Determination of humus content in the soil GOST. Determination of the degree of humification of soil organic matter by the method of Robinson and Joies
The method of I. V. Tyurin is based on the oxidation of soil organic matter with chromic acid to the formation of carbon dioxide. The amount of oxygen consumed for the oxidation of organic carbon is determined by the difference between the amount of chromic acid taken for oxidation and the amount of it remaining unused after oxidation. As an oxidizing agent, 0.4 N. solution of K2Cr2O7 in sulfuric acid, previously diluted with water in a ratio of 1:1.
The oxidation reaction proceeds according to the following equations:
2K2Cr2O7 + 8H2SO4 = 2K2SO4 + 2Cr2(SO4)3 + 8H2O+3O2,
ZS + ZO2 = ZSO2.
The rest of the chromic acid not consumed for oxidation is titrated with 0.1 N. Mohr's salt solution with indicator diphenylamine or phenylanthranilic acid. Titration with Mohr's salt, which is a double salt ammonium sulfate and ferrous sulphate - (NH4) 2SO4 * FeSO4 * 6H2O, goes according to the equation
K2Cr2O7 + 7H2SO4 + 6FeSO4 = 7H2O + K2SO4 + Cr2(SO4)3 + SFe2(SO4)3.
The completeness of the oxidation of organic matter, subject to all the conditions of the method indicated below, is 85-90% of the value of humus oxidation by the dry combustion method according to Gustavson. The use of silver sulfate as a catalyst increases the completeness of oxidation to 95%.
To obtain reliable results, it is necessary to pay attention to: 1) careful preparation of the soil for analysis and 2) exact observance of the duration of boiling of organic matter; the boiling of the oxidizing mixture itself should proceed calmly.
The method gives good convergence of parallel analyses, is fast, does not require special equipment (in connection with which it can be used in expeditionary conditions) and is currently generally accepted, especially in mass analyses.
When preparing the soil for analysis on the content of humus, special attention should be paid to the removal of roots and various organic residues of plant and animal origin from the soil.
From a soil sample taken in the field and brought to an air-dry state, an average sample of 50 g is taken, the roots and organic residues visible to the eye (insect shells, seeds, coals, etc.) are carefully selected with tweezers, soil clods are crushed with a wooden pestle with rubber tip and again carefully select the roots, using a magnifying glass.
Then the soil is ground in a porcelain mortar and passed through a sieve with holes with a diameter of 1 mm, after which an average sample weighing 5 g is again taken from it and the selection of roots is repeated using the following method: a dry glass rod is vigorously rubbed with a dry cloth or woolen cloth and quickly carried out at a height of about 10 cm above the soil, distributed in a thin layer over the surface of wax paper or parchment paper. Thin small roots and semi-decayed plant residues, which could not be selected before due to their small size, adhere to the surface of the electrified stick and are thus removed from the soil. They are removed from the stick when it is rubbed again. It should not be held too low with a stick above the soil surface in order to avoid the removal of not only organic residues from the soil, but also fine earth.
In the process of selecting roots, it is necessary to repeatedly mix the soil and again distribute it in a thin layer. The operation should be carried out until only single roots are found on the stick. The purity of the selection of roots is controlled, in addition, by viewing the soil in a magnifying glass.
At the end of the selection of roots, the soil is again ground in a porcelain, jasper or agate mortar and passed through a sieve with holes with a diameter of 0.25 mm. The entire 5 g sample should be prepared in the manner described above.
The soil prepared for analysis should be stored in parchment paper or wax bags or in test tubes with stoppers.
Analysis progress. A sample of air-dry soil for humus analysis is taken on an analytical balance. The sample size depends on the expected humus content in the soil, taking into account the type of soil (chernozem, podzolic, etc.) and the sampling depth.
With a humus content of 7 to 10%, I. V. Tyurin recommends a sample of 0.1 g, at 4-7% - 0.2 g, at 2-4% - 0.3 g, less than 2% -0.5 d. In the case of sandy soils with a low humus content, the sample can be increased to 1 g.
It is better to take the exact weights - 0.1; 0.2 g, which facilitates further calculations. To take accurate weights, you can use a calibrated watch glass with a diameter of 2.5-3 cm, from which the whole weight is transferred into a flask for combustion using a small spatula and a brush for watercolor paints. The determination of humus according to Tyurin can be carried out simultaneously in 20-30 samples.
Samples are placed in dry conical flasks of 100 ml of ordinary glass, powdered silver sulfate is added there at the tip of a knife. In mass analyzes, silver sulfate is not used. To be able to compare the results obtained in this case with the dry combustion method, IV Tyurin gives a coefficient of 1.17. Then, 10 mg of a 0.4 p solution of K2Cr2O7 prepared on a mixture of one part of H2SO4 ( specific gravity 1.84) and one part distilled water.
Potassium bichromate solution should be poured from the burette slowly, measuring the required volume each time from zero and allowing the liquid to drain always at the same rate. You can also use a pipette, but always equipped with safety balls in the upper part. In this case, a separating funnel made of refractory glass, adapted to work with strong acids, is very convenient. The use of such a funnel greatly speeds up the work and makes it safe.
After adding the K2Cr2O7 solution, funnels with a diameter of about 4 cm are inserted into the neck of the flasks, the contents of the flasks are gently mixed (making sure that the soil does not stick to their walls), after which the flasks are placed on an already hot eternite tile or sand bath or on a tile with an exposed spiral, but covered with asbestos. You can also use gas burners, and in expeditionary conditions - with a primus stove or a kerosene stove, placing a heating device under a sand bath (a frying pan with calcined quartz sand).
The contents of the flasks are brought to a boil and boiled for exactly 5 minutes. It is necessary to note the beginning of the liquid boiling, without mixing it with the appearance of small air bubbles at the beginning of heating. Boiling should be uniform and moderate; the release of steam from the funnel and the bouncing of the latter are unacceptable. Vigorous boiling should be avoided so as not to change the concentration of sulfuric acid, an increase in which can cause decomposition of chromic acid. To avoid over-boiling boiling on tiles with a naked spiral is unacceptable.
