234 Gooch and Austin — Determination of 



In discussing these methods of precipitation, McKenna* 

 points out that both give good and accordant results, and that 

 the process may be carried on in glass as well as in platinum, 

 if the time of crystallization is made short enough. 



When a manganous salt is precipitated in the cold by an 

 excess of an alkaline phosphate, it falls, as Heintzf has shown, 

 in the form of the trimanganous phosphate of the formula 

 Mn 3 P 2 8 . This same phosphate constitutes, as we have found, 

 the greater part of the precipitate which forms when a man- 

 ganous salt reacts in the cold in the presence of ammonium 

 chloride with microcosmic salt and ammonia in slight excess. 

 Boiling or even subsequent standing may, as is well known, 

 effect a more or less complete conversion of the manganese 

 phosphate to the ammonium manganese phosphate. Thus, in 

 one experiment in which an amount of manganous chloride 

 enough to produce 0-2214 grams of the pyrophosphate was 

 precipitated in the cold by 5 cm3 of a saturated solution of 

 microcosmic salt, with the subsequent addition of ammonia in 

 excess, in a volume of 200 CI " 3 containing also 5 grams of ammo- 

 nium chloride, the residue after ignition weighed 0*1904 grams. 

 Presuming this residue to consist entirely of the pyrophosphate 

 and the trimanganous orthophosphate, the proportion of the 

 former to the latter calculated from the relation of symbols, 

 and the weights taken and found, is nearly one to six. That 

 is to say, about six-sevenths of the precipitate fell in this 

 experiment in the form of the tribasic orthophosphate. In 

 another experiment made exactly similarly, excepting that the 

 liquid was heated to boiling, the proportion of the manganese 

 pyrophosphate to the trimanganous orthophosphate in the 

 only partially crystallized precipitate proved to be two to one. 

 That is, in this case, two-thirds of the precipitate was in the 

 form of the pyrophosphate. In the former of the experiments 

 a small amount of manganese was found in the filtrate, but not 

 enough to change materially the ratio recorded. The slight 

 solubility appears to be connected with the incomplete conver- 

 sion Of the trimanganous phosphate to the ammonium man- 

 ganese phosphate, for as will appear later, the manganese 

 found in the filtrate, when the conversion is known to be nearly 

 complete, is inappreciable unless extraordinary amounts of the 

 ammonium salt are present. The success of the analytical pro- 

 cess under discussion turns, therefore, upon the change of the 

 trimanganous phosphate Mn,P 2 8 to the ammonium manganese 

 phosphate NH 4 MnP0 4 . In the work to be described the 

 attempt was made to learn the conditions under which this 

 conversion may be best and most completely accomplished. 



* Jour. Anal. Cliera., v, 141. f Pogg. Ann., Ixxiv, 449. 



