SU310175A1 - METHOD OF SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHORUS - Google Patents

Description

The invention relates to methods for the quantitative analysis of substances.

A spectrophotometric method for the determination of phosphorus is known, based on measuring the optical density of blue phosphor-blended heteropoly complex (PMK) solutions. It includes the preparation of the test hood or solution; sampling; its neutralization by phenolphthalein; addition of a 1.2% solution of molybdate in 12% sulfuric acid; recovery of PMK in two stages: first with a 0.12% solution of hydrazine sulfate in 12 ° / o-sulfuric acid with a pH of 1.0-1.5 n. sulfuric acid and then, after diluting the sample with water to a certain volume, in order to reduce the acidity to 0.2-0.3 n. hydrochloric acid 0.5% solution of tin dichloride; bringing the volume of the solution in the flask to the mark with water; shaking, settling for 1 hour; photometry and and calculation of phosphorus content in the analyzed object from photometry data using a pre-constructed calibration graph.

The known method is not accurate enough, the sensitivity and reproducibility of the results of photometric analysis of blue PMK solutions are insufficient in connection with the restoration (at 0.2-0.3 N. of the acidity of the medium) of free molybdate, which is in excess, to molybdenum blue, the spectrum of which is superimposed on spectrum of blue PMK obtained using tin dichloride as a reducing agent.

In addition, at low acidity of the medium, hydrolysis of tin chloride reductant is possible, and turbidity of the test solution. The appearance of turbidity in a photometric solution always serves as a source of large errors in photometric analysis.

The technological cycle is extended and the analysts labor productivity is accordingly lowered, which is due to the two-stage addition of two solutions of reducing agents to the analyzed sample — first hydrazine sulphate, and then, after diluting the solution with water, tin dichloride, and the effect of these reducing agents appears with two different the acidity of the medium.

The aim of the invention is to eliminate the listed disadvantages.

According to the proposed method, one (single) working solution of a reducing agent is used, containing two components: tin dichloride as a reducing agent and a specific reagent — sulphate or hydrazine chloride or hydroxylamine — as an accelerator of the complex formation reaction and inhibitor (retarder) of the process of formation (bleaching) During the reaction of the blue PMKU, during the reaction, the optimum acidity of the medium in which the blue PMK formation process takes place is at the level of 0.6-0.8N. sulfuric acid, mainly in the region of 0.7 n. It is precisely at this end of the hydrogen ion free ion: the reagent, molybdate, is not reduced to molybdenum blue and does not appear to be cloudy.

The mechanism of action of a specific reagent, sulfate or chloride of hydrazine or hydroxylamine, as an accelerator of the complex formation reaction, on the one hand, and an inhibitor of the process of destroying the blue PMK, on the other, consists in the following.

With the interaction of divalent tin and yellow PMK, blue PMK and tetravalent tin are formed:

N. P (W, LM + 2Sn2 + + 4H- 4:

4: H, P (MO, o, OH) + 2Sn + 4.

This reaction is of equilibrium nature and with the accumulation of tetravalent tin in the solution, which has a relatively high oxidative potential (+0.15 V), there is a gradual oxidation of the formed blue PMK and its destruction (discoloration).

In order to speed up and shift the equilibrium reaction to the right, it is necessary to eliminate tetravalent tin from the reaction sphere. This is achieved by introducing a specific reagent into the solution, which is not only capable of rapidly reducing tetravalent tin ions, but also irreversibly destroyed during this reaction. Such specific reagents can be hydrazine chloride or sulfate, hydroxylamine. As is known, hydrazine is irreversibly destroyed during the course of a redox reaction to form molecular nitrogen;

N, + 4H ++ 4e.

The interaction of hydrazine with tetravalent tin ions proceeds according to the following scheme:

 + Na + N, + 4H +

Thus, during the reaction of the formation of blue PMK, due to the action of hydrazine or hydroxylamine, one of the reaction products, tetravalent tin, capable of destroying the blue PMKI, is continuously reduced, and a specific reagent, for example hydrazine or hydroxylamine, changes the direction of the blue PMK formation reaction, shifting it to the right, and thus speeds up the process of "ripening blue PMK (what is sometimes called the process of developing color).