Titration with diphenylamine. After 5 minutes of boiling, the flasks are removed from the heating device, allowed to cool, the funnels above the flasks are washed from the inside and outside with distilled water from the wash and the contents of the flasks are quantitatively transferred into 250 ml conical flasks, the flask in which the oxidation was carried out is thoroughly rinsed several times. . The volume of liquid after transfer to a 250 ml flask should be 100-150 ml. The color of the liquid is orange-yellow or greenish-yellow; its greening indicates a lack of an oxidizing agent; the analysis in this case must be repeated, reducing the sample of soil.
8 drops of diphenylamine solution, which is an indicator, are added to the liquid, and the chromic acid remaining unused after the oxidation of the organic substance is titrated with 0.1 N. Mohr's salt solution. The indicator should be added immediately before titration. Titration is carried out at room temperature. The red-brown color of the liquid, which appears after the addition of diphenylamine, when titrated with a solution of Mohr's salt, gradually turns into an intense blue, and then into a dirty violet. From this point on, the titration is carried out carefully, adding Mohr's salt 1 drop at a time and thoroughly mixing the contents of the flask. The end of the titration - a change in the dirty purple color of the solution to bottle green; after some standing (10-15 min.), the color of the liquid becomes green. The appearance of a bright green color during titration indicates an excess of Mohr's salt, i.e., that the solution has been overtitrated; analysis in this case must be repeated.
To eliminate the effect of ferric ions, which oxidizes the indicator and causes a premature change in the color of the solution, and for a more pronounced end of the titration, 85% orthophosphoric acid is added to the flask before titration in an amount of 2.5 ml; the color change at the end of the titration in the presence of phosphoric acid is very abrupt and is caused by 1-2 drops of Mohr's salt solution.
Titration with phenylanthranilic acid. In 1957 prof. V.N. Simakov suggested using phenylanthranilic acid instead of diphenylamine as an indicator in the titration of a chromium mixture with Mohr's salt solution.
Numerous determinations have shown that the results of titration with phenylanthranilic acid are in complete agreement with the results of titration in the presence of diphenylamine. However, the use of phenylanthranilic acid has great advantages over diphenylamine.
Thus, the color change at the end of the titration of potassium dichromate with a solution of Mohr's salt in the presence of phenylanthranilic acid is more pronounced than with diphenylamine, and comes from one excess drop of even such a diluted solution of Mohr's salt as 0.02 N, which indicates a high sensitivity of the indicator and allows the determination of small amounts of humus with great accuracy (for example, in aqueous extracts and in the analysis of natural waters).
Another advantage is the following. When using phenylanthranilic acid as an indicator in the titration of potassium dichromate with Mohr's salt solution, a high concentration of sulfuric acid in the titrated solution is required - about 15-20 N. This allows the liquid to be only slightly diluted before titration (after combustion) and thus to carry out the titration in the same 100 ml conical flasks in which the combustion was carried out (without transferring their contents to dilution into large flasks), as in the case of diphenylamine.
Thus, the analysis time, the consumption of glassware and distilled water is reduced.
The procedure for determining the content of humus with phenylanthranilic acid is as follows: the combustion of organic matter in soil samples is carried out exactly according to the Tyurin method in 100 ml conical flasks with the same reagents. After 5 minutes of boiling and cooling of the flasks, the funnels above the flasks are thoroughly washed with a minimum amount of water, 3-5 drops of a 0.2% solution of phenylanthranilic acid are added and titrated in the same flasks with 0.1 N. Mohr's salt solution until the color changes from cherry-violet to green. Since the color transition is very sharp - instantaneous, Mohr's salt solution should be added at the end of the titration drop by drop.
Simultaneously with the main analyzes in the same sequence, a blank is carried out (in triplicate) to establish the ratio between 10 ml of a chromium mixture solution and Mohr's salt solution. For uniform boiling of the liquid during blank analysis into the flask before pouring the solution of the chromium mixture be sure to add about 0.1-0.2 g of calcined pumice or soil ground into powder. Otherwise, the inevitable happens when boiling pure solution overheating, which can cause decomposition of chromic acid. The rest proceed according to the described course of analysis.
When conducting large batches of analyzes for the content of humus according to the Tyurin method (30-60 analyzes at the same time), you can take breaks at the following stages of work: taking samples - one day; oxidation, transfer to titration flasks and titration - the next day. Or, less desirable, sampling and oxidation on the same day, titration the next. In the latter case, the contents of the flasks after incineration must be diluted and transferred to titration flasks. Titration of blanks in this case should also be left until next day. Each batch should always be titrated at same conditions lighting (in daylight or electric light).
Calculation of analysis results. The number of milliliters of Mohr's salt solution used for titration after the oxidation of humus corresponds to the amount of chromic acid that remained unused during the oxidation process.
I. V. Tyurin indicates that stable results are obtained when at least 20 ml of 0.1 N is used for titration of the chromic acid residue after the oxidation of organic matter. Mohr's salt solution, in other words, the concentration of CrO3 should not be below 0.2 N by the end of oxidation.
The amount of Mohr's salt corresponding to the amount of chromic acid used for the oxidation of the humus of the sample is determined by subtracting the results of the titration after the oxidation of the humus from the blank titration results.
When calculating the content of organic carbon and humus, the following values were taken: 1 ml 0.1 N. Mohr's salt solution corresponds to 0.0003 g of organic carbon or 0.000517 g of humus (1 g of carbon corresponds to 1.724 g of humus).
