In the year I882, I described a process for determining phosphoric acid, volumetrically, by ammonium molybdate, on the principle of Wildenstein's sulphuric acid determination, or of Gay Lussae's silver method. An aqueous solution of ammonium molybdate is run into the solution of the phosphate until no further precipitate is formed.
But it is not of this process that I now have to speak. It is referred to here, in order to draw attention to the concluding paragraph of the paper, as follows :
"I have obtained very sharp and accurate results by determining the amount of yellow precipitate (formed as above, after thorough washing), by means of a standard solution of caustic alkali, using litmus as an indicator; a description of which I hope to present in a future paper. I mention it here simply to place the fact on record." Iffourlz. Franh. Inst., 113, 193 ; Chem. News, 46, 7.) At that time I did a considerable amount of work upon the last-mentioned process, with very satisfactory results. A study was made of the conditions most favorable to obtaining a phospho-molybdate precipitate of constant composition, using solutions of di-sodie hydrie phosphate of known strength, and also a solution of apatite, the determinations being checked by standard methods.
Before the process was in shape for publication, however, my attention was called to work of an entirely different nature, and no description of the method was published other than that embodied in the above-quoted paragraph.
Since that time several chemists have described proeesses based upon the same principle.
Nov., I893.] Chemical Section. 363 E. Thilo, in the analysis of Thomas slag (Chem. Ztg., 11, I93), dissolves the yellow precipitate in standard ammonia, and titrates back with acid, using litmus as an indicator.
Franz Hundeshagen (Zezt. Anal. Chem., 28, I7I) uses standard sodium hydrate in excess, and titrates back with nitric acid, using phenolphthalein as an indicator.
C. E. Manby (.four. Anal. and Apfld. Chem., 6, 82) determines the phosphorus in steel, iron and iron ones, by dissolving the yellow precipitate in ammonia, acidifying with nitric acid, evaporating to dryness, and heating gently to expel nitric acid and ammonium nitrate. He then titrates, using the same solution and indicator as Hundeshagen employs.
James O. Handy (Jour. Anal and A~Opd. Chem.,6, 2o4) avoids the evaporation and heating and titrates directly as Thilo and Hundeshagen do, using standard soda and phenolphthalein. M. Rothberg and W. A. Auchinvole (Jour. Anal. and Appd. Chem., 6, 243) also describe the same