On the other hand, hydrazine or hydroxylamine removes tetravalent tin ions from the reaction sphere and inhibits the destruction (decolouration) of the detectable blue PMK.

The following facts provide convincing evidence of the correctness of the evaluation of the function and role of each of the two named components - tin chloride and hydrazine or hydroxylamine in the complexation process.

Maximum light absorption of blue PMK recovered by a solution containing both

These components are located in the region of 720–725 N. M, which exactly corresponds to the spectral characteristic of the blue PMK, reduced by tin chloride alone. lack of hydrazine. Between tem

The blue PMK, reduced, for example, by hydrazine sulfate alone in the respective optimal conditions, has a maximum absorption light at 825 nm.

The value of the optical density of the blue FMK solution becomes constant after 5–10 min after mixing of the reagents and remains unchanged for 24 hours or more. The essence of the proposed

method on the example of the analysis of soil stretching.

Example. Object of study - 0.2 n. salt extracts from soil.

Reagents

1.2,5 / o-ny solution of ammonium heptamolybdate in 7 n. sulfuric acid.

2. Reducing solution: 5% solution of hydrochloric or, sulfuric acid, 0.1259 / o-ny on tin dichloride and 0.5% on hydrazine sulfate. The latter can be replaced by hydrazine chloride, hydroxylamine chloride or sulphate, and tin chloride by stannous sulphate.

3. Standard phosphorus solution containing 0.1 mg 1 ml PjOs. Test Method A 50 ml reactor was injected with an aliquot (e.g., 5 ml) with a dispenser of the test extract and neutralized with p-dinitrophenol alkali. Then the volume of the solution in the reactor is brought to 40 ml with distilled water, after which 5 ml of molybdate solution (reagent 1) is added to the reactor, agitated and after 10 minutes 2 ml of reducing agent solution (reagent 2) is added, agitated, volume liquids in the reactor with water up to 50 ml and shaken again. At the same time, a control solution containing all reagents, except for the determined phosphorus, is prepared in the same way. The solutions are left for 30 minutes in a place protected from direct sunlight. Photometry of the solutions is carried out using an SF-4A spectrophotometer (Khmax 720-725 nm) or a photoelectric colorimeter of the type FEC-M, FEC-56, etc., with a red light filter. The phosphorus content in the analyzed soil is calculated from the photometric results using a pre-built calibration graph. The calibration graph, expressing the relationship between optical density and the concentration of PMK, is linear in the concentration range O — 0.1 mg of PgO3 per 50 ml. The magnitude of the molar extinction coefficient of the PMK measured at Hmax 720 nm is 3.55-10-Å, which corresponds to the literature data for the PMK solutions reduced by tin dichloride. The sensitivity of the determination with a thickness of a layer of liquid in a cuvette of 5 cm is 0.001 mg / 50 ml of PsOs. Optical density (color) of solutions is stable for 24 hours or more. A comparative test of the method was carried out. The spectrophotometric method for determining phosphorus in the form of a blue PMK with the use of hydrazine sulfate as a reducing agent was used as a reference. Some data obtained during tests of the proposed method are presented in Table. 1-3. As can be seen from the above data, the proposed method has sufficient accuracy and good reproducibility. The high stability of the optical density (color) of the solutions permits the introduction of this method into the technology of mass analysis and the transfer of all technological operations to a flow chart using the existing technological equipment. The application of the method will increase the productivity of analysts, improve the organization of labor and the workplace. Table 3

The results of comparative tests of the proposed method for determining the gross P205 in plant objects

Subject invention

Claims (2)

1. A method for the spectrophotometric determination of phosphorus predominantly in extracts from agrochemical objects, including the preparation of a extract, neutralization with a sample with an alkali, dilution with water, adding to the sample an acidified solution of ammonium molybdate and a solution of divalent tin and photometry, characterized in that, in order to improve accuracy analyzing and increasing the labor productivity of analysts, the restoration of the phosphomolybdenum complex is carried out in one stage with a solution containing, in addition to two more valent tin and specific reactant - hydrazine sulfate or chloride, or hydroxylamine. 2. A method according to claim 1, characterized in that the reduction of the phosphomolybdenum heteropoly complex is carried out at a constant acidity of the medium of 0.6-0.8N. (for sulfuric acid), preferably at 0.7 N. hydrogen ion concentrations.