Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Volume 164　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1967

On the Conditions for Carrying Out Analysis in the Determination of Trace Amounts of Impurities in High-Purity Materials by Amalgam Polarography with Accumulation

L. F. Trushina, M. S. Zakharov

(Presented by the Scientific-Methodological Seminar of the CTF)

In determining trace amounts of various elements in high-purity materials, both contamination of the sample being analyzed and losses of it during the course of analysis may occur [1].

The analysis of high-purity substances by the method of amalgam polarography with accumulation, as with most other methods, may be regarded as a combination of the following stages:

1. Preparation of the sample (specimen) for analysis.
2. Treatment of the sample.
3. Direct determination of the impurity in high-purity materials.

At all stages of analysis it is very important to avoid the possibility of contamination of the substance being analyzed or its losses. As is known [2, 3, 4], sources of contamination may include:

1) the reagents used, and the water used for dissolving the sample and preparing standard solutions;
2) the vessels used at all stages of analysis;
3) laboratory air, containing microimpurities of many elements: Zn, Pb, Fe, Cu, etc.

When working with small quantities of substances, losses of impurities are also possible, caused by anomalous volatility and by their adsorption on the walls of the vessels [5—6].

In this connection, it becomes necessary to carry out all stages of the analysis under conditions that eliminate or reduce the possibility of contamination of the sample being analyzed or loss of the impurity being determined [7].

Various methods of treating the sample during preparation of the substance for analysis may be accompanied by considerable contamination or losses [8]. An important role here is played by the duration of the operations, as well as by the aggressiveness of the substances used in the analysis in combination with high temperatures. When preparing a sample for analysis (crushing, grinding, mixing, weighing solid samples, pouring liquids), it is necessary to eliminate the possibility of contamination introduced during sampling through contact of the sample with the material of the cutting tool, pipettes, etc. As has already been indicated earlier [9, 10], for grinding soft samples (high-purity In, Sn) it is advisable to use scissors and mortars made of plastics (organic glass, fluoroplast), and foil pads. It should be taken into account that ordinary glass, from which pipettes widely used in microanalysis are made, contains

a very large number of different elements (such as Zn, Cu, Pb, Al, Cr, etc.); the presence of foreign polyvalent cations on the glass surface increases its adsorption capacity [11]. Therefore, wherever possible, we recommend eliminating the use of pipettes and other intermediate vessels. Thus, in the analysis of traces by the method of amalgam polarography with accumulation, the evaporation process is almost always carried out in small volumetric beakers—up to 5–10 ml—and therefore it is possible simply to add the necessary solutions up to the mark in an isolated atmosphere. Where such procedures are impossible, the walls of vessels and pipettes should be coated with thin sorbents (chlorosilane, Desicote) to eliminate losses of the impurities being analyzed due to adsorption on the vessel walls. During evaporation of the sample being analyzed, which is performed in order to concentrate impurities in a small volume [12] and separate them from the matrix, contamination from the air may have the strongest effect, since the evaporation process may be carried out for 2–3 hours or more, depending on the evaporation temperature, the volume of the solution being evaporated, and its boiling point. When evaporating a sample, repeated evaporation in the same vessels is more appropriate than using a large number of them. In determining trace impurities \((10^{-7}—10^{-8}\%)\) by the method of amalgam polarography with accumulation, we can recommend for evaporation of the analyzed sample an evaporation chamber made as a single unit with a box, shown in Fig. 1.

Fig. 1. Evaporation chamber

A sealed box made of organic glass is tightly joined on the outside by the evaporation chamber. The latter is a Pyrex or quartz beaker in which there are two branch tubes for introducing nitrogen, purified according to [13], and for removing the evaporation products. We carried out a series of experiments on evaporation of the sample in open air, in the chamber proposed in [14], for determination of \(10^{-5}—10^{-6}\%\) impurities, and in the chamber proposed by us. The results of determining copper in pure water, carried out under otherwise identical conditions (temperature, solution volume, concentration of the substance analyzed, electrolysis time, etc.), are presented in the table.

Table

Results of determination of copper in pure water

As can be seen from the table, the effect of using such a chamber is quite significant. Contamination of the analyzed sample during evaporation in the indicated box is reduced to a minimum. It is advisable to pass purified air or nitrogen through the box [15–17]. For evaporation it is use—

...are sometimes used as containers made of heat-resistant or quartz glass, isolated from the surrounding atmosphere [18, 19].

In the proposed chamber it is possible to evaporate solutions and substances having boiling points not higher than 250–300°. For treating a sample with a low-boiling solvent, it is best to use infrared lamps or water baths of special design [20–22].

