ON THE PROSPECTS FOR APPLYING A NEW HIGHLY SENSITIVE APN METHOD IN GEOCHEMISTRY

N. A. KOLPAKOVA, A. G. STROMBERG, A. A. KAPLIN, N. I. BELOUSOVA (TPI)

One of the main tasks posed by geochemistry is the development of simple, rapid, and highly sensitive methods for determining elements in various geological objects. It is not only geochemistry that needs highly sensitive methods of analysis. Nearly all branches of new technology—the nuclear industry, semiconductor devices, rocket technology, quantum generators, etc.—use materials of very high purity. Therefore, the task of determining \(10^{-5}\)—\(10^{-8}\) percent and smaller amounts of impurities in various objects is a central task of modern analytical chemistry, and the efforts of many groups of chemists, physical chemists, and physicists are concentrated on solving it.

The increase in the sensitivity of analytical methods is proceeding in two directions. First, in the direction of improving methods for concentrating elements through the use of extraction, chromatography, coprecipitation, and other methods of separating and concentrating elements. From the combination of a concentration method and an analytical method there arose new names for combined methods: chemical-spectral, extraction-photometric, chromatopolarographic, and others.

Second, the increase in the sensitivity of analysis has proceeded in the direction of improving the physicochemical methods of analysis themselves and creating new, chiefly physical, methods of analysis. In this connection, new highly sensitive analytical methods have appeared: amalgam and film polarography with accumulation, the luminescent method, kinetic, mass-spectral, and radioactivation (or neutron-activation) methods, and others.

In the present communication we shall dwell on a new highly sensitive method of analysis—the method of amalgam polarography with accumulation (APN)—and, to a lesser extent, on one of its varieties, the method of film polarography with accumulation (PPN).

In a number of works [1, 2] it has long been pointed out that the polarographic method of analysis is promising for hydrogeochemical prospecting and exploration. The essence of the polarographic method consists in the fact that from very dilute solutions (\(10^{-2}\)—\(10^{-3}\) μg/l) the element being determined is accumulated by electrolysis, over the course of 5–10 min, on an electrode (a mercury drop or a mercury film on the surface of platinum

or silver), and then, when the potential is changed linearly toward more positive values, anodic waves are recorded on the polarogram, the depth of which is proportional to the concentration of the element in the mercury drop and in the solution (with all analysis conditions held constant), while the peak potential indicates the nature of the element.

Thus, in contrast to the ordinary polarographic method, a stationary mercury electrode rather than a dropping one is used, and what is recorded is not a cathodic polarogram during deposition of the element on the electrode, but an anodic polarogram during anodic dissolution of the sought element from the amalgam obtained in the preliminary electrolysis. The increase in the sensitivity of the APN method is due to the fact that, in addition to chemical methods of concentration, there is, within the method itself, one more stage of concentration: the process of preliminary electrolysis. This makes it possible to increase the sensitivity of the APN method by 2–4 orders of magnitude in comparison with the ordinary polarographic method and places it among the most sensitive methods of analysis.

Let us consider which elements can be determined by the APN method. From D. I. Mendeleev’s periodic system it is evident that the elements most readily determined by the APN method are those located close to mercury in the I–V side subgroups and in groups 4–6. These 11 elements include: copper, zinc, gallium, germanium, cadmium, indium, tin, antimony, thallium, lead, and bismuth. All these metals are comparatively readily soluble in mercury at room temperature and are cathodically deposited on mercury, or are anodically dissolved from the amalgam, at intermediate potentials on the sloping portion of the residual current, where the discharge of the background cation or ionization of mercury from the electrode is not superimposed on the electrode process. It should be noted here that the possibility of determining gallium and germanium by the APN method was demonstrated 2–3 years ago in our laboratory.

But this does not exhaust the list of elements determined by the APN method. The possibility has been shown of determining by the APN method three more elements: uranium, chromium, and titanium. These three elements are soluble in mercury with formation of amalgams, but, in order to prevent their hydrolysis, they have to be deposited electrolytically from a very acidic solution (\(2M H_2SO_4\)) and at a sufficiently negative potential, in the region of vigorous evolution of hydrogen from the acid, which is associated with certain additional complications that we have managed to overcome. Unstable anodic waves over a narrow concentration interval are also given by nickel, iron, and manganese when they are determined by the APN method. For the determination of these elements, the AKPN method (anodic-cathodic polarography with accumulation) has been developed and is being successfully applied. Its essence is as follows. Elements that do not form amalgams during the process of preliminary electrolysis are deposited in the form of metal on the surface of a mercury-film electrode (the process of electrochemical concentration).

