A new method for the direct photometric determination of silicon in copper-base alloys offers considerable advantage over the regular gravimetric method in speed and simplicity of operation. Determinations of silicon in a group of manganese bronze samples containing up to 0.1 5% of silicon show good agreement with results obtained b y the regular gravimetric method. The optimum conditions for the development and evaluation of the silicomolybdate color complex in the presence of copper are discussed. A method for overcoming the interference of phosphorus is described.HE usual gravimetric determination of silicon in copper-base T alloys (2) is a rather lengthy and exacting procedure. Two dehydrations must be made to ensure complete separation of the silicon and where, as is often the case, insoluble silicides remain undecomposed after the first dehydration, these must be fused with sodium carbonate and carried through two more dehydrations, These multiple dehydrations of the large amount of salts resulting from a 5-gram sample must be made slowly and carefully to avoid spattering. Besides the time consumed by dehydration, considerably more time must be spent in filtering, igniting, weighing, and volatilizing the separated silica and reweighing the platinum crucible.I n the present emergency, when large volumes of samples must be handled daily and results reported quickly, a simpler and more rapid method for this determination is highly desirable. A photometric method seemed to offer the best possibility along these lines.A survey of the literature revealed that practically all the proposed colorimetric methods for the determination of silicon (17,20) depend upon the formation of a silicomolybdate complex by the reaction of silicic acid and ammonium molybdate in a moderately acid solution. This method is often credited. to Dienert and Wandenbulcke (3) in the recent literature, but the reaction was used for the colorimetric determination of silica &s long ago as 1898 by Jolles and Neurath (6). The method in various modifications has been used for the determination of silicon in fresh water (3, 6, 9), ir. sea water (13, 18, f9), in boilerfeed water (15), in tissue (8), in iron and steel (ff), and in aluminum-and magnesium-base alloys (1, 4,5,12). No reference could be found to the use of this reaction for the determination of silicon in copper-base alloys.T h a t the reaction has not been so employed is probably due to difficulty in obtaining complete solution of the silicon when ordinary acids are used for dissolving these alloys. The thought occurred that if the samples were dissolved by adding a little hydrofluoric acid to the regular dissolving acid, and the excess hydrofluoric acid was inactivated by the addition of boric acid (f4), it should be possible to obtain a solution of the sample which could be treated directly with ammonium molybdate t o develop the silicomolybdate color complex. Experiment showed this to be true. Solution of the sample must, of course, take place in a platinum container and all contact...

No abstract