Citric Acid Determination

Three types of methods have been proposed for the determination of citric acid in urine. Salant and Wise (1) used the unspecific mercuric sulfate reagent of Deniges (2) in a study of the toxicity of citrates. Thunberg (3) developed an enzymatic method employing a dehydrogenase said to be specific for citrate. The remaining methods depend on the so called pentabromoacetone method of Stahre (4), a qualitative test published in 1897 which Kunz (5), in 1914, used as the basis of the first quantitative pentabromoacetone method. It has been generally overlooked that in 1847 Cahours (6) prepared a bromination product from several citrates which he called “bromoxaforme” which according to his melting point and solubility data is without doubt identical with pentabromoacetone. Cahours was aware of the specificity and analytical applicability of the reaction. (“Le brome peut done servir a reconnaftre de petites quantit& d’acide citrique mhlangbes a l’acide tartrique.“) Kunz’s technique was adapted for use with urine by Amberg and McClure (7). Fasold (8) with this method was the first to isolate citric acid from normal urine. Kometiani’s (9) modification of this method was applied to urine by Stillmann and Schaerer (10). Pucher, Sherman, and Vickery (11) developed a spectrophotometric method based on the color formed when pentabromoacetone is treated with sodium sulfide. Our procedure is another modification of this method. It is precise, accurate, and timesaving. The “pentabromoacetone reaction” has been clarified and new reagents with certain advantages have been introduced. While the procedure was designed for 15 cc. samples of urine, it is also applicable to 2 cc. samples, as well as to other fluids of comparable citrate content. The range is from 1 to 40 mg., although with care, useful results are obtainable down to 0.2 mg. The method has already been used in a study of the variation of urinary citrate with the menstrual cycle (12).