The formula for calculating the humus content (in % of air-dry soil) is as follows:
Humus \u003d (a - b) * K * 0.000517 * 100 / P,
where a is the amount of 0.1 n. Mohr's salt solution used for titration 10 ml 0.4 I. K2Cr2O7 solution in blank analysis, ml; b - the amount of 0.1 n. Mohr's salt solution used for titration after humus oxidation, ml; (a - b) - the amount of 0.1 n. Mohr's salt solution corresponding to the amount of chromic acid used for humus oxidation, ml; K - correction factor to the titer of Mohr's salt solution; 0.000517 - the amount of humus corresponding to 1 ml of 0.1 n. Mohr's salt solution, g; P - sample of air-dry soil, g.
If it is desirable to express the results of the analysis in carbon, then instead of 0.000517 (the conversion factor for humus), the value 0.0003 should be put, since 1 or 0.1 n. Mohr's salt solution corresponds to 0.0003 g of organic carbon.
To recalculate the humus content as a percentage of the soil dried at 105 ° C, determine the moisture content in a separate sample of soil and enter the appropriate coefficient into the calculation.
The following notation is convenient for work:
1. Blank analysis. For titration 10 ml 0.4 N. K2Cr2O7 solution is 45.0 ml of Mohr's salt solution 0.102 N. (average of three definitions).
2. Soil analysis results:
Reagents. 1. A solution of potassium dichromate 0.4 N. (oxidizing agent): 40 g of R2Cr2O7 crushed in a mortar are dissolved in distilled water, filtered through a paper filter into a 1 liter volumetric flask, brought to the line, and then transferred to a 3-5 liter refractory glass flask or into a large porcelain cup, where mixed with 1 l of H2SO4 (sp. gr. 1.84). To avoid strong heating of the liquid during mixing sulfuric acid should be poured into an aqueous solution of K2Cr2O7 little by little, at intervals of 15-20 minutes. and with gentle stirring. The mixture is allowed to cool, again thoroughly mixed and poured into a flask with ground glass stopper for storage.
2. Mohr's salt solution 0.1 N. prepared from the calculation: 40 g of Mohr's salt in 1 liter of water containing 20 ml of sulfuric acid (sp. weight 1.84). A portion of commercial Mora salt is dissolved in the cold in a certain amount of distilled water, the solution is filtered through a pleated filter. To the filtrate add the amount of sulfuric acid required by calculation for a given sample, bring the solution with distilled water to a predetermined volume, and then mix thoroughly. Mohr's salt solution is stored in a bottle equipped with a siphon with a glass tap for supplying the solution to the burette and a Tishchenko flask with an alkaline solution of pyrogallol to protect the Mohr's salt solution from oxidation by atmospheric oxygen.
The titer of Mohr's salt is set at 0.1 N. KMnO4 solution as follows: 1 ml of H2SO4 (specific weight 1.84) is added to a 100 ml conical flask using a graduated cylinder, then 10 ml of Mohr's salt solution is poured from a burette; the contents of the flask are diluted with distilled water to 30-40 ml and immediately titrated with 0.1 N. KMnO4 solution to a slightly pink color that does not disappear within 1 min. The titer of Mohr's salt solution is expressed as a decimal fraction or as the so-called humus number, assuming, as mentioned above, that 1 ml of 0.1 N. Mohr's salt solution corresponds to 0.000517 g of humus.
Due to the fact that Mohr's salt contains ferrous iron, the titer of its solution, despite the pyrogallol fuse, is relatively unstable, and it must be checked every time before starting work.
3. Pyrogallol solution (protection against oxidation of Mohr's salt solution by atmospheric oxygen). 12 g of pyrogallol are dissolved in 50 ml of water; 180 g of KOH are dissolved in 300 ml of water. Both solutions are mixed, placed in a Tishchenko flask and attached with rubber and glass tubes to a bottle with Mohr's salt solution.
4. Diphenylamine solution (indicator when titrated with Mohr's salt solution): 0.5 g of diphenylamine is dissolved in 100 ml of H2SO4 (sp. wt. 1.84); to the solution gradually, with great care, add 20 ml of distilled water.
5. Solution of phenylanthranilic acid (indicator for titration with Mohr's salt solution): 0.2 g of it is dissolved in 100 ml of 0.2% Na2CO3 aqueous solution; for better wetting of the powder of phenylanthranilic acid, a portion of it is first rubbed with a glass rod in a porcelain cup with a small amount of 0.2% soda solution to a creamy state, and only after that the rest of the soda solution is added with thorough mixing.
6. Silver sulfate (catalyst). Used in powder form. However, in mass analyzes, the determination of carbon by the Tyurin method is carried out, as mentioned above, without silver sulfate.
7. 0.1 n. KMnO4 solution (for setting the titer of Mohr's salt solution). The solution is prepared in the usual manner; its titer is set to 0.1 n. a solution of recrystallized sodium oxalate (Na2C2O4).
8. Calcined pumice or soil (for uniform boiling of the oxidizing mixture during blank analyses). Low-humus soil or commercial pumice is ground in a porcelain mortar, sifted through a sieve with 1 mm holes and calcined in a porcelain cup in a muffle furnace at red heat for 1-1.5 hours. with occasional stirring to avoid sintering.
9. Orthophosphoric acid 85%, chemically pure. It is used to eliminate the influence of iron oxide ions with the indicator diphenylamine.
Finishing the description of the method, it should be noted that the presence of carbonates in the soil, even in such large quantities, as in gray soils, does not interfere with the determination of humus according to Tyurin and does not require any changes during the analysis.
The method is not applicable when the content of humus in the soil is more than 15-20%, since the completeness of oxidation is not achieved in this case.
For soils containing chlorides over 0.6%, ferrous compounds of iron and manganese, the method is not suitable, since part of the chromic acid is spent on the oxidation of these compounds, which distorts the results of the analysis. For such soils, the Knop or Gustavson method should be used.
