POLAROGRAPHIC DETERMINATION OF ORGANIC IMPURITIES IN WATER

A. A. KAPLIN, L. M. POSKREBYSHEVA

(Presented by the scientific seminar of the Department of Physical Chemistry)

The analysis of organic impurities lags significantly behind the analysis of inorganic impurities in sensitivity, accuracy, and selectivity; for the latter, the absolute sensitivity of a number of methods already reaches \(10^{-10}\)—\(10^{-11}\) g.

As the basis for studies on the determination of organic microimpurities, we have taken the dependence of the magnitude of the current of ion reduction or metal-atom oxidation on the nature and amount of surface-active substances adsorbed on the electrode. The paper proposes the use, for the determination of surface-active organic substances, of alternating-current polarography without preliminary concentration, using a TsLA vector polarograph.

The element used as the standard for the quantitative estimation of the content of organic substances is selected on the basis of the following considerations:

1. Reduction in the region of the organic impurities being analyzed.

2. Maximum dependence of electrokinetic parameters on the nature and amount of adsorbed surface-active substances.

When working with alternating current, we selected cadmium; cadmium reduction occurs in the region of the electrocapillary zero; its sensitivity to surface-active substances is the highest among the elements we studied (copper, lead, antimony, bismuth, zinc). In view of the fact that among the organic substances to be determined in solution there may be neutral molecules as well as positively and negatively charged particles adsorbing at different potentials, we propose using 3–4 elements as standard metals, reduced over a wide range of potentials (for example, copper, cadmium, zinc).

As the standard organic substance in determining impurities of individual organic substances of known composition, the same substance of known concentration is used. In determining the sum of organic substances in deionized water, we used as the standard the tetrabutylammonium bromide (TBAB) recommended in [1]. It should be noted that in this case the selection of the standard organic substance is one of the most difficult methodological tasks.

Experimental Section

The measurements were carried out on a TsLA vector polarograph in a polarographic cell made of fluoroplastic or quartz. The anode was bottom mercury, and the cathode was a stationary mercury-film electrode. A 1 M KCl solution was used as the supporting electrolyte.

Into the working beaker were introduced 6 ml of a \(1 \cdot 10^{-5}\) M cadmium solution. The solution was stirred with nitrogen for 1–2 min, after which a series of cathodic cadmium peaks was recorded. For calculations the mean value of 8–10 measurements was used. After this, the first addition of TBAB was introduced into the solution and a series of cadmium peaks was recorded. Then the next addition was made, and so on, and a calibration plot was constructed for the dependence of the percentage decrease in the cadmium peak on the TBAB concentration. The graph qualitatively coincides with the Langmuir isotherm.

To evaluate the sensitivity of the method using alternating-current polarography, the results obtained on the vector polarograph were compared with data obtained on an LP-60 polarograph with a linearly varying potential, under the condition of the same initial height of the cadmium peaks. With the aid of a changeover switch, the polarograms were recorded practically simultaneously on both polarographs in the same cell. From the results obtained it follows that the sensitivity of alternating-current polarography to surface-active substances is higher.

We studied the effect of water with different contents of organic impurities on the cathodic cadmium peaks. Such water was obtained by saturation on KU-2 resin in a column with \(d = 50\) mm and \(h = 200\) mm for different periods of time. Dependences were obtained of the percentage decrease in the cathodic cadmium peaks on the number of additions of water with different contents of organic impurities. As the contact time of the water with the resin increases, the amount of organic impurities in the water increases.

The comparatively high sensitivity of the method makes it possible to use it for monitoring the content of organic microimpurities in deionized water used in semiconductor technology.

For monitoring the total content of organic impurities in water, the chemical method of oxidizability is used; this method has low sensitivity, which requires the use of large volumes of the water being analyzed (up to 1 l), and is labor-intensive and time-consuming.

We carried out a comparison of the polarographic method with the oxidizability method. From the comparison it was found that water in which organic impurities are not detected by the oxidizability method gives a decrease in the cadmium peak of 60 mm upon addition of 1 ml of this water to an initial cadmium-solution volume of 5 ml.

The proposed method for determining organic impurities in water has a number of advantages:

1. Comparatively high sensitivity of determination (down to \(10^{-9}\)—\(10^{-8}\) g/ml), and the possibilities for increasing the absolute sensitivity have not been fully utilized.
2. Less time required to carry out the analysis and a smaller number of reagents.
3. Possibility of automating the water analysis.

References

1. N. P. Berezina, N. V. Fedorovich, Zh. “Elektrokhimiya”, vol. III, No. 1, 1967, p. 3.