Extraction-Photometric Determination of the Total Heavy Metals in Demineralized Water

V. B. Sokolovich, Yu. L. Lelchuk, G. A. Detkova

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Among group reagents for heavy metals, the sodium salt of diethyldithiocarbamic acid—sodium carbamate [1]—is of considerable interest. With the ions of more than twenty metals, sodium carbamate forms complex compounds—carbaminates—soluble in organic solvents. Solutions of many carbaminates are colored.

Metal carbaminates differ in solubility and stability, on which the quantitative conversion of the carbaminates of some metals into others is based [2, 3]. Sodium carbamate is used in the determination of individual metals in various materials [4, 5], in the concentration of heavy metals [6, 7], and in determining their total amount [8]. One of the advantages of the carbaminate method is the possibility of calculating results in mg-eq or in eq percent. In determining the total heavy metals with dithizone or thioacetamide, however, the analytical result is conventionally expressed as calculated for some particular metal, which does not give a complete idea of the actual total amount of metallic contaminants.

We studied the possibility of using sodium carbamate for determining the total heavy metals in demineralized water, taking as a basis the procedure used by Blank [8] in the analysis of alkaline salts. Taking into account literature data and the preliminary experimental data obtained by us, a method was developed for determining the total heavy metals in demineralized water.

The proposed method is based on binding the heavy metals contained in water into carbaminates at pH 7–7.5, extracting the latter with chloroform, and converting the heavy-metal carbaminates into copper carbaminate by displacing the heavy metals from their carbaminate complexes in chloroform with copper ions in the aqueous phase. The chloroform extracts of copper carbaminate are colorimetered on an FEK-N-57 with blue light filter No. 3, having a maximum transmittance of 453 mμ, in 50 mm cuvettes.

The result is expressed in μg-eq/l. It is possible to determine 0.04 μg-eq/l and more of heavy metals with a relative error of up to ±20%. The time required for one determination is about 1.5 hours. Seven parallel determinations can be performed in 6 hours.

Experimental Part

In the work, water distilled three times was used (the last time from a quartz distillation apparatus), as well as “medical” chloroform, twice distilled at a temperature of 60°C. Reagent solutions were prepared from chemically pure and analytical-grade reagents. Sodium diethyldithiocarbamate was used as a 10% solution of the “pure” reagent. The initial standard copper solution with a content of 100 μg/ml was prepared from chemically pure CuSO₄·5H₂O. The titer of the solution was checked titrimetrically. The working standard copper solution with a copper content of 1 μg/ml was prepared by appropriate dilution of the initial solution immediately before use.

Construction of the calibration curve. Into separatory funnels are introduced 10 ml portions of tridistillate and a standard copper solution corresponding to 0.0; 1.0; 2.5; 5.0; 7.5; and 10.0 μg of copper; 5 ml of citrate buffer mixture with pH = 7.2 and 3 ml of 1% sodium carbamate solution are added. Everything is shaken and left for 15 min. The carbaminates formed at pH = 7.5 are then extracted twice with chloroform in portions of 10 and 5 ml with two-minute shaking. After separation, the chloroform extracts are drained into another series of funnels, where they are washed with 50 ml of 0.01% caustic soda solution by shaking for two minutes, and transferred to a third series of separatory funnels, into which 5 ml portions of 5% copper sulfate solution are introduced, and everything is shaken for three minutes. The separated chloroform extracts of copper carbaminate are drained into 25 ml volumetric flasks. The aqueous layer remaining in the funnels is washed with 5–7 ml of chloroform, which is combined with the first portions of extract. The solution in each flask is brought to the mark, shaken, filtered through a dry ashless filter into 50 mm cuvettes, and colorimetered on an FEK-N-57 with blue light filter No. 3, having a transmission maximum of 453 mμ. Chloroform serves as the reference solution.

Table 1

Dependence of optical density on the concentration of copper carbaminate

The obtained data, recalculated in μg-eq, are given in Table 1; the calibration curve is in Fig. 1. The equivalent of copper is 31.77.

Preparation of water for analysis. A sample of the water to be analyzed is taken into a polyethylene bottle. Its pH is checked and adjusted to 5.8–6.3 by adding ammonia or hydrochloric acid solutions obtained by saturating separate portions of the analyzed water with gaseous NH₃ or HCl. The pH is monitored with a pH meter or with the indicator methyl red.