Tyurin indicates the possibility of partially eliminating the effect of chlorides by first adding 0.2 g of Ag2SO4 with 5 ml of H2SO4, diluted with water in a ratio of 1:1, to a sample of soil taken. After adding these reagents, the mixture is left for an hour with occasional stirring to convert the chlorides contained in the soil into AgCl. However, this technique does not completely eliminate the effect of chlorine, since during further boiling of the oxidizing mixture, some decomposition of silver chloride occurs with the release of chlorine. Therefore, for soils salinized with chlorides over 0.6%, it is possible to recommend preliminary washing of the latter with distilled water. To do this, proceed as follows: 100-200 g of air-dry soil, passed through a sieve with holes of 1 mm, is placed in a beaker with a capacity of 500-800 ml, poured to the top with distilled water, slightly acidified with a few drops of 1.0 N. H2SO4, and stir repeatedly throughout the day. In the morning, the settled clear solution is drained by decantation, without stirring up the sediment of the soil, which is then again poured with acidified water. If the solution above the soil sediment continues to remain cloudy by morning, the water should be acidified with sulfuric acid somewhat more strongly. The washing operation is repeated until the chlorine ion disappears in the washing water (qualitative test for the presence of chlorine with 1% AgNO3 solution).
At the end of washing, all the soil from the beaker is transferred with distilled water into a tared porcelain cup, placed in a boiling water bath and evaporated to dryness. The soil in the cup is left near the scales for a day to establish an air-dry state, after which it is weighed (weighing is carried out on technochemical scales). Thus, the ratio between the weight of the original soil taken for washing and the weight of the soil after washing is determined. From the washed, dried and weighed soil, an average sample weighing 5 g is taken, ground in a mortar and passed through a sieve with holes with a diameter of 0.25 mm, after which ordinary samples are taken to determine humus. Knowing the ratio between the weight of the soil before and after washing, the calculation of the humus content is carried out on the original soil.
GOST 26213-91
Group C09
STATE STANDARD OF THE UNION OF THE SSR
Methods for determining organic matter
soils. Methods for determination of organic matter
OKSTU 9709
Introduction date 1993-07-01
INFORMATION DATA
1. DEVELOPED AND INTRODUCED by the All-Union Production and Scientific Association "Soyuzselkhozkhimiya"
DEVELOPERS
L.M.Derzhavin, S.G.Samokhvalov (head of development), N.V.Sokolova, A.N.Orlova, K.A.Khabarova, V.G.Prizhukova, S.Ya.Privalenkova
2. APPROVED AND INTRODUCED BY Decree of the Committee for Standardization and Metrology of the USSR of December 29, 1991 N 2389
3. Term of verification - 1996
4. REPLACE GOST 26213-84
5. REFERENCE REGULATIONS AND TECHNICAL DOCUMENTS
Number |
|
2.2, 2.3, 2.4, 2.5.1, 2.5.3 |
|
Introduction |
|
TU 6-09-5313-87 |
This International Standard specifies photometric and gravimetric methods for the determination of organic matter in soils, overburden and wall rocks.
General requirements for analysis - according to GOST 29269.
1. DETERMINATION OF ORGANIC MATTER BY THE TYURIN METHOD IN THE CINAO MODIFICATION
1. DETERMINATION OF ORGANIC SUBSTANCE BY THE METHOD
TYURINA IN MODIFICATION TsINAO
The method is based on the oxidation of organic matter with a solution of potassium dichromate in sulfuric acid and the subsequent determination of trivalent chromium, equivalent to the content of organic matter, on a photoelectric colorimeter.
The method is not suitable for samples with mass fraction chloride more than 0.6% and samples with a mass fraction of organic matter more than 15%.
The limit values of the relative error of the analysis results for a two-sided confidence level of 0.95 are in percent (rel.):
20 - with a mass fraction of organic matter up to 3%;
15 - St. 3 to 5%;
10 - St. 5 to 15%.
1.1. Sample selection
1.1.1. Sampling is carried out according to GOST 28168, GOST 17.4.3.01 and GOST 17.4.4.02 - depending on the objectives of the research.
1.1.2. A representative sample weighing 3-5 g is taken from the ground soil or rock for fine grinding. Before grinding, undecomposed roots and plant residues visible to the naked eye are removed from the sample with tweezers. Then the sample is completely crushed and passed through a wicker sieve with holes with a diameter of 0.25 mm. For fine grinding, mortars and grinding devices made of porcelain, steel and other hard materials are used.
1.2. Equipment and reagents
Photoelectrocolorimeter.
Bath water.
Torsion scales or others with an error of not more than 1 mg.
Heat-resistant glass test tubes with a capacity of 50 cm3 in accordance with GOST 23932.
Stand for test tubes.
Burette or dispenser for measuring 10 cm of chrome mixture.
Glass sticks 30 cm long.
Cylinder or dispenser for measuring 40 cm of water.
A rubber bulb with a glass tube or a device for barbation.
Burette with a capacity of 50 ml.
Volumetric flasks with a capacity of 1 dm.
Porcelain mug with a capacity of 2 dm.
Flask conical with a capacity of 1 dm.
Conical flasks or technological containers with a capacity of at least 100 ml.
Ammonium-iron (II) sulfate (Mohr's salt) according to GOST 4208 or iron (II) sulfate 7-water according to GOST 4148.
Potassium hydroxide according to GOST 24363.
Potassium dichromate according to GOST 4220.
Potassium permanganate, standard titer for preparing a solution of concentration (KMnO) = 0.1 mol / dm (0.1 N).
Sodium sulfite according to GOST 195 or sodium sulfite 7-water according to TU 6-09 5313.
Sulfuric acid according to GOST 4204 concentrated and concentration solution (HSO)=1 mol/dm.
Distilled water.
Ashless filters, with
blue tape.
