Method for Purifying Tetrabutylammonium Iodide for the Polarographic Determination of Electronegative Elements
Abstract Generated abstract
A simple recrystallization method is described for purifying commercial tetrabutylammonium iodide used as a supporting electrolyte in polarographic determination of electronegative elements. The authors examine dimethylformamide as a solvent, determine the temperature dependence of salt solubility, and propose dissolving the reagent in freshly redistilled dimethylformamide, crystallizing at room temperature, filtering, and drying. The procedure gives a reported yield of 75 to 83 percent, produces a salt suitable for preparing a stable 0.05 M dimethylformamide background, and provides an acceptable polarographic working range from approximately minus 1.5 to minus 2.3 V after minor compensation.
Full Text
METHOD FOR PURIFYING TETRABUTYLAMMONIUM IODIDE FOR THE POLAROGRAPHIC DETERMINATION OF ELECTRONEGATIVE ELEMENTS
V. K. IVANOV, E. V. TIKHOMIROVA
(Presented by the Scientific-Methodological Seminar of the Problem Laboratory of Microimpurities)
Solutions of tetrabutylammonium iodide prepared in various solvents are used as indifferent electrolytes in the polarographic determination of electronegative elements. The commercial reagent of this salt does not always satisfy the requirements imposed on polarographic supporting electrolytes. The reason for this is evidently the insufficient purity of the salt, as well as the presence of impurities of decomposition products of the tetrabutylammonium iodide itself. Therefore, the use of commercial salt for preparing the supporting electrolyte without preliminary purification is not recommended.
In the literature, numerous methods are proposed for purifying tetrabutylammonium iodide. Methods are known that are based on recrystallization of this salt from ethyl acetate [1], from anhydrous dioxane [2], and from a mixture of benzene with dimethylformamide [3]. However, purification of the reagent by such procedures is often associated with complicated apparatus and repeated recrystallization, while the yields of pure salt are very low, which leads to large losses of material.
Fig. 1. Dependence of the solubility of tetrabutylammonium iodide in dimethylformamide on temperature
In order to improve the quality of purification of tetrabutylammonium iodide, numerous experiments were carried out in our laboratory on selecting a suitable solvent and increasing the yield of the recrystallized salt.
As the solvent for purification we used freshly distilled dimethylformamide. We obtained the dependence of the solubility of tetrabutylammonium iodide in dimethylformamide (Fig. 1).
The purification method we propose is as follows: 30 g of commercial tetrabutylammonium iodide is dissolved, with heating, in 20 ml of freshly redistilled dimethylformamide until complete dissolution. If necessary, the hot solution is filtered, after which it is left for 24 hours at room temperature. The precipitated crystals are collected on a glass filter and dried at 100°C for 2–3 hours. This gives 23–25 g of recrystallized salt. In our experiments the yield varied from 75 to 83%. The salt purified in this way keeps well under ordinary conditions, and the background prepared from it—0.05 M tetrabutylammonium iodide in dimethylformamide—remains without any noticeable change in properties for several months.
Fig. 2. Cathodic polarograms of 0.05 M tetrabutylammonium iodide in dimethylformamide: 1—commercial reagent, 2—after a single recrystallization, 3—dirty reagent.
Figure 2 shows cathodic polarograms of the background, recorded with a film electrode in the potential interval from −1 to −2.5 V. The reference electrode was bottom mercury. As can be seen from the figure, by applying a small compensation we can easily obtain a fully acceptable working segment on the background line in the interval from −1.5 to −2.3 V.
Conclusions
A new, simple method has been proposed for purifying tetrabutylammonium iodide.
The high yield, with a relatively large reagent deficiency, favorably distinguishes this method from those used previously.
References
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- P. Zaurent, Bull. Soc. Chem. France, 848 (1960).
- L. Silverman, W. Bradshaw, Analyt. Chem., 31, 1672 (1959).