The first suggestion that iodine can be determined by measuring its accelerating effect on the interaction of two substances which otherwise are more or less inert toward each other, appears to have been made by R . L a n g , who at the end of a paper on the titration of trivalent arsenic" with potassium permanganate in the presence of iodate or iodide as catalygt makes the statement, , Z u m Schluß sei noch auf die Möglichkeit hingewiesen, Spuren von Jodverbindungen, die mit sonstigen analytischen Methoden nicht erfaßbar sind, durch Zuhilfenahme der Jodkatalyse der Reaktion Mnm-salz--arsenige Säure nicht nur qualitativ nachweisen, sondern auch, da ja im allgemeinen meist einfache Proportionalität zwischen Geschwindigkeitskonstante und Katalysatorkonzentration besteht, quantitativ bestimmen zu können. ''1 A number of years ago H . B a i n e s ~ described a chronometric method for the determination of iodine as iodide, "depending on the pseudocataIysis by iodide of the oxidation of thiosulphate by nitrous acid". The procedure given was suitable for the determination of amounts of iodine corresponding to 5--50 mg. of potassium iodide.In a preliminary note a it was shown that it is possible to estimate small amounts of iodine (as iodide) by measuring its catalytic effect on the reaction between quadrivalent cerium and trivalent arsenic in sulfuric acid solution. Eren at the boiling point in dilute Sulfuric acid medium, the reaction between cerie ~cerium and excess arsenious oxide is incomplete a in the absence of a catalyst, and at room temperatur~ the velocity is ex-1 R. Lang: Ztschr. anorgan, allg. Chem. 152, 206 (1926).

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