V. N. NEUSTROEVA, N. N. SUDAKOVA

DETERMINATION OF HEXAVALENT CHROMIUM IN THE ELECTROLYTE OF AN ACID CHROMIUM-PLATING BATH BY THE ELECTRICAL CONDUCTIVITY OF SOLUTIONS

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

The operating regime of the electrolyte of a chromium-plating bath presupposes a constant concentration of the principal components: $\mathrm{CrO_3} = 250\ \mathrm{g/l}$, $\mathrm{H_2SO_4} = 2.5$–$10\ \mathrm{g/l}$.

The maximum permissible concentrations of impurities are: $\mathrm{Cr^{\cdots}} = 7\ \mathrm{g/l}$; $\mathrm{Fe^{\cdots}} = 8\ \mathrm{g/l}$; $\mathrm{Cu^{\cdots}} = 5\ \mathrm{g/l}$; $\mathrm{NO_3'} =$ traces.

A survey of the literature has shown that at the present time analysis of an electrolyte for its content of hexavalent chromium is carried out mainly by chemical methods. As is known, electrometric methods of analysis have a number of advantages over chemical methods. These are, above all, rapidity of execution and the possibility of determining small amounts of elements. With electrometric methods of analysis, broad possibilities are created for automating production control. Unfortunately, the literature contains very little information on electrometric methods for monitoring chromium-plating baths. The existing works on the determination of hexavalent chromium concern the determination of it in steels, alloys, or solutions that do not contain impurities of other ions.

These works, from our point of view, may be divided into two groups.

1. Optical methods of analysis (colorimetric, photometric—1, 2, 3, 4, 6, 7). By these methods trivalent and hexavalent chromium is determined under the condition that other colored ions are absent from the solutions, and, first of all, iron and copper ions (1, 4). In a number of works, ions interfering with the determination of chromium (2, 7), or chromium itself (3, 6), are separated chromatographically, or it is extracted into a layer of organic solvent. The work of separating ions is very labor-consuming, is associated with a large number of analytical operations, and requires much time.

2. Electrochemical methods of analysis (conductometry, potentiometry, polarography—8, 9, 10, 11, 12). Works in this area are connected either with the determination of small amounts of chromium (9, 11), or are carried out in solutions that contain no other ions. In all cases, preliminary treatment of the sample is required, with a large expenditure of time and reagents. The percentage error is 2–3% (8, 10, 12).

In the electrolyte of a chromium-plating bath there is a high concentration of hexavalent chromium. The solution is chromic acid. The acid is strong, $K_{1,\mathrm{diss.}} = 1.8 \cdot 10^{-1}$; $K_{2,\mathrm{diss.}} = 3.2 \cdot 10^{-7}$. The concentration

the sulfuric acid and impurities of iron, copper, and trivalent chromium are small in comparison with its chromic acid content. Under such conditions it could be assumed that the electrical conductivity of the electrolyte would be determined by the content of hexavalent chromium and would depend only slightly on the presence of small amounts of other substances. To measure the electrical conductivity of solutions and to study its dependence on the concentration of all components of the chromium-plating bath electrolyte, we assembled an apparatus. The electrical circuit of this apparatus is shown in Fig. 1. In the apparatus, in order to avoid

Fig. 1. Schematic of the apparatus for studying the electrical conductivity of solutions: \(R_x\)—vessel with electrolyte solution; \(R'_m\)—fixed resistance; \(R_m\)—resistance box; \(Oc\)—oscilloscope; \(Zg\)—audio-frequency generator.

electrolysis, which entails a change in electrolyte concentration, an audio-frequency generator of the ZG-10 type was used. The investigations were carried out at an alternating-current frequency of 1000 Hz. An oscilloscope was used as the null instrument, registering the smallest current in the circuit. At the smallest current in the circuit, the resistance of the solution being studied is practically equal to the resistance selected on the resistance box, and on the oscilloscope screen the amplitude of the sinusoid becomes minimal. The vessel for measuring electrical conductivity was chosen of an appropriate design, taking into account the smaller consumption of electrolyte for carrying out the analysis (Fig. 2). The capacity of the vessel was 10 ml. Platinum electrodes were used. The distance between them and the size of the electrodes were selected so that the resistance of the solution under study was on the order of several tens of ohms.

Course of analysis. The solution under investigation, diluted the appropriate number of times, was poured into the vessel for measuring electrical conductivity. The vessel was placed in a thermostat maintained at \(25^\circ\mathrm{C} \pm 0.1^\circ\mathrm{C}\). The thermostat was water-filled and equipped with a heater, a temperature regulator, and a stirrer. The vessel with the solution under investigation was kept in the thermostat for 5 minutes so that the solution acquired the temperature of the thermostat and po

After this, the resistance of the solution was measured. For this purpose, by rotating the knobs of the resistance box, a resistance was selected that corresponded to the minimum amplitude of the sinusoid on the oscilloscope screen.