The resulting deposit is anodically transferred into a solution of very small volume (\(<0.1\) ml), and the cathodic polarogram of the solution is recorded immediately. The ratio between the anodic and cathodic waves is 0.8 [1]. Thus, at present about 16 elements can be determined by the APN method. In addition, there is every reason to believe that 11 alkali and alkaline-earth metals will soon also be determinable by the APN method. Thus, for example, we have already obtained anodic waves of sodium, potassium, and lithium. Silver and gold are not yet determined by the APN method, since it has not been possible to select a complex-forming compound (ligand) that would shift the potential of anodic dissolu-

silver and gold from the amalgam by several tenths of a volt more negative than the potential of anodic dissolution of mercury from the electrode.

A valuable supplement to the APN method is the PPN method (film polarography with accumulation). In this variety of polarography with accumulation, the substance is concentrated on an electrode (usually graphite) in the form of a solid film, and is then anodically dissolved, likewise with the production of anodic peaks. In this way many metals are determined, but we shall mention only those among them that are poorly determined by the APN method, yet are of great practical importance in hydrogeochemistry. These are gold, silver, nickel, mercury, and many others. We have developed a procedure for determining silver and mercury in ores and in natural waters of low and high mineralization by the PPN method on a graphite electrode [3]. The sensitivity of the method described is \(5 \cdot 10^{-9}\%\). The electrolysis time is 10 min. Simultaneously with mercury and silver, equivalent quantities of copper and lead can be determined.

In addition to metals, the PPN method can also be used to determine a number of anions (chlorine, bromine, iodine, sulfide ion, and others) that form sparingly soluble compounds with the electrode material (for example, mercury), but we shall not dwell on this.

In the practice of polarographic analysis, foreign polarographs are mainly used (the Hungarian OH-101, the Czech LP-60). In the problem laboratory of trace impurities of the TPI, a new polarograph weighing 3 kg is now being created for geologists, on which polarographic analysis can be carried out directly under field conditions. A new vector polarograph has been developed, with the aid of which organic impurities in natural water can be determined.

Let us draw attention to the fact that the elements determined by the APN and PPN methods are precisely the microelements most frequently encountered that must be determined in geochemical prospecting and exploration. Therefore the APN (and PPN) method should find broad application in geochemistry.

For a more complete characterization of the polarography-with-accumulation method, we present a procedure for determining \(10^{-8}\%\) gold in rocks, minerals, and natural waters by the PPN method.

Course of analysis

A weighed portion of ground ore (1–25 g) is roasted in a muffle furnace at a temperature of 560–600°C. In this process the carbon burns out completely, and iron is converted into an insoluble form. The sample is then dissolved in aqua regia (\(\mathrm{HCl} : \mathrm{HNO_3} = 3 : 1\)) and filtered through an ordinary filter, washing the precipitate with water. The insoluble residue is discarded. The filtrate solution is brought to 100 ml. Further analysis of ore and natural water is analogous. To 100 ml of the sample are added 10 ml of \(1N\ \mathrm{H_2SO_4}\), 1 ml of \(1N\) Trilon B, 1 ml of \(1N\ \mathrm{KSCN}\) (if silver is not to be determined beforehand), and it is shaken successively with several portions of a 0.005% solution of dithizone in carbon tetrachloride. The organic extracts, together with the floating film, are collected in a beaker for polarography. The beaker is first checked polarographically for purity.

The solution is evaporated and ignited at a temperature of 400–500°C. In this process mercury volatilizes from the extract and the organic phase burns off. Into the hot beaker, dropwise, are added a \(1M\) solution of KBr and 0.1 ml of \(2N\ \mathrm{HBr}\). The solution is boiled for better dissolution of the gold. Electrolysis of the gold is carried out at a potential of \(-0.2\) V versus the saturated calomel electrode for 3–10 min. The peak potential of gold is \(+0.5\) V versus the saturated calomel electrode. Table 1 gives

Results of determining gold in natural waters and certain ores.
$S = 1 \cdot 10^{-9}$ g/ml, $\tau = 5$ min, $W = 200$ mg/min, $\varphi^{\mathrm{e}} = -0.2$ V, rel. to s.c.e.

Table 1

the results of analyses of ores and natural waters for gold by the method described above. The sensitivity of the determination of gold in ores is \(1 \cdot 10^{-8}\%\). The relative error of a single measurement is 10%.

REFERENCES

1. I. Yu. Sokolov, A. A. Brodsky, A. D. Miller, A. A. Reznikov. Bull. Sci.-Tech. Inform. Min. Geol. and Mineral Res. USSR, No. 10, 90 (1962).
2. Wahler William. Chem der Erde, No. 3, 23 (1964).
3. N. A. Kolpakova, A. G. Stromberg, A. I. Kartushinskaya. Proceedings of the Ural Scientific-Technical Conference. Sverdlovsk, 1967.