1.3. Preparation for analysis
1.3.1. Preparation of the chromium mixture
(40.0 ± 0.1) g of finely ground potassium dichromate is placed in a volumetric flask with a capacity of 1 dm3, dissolved in water, bringing the volume to the mark, and poured into a porcelain mug. To the prepared solution is poured in portions of 100 cm 3 with an interval of 10-15 minutes 1 dm of concentrated sulfuric acid. The mug with the solution is covered with glass and left to cool completely.
The solution is stored in a dark glass bottle.
1.3.2. Preparation of a reducing agent solution - Mohr's salt solution of concentration [(NH)SOFеSO 6HO] ± 0.1 mol / dm or a solution of iron (II) sulfate 7-aqueous concentration (FeSO 7HO) = 0.1 mol / dm
(40.0 ± 0.1) g of Mohr's salt or (27.8 ± 0.1) g of 7-aqueous iron (II) sulfate is dissolved in 700 ml of sulfuric acid solution with concentration (HSO) = 1 mol / dm, filtered through double pleated filter into a 1 liter volumetric flask and dilute to volume with water.
The concentration of the solution is checked by titration against a solution of potassium permanganate concentration (KMnO) = 0.1 mol / dm, prepared from a standard titer. For titration in three conical flasks, 10 cm3 of the prepared reducing agent solution is measured with a burette, 1 cm3 of concentrated sulfuric acid and 50 cm3 of water are added, and the concentration is titrated with a solution of potassium permanganate until a faint pink color appears, which does not disappear within 1 min. To calculate the correction factor, use the arithmetic mean of the results of three titrations.
The correction factor () is calculated by the equation
where is the volume of potassium permanganate solution used for titration, cm;
- the volume of the reducing agent selected for titration, see
The solution is stored in a dark glass bottle, to which a burette is attached using a siphon. To protect the solution from oxidation by air oxygen, a Tishchenko bottle with an alkaline solution of sodium sulfite is attached to the bottle. The correction factor is checked at least every 3 days.
1.3.3. Preparation of an alkaline solution of sodium sulfite
(40.0±0.1) g of anhydrous or (80.0±0.1) g of 7-aqueous sodium sulphate is dissolved in 700 cm3 of water. (10.0±0.1) g of potassium hydroxide is dissolved in 300 ml of water. The prepared solutions are mixed.
1.4. Conducting an analysis
1.4.1. Oxidation of organic matter
The mass of a soil or rock sample for analysis is determined based on the expected content of organic matter, according to Table 1.
Table 1
Sample weight for analysis, mg |
|
Soil or rock samples are weighed with an error of not more than 1 mg and placed in test tubes installed in racks. 10 cm3 of chromium mixture is added to the samples. A glass rod is placed in each test tube and the sample is thoroughly mixed with the chromium mixture. The test tube racks are then lowered into a boiling water bath. The water level in the bath should be 2-3 cm higher than the level of the chromium mixture in the test tubes. The duration of suspension heating is 1 hour from the moment the water boils in the bath after the test tubes are immersed in it. The contents of the tubes are stirred with glass rods every 20 minutes. After 1 hour, the racks with test tubes are placed in a water bath with cold water. After cooling, 40 ml of water is poured into the test tubes. Then the sticks are removed from the test tubes, the suspensions are thoroughly mixed by air bubbling and left to settle solid particles and completely clarify the supernatant part of the solution. Instead of settling, it is allowed to filter suspensions through ashless filters (blue tape).
1.4.2. Preparation of reference solutions
Pour 10 ml of chromium mixture into nine test tubes and heat them for 1 hour in a boiling water bath together with the analyzed samples. After cooling, the volumes of distilled water and reducing agent solution indicated in Table 2 are poured into the test tubes. The solutions are thoroughly mixed by air bubbling.
table 2
Characteristic | Reference solution number |
||||||||
Water volume, cm | |||||||||
The volume of the reducing agent solution, cm | |||||||||
Mass of organic matter equivalent to the volume of the reducing agent in the reference solution, mg | |||||||||
1.4.3. Photometry of solutions
Photometry of solutions is carried out in a cuvette with a translucent layer thickness of 1-2 cm relative to reference solution N 1 at a wavelength of 590 nm or using an orange-red light filter with a maximum transmission in the region of 560-600 nm. The solutions are carefully transferred into the cuvette of the photoelectric colorimeter without disturbing the sediment.
1.5. Results processing
1.5.1. The mass of organic matter in the analyzed sample is determined by the calibration curve. When constructing a calibration graph, the mass of organic matter in milligrams corresponding to the volume of the reducing agent in the reference solution is plotted along the abscissa axis, and the corresponding reading of the instrument is plotted along the ordinate axis.
1.5.2. The mass fraction of organic matter () in percent is calculated by the equation
where is the mass of organic matter in the analyzed sample, found from the graph, mg;
- correction factor for the concentration of the reducing agent;
- sample weight, mg;
100 - conversion factor to percent.
1.5.3. Permissible relative deviations from the certified value of the standard sample for a two-sided confidence level of 0.95 are shown in Table 3.
Table 3
Mass fraction of organic matter, % | Permissible deviations, % (rel.) |
St. 3 to 5 | |
2. GRAVIMETRIC METHOD FOR DETERMINING THE MASS FRACTION OF ORGANIC MATTER IN PEAT AND PEAT SOIL HORIZONS
The method is based on determining the mass loss of a sample after calcination at a temperature of 525 °C.
2.1. Sample selection
Sampling for analysis is carried out according to GOST 28168, GOST 27784.
2.5.2. The mass fraction of organic matter in percent is calculated by the formula
where is the mass fraction of ash content,%.
2.5.3. Control of the accuracy of the results of analyzes - according to GOST 27784.
The text of the document is verified by:
official publication
M.: Publishing house of standards, 1992
STATE STANDARD
UNION SSR
SOILS
METHODS FOR THE DETERMINATION OF ORGANIC SUBSTANCE
GOST 26213-91
USSR STANDARDIZATION AND METROLOGY COMMITTEE
Moscow
STATE STANDARD OF THE UNION OF THE SSR
Introduction date 01.07.93
This International Standard specifies photometric and gravimetric methods for the determination of organic matter in soils, overburden and wall rocks.
