another, it is impossible to carry out direct titration of the cation being determined, the back titration method is used. An accurately measured volume of a complexone standard solution is added to the analyzed solution, heated to boiling to complete the complexation reaction, and then the excess of the complexone is titrated in the cold with a titrated solution of MgSO 4 or ZnSO 4 . To establish the equivalence point, an indicator metal is used that reacts to magnesium or zinc ions.

The back titration method is used in cases where there is no suitable indicator for the cations of the metal to be determined, when cations form a precipitate in a buffer solution, and when the complexation reaction proceeds slowly. The back titration method also determines the content of cations in water-insoluble precipitates (Ca 2+ in CaC 2 O 4, Mg 2+ in MgNH 4 PO 4, Pb 2+ in PbSO 4, etc.).

Substituent titration method. In some cases, instead of the methods described above, the substituent titration method is used. The method of complexometric titration of a substituent is based on the fact that Mg 2+ -ions give a less stable complex compound with a complexone (pK = 8.7) than the vast majority of other cations. Therefore, if you mix the cations of the metal being determined with the magnesium complex, then an exchange reaction will occur.

For example, this reaction is used to determine thorium ions when magnesium complexonate MgY 2 - is introduced into the analyzed solution, and then the released Mg 2+ ions are titrated with a standard EDTA solution (b);

Th4+ + MgY2 -
Mg 2+ + H 2 Y 2 -		MgY 2 - +2H +

Due to the fact that Th IV forms a more stable complex compound with complexone than Mg 2+ , the equilibrium of reaction (a) shifts to the right.

If, after the completion of the displacement reaction, Mg 2+ is titrated with a standard EDTA solution in the presence of eriochrome black T, then the content of Th IV ions in the test solution can be calculated.

Methodacid-base titration. In the process of interaction of the complexone with certain metal cations, a certain amount of equivalents of hydrogen ions is released.

The hydrogen ions formed in this case in an equivalent amount are titrated by the usual alkalimetric method in the presence of an acid-base indicator or by other methods.

There are other methods of complexometric titration, the description of which is beyond the scope of our task.

Setting the titer of EDTA solution

To prepare a standard (titrated) solution of EDTA, disodium salt of ethylenediaminetetraacetic acid is used, which crystallizes with two molecules of water; its composition corresponds to the formula Na 2 C 10 N 14 O 8 N 2 2H 2 O.

If the disodium salt containing water of crystallization is dried at 120-140°C, then an anhydrous salt is obtained, the composition of which corresponds to the formula Na 2 C 10 H 14 O 8 N 2 .

Both salts can serve as starting materials for the preparation of an EDTA standard solution.

For the preparation of 1 l 0.1 N. EDTA solution should be taken:

M Na 2 C 10 H 14 O 8 N 2 2H 2 O╱2 10 = 372.24╱ 2 10 = 18.61 g

M Na 2 C 10 H 14 O 8 N 2 ╱2 10 = 336.21╱ 2 10 = 16.81 g

To set the titer of EDTA, x is used. h. calcium carbonate, x. h. ZnO or x. including metallic zinc, the calculated sample of which is dissolved in x. including hydrochloric or sulfuric acid, neutralized with sodium hydroxide or ammonia, diluted with ammonia buffer solution and titrated with a standard EDTA solution in the presence of the required indicator. Towards the end, titrate slowly.

The titer of the solution can also be determined by fixanal of the magnesium salt (0.01 and 0.05 N solutions of magnesium sulfate are commercially available).

According to the results of titration, T is calculated, N and To EDTA solution.

Determination of calcium content

Methods for the quantitative determination of calcium. There are various methods for determining calcium.

gravimetric methods.

1. Precipitation in the form of CaC 2 O 4 -H 2 O oxalate and weighing as CaCO 3 or CaO (see "Gravimetric analysis").

2. Precipitation in the form of sulfate CaSO 4 from an alcohol solution.

3. Precipitation in the form of picrolonate Ca (C 10 H 7 O 5 N 4) 2  8H 2 O.

titrimetric methods.

1. Precipitation as calcium oxalate and subsequent determination of the calcium-bound oxalate ion by permanganatometry or cerimetry.

2. Precipitation in the form of CaMoO 4 molybdate, molybdenum reduction and titration with ammonium vanadate.

3. Complexometric method.

The gravimetric method for determining calcium has very significant drawbacks.

1. Determination of the calcium content in various technical objects by the gravimetric method is a very lengthy operation.

2. Precipitation of calcium ions in the form of CaC 2 O 4 is associated with great difficulties due to the impossibility of achieving a quantitative isolation of calcium oxalate;

3. Calcium oxalate precipitate is often contaminated with foreign impurities, and it is difficult to isolate it in a chemically pure form.

4. Obtaining the weight form (CaO) is associated with the use of a relatively high temperature required for thermal decomposition of calcium oxalate.

5. The resulting weight form (CaO) is unstable and exposed to moisture and carbon dioxide in the air, as a result of which its mass varies depending on the conditions of production and storage.

Therefore, at present, the gravimetric method for determining calcium has lost its former significance and has been supplanted by more advanced titrimetric methods of analysis.

The permanganometric method for determining calcium has a number of advantages over the gravimetric method of analysis. One such advantage is faster completion of the determination operation. However, the permanganometric method for determining calcium, based on the precipitation of calcium ions in the form of oxalate and the subsequent titration of oxalate ions with permanganate, has many disadvantages of analysis associated with the impossibility of complete quantitative precipitation and separation of calcium oxalate.

