C. Fairbanks — Determination of Phosphorus in Iron. 181 



Art. XXII. — An lodometric Method for the Determination 

 of Phosphorus in Iron ; by Charlotte Fairbanks. 



[Contributions from the Kent Chemical Laboratory of Tale University, LIV.] 



A RECENT paper from this laboratory^ showed that molybdic 

 acid was reduced by hydriodic acid to the condition of oxida- 

 tion represented by the symbol Mo^O^ in acid solution, while in 

 an alkaline solution the reduced product was reoxidized by 

 standard iodine. 



In acid solution, 2Mo03-|-2lII=Mo,0, + l2 + Hp 

 In alkaline solution Mo,0^ + I, + Iip = 2Mo03H- 2HI 



These reactions obviously present the possibility of develop- 

 ing a method for the determination of phosphorus in the phos- 

 pho-molybdates. 



The very careful work of Messrs. Blair and Whitfieldf shows 

 conclusively that the ammonium phospho-molybdate, precipi- 

 tated under the conditions ordinarily prescribed for the deter- 

 mination of phosphorus in iron or iron ores, is of definite con- 

 stitution. They find that 24M0O3, P,0„ 3(I^HJ,0, 2H,0 

 expresses the composition of this phospho-molybdate, which 

 therefore contains 1*794 parts of phosphorus to every 100 parts 

 of molybdic anhydride. 



This being the case, it is plain that if advantage can be taken 

 of the fact that the amount of iodine necessary to reoxidize 

 reduced molybdic acid is large, as well as of the fact that the 

 amount of molybdic acid compared to the phosphorus con- 

 tained in the phospho-molybdate is also large, a method of 

 great theoretical accuracy s'hould result. 



I have experimented with a process which consists essentially 

 in treating the ammonium phospho-molybdate, produced by 

 the accepted methods of iron analysis, with potassium iodide 

 in the presence of strong hydrochloric acid and under carefully 

 regulated conditions ; in neutralizing the residue thus obtained 

 by acid sodium carbonate ; and in reoxidizing the reduced 

 molybdenum compound to the condition of molybdic acid by 

 titration with iodine. 



The directions prescribed by Messrs. Blair and Whitfield for 

 making the ammonium molybdate solution were followed : 100 

 grams of molybdic anhydride are intimately mixed with 400^° 

 of distilled water and 80^^ of strong ammonia, and after filtering, 

 this liquid is poured into a mixture of 300^*^ of nitric acid 



* Gooch and Fairbanks, this Journal, vol. ii, p. 156. 

 f Jour. Am. Chem. Soc, vol. xvii (1895). 