General requirements for analysis - according to GOST 29269.
1. DETERMINATION OF ORGANIC MATTER BY THE TYURIN METHOD IN THE CINAO MODIFICATION
The method is based on the oxidation of organic matter with a solution of potassium dichromate in sulfuric acid and the subsequent determination of trivalent chromium, equivalent to the content of organic matter, on a photoelectric colorimeter.
The method is not suitable for samples with a mass fraction of chloride greater than 0.6% and samples with a mass fraction of organic matter greater than 15%.
Limit values of the relative error of the results of the analysis for the two-sided confidence level R= 0.95 are as a percentage (rel.):
20 - with a mass fraction of organic matter up to 3%;
15 - St. 3 to 5%;
10 - St. 5 to 15%.
Photoelectrocolorimeter.
Bath water.
Torsion scales or others with an error of not more than 1 mg.
Heat-resistant glass test tubes with a capacity of 50 cm 3 according to GOST 23932.
Stand for test tubes.
Burette or dispenser for measuring 10 cm 3 of the chromium mixture.
Glass sticks 30 cm long.
Cylinder or dispenser for measuring 40 cm 3 of water.
A rubber bulb with a glass tube or a device for barbation.
Burette with a capacity of 50 cm 3 .
Volumetric flasks with a capacity of 1 dm 3 .
Porcelain mug with a capacity of 2 dm 3.
Flask conical with a capacity of 1 dm 3.
Conical flasks or technological containers with a capacity of at least 100 cm 3.
Ammonium iron (II ) sulfate (Mohr's salt) according to GOST 4208 or iron ( II ) sulfate 7-water according to GOST 4148.
Potassium hydroxide according to GOST 24363.
Potassium dichromate according to GOST 4220.
Potassium permanganate, standard titer for preparing a concentration solution with(1/5 KMnO 4) \u003d 0.1 mol / dm 3 (0.1 n.).
Sodium sulfite according to GOST 195 or sodium sulfite 7-water according to TU 6-09.5313.
Sample weight for analysis, mg
1.4.2. Preparation of reference solutions
Pour 10 cm 3 of the chromium mixture into nine test tubes and heat them for 1 hour in a boiling water bath together with the analyzed samples. After cooling, the test tubes are poured into the test tubes indicated in the table. volumes of distilled water and reducing agent solution. The solutions are thoroughly mixed by air bubbling.
table 2
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Reference solution number |
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The volume of water, cm 3 |
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The volume of the reducing agent solution, cm 3 |
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Mass of organic matter equivalent to the volume of the reducing agent in the reference solution, mg |
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1.4.3. Photometry of solutions
Photometry of solutions is carried out in a cuvette with a translucent layer thickness of 1–2 cm relative to reference solution No. 1 at a wavelength of 590 nm or using an orange-red light filter with a maximum transmission in the region of 560–600 nm. The solutions are carefully transferred into the cuvette of the photoelectric colorimeter without disturbing the precipitate.
1.5. Results processing
1.5.1. The mass of organic matter in the analyzed sample is determined by the calibration curve. When constructing a calibration graph, the mass of organic matter in milligrams corresponding to the volume of the reducing agent in the reference solution is plotted along the abscissa axis, and the corresponding reading of the instrument is plotted along the ordinate axis.
1.5.2. Mass fraction of organic matter (X) in percent is calculated according to the equation
where m- mass of organic matter in the analyzed sample, found from the graph, mg;
To- correction factor for the concentration of the reducing agent;
m 1 - sample weight, mg;
100 - conversion factor to percent.
1.5.3. Permissible relative deviations from the certified value of a reference material for a two-sided confidence level R= 0.95 are indicated in Table. .
Table 3
2. GRAVIMETRIC METHOD FOR DETERMINING THE MASS FRACTION OF ORGANIC MATTER IN PEAT AND PEAT SOIL HORIZONS
The method is based on determining the mass loss of a sample after calcination at a temperature of 525 °C.
Sampling for analysis is carried out according to GOST 28168, GOST 17.4.3.01 and GOST 17.4.4.02 - depending on the objectives of the research.
2.2. Equipment and reagents - by GOST 27784.
2.3. Preparation for analysis - by GOST 27784.
2.4. Carrying out analysis - by GOST 27784.
2.5. Results processing
Method I.V. Tyurin is based on the oxidation of soil organic matter with chromic acid to the formation of carbon dioxide. The amount of oxygen consumed for the oxidation of organic carbon is determined by the difference between the amount of chromic acid taken for oxidation and the amount of it remaining unused after oxidation. As an oxidizing agent, 0.4 N. solution of K2Cr2O7 in sulfuric acid, previously diluted with water in a ratio of 1:1.
The oxidation reaction proceeds according to the following equations:
- 1) 2K 2 Cr 2 O 7 + 8H 2 SO 4 \u003d 2K 2 SO 4 + 2Cr 2 (SO4) 3 + 8H 2 O + 3O 2
- 2) 3C + 3O 2 \u003d 3CO 2
The rest of the chromic acid not consumed for oxidation is titrated with 0.1 N. Mohr's salt solution with indicator diphenylamine. Titration with Mohr's salt, which is a double salt of ammonium sulphate and ferrous sulphate - (NH4) 2SO4. FeSO4.GH2O, follows the following equation:
K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 \u003d 7H 2 O + K 2 SO 4 + Cr 2 (SO4) 3 + 3Fe 2 (SO4) 3
The use of silver sulfate as a catalyst increases the completeness of oxidation to 95% (Komarov).
To get reliable results, you need to pay attention to:
- 1) careful preparation of the soil for analysis;
- 2) exact observance of the duration of boiling during the oxidation of organic matter; the boiling of the oxidizing mixture itself should proceed calmly.