Procedure of analysis. Into a 300 ml separatory funnel are introduced 10 ml of tridistillate and an exact volume of the demineralized water prepared for analysis (100, 150, 200 ml, depending on the purity of the water). To the water are added 5 ml of citrate buffer mixture with pH 7.2 and 3 ml of 1% sodium carbamate solution; it is shaken and left for 15 min.

The carbaminates of heavy metals that have formed are extracted twice with 10 and 15 ml of chloroform with shaking for two minutes. The chloroform extracts are drained into another separatory ...

funnel and, after washing with 50 ml of 0.01% caustic soda solution, are drained into a third funnel, where they are shaken for three minutes with 5 ml of a 5% copper sulfate solution in order to convert the carbaminates of the heavy metals into copper carbaminate.

The chloroform extract of copper carbaminate is run into a 25 ml volumetric flask. The remaining aqueous layer is washed in the funnel with 10 ml of chloroform, which is then combined with the main extract. The contents of the flask are brought to the mark with chloroform, shaken, and filtered through a dry ashless filter into a 50 mm cuvette.

Fig. 1. Calibration curve for the colorimetric determination of the sum of heavy metals in demineralized water. Filter No. 3, 50 mm cuvette, reference solution—chloroform

The optical density of the solution is measured on an FEK-N-57 with blue filter No. 3, with a transmittance maximum of 453 mµ. Chloroform is used as the reference solution. A blank test is carried out in parallel.

The sum of heavy metals in the analyzed aliquot volume is determined from the calibration curve and recalculated in µg-equiv/l according to the formula:

\[ \sum TM = \frac{B \cdot 1000}{v}\ \text{µg-equiv/l}, \]

where \(\sum TM\) is the sum of heavy metals in 1 l of demineralized water, in µg-equiv/l;
\(B\) is the sum of heavy metals in the analyzed volume of demineralized water, in µg;
\(V\) is the analyzed volume of demineralized water, in ml.

Using the method described above, the sum of heavy metals was determined in various samples of demineralized waters. Some of the data obtained by us are given in Table 2.

Table 2

Results of determination of the sum of heavy metals in water of various degrees of purity

Conclusions

A procedure is proposed for the extraction-photometric determination of the sum of heavy metals in demineralized water using sodium carbamate.

The procedure makes it possible to determine 0,04 μg-equiv/L and higher contents of heavy metals with a relative error of up to ±20%, with a time expenditure for 7 parallel determinations of about 6 hours.

REFERENCES

1. Yu. A. Chernikhov, B. M. Dobkina. Use of sodium diethyldithiocarbamate in analytical chemistry. Zavodsk. laborator., 15, 1, 10, 1949.

2. A. M. Bulgakova, A. M. Volkova. Extraction-photometric determination of traces of lead in reagents of special purity. Journal of Analytical Chemistry, 15, 591, 5, 1960.

3. S. E. Kreimer, A. S. Lomekhov, A. V. Stogova. Determination of silver with copper diethyldithiocarbaminate. Journal of Analytical Chemistry, 17, 674, 1962.

4. S. E. Kreimer, L. P. Butylkin. Determination of copper with lead diethyldithiocarbaminate. Zavodsk. laborator., 24, 132, 2, 1958.

5. K. P. Stolyarov. Photometric determination of small amounts of cobalt in the form of the diethyldithiocarbamate of trivalent cobalt. Journal of Analytical Chemistry, 16, 452, 4, 1961.

6. I. T. Klimov. Chemical-spectral determination of microelements (heavy metals) using extraction enrichment. In: Modern Methods of Analysis of Natural Waters, Moscow, Academy of Sciences of the USSR, 67–87, 1962.

7. V. P. Belyaev, V. R. Kalinchenko, N. M. Kuzmin. Chemical-spectral method of analysis of water of special purity. Zavodsk. laborator., 28, 685, 1962.

8. A. B. Blank, N. T. Sizonenko, A. P. Bulgakova. Extraction-photometric method for determining the sum of heavy metals using sodium diethyldithiocarbamate. Journal of Analytical Chemistry, 18, 1046–1050, 9, 1963.