Of the titrimetric methods of analysis, the most accurate and fastest method for determining calcium is undoubtedly the complexometric titration of calcium ions with EDTA.

Complexometric method for the determination of calcium. The complexometric determination of calcium is based on the direct method of titration of its ions with a standard EDTA solution in the presence of murexide or acid chromium dark blue. The indicator forms a red complex with calcium ions. When titrating an EDTA solution at the equivalence point, the red color changes to the color characteristic of the free indicator.

As a result of titration of calcium salts with EDTA, a complex of CaY 2 - and acid is formed:

Ca 2+ + H 2 Y 2 - ⇄ CaY 2 - + 2Н +

The resulting CaY 2 complex is relatively unstable:

╱ =310 -11

The formation of a free acid during the reaction or its addition to the titrated solution before titration shifts the indicated equilibrium to the left, i.e., towards the destruction of the complex.

EDTA is a tetrabasic acid characterized by the following constants: pK 1 = 2; RK 2 = 2,7; RK 3 = 6,2; RK 4 \u003d 10.3 and is a relatively weak acid, therefore the pH of the solution of its complex with Ca 2+ should not be lower than 10.3. If the pH is lower, then Y 4 - with H + forms the corresponding hydroanions: HY 3 -, H 2 Y 2 -, H 3 Y - and acid H 4 Y. In this case, the CaY 2 - complex is destroyed or not formed at all.

Thus, the stability of the intra-complex salt formed by calcium ions with EDTA depends on the pH of the solution. To ensure the optimal course of the CaY 2 complex formation reaction, titration of calcium salts with an EDTA solution should be carried out in a strongly alkaline medium at pH > 12. In this case, the free acid formed during the titration is completely neutralized and the maximum jump in the titration curve is observed.

Volume of EDTA, ml

Rice. 6.1 Calcium ion titration curves by the complexometric method at various pH values ​​of the solution:

1 – pH =6; 2 – pH = 8; 3 – pH = 10; 4 – pH = 12

b decreases with increasing calcium concentration). An increase in pH leads to a decrease in sensitivity (Fig. 21). The optimal amount of alkali is 5 ml of a 10% NaOH/SO solution. In the photometric determination of calcium, a 0.02% aqueous solution of acidic chromium dark blue is used. Aqueous solutions of the reagent are stable for several weeks. Definition

interfere with Al, Fe, Co, Ni, Mn. The influence of these elements is eliminated by masking with triethanolamine with the addition of sodium fluoride, or 1% sodium cyanide solution.

The photometric method for the determination of calcium with acid chromium dark blue is used in the analysis of cement raw mixtures and clinkers. The method has been proposed for the determination of large amounts of calcium (40 -45% CaO). At the same time, most of the calcium is bound to a colorless complex with complexone III, and the remaining calcium (~6%) is determined by a color reaction with acid chromium dark blue.

0.15 g of the analyzed material is fused with 1 part of the mixture (1 g of borax and 2 parts of soda), the melt is dissolved in 100 ml of HG1 (1: 3) and diluted with water to 500 ml. From the resulting solution, 20 ml are taken into a 100 ml volumetric flask, 5 ml of a solution containing 1% triethanolamine and 0.5% NaF, 20 ml of 0.00450 g7 solution of complexope III are added, neutralized by methyl red with 1% NaOH solution and added excess 5 ml. Then add 10 ml of a 0.02% aqueous solution of acidic chromium dark blue, dilute with water to the mark and photometer on FEK-M with a yellow light filter (L = 595 nm) in a cuvette with I = 1 cm.

Acid chromium dark blue is also used for the photometric determination of calcium in biological objects, cast iron, metallic titanium. Also used for indirect photometric determination of calcium eriochrome black T.

Determination of calcium with other reagents]

Calcium is precipitated as phosphate, molybdate, or tungstate. The precipitate is separated by filtration, dissolved in acid, and the phosphate ion, molybdenum, and tungsten are determined by appropriate methods. Loretin is used to precipitate calcium, then iron loretinate is photometered.

In a number of colorimetric methods, calcium is precipitated in the form of K2Ca, and then NO2-ion, nickel with dimethylglyoxime are determined, or the green color that appears during the interaction of K2Ca and sodium naphthyl hydroxamate is colorimetric.

Ce(IV) sulphate is used for the colorimetric olre-division of calcium after its precipitation with oxalate. The precipitate of the latter is dissolved in sulfuric acid, an excess of Ce(SO4)2 is added, and the color intensity is measured. The following indirect determination of calcium is also possible: after dissolving calcium oxalate in sulfuric acid and adding an excess of Ce (SO4) 2 and potassium iodide, photometry is carried out by yellow color of free iodine or by blue color after addition of starch.

Calcium can be determined with high accuracy by photometry of the color of potassium permanganate added in excess to calcium oxalate dissolved in acid.

When chloranilic acid is added to calcium oxalate and calcium chloranilate is precipitated, the latter is determined by measuring the optical density of the mother liquor. A calibration graph is built for 0-0.2 mg Ca.

One of the colorimetric options for determining calcium in the form of oxalate is based on the discoloration of the red color of an iron thiocyanate solution by oxalates)