RU2232718C1 - Extraction method for pretreating analytical samples - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: objective of invention is pretreating solid samples of environmental objects to enable quantitative determination of cadmium by physicochemical techniques. According to invention, cadmium is removed from the surface and bulk of natural object using a stratifying system composed of thiopyrine, 0.02-0.03 mole, trichloroacetic acid, 1.2-1.6 mole, orthophosphoric acid, 0.03-0.05 mole, and distilled water to 1 l total volume. Natural material is treated with above-defined system at 25^оС for 30 min.

EFFECT: enabled maximum removal of cadmium for a short time and at room temperature.

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Description

The invention relates to the field of preparation of solid samples of environmental objects (solid particles of snow cover, suspended matter of rivers and lakes, soil, bottom sediments and other solid samples) for the quantitative determination of cadmium by optical, polarographic or other analysis method.

Known methods of dry [GOST 26929-94. Raw materials and food products. Sample preparation. Mineralization for the determination of toxic elements] and wet ashing [Bock R. Decomposition methods in analytical chemistry. - M .: Chemistry, 1984. - 427 p. Chemical analysis / A series of monographs on analytical chemistry // Selected methods of trace metal analysis: Biological and Environmental Samples / Jon C. Van Loon. 1985 / NY / Toronto / Vol.80. - 351 pp] solid particles of samples of natural objects in order to extract (desorb) toxic elements, such as cadmium, from the surface and from the volume of particles into a mixture of mineral acids. As a rule, a mixture of sulfuric, nitric and hydrochloric concentrated acids in various volume ratios depending on the object of analysis is used as extractants of cadmium and heavy metals. Sulfuric and nitric acid act on the solid component of the sample as the destructive and oxidizing agents of the acid mixture, destroying and oxidizing the natural compounds of organic and inorganic composition. Hydrochloric acid acts in the acid mixture as a complexing agent for cadmium and heavy metals, which binds heavy metal ions to chloride complexes. “Royal” vodka (a mixture of nitric and hydrochloric acids in a volume ratio of 3: 1) acts on metallic gold in the same way when it is dissolved. However, dry ashing requires careful monitoring of the temperature of ashing. So, with an increase in temperature of more than 550 ° C, cadmium and other easily volatile metals (bismuth, arsenic, mercury, lead, tin) are irreversibly lost. In addition, refractory expensive quartz dishes are needed. Wet ashing partially eliminates the loss of metals during the "opening" of solid samples. However, the acid mixture used in the wet ashing method requires special care (fume hood, individual protective equipment), additional time and labor costs.

The proposed method of sample preparation eliminates the above listed disadvantages, providing the required degree of cadmium recovery close to 100%.

Of the known technical solutions, the closest in purpose and technical nature to the claimed object is the method of sample preparation according to GOST 26929-94. Raw materials and food products. Sample preparation. Mineralization to identify toxic elements. Common to the prototype and the claimed invention are the use of acids for the desorption of cadmium in solution from the surface and from the volume of solid particles of the sample. The main disadvantages of the prototype should be listed:

- the use of aggressive mineral acids (sulfuric, nitric, hydrochloric);

- prolonged (more than 90 min) heating of the analytical sample.

The proposed method differs from the prototype in that aggressive (sulfuric, nitric, hydrochloric) acids are replaced in the application with phosphoric (salting-out agent) and trichloroacetic (solid organic matter) acids. Acids provide safe extraction of cadmium in the form of a complex with an organic reagent - thiopyrin (not in the prototype), converting the metal into the organic phase from an aqueous solution of acids during the delamination of the four-component system (water, trichloroacetic and phosphoric acids, thiopyrin). In the inventive method does not require prolonged heating, which can cause irreversible loss of cadmium. Unlike the prototype, the claimed method provides the necessary extraction of cadmium in less time for 30 minutes at room temperature. The extraction of cadmium from a solid sample is provided by a sulfur-containing organic nitrogenous reagent - thiopyrine.

The essence of the invention lies in the fact that for the extraction of cadmium from the surface and from the volume of solid particles of a natural object (with a total mass of 0.5-1 grams) with a degree of extraction of 98-100%, a four-component exfoliating system with a volume of 10 ml consisting of a reagent - thiopyrin ( 0.00002-0.00003 mol), trichloroacetic acid (0.012-0.016 mol), phosphoric acid (0.0003-0.0005 mol) and distilled water - up to 10 ml. Unlike the prototype, in order to achieve a technical result and obtain a reliable expert assessment close to 100%, the cadmium desorption process is carried out at room temperature 25 ° C for 30 minutes. The required degree of cadmium recovery is achieved by extracting the metal into the organic phase with a volume of 1 ml formed during the delamination process. The resulting organic phase with recovered cadmium is used for its quantitative physicochemical determination (atomic absorption, polarography, emission spectrometry, spectrophotometry, etc.).

The implementation of the invention is achieved as follows. A sample weighing 0.5-1 grams of natural material, for example, solid particles of snow water obtained after filtration through an MFA-MA membrane filter No. 6 (TU 6-05-1903-81), is weighed on an analytical balance with an accuracy of ± 0,0002 g and placed in a clean 15 ml extraction tube equipped with a ground stopper. The extract is prepared as follows. 4 ml of a 4M solution of trichloroacetic acid (ChP) plus 4 ml of 0.1M orthophosphoric acid (ChP) plus 0.4546 g of thiopyrin plus distilled water up to 10 ml are mixed. The prepared sample of natural material was treated with the prepared extract for 30 minutes at a temperature of 25 ° C and then settled until the formation of the organic phase (≈1 ml). Next, 1 ml of the organic phase was pipetted, diluted with double-distilled water in a 25 ml volumetric flask, and analyzed for the total cadmium content by two methods: inverse voltammetry on a mercury film electrode and atomic absorption spectrometry. As a comparative prototype, parallel samples of the solid components of the snow cover were subjected to wet ashing in a mixture of nitric, sulfuric and hydrochloric acids, prepared in a volume ratio of 2: 1: 1, and also analyzed.

The table provides comparative data for both methods of preparing the same snow samples. From the table it follows that regardless of the degree of contamination of solid aerosols with cadmium, the metal concentrations desorbed by the claimed and known method coincide within the random statistical errors of the analysis. Moreover, the proposed sample preparation technology is more attractive and technologically advanced (pH 2-3, temperature 25 ° С, time 30 minutes, safety, water-organic system with low chemical consumption, water after cadmium extraction can be easily purified).

The prototype, which is a mixture of concentrated sulfuric, nitric and hydrochloric acids, requires additional care and mandatory personal protective equipment for the analyst, as well as good exhaust ventilation to remove toxic gaseous emissions: sulfur dioxide and nitrogen oxides formed during the processing of solid particles of natural objects. Sample preparation in the prototype requires additional heating to 250-300 ° C, heat-resistant flasks equipped with reflux coolers, the decomposition time of samples is 5-6 times longer (2.5-3 hours) compared with extraction, which provides a quantitative transition of metal from the analyzed solid object analysis in the liquid phase for 30 minutes.

Claims (1)

The extraction method for preparing analytical samples of natural objects, which consists in treating cadmium from solid particles of natural material with a mixture of acids, characterized in that an aqueous-organic delaminating system containing a reagent, thiopyrin, in an amount of 0.002-0.003 is used to extract the metal mol and organic - trichloroacetic - 1.2-1.6 mol and inorganic - phosphoric - 0.03-0.05 mol of acid and distilled water to a liter, which at room temperature 25 ° C for 30 min yvayut natural material.