Particularly great attention should be paid to the cleanliness of the beakers in which the sample being analyzed is treated.

As our studies have shown, it is best to carry out the treatment of the sample being analyzed in beakers made by pressing from fluoroplastic-4. Fluoroplastic is not wetted by aqueous solutions, contains no metal impurities, and is resistant to aggressive media when heated [23].

For storing fluoroplastic and quartz beakers with a capacity of 5–10 ml, we recommend using small intermediate boxes made of organic glass. In addition, it is convenient to carry the beakers in them. For washing the beakers we propose a steamer improved by us, described earlier in [24], which is quite simple in design (Fig. 2). To an ordinary Pyrex flask with a capacity of 1–2 liters there is ground on a reflux tube ending in forked tubes for the beakers. In order that during washing on the steamer the beakers not be contaminated by impurities from the surrounding medium, a chamber with a hydraulic seal is soldered to the reflux tube. During washing, the beakers hang on the prongs in the chamber.

Experiments carried out by us on washing beakers on the proposed steamer and on a steamer without a hydraulic seal showed that the most effective cleaning of the beakers is achieved when, during washing, they are isolated from the surrounding atmosphere.

Fig. 2. Steamer with a hydraulic seal

At the stage of the direct determination of a microimpurity, which consists in concentrating it on a mercury stationary electrode at a constant potential and in the subsequent dissolution of the amalgam formed at a continuously varying potential, it is necessary to observe the same precautions as in the preceding stages.

To avoid prolonged contact of the working solution with the laboratory atmosphere, it is desirable to carry out both the removal of oxygen from the solution and the polarographic measurement in a closed box through which purified air is passed. The amount of impurity in the analyzed high-purity material is usually determined by the method of additions.

In this connection, questions of preparing the initial standard solutions, the method of their storage, the material of the vessels in which they are stored, and the stability of standard solutions of various elements as a function of their storage time acquire great importance.

Cases are known in which the source of errors caused by adsorption of the substance being analyzed on the walls of vessels is the use of polyethylene ware. This applies mainly to saturated acid solutions (H₂S, HNO₃, etc.) [25].

However, in a number of works [4, 23] it is indicated that polyethylene ware has the lowest adsorption capacity with respect to metal cations.

Taking into account that in the analysis of trace amounts of various elements one has to deal with standard solutions of small...

concentrations—$10^{-8}$—$10^{-9}$ g-ion/liter, when phenomena associated with adsorption of the substance being analyzed can introduce significant errors into the results of the analysis, it is advisable to determine the possibility of adsorption of various elements in their standard solutions of low concentration.

We investigated the stability of standard solutions of Pb, Cd, and Zn at concentrations of $1 \cdot 10^{-6}$ g-ion/liter and $1 \cdot 10^{-8}$ g-ion/liter as a function of storage time and vessel material.

All glassware used in these investigations had previously been used for storing various solutions. For the indicated experiments, the vessels were thoroughly washed and checked for cleanliness; for this purpose, 50 ml of a 0.1 N solution of $\mathrm{Na_2SO_4}$ was poured into the vessel, and the solution was shaken in the vessel for 30 min. For polarography, 10 ml of the solution was taken. If no anodic peaks appeared on the polarogram, the vessel was considered clean.

Standard solutions of lead, zinc, and cadmium at concentrations of $10^{-2}$ g-ion/liter were prepared by dissolving the metals in nitric acid purified by repeated distillation in quartz apparatus. Solutions with concentrations of $10^{-6}$—$10^{-8}$ g-ion/liter were prepared by successive dilution of more concentrated solutions. All glassware used for preparing the standards was first thoroughly washed and checked for cleanliness. The pH of the standard solutions studied ($10^{-6}$—$10^{-8}$ g-ion/liter) was equal to four; N was created by adding pure nitric acid.

The concentration of Pb, Cd, and Zn in the solutions was determined by the method of amalgam polarography with accumulation. Each determination was repeated three times,

Fig. 3. Stability graph of a standard zinc solution (pH = 3–4) as a function of storage time and vessel material. Curves 1, 4—polyethylene vessel; curves 2, 5—quartz vessel; curves 3, 6—ordinary-glass vessel; curves 1, 2, 3—for a solution with concentration $1 \cdot 10^{-7}$ g-ion/liter, and curves 4, 5, 6 for a solution with concentration $1 \cdot 10^{-6}$ g-ion/liter.

and the average of the three measurements was taken as the desired value.

