882 M. Scheerer on the determination of Magnesia. 
precipitate is formed. The precipitate which separates is of a 
beautiful white, is very dense, and settles readily, more especially 
when warmed; the whole liquid is heated to boiling, filtered and 
washed with boiling water. It is then dried and heated. It 
has the composition BiO%, PO, which is that of a neutral phos- 
phate, for bismuth is triatomic, and replaces three atoms of water 
in tribasie phosphoric acid. 
Pyrophosphoric acid is also precipitated by acid nitrate of 
bismuth as neutral pyrophosphate of bismuth, 2Bi0%, 3p PO**. 
The precipitate is white, but much more voluminous than with 
tribasic phosphoric acid. Metaphosphoric acid is likewise pre- 
cipitated by nitrate of bismuth. Both these precipitates, however, 
are converted into the tribasic phosphate of bismuth when boiled 
with excess of the bismuth solution ; the metaphosphate requires 
a somewhat more prolonged ebullition. It is therefore of little 
importance in what modification the phosphoric acid exists in 
the solution to be determined. 
This method affords a very delicate means of determining 
phosphoric acid. Chancel was able to determine and separate a 
milligramme of phosphoric acid in the presence of 120 milli- 
grammes of alumina; and the precipitation is so rapid that it 
will be possible to estimate phosphoric acid by means of a 
standard solution of acid nitrate of bismuth. 
To estimate magnesia in the presence of the alkalies, Scheerer+ 
proceeds as follows :— 
The mixture of magnesia and the alkalies being given, is con- 
verted into neutral sulphates and weighed. This mass is dis- 
solved in a small quantity of water, and the solution divided 
into two parts of known weight. In the one the magnesia is 
determined by the addition of phosphate of soda, and in the 
other the potass is precipitated by bichloride of platinum. The 
soda is estimated from the difference. 
According to Rose {, when silica of the density 2-2 is mixed 
with fluoride of ammonium and the mixture heated, the silica is 
completely volatilized as fluoride of silicon. With quartz and 
sand the action is less energetic. Rose recommends the use of 
fluoride of ammonium as convenient for decomposing silicates. 
It may readily be prepared by adding ammonia in excess to the 
commercial acid, and then a little carbonate and sulphide of 
ammonium. ‘The liquid is allowed to stand, the clear liquid 
* Chancel represents ordinary or tribasic phosphoric acid as PO*, bibasic 
phosphoric or pyrophosphoric acid as pPO’, and monobasic or meta- 
phosphoric acid as mPO’. 
+ Liebig’s Annalen, November 1859. 
{ Poggendorff’s Annalen, September 1859. 
