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An investigation was carried out on the application of lead-selective ion electrode for the determination of sulphate in food coal-tar dyes with the aim of adopting it as the routine method for product examination. Promising results were obtained. Coal-tar dye solution was titrated with 0.01M lead nitrate solution to form precipitate of lead sulphate and then the presence of an excess of lead was selectively detected with lead electrode. Since lead sulphate was slightly soluble in water which might lead to an indistinct end-point, 50% dioxane solution was chosen as the suitable solvent. Since three xanthene dyes (Food Red No. 3, No. 104 and No. 105) formed precipitate with lead nitrate solution, removal of these dyes was indispensable before titration; and so an aliquot of the filtrate obtained by the filtration of dye acid precipitate formed by the addition of nitric acid was used. Remarkable jumping of electrode potential was observed when the pH was adjusted neutral before titration. Presence of copper and chloride was known to show interference on the titration, but the permissible level of the presence of them in food coal-tar dyes was not so deadly effective as to disturb the reading of the end-point. The methods finally adopted were compared with the contemporarily adopted routine method (turbidimetry) on 8 kinds of food coal-tar dyes, from the results it became clear that the ion electrode titrimetry was superior to turbidimetry both in recovery and repeatability.

Determination of sulphates in food coal-tar dyes by use of lead-selective ion electrode.