DETERMINATION OF NANOGRAM QUANTITIES OF MERCURY IN NATURAL WATERS BY THE PPN METHOD

A. I. KARTUSHINSKAYA, N. A. KOLPAKOVA

(Presented by the scientific-methodological seminar of the Problem Laboratory of Microimpurities)

In studying the geochemistry of mercury, of great interest is the clarification of such questions, little covered in the literature, as its content in the hydrosphere. The first quantitative determinations of mercury in waters were published in 1934 by A. Stock and F. Cucuel, who established the minimum mercury content in water at the level of \(n \cdot 10^{-10}—n \cdot 10^{-11}\ \text{g/ml}\), which has been confirmed by a number of subsequent works.

To establish the presence in water of such an amount of mercury, it is necessary to use simple and rapid methods of very high sensitivity, permitting mass determinations to be carried out.

It is important that the method be applicable near the sampling site, that is, under field-laboratory conditions. The literature [1—3] describes spectral and colorimetric methods for the analysis of natural waters of relatively low mineralization (fresh and slightly saline waters with mineralization of \(0—5\ \text{g/l}\)). Owing to the insufficient sensitivity of these methods, all of them provide for a certain concentration of mercury from solution, which substantially distorts the results of the analysis and lengthens the time required for carrying it out.

Table 1

In addition, these methods are completely unsuitable for the analysis of highly mineralized waters and brines (mineralization up to 300 g/l). Table 1 gives data on the sensitivity of a number of methods for mercury and on the method of its concentration used in the present procedure.

A new instrumental method of analysis—polarographic analysis with accumulation on a stationary electrode—is considerably simpler and more accurate when applied to water analysis; it has a simple apparatus design and, for a broad range of elements, possesses high sensitivity.

A number of works [7–9] have long pointed out the prospects for applying the polarographic method of analysis in hydrogeochemical prospecting and exploration, and even present data on the use of this method in expeditionary and field laboratories.

The aim of the present work was to study the possibility of applying polarography to the analysis of waters of high mineralization (up to 300 g/l) and to develop a procedure for determining mercury in such waters.

The method consists in the electrochemical concentration of the ion being determined in the form of a film on the surface of a graphite electrode, followed by its anodic dissolution at a continuously varying potential.

Fig. 1. Dependence of the depth of the anodic notch on the concentration of mercury in a solution of 0.1 M KNO\(_3\) + 0.1 M KCNS; \(t_{\mathrm{e}} = 10\) min, \(\varphi_{\mathrm{e}} = -0.7\) V (relative to sat. c. e.), \(K_{\mathrm{e}} = 100\), \(D \cdot Z\) a·cm/g-ion.

It was established experimentally that the depth of the notch during the electrodissolution of mercury is directly proportional to the concentration of the reducible ions in solution [Fig. 1], which is consistent with the theoretical conclusions for the electrodissolution of thin films from the surface of a solid electrode [4].

A peculiarity of the analysis of natural waters is that these waters always contain a large amount of iron. When the solution is alkalized, the formation of a colloidal solution of iron hydroxide is observed, capable of adsorbing on its surface the ions present. Therefore, the polarographic determination of mercury should be carried out from acidic solutions (pH = 1).

For the purpose of determining mercury in the presence of large amounts of copper, which is reduced near the potential of mercury deposition on acidic backgrounds, among the backgrounds studied (0.1 M KNO\(_3\) and 0.1 M HNO\(_3\), etc.).

Table 2

We selected a background of 0.1 M KNO\(_3\) + 0.1 M KCNS, on which the difference between the potentials of the mercury and copper peaks is maximal, \(\Delta \varphi = 0.4\) V (on other backgrounds \(\Delta \varphi = 0.1\)—0.2 V).

