IOWA ACADEMY OF SCIENCES 
1G9 
The precipitate from the ammonium oxalate, consisting of the cal- 
cium and possibly a small amount of magnesium need not be washed 
as the small quantity of magnesium that is present will not be precipi- 
tated a second time with the calcium. For the same reason in a speci- 
men of rock where the calcium is greatly in excess of tlie magnesium, a 
second precipitation is not necessary. 
In the case of dolomite, the precipitate is dissolved on the paper with 
a small quantity of warm dilute hydrochloric acid, and the calcium is 
a second time precipitated with ammonia and a little ammonium oxalate, 
the magnesium now remaining in solution. The filtrate can be added to 
the main filtrate which contains the bulk of the magnesium. These fil- 
trates should be concentrated if necessary. Care must be exercised in 
washing the precipitated calcium oxalate, which we have found to be 
appreciably soluble in hot water, less so in cold water, but it is best 
washed in cold water, to which a little ammonium oxalate has been 
added. On account of this solubility, it is better to wash the calcium 
oxalate on the filter rather than by decantation. In the analysis of Ice- 
land Spar, washing the precipitated calcium oxalate is unnecessary as 
there are present with the precipitate only ammonium salts which the 
heat will afterwards volatilize. 
The somewhat concentrated filtrate containing the magnesium is 
precipitated, while cold with a salution of disodium phosphate with con- 
stant stirring, and about 25 cc. of ammonia water is added. It is allowed 
to stand 24 hours before filtering, ^Lawrence Briant found that by vio- 
lently shaking for ten minutes in a stoppered bottle all would be precipi- 
tated. Time, however, is not usually an object in such analyses, as 
other work can be in progress while the precipitate is standing. 
Should there be present with the normal magnesium ammonium phos- 
phate more or less of the other salt (NH4)4Mg(P04)2,it would only be 
necessary to dissolve in hydrochloric acid and re-precipitate with 
ammonia. 
We have found in hundreds of analyses of pure magnesium sulphate 
that very seldom do we get too high a result by precipitating as before 
described with disodium phosphate, and the same appears to be true of 
dolomite, although in the latter substance it is more difiBcult to know 
what is the real content. The occasional high result can be explained by 
imperfect washing of the precipitate. 
In estimating the phosphoric acid in disodium phosphate with “mag- 
nesia mixture”, the average student will more frequently obtain too high 
a result, but this seems to be the case when a large excess of the mix- 
ture has been carelessly added and the precipitate has been imperfectly 
washed. Our experience on the whole wmuld tend to show but little 
necessity for a second precipitation. 
Microcosmic salt is considerably more expensive than disodium phos- 
phate, but doubtless the difference in the cost would ordinarily not ba 
considered. 
“'Crooks Select Methods, page 51. 
