242 Gooch and Austin — Determination of 



seems essential that the amount of ammonium chloride should 

 reach a certain limit. According to our experience the propor- 

 tion of ammonium chloride to the pyrophosphate should be at 

 least 50 : 1 ; or, speaking approximately, more than 200 mole- 

 cules of ammonium chloride must be present in the liquid 

 100 cmS or 200 c,n3 ) to every molecule of the ammonium man- 

 ganese phosphate to be formed. However, the quantity of the 

 ammonium salt may be increased almost to the point of satura- 

 tion of the liquid without causing more than a trifling solubility 

 of the ammonium manganese phosphate in the presence of an 

 excess of the precipitant. The statement of Fresenius and 

 Munroe that ammonium manganese phosphate is soluble in 

 ammonium chloride does not hold if there is an abundance 

 of the soluble precipitating phosphate present. Further, our 

 experience goes to show that the precipitate may be washed 

 with perfect safety with pure water as well as with slightly 

 ammoniacal water, or with ammoniacal water containing am- 

 monium nitrate, if the filtration is performed rapidly and the 

 precipitate is gathered in small space, as is the case when 

 the phosphate is collected on asbestos in a perforated crucible. 

 The finely granular precipitate which may be obtained by 

 slow action of dilute ammonia added gradually to the hot 

 solution of the manganese salt apparently includes a portion of 

 unconverted phosphate which resists the replacement of the 

 manganese by ammonium. On the other hand, the precipitate 

 of flocky condition thrown down in the cold passes easily to the 

 silky and crystalline condition when heated with the proper 

 amount of ammonium salt and possesses a constitution approach- 

 ing the ideal under such conditions. The conversion of the 

 flo<!ky manganous phosphate is so rapid that the precipitation 

 may be carried on safely in glass vessels. If the ammonium 

 chloride in the solution were to be included in the precipitate 

 it would volatilize entirely during the ignition, leaving no trace 

 unless, possibly, a portion of its chlorine were to combine with 

 the manganese. Tests for chlorine in the residue of pyro- 

 phosphate resulted negatively — no more than a mere trace 

 being found in any case, so that the contaminating effect of 

 the ammonium chloride proves to be insignificant and the 

 responsibility for the increase in weight above the theory must 

 apparently rest with the included microcosmic salt. 



In the practical determination of manganese by the phos- 

 phate method of Dr. Gibbs, therefore, we advocate strongly 

 the presence of large amounts of ammonium chloride. Good 

 results may be obtained by the method of precipitation origi- 

 nally laid down by Dr. Gibbs, or by the modification proposed 

 by Blair, if the ammonium salt is present in sufficient quantity. 

 On the whole trustworthy results are obtained most easily and 