Preparation of the solution under study (dilution, holding in a thermostat) takes 8–10 minutes of working time. The actual measurement of the resistance of the solution under study takes 1–2 minutes.

Table 1

Effect of sulfuric acid on the electrical conductivity of a CrO₃ solution

We carried out a study of the electrical conductivity of chromic acid solutions as a function of its concentration (Table 1, No. 1), as a function of the concentration of sulfuric acid in the solution (Table 1), of the presence of impurities of iron, copper, and trivalent chromium salts (Table 2, Nos. 1, 2), and of both simultaneously (Table 2, Nos. 3, 4, 5). Moreover, these data were checked for various degrees of dilution of the solutions (Tables 1, 2).

Fig. 2. Vessel for measuring the electrical conductivity of solutions: 1—platinum electrodes; 2—stopcock for draining solutions.

In the study, the quantity inverse to the electrical conductivity of the solution—the resistance of the solution—was recorded. On the basis of this study, the following was established:

1. When sulfuric acid is added, the electrical conductivity of the chromic acid solution increases somewhat, and this effect is less pronounced at high dilution of the solutions.

2. The addition of salts somewhat lowers the electrical conductivity of the solutions (the resistance of the solution increases), but almost within the limits of measurement accuracy. At very high dilution of the electrolyte (160-fold), the effect of salts on the change in the electrical conductivity of chromic acid is greater.

3. When the electrolyte is diluted 40–50 times, the influence of sulfuric acid and salt impurities on the electrical conductivity of the electrolyte is the smallest. Under these conditions, the electrical conductivity is determined mainly by the content of hexavalent chromium in the solution.

Table 2

Effect of salts and sulfuric acid on the electrical conductivity of solutions

On the basis of the foregoing, we measured the electrical conductivity of the electrolyte of seven artificially prepared acid chromium-plating baths with the sample diluted 40-fold. In these baths the concentration of hexavalent chromium varied from 260 to 230 g/L CrO₃. This, on average, corresponds to the possible change in the concentration of hexavalent chromium during operation of the bath. The concentration of free sulfuric acid was constant and equal to 5 g/L. The concentrations of impurities of iron, copper, and trivalent chromium were taken in amounts corresponding to one-half of their maximum permissible norm in chromium-plating baths (Table 3, No. 1).

The choice of such an electrolyte regime is explained by the fact that deviations from it toward a higher or lower content of free

sulfuric acid or impurities will have little effect on the magnitude of the electrical conductivity of the solutions. The latter will be determined mainly by the content of hexavalent chromium in the solution.

Table 3

Electrical conductivity of the electrolyte of artificially prepared chromium-plating baths

Table 4

Determination of CrO₃ content by the electrical-conductivity method and analytically

The obtained dependence of the resistance of the solutions (a value inversely proportional to electrical conductivity) on the content of chromic anhydride in them is shown graphically in Fig. 3.

Using this dependence, it is possible to determine the concentration of hexavalent chromium in any galvanic baths of acid chromium plating.

To verify the accuracy of determining the concentration of hexavalent chromium from the electrical conductivity of solutions, we prepared a series of electrolytes with different contents of chromic anhydride.

(within the range 230–260 g/L CrO₃), free sulfuric acid, and impurities. Analysis for the content of hexavalent chromium in them was performed by the chemical method and by the electrical conductivity of the solutions.

Fig. 3. Graph for Table 3. Resistance of solutions as a function of the CrO₃ content.

Hexavalent chromium was determined by the chemical method by titrating the sample with hydrogen peroxide (5). For analysis by the electrical conductivity of the solutions, 5 mL of electrolyte was diluted 40-fold (in a 200 mL volumetric flask), and the resistance of the solutions was measured in the vessel shown in Fig. 2. From the graph (Fig. 3), the content of chromic anhydride in the electrolyte was determined. The results of the analysis by both methods are given in Table 4.

Comparison of the analytical data shows that the error in determining hexavalent chromium by the electrical conductivity of solutions is, on average, not more than 1%.

Conclusions

1. The electrical conductivity of chromic acid solutions of various concentrations was measured, and its dependence on the presence of small amounts of sulfuric acid and impurity ions Fe···, Cr···, Cu·· (within the permissible norm for these impurities in chromium-plating baths) was studied.

1. It has been shown that, under the existing operating conditions of the electrolyte in an acid chromium-plating bath, the electrical conductivity depends mainly on the concentration of \(\mathrm{CrO_3}\) in the solution.

2. It has been established that, on the basis of measurements of the electrical conductivity of the electrolyte, the concentration of hexavalent chromium in an acid chromium-plating bath can be determined. The experimental error averages 1%. The time required to carry out the analysis is no more than 10 minutes.

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