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It is apparent that significantly discrepant results occur only in the case of aluminum alloyed with magnesium silicide and then only when the metal is in a heat-treated condition.In laboratories wherein aluminum and aluminum alloys are analyzed on a routine basis, the filtrate from the silicon determination is used for the determination of other elements. When the tri-acid method is used for silicon, this filtrate is a sulfate solution which is convenient for further work. The filtrate, when the sodium hydroxide method is used, may consist of either sulfates or perchlorates according to which acid is used for dehydrating silica. However, the solution contains large amounts of sodium salts which are somewhat undesirable.The tendency toward low results found when perchloric acid is used as the decomposition reagent is somewhat disappointing; the use of this acid is usually desirable in determining silica, since dehydration of silica is satisfactory and the subsequent re-solution-of salts is more easily effected than when sulfuric acid is used.The data shown are representative of many other data, allof which indicate that while the tri-acid method is satisfactory in most cases the sodium hydroxide method should be used when aluminum-magnesium silicide alloys are analyzed for silicon.The following methods give satisfactory service in determining silicon in aluminum and aluminum alloys. Tri-Acid Solution MethodPlace 1 gram of sample in a 250-cc. beaker. Keeping the beaker covered as much as possible, cautiously add 35 cc. of acid mixture No. 1 (485 cc. of water, 115 cc. of sulfuric acid, 200 cc. of hydrochloric acid, and 200 cc. of nitric acid). When no further action can be seen, evaporate till heavy fumes of sulfuric acid are evolved for 15 minutes, cool, add 10 cc. of 1 to 3 sulfuric acid and 100 cc. of hot water, and boil until salts are dissolved. Stir in some paper pulp, filter through a close-textured paper, and wash well with hot water. Evaporate the filtrate to fumes, cool, dissolve in water, add pulp, filter, and wash as before. Ignite the residues in a platinum thimble. After cooling, mix with 1 to 8 grams (depending on amount of residue) of sodium carbonate.Fuse cautiously until nearly quiet, then finish with a strong heat.