hillebrand.] MAGNESIUM, TITANIUM. 67 
pletely in hydrochloric arid, but that sometimes a white residue [g Left 
in light lumps which appears to be quite insoluble in acids. This 
residue contains no silica, but only the constituents of a magnesian 
phosphate, and it may be a peculiar metaphosphate. Whether its ap- 
pearance is due to an abnormal composition of flic original magnesian 
precipitate or to conceivable change during evaporation in the cru- 
cible with nitric acid remains to be determined. Until this is done 
the employment of this method of igniting is not to be recommended. 
CONTAMINATION BY AND REMOVAL OF BARIUM AND CALCIUM. 
Barium phosphate will not contaminate the second magnesian pre- 
cipitate unless there are notable amounts of barium in the rock, in 
which case it must be removed by sulphuric acid prior to the final 
precipitation of the magnesium. Calcium, however, is probably never 
absent, and has to be estimated and allowed for as follows: 
To the ignited pyrophosphate, dissolved in but slight excess of 
hydrochloric acid, is added ammonia to alkalinity, and then acetic acid, 
drop by drop, till the solution, which should not be hot, clears. It 
now and then happens that a little flocculent matter fails to dissolve. 
This is to be removed, ignited, and subtracted from the original weight. 
It is likely to consist, in great part or wholly, of phosphates of iron 
or manganese, or both, and shows often a reddish color on ignition. 
If an excess of acetic acid has been used, this is cautiously removed by 
ammonia. Then a drop or two of solution of ammonium oxalate is 
added, and the small beaker is set aside for twelve hours if necessary. 
Almost invariably a small precipitate soon shows itself, which if fine- 
grained and nonadherent to the glass may be regarded as pure cal- 
cium oxalate; otherwise it contains, or may largely consist of, mag- 
nesium oxalate. It is in that case to be collected, ignited, redissolved, 
and repreeipitated. Its final weight, averaging perhaps one-half milli- 
gram, is to be added to that of the lime already found, and subtracted 
as tricalcium phosphate from that of the magnesium pyrophosphate in 
order to arrive at the true figure for magnesia. This separation, to be 
satisfactory, requires great care. 
XII. TITANIUM. 
COLORIMETRIC ESTIMATION WITH HYDROGEN PEROXIDE (WELDER'S 
METHOD). 1 
The method consists in comparing the color of a known bulk of solu- 
tion to be tested with that of a standard solution of titanium sulphate, 
both having been fully oxidized by hydrogen peroxide. The strength 
iBer. deutsch. chem. Gesell., Vol. XV, p. 2593, 1882. 