Soil preparation for analysis. When preparing the soil for analysis for humus content, special attention should be paid to the removal of roots and various organic residues of plant and animal origin from the soil.
From a soil sample taken in the field and brought to an air-dry state, an average sample of 50 g is taken, the roots and organic residues visible to the eye (insect shells, seeds, coals, etc.) are carefully selected with tweezers, soil clods are crushed with a wooden pestle with a rubber with a tip and again carefully select the roots, using a magnifying glass.
Then the soil is ground in a porcelain mortar and passed through a sieve with a hole diameter of 1 mm, after which an average sample weighing 5 g is again taken from it and the selection of roots is repeated using the following method. A dry glass rod is vigorously rubbed with a dry cloth or woolen cloth and quickly passed at a height of about 10 cm above the soil, distributed in a thin layer over the surface of wax or parchment paper.
Thin small roots and semi-decayed plant residues, which could not be selected before due to their small size, adhere to the surface of the electrified stick and are thus removed from the soil. They are removed from the stick when it is rubbed again. It should not be held too low with a stick above the soil surface in order to avoid the removal of not only organic residues from the soil, but also fine earth.
In the process of selecting roots, it is necessary to repeatedly mix the soil and again distribute it in a thin layer. The operation should be carried out until only single roots are found on the stick. The purity of the selection of roots is controlled, in addition, by viewing the soil in a magnifying glass.
At the end of the selection of roots, the soil is again ground in a porcelain, jasper or agate mortar and passed through a sieve with a hole diameter of 0.25 mm. The entire 5g sample should be prepared in the manner described above. It is impossible to discard a part of the sample that is difficult to grind.
The soil prepared in the above way for analysis should be stored in parchment paper or wax bags or in test tubes with stoppers.
Of the indirect methods for determining humus, the method of I.V. Tyurin, based on the oxidation of carbon of the organic matter of the soil with a sulfate solution of potassium dichromate, the excess of which is titrated with a solution of Mohr's salt, is most widely used. In fact, this method determines the oxidizability of humus. If we assume that the interaction of a solution of potassium dichromate with soil only oxidizes the carbon of humus and restores Cr 2 O 7 2- to Cr 3+, then the reaction can be schematically expressed by the following equation:
3С + 2K 2 Cr 2 O 7 + 8H 2 SO 4 → 3CO 2 + 2Cr 2 (SO 4) 3 + 2K 2 SO 4 + 8H 2 O
Since the potassium dichromate solution is poured into the sample of soil in excess, some part of it remains unused after the completion of the carbon oxidation reaction. The unreacted excess of Cr 2 O 7 2- is titrated with Mohr's salt solution (NH 4) 2 SO 4 ∙ FeSO 4 ∙ 6H 2 O:
K 2 Cr 2 O 7 + 6FeSO 4 + 7H 2 SO 4 → Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
The volume of Mohr's salt solution used for titration is used to calculate the carbon content of the soil.
When interacting with humus, the Cr 2 O 7 2- ion reacts not only with carbon, but also with hydrogen, which is part of organic compounds:
12Н + 2K 2 Cr 2 O 7 + 8H 2 SO 4 → 2Cr 2 (SO 4) 3 + 2K 2 SO 4 + 14H 2 O
Since the product of hydrogen oxidation is water, it will not affect the results of carbon determination only if the ratio of hydrogen and oxygen atoms in the composition of soil humus is 2:1, as in H 2 O. If the ratio H:O >2 in humus, then more K 2 Cr 2 O 7 is consumed for its oxidation than is required for carbon oxidation, and the results are overestimated. With a ratio of H:O< 2 на окисление гумуса K 2 Cr 2 O 7 израсходуется меньше, чем необходимо для окисления углерода. В этом случае результаты будут заниженными.
Sulfuric acid solution of potassium dichromate reacts not only with humus, but also with some mineral components of the soil.
When analyzing soils containing free carbonates, sulfuric acid is partially neutralized, but this does not affect the results of determining humus carbon.
If the soils are saline and contain chloride ions, then the results of determining the total humus turn out to be overestimated, because along with the oxidation of carbon, Cr 2 O 7 2- is also consumed for the oxidation of chloride ions. The presence of reduced iron and manganese ions in hydromorphic soils also leads to overestimated results, since a part of Cr 2 O 7 2- goes to the oxidation of these ions. However, restrictions on the use of the Tyurin method for determining the humus content in hydromorphic soils apply only to freshly taken samples. It has been repeatedly noted in the literature that when analyzing samples of hydromorphic soils dried to an air-dry state, the results of determining humus obtained by the Tyurin method practically do not differ from the results obtained by the Knopp-Sabanin method. Therefore, the Tyurin method can also be used for the analysis of air-dry samples of hydromorphic soils.
The shortcomings of the Tyurin method include incomplete oxidation of organic matter, especially when analyzing samples from peaty horizons or enriched in decomposed plant remains. The humus content found by the Tyurin method is 85-95% of the amount determined by the dry combustion method according to Gustavson. For a more complete oxidation of carbon in organic compounds with a solution of potassium dichromate, I.V. Tyurin recommended using 0.1-0.2 g of Ag 2 SO 4 as a catalyst. In this case, 95–97% of the carbon of organic compounds is oxidized; however, in the practice of mass analyzes, a catalyst is usually not used.
Analysis progress. On an analytical (or torsion) balance, a sample of soil prepared for determining the total humus is taken, accurate to the third decimal place. It is recommended to adhere to the following weights (V.V. Ponomareva, T.A. Plotnikova, 1980):
Samples of soils are transferred into dry, clean 100 ml conical flasks and exactly 10 ml of a 0.4 N solution of a chromium mixture are added to them from a burette. It is a thick, viscous liquid, and if it is added quickly, some of the reagent will remain on the walls of the burette, which will lead to a large inaccuracy in the results of the analysis. The chromium mixture must be added slowly, at such a speed that falling drops can be seen. The nose of the burette should touch the neck of the flask to avoid splashing of the reagent when the drops fall freely.
