On Reduction of Ammonium Molybdate in Acid

Ammonium molybdate, in acid solution, is reduced to an oxide stage, with the production of a blue color, by many substances of biological importance. Our attention was first attracted to this subject through an accidental observation. We were engaged in the attempt to work out a technique for the nephelometric estimation of phosphoric acid, using a heavy solution of cane sugar for the medium of suspension. When the mixture containing acid, ammonium molybdate and cane-sugar was heated, a deep blue color was rapidly produced. The same reaction was slowly accomplished in the cold. Further study indicated that this reaction was due to the levulose split off from the cane sugar by the acidity of the solution. Later we found that this reaction had been described as a test for levulose by Pin0ff.l Later study of the reaction has yielded instructive results. The reaction is in some ways a rather specific one, since comparatively few reducing substances, otherwise active, are able to effect in acid solution the transfer of the metal to the stage of blue oxide. A review of the literature dealing with this metal indicates firstly that more than one oxide stage presents a blue color, and secondly that the several oxide stages are so unstable and interchangeable that a qualitative relation of a particular reagent to a particular oxide is not now demonstrable. The solution we have most commonly used consists of 30 grams of ammonium molybdate and 25 cc. of sulphuric acid dissolved in 1 liter of distilled water. The test is made by adding to a few cubic centimeters of the reagent about an equal volume of the