Analytical procedure

The analysis was carried out on an OH-101 polarograph (Hungary) with an electrolyzer described in [5]. A graphite electrode served as the working electrode; its preliminary preparation was similar to that described earlier [6]. A mercury sulfate electrode was used as the reference electrode. The salt bridge was filled with a half-saturated solution of KNO\(_3\) and, on the side of the solution being analyzed, was closed with a filter-paper plug. The solution to be analyzed was preliminarily acidified with triply distilled concentrated nitric acid until the precipitated iron was completely dissolved (pH-1). Then an aliquot portion of the solution, 5–10 ml, was taken into a quartz beaker, into which a definite amount of KNO\(_3\) and KCNS was introduced so that the concentration of salts in the solution was 0.1 M. The solution was stirred with a stream of nitrogen. The concentration of mercury ions in the solution was determined by the method of additions. Electrolysis was carried out at a potential of \(-0.7\) V (vs. sat. calomel electrode).

Table 3

Table 4

Avg. 14%.

The results of the analysis of a series of highly mineralized natural waters are summarized in Table 2. To evaluate the accuracy of the method, a statistical treatment of the experimental data obtained was carried out.

The procedure can be used under field conditions. The determination is not interfered with by \(10^{-7}\) g/ml silver and all other ions. The sensitivity of the method is \(1 \cdot 10^{-9}\) g/ml mercury, the analysis time is 10 minutes, the electrode surface is \(S = 3 \cdot 10^{-2}\) cm\(^2\), \(K_e = 1000\) g a.s.m./g-ion. To determine lower concentrations of mercury in solution, we investigated the possibility of concentrating it by evaporating the solution being analyzed at a temperature of 90°–100°. Using the analysis of carbonate natural waters with artificially introduced mercury as an example (mineralization does not exceed 5.0 g/l), we demonstrated the possibility of such concentration. In this case, the error of determination does not exceed the experimental error. The data of such an analysis with concentration of the solution by a factor of 25 are given in Table 4.

This procedure is applicable to fresh and slightly saline waters (mineralization up to 5 g/l).

Conclusions

1. A procedure has been developed for the polarographic determination of mercury in natural waters of any mineralization. The sensitivity of the described method is \(1 \cdot 10^{-9}\) g/ml. Electrolysis time is 10 min.

2. The possibility has been shown of determining lower mercury concentrations by evaporating the solution.

3. To ascertain the accuracy of the method, statistical processing of the experimental results was carried out. The greatest possible error of an individual measurement does not exceed 20%.

4. The determination of mercury is not interfered with by \(Ag^+\) ions at a concentration lower than \(10^{-7}\) g/ml, nor by other ions.

5. The method can be used under field conditions.

References

1. N. Kh. Aidinyan. Proceedings of the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Academy of Sciences of the USSR, issue 46, 1960.

2. P. A. Udodov, I. P. Onufrienok. Proceedings of the First All-Union Conference on Geochemical Methods for Prospecting Ore Deposits. Gosgeoltekhizdat, 1957.

3. Methodological Guide to the Determination of Microcomponents in Natural Waters in the Prospecting of Ore Deposits. Gosgeoltekhizdat, Moscow, 1961.

4. Kh. Z. Brainina, E. M. Roizenblat, V. B. Belyavskaya. Zavodskaya laboratoriya, XXVIII, 9, 1047, 1962.

5. A. G. Stromberg, M. S. Zakharov, V. E. Gorodovykh, L. F. Zaichko. Zavodskaya laboratoriya, XXVII, 517, 1961.

6. Kh. Z. Brainina and N. K. Kiva. Zavodskaya laboratoriya, XXVIII, 5, 1963.

7. I. Yu. Sokolov, A. A. Brodsky, A. D. Miller, A. A. Reznikov. Bulletin of Scientific and Technical Information. Ministry of Geology and Protection of Mineral Resources of the USSR, No. 10 (44), 90, 1962.

8. Methodological Guide to the Determination of Microcomponents in Natural Waters in the Prospecting of Ore Deposits. Gosgeoltekhizdat, Moscow, 1961.

9. Wahler Willian, Chem. der Erde, No. 3, 23, 1964.