The flasks are closed with small funnels or a stopper - a refrigerator and placed on a preheated tile. From the moment large bubbles of gas appear, the solution should boil moderately for exactly 5 minutes. It should not be taken as the beginning of boiling the intense release of small bubbles of air absorbed by the soil, which occurs even before boiling. The boil should always be more or less the same in intensity, neither too violent nor too weak, and the bubbles a little larger than a poppy seed. Boiling should not be accompanied by the release of steam from the funnel.
In the process of boiling, the solution of the chromium mixture changes its color from reddish-brown to brownish-brown, and sometimes green. The green color of the chromium mixture after the end of boiling indicates that potassium dichromate was not enough for the complete oxidation of soil humus. In this case, the analysis should be repeated with a smaller sample of soil.
After the boiling time, the flasks are removed from the stove and cooled. The funnel or stopper-refrigerator, as well as the walls of the flask, are washed from the rinse with distilled water, diluting the solution in the flask by 2-3 times. Add 5-6 drops of an indicator (0.2% solution of phenylanthranilic acid) and titrate the unreacted residue of the chromium mixture with 0.2 N. Mohr's salt solution until the brownish-brown color changes first to purple and then to green. The color of the chromium mixture, especially at the end of the titration, changes very sharply, so the titration must be done carefully and vigorously stir the contents of the flask all the time in a circular motion. The transition from purple to green comes from one drop of Mohr's salt. Reliable results are obtained when at least 10 ml of 0.2 N Mohr's salt solution is used to titrate the potassium dichromate residue.
Under strictly similar conditions, a blank determination is carried out in 2-fold repetition, adding about 0.1 g of calcined soil or pumice to the flask instead of the analyzed soil.
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where V 1 is the amount of Mohr's salt solution used for titration of 10 ml of chromium mixture in a blank experiment, ml; V 2 is the amount of Mohr's salt solution used for titration of the chromium mixture of the analyzed sample, ml; n is the normality of the Mohr salt; 0.003 – molar mass of carbon equivalent, g/mol; m is the sample of soil, g; Kn 2 o - conversion factor for absolutely dry soil; 100 is a multiplier for conversion to 100 g of soil.
Calculation example. The sample of soil taken to determine humus is 0.305 g. 25.8 ml of Mohr's salt solution was used for titration of a blank sample, 22.3 ml of Mohr's salt solution was used for titration of the analyzed sample. The normality of Mohr's salt solution is 0.204. The conversion factor for absolutely dry soil is 1.072. The organic carbon content is:
Humus \u003d 0.96 ∙ 1.724 \u003d 1.66%.
The following reagents are used for analysis:
1. 0.4 n. solution of K 2 Cr 2 O 7 in dilute (1:1) sulfuric acid. 40 g of K 2 Cr 2 O 7 are dissolved in 500-600 ml of distilled water and filtered through a paper filter into a 1-liter volumetric flask. The solution is brought to the mark with distilled water and poured into a heat-resistant container with a capacity of 2.5-5 liters. To a solution of K 2 Cr 2 O 7 in a fume hood, poured in small portions (approximately 100 ml each) with careful and repeated stirring, 1 liter of concentrated H 2 SO 4 (pl. 1.84). When the solution is mixed with sulfuric acid, a strong heating of the liquid occurs, so you need to perform operations very carefully and use only heat-resistant dishes.
The prepared solution is closed with a funnel or glass and left to stand for complete cooling until the next day, then poured into a bottle with a ground stopper and stored in a dark place.
2. 0.2 n. Mohr's salt solution. Take 80 g of salt (NH 4) 2 SO 4 ∙ FeSO 4 ∙ 6H 2 O ( only blue crystals are used, brown ones are discarded) is placed in a flask filled with 650-700 ml of 1 N H 2 SO 4 solution and the solution is shaken until the salt is completely dissolved. The solution was then filtered into a 1 L volumetric flask and made up to the mark with distilled water. Mohr's salt solution is stored in a bottle insulated from the air with a Tishchenko flask with an alkaline solution of pyrogallol or a tube with Mohr's salt crystals.
The normality of Mohr's salt solution is established and checked by 0.1 N. KMnO 4 solution. Due to the fact that the normality of Mohr's salt changes rapidly, it should be checked after 1-2 days. To do this, 1 ml of H 2 SO 4 (density 1.84) is poured into a 250 ml conical flask with a measuring cylinder, 10 ml of Mohr's salt solution is measured with a burette, 50 ml of distilled water is added and titrated with 0.1 N. KMnO 4 solution (prepared from fixonal) to a slightly pink color that does not disappear within 1 min. The titration is repeated and the average value is taken. The normality of a solution of Mohr's salt is found by the formula:
V 1 ∙ N 1 = V 2 ∙ N 2
where V 1 and N 1 are the volume and normality of the Mohr salt solution, V 2 and N 2 are the volume and normality of the KMnO 4 solution.
3. 0.2% solution of phenylanthranilic acid C 13 H 11 O 2 N. Phenylatranilic acid is insoluble in water, so the indicator is prepared in a soda solution, for which 0.2 g of phenylanthranilic acid is dissolved in 100 ml of a 0.2% solution of anhydrous soda ( Na 2 CO 3). For better dissolution, a sample of phenylanthranilic acid is pre-moistened in a porcelain cup with a 0.2% soda solution to a creamy state and, in this form, is thoroughly mixed with a glass rod. After that, the rest of the soda solution is poured.
4. 1 n. H 2 SO 4 solution. In a 1 L volumetric flask filled with ~ 500 ml of distilled water, add 28 ml of concentrated H 2 SO 4 measured with a cylinder and mix. Allow the flask to cool to room temperature, dilute to the mark with distilled water and mix thoroughly.