The results of the investigations are presented in Figs. 3, 4, 5. Using Figs. 3, 4, and 5, we shall discuss the behavior of each element in vessels made of different materials.

From Fig. 3 (curve 4) it is evident that, when a zinc solution with concentration $1 \cdot 10^{-6}$ g-ion/liter is stored in a polyethylene vessel, the content

the latter in solution over the course of 30 days practically did not change. However, when storing a solution with a concentration of \(1 \cdot 10^{-7}\) g-ion/l, with time the concentration of zinc in the solution increased (Fig. 3, curve 1). This can be explained by desorption of zinc from the walls of the polyethylene bottle. As already indicated above, the polyethylene bottles used for the study had previously been used for storing

Fig. 4. Stability of a standard lead solution (pH = 3–4) as a function of storage time and vessel material. Curves 1, 4—polyethylene vessel; curves 2, 5—quartz vessel; curves 3, 6—vessel made of ordinary glass; curves 1, 2, 3—for a solution with a concentration of \(5 \cdot 10^{-8}\) g-ion/l; curves 4, 5, 6—for a solution with a concentration of \(1 \cdot 10^{-6}\) g-ion/l.

various solutions in which the zinc concentration was considerably higher than \(1 \cdot 10^{-7}\) g-ion/l.

Storage of zinc solutions with concentrations of \(1 \cdot 10^{-7}\) and \(1 \cdot 10^{-6}\) g-ion/l for 30 days in quartz vessels practically did not lead to a change in the zinc content in the solution (Fig. 3, curves 2 and 5).

A different picture was observed when zinc solutions were stored in vessels made of ordinary glass. In this case, with time the concentration of zinc in the solution gradually increased, which can be explained by leaching of zinc from the glass.

Thus, zinc solutions with a concentration of \(1 \cdot 10^{-6}\) M are best stored in quartz or polyethylene vessels, but not in vessels made of ordinary glass.

When lead solutions with concentrations of \(1 \cdot 10^{-6}\) and \(5 \cdot 10^{-8}\) M were stored in polyethylene vessels, the lead content in the solution continuously increased, which can be explained by desorption of lead from the vessel walls (Fig. 4, curves 1 and 4). Practically no change was observed in the concentration of lead in solutions stored in quartz vessels (Fig. 4, curves 2 and 5). A slight decrease in the lead content occurred in solutions stored in vessels made of ordinary glass (Fig. 4, curves 3, 6). Consequently, the experimental data obtained show that lead solutions at concentrations of \(1 \cdot 10^{-6}\) g-ion/l and lower remain most stable when stored in quartz vessels.

It is evident from Fig. 5 that, when cadmium solutions with concentrations of \(1 \cdot 10^{-6}\) and \(5 \cdot 10^{-8}\) g-ion/l are stored in polyethylene vessels, the cadmium content in the solution increases with time (curves 1 and 4).

In solutions stored in quartz vessels, a slight increase in the concentration of cadmium was observed (Fig. 4, curves 2 and 5). When cadmium solutions with a concentration of \(5 \cdot 10^{-8}\) M were stored in vessels made of ordinary glass, an increase in the cadmium content in the solution was observed over time, whereas in solutions with a concentration of

Fig. 5. Dependence of the height of the anodic peak of cadmium on the storage time of the standard Cd solution and on the vessel material (\(\mathrm{pH}=3\text{--}4\)). Curves 1, 4—polyethylene vessels; curves 2, 5—quartz vessels; curves 3, 6—vessels made of ordinary glass; curves 1, 2, 3 refer to a solution with cadmium concentration \(5 \cdot 10^{-8}\) g-ion/l; curves 4, 5, 6—to a solution with concentration \(1 \cdot 10^{-6}\) g-ion/l.

\(10^{-6}\) g-ion/l there was a decrease in the concentration of the element. The observed phenomena can be explained by adsorption and desorption effects and by leaching.

Thus, solutions of cadmium, lead, and zinc with a concentration of \(10^{-6}\) g-ion/l are best stored in quartz vessels.

Conclusions

1. Studies were carried out to determine the conditions for measuring trace amounts of impurities. It was established that the accuracy of determination can depend strongly on the conditions under which the analysis is performed.

2. To obtain satisfactory analytical results, it is necessary to carry out all analytical operations in hermetically sealed chambers and boxes.

3. Our studies showed that solutions of the cations Pb, Zn, Cd with concentrations of \(1 \cdot 10^{-6}\) g-ion/l and lower are best stored in quartz vessels.

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