Gooch and Austin — Determination of Manganese, etc. 233 



Art. XXIV. — On the Determination of Manganese as the 

 Pyrophosphate ; by F. A. Go.och and Martha Austin. 



[Contribution from the Kent Chemical Laboratory of Tale University — LXXIIL] 



For the estimation of manganese in a gravimetric way when 

 accuracy is a consideration, recourse is usually taken to the 

 excellent method of Prof. Wolcott Gibbs.* This method con- 

 sists in the precipitation of a manganous salt by an alkaline 

 phosphate, the conversion of the tribasic phosphate into the 

 ammonium manganese phosphate, and the weighing of the 

 product of ignition as the pyrophosphate. 



By Dr. Gibbs' original method the orthophosphate of man- 

 ganese was precipitated by hydrogen disodium phosphate in 

 large excess above the quantity required to cause the precipita- 

 tion. The flocky white precipitate was dissolved either in 

 sulphuric or hydrochloric acid, and precipitated again at the 

 boiling temperature by ammonia in excess. This semi-gelati- 

 nous precipitate on boiling or long standing even in the cold 

 becomes crystalline, the crystals forming beautiful talcose scales 

 which have a pearly luster and a pale rose color. The preci- 

 pitate was filtered off, washed with hot water, dried and ignited. 

 The results obtained by Dr. Gibbs' students for the pyrophos- 

 phate accord closely with the theory. 



Freseniusf showed subsequently that ammonium manganese 

 phosphate dissolves in cold water, in hot water, and in an 

 aqueous solution of ammonium chloride [1:70] to the extent of 

 1 part in 32,000, 1 part in 20,000, and 1 part in 18,000, respec- 

 tively. It is clear, however, that the solubility of this precipi- 

 tate is not indicated necessarily by the proportions given so 

 long as an excess of the precipitant is present during the wash- 

 ing, though Fresenius did tind in the filtrate traces of manganese 

 which to his mind were sufficient to account for losses indicated 

 by his test analyses, viz., one to three milligrams of oxide, or 

 from two to six milligrams of phosphate. 



Another mode of manipulation has been advocated by Blair;): 

 in order that the precipitate may be obtained more easily in 

 crystalline condition. According to this method dilute ammo- 

 nia is added drop by drop to the hot acid solution until the 

 precipitate begins to form, the boiling and stirring are con- 

 tinued until the small amount of flocky precipitate is converted 

 completely to crystalline condition, and the process of adding 

 ammonia drop by drop is repeated until the manganese is all 

 down in crystalline condition. The dilute ammonia is added 

 in excess and the liquid filtered after cooling in ice water. 



* This Journal, xliv. 216. f Zeitschrift fur Anal. Chera., vi, 415. 



% The Chemical Analysis of Iron, 106. 



Am. Jour. Sci.— Fourth Series, Yol. VI, No. 33- September, 1898. 

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