336 
ELEMENTARY CHEMICAL MICROSCOPY 
minutes will usually permit the crystals to develop and their 
appearance will then be such as to avoid error. If, however, the 
analyst is still in doubt he may proceed as follows: After allow¬ 
ing sufficient time for the separation of almost all the calcium as 
CaS04 • 2 H2O, draw off the supernatant liquor, add to the residue 
a solution of ammonium carbonate, the crystals of calcium 
sulphate will be dissolved and highly refractive rhombs and 
grains of calcium carbonate will appear; these are easily found 
by examining the preparation between crossed nicols. A high 
power is generally required. 
A serious interference is that of the chlorides of the trivalent 
metals. In the presence of these salts in large amounts it is 
generally advisable td proceed thus: Add to the somewhat 
dilute solution, ammonium acetate, heat to boiling, but avoid 
long or violent ebullition, since in the latter case the precipitate 
formed often refuses to settle. The clear liquid is then sepa¬ 
rated from the precipitate (by drawing-off on the slide, filtration, 
or by means of the centrifuge), concentrated if necessary, and 
tested for calcium with sulphuric acid. 
Behrens states that calcium cannot satisfactorily be detected 
in the presence of borates; this appears to be true when only a 
minute quantity of calcium is present with a high percentage 
of boron and other salts; in such an event test by Method 5 . 
Strong mineral acids, in excess, so increase the solubility of 
calcium sulphate as to require evaporation almost to complete 
dryness before the crystals of this salt appear. The addition 
of a fragment or two of sodium acetate or of ammonium acetate 
is always necessary in such cases before the sulphuric acid drop 
is allowed to flow in. This method of mitigating the action of 
the free acids, also reduces the delicacy of the reaction because 
of the formation of more soluble double sulphates of calcium and 
sodium or ammonium. Hence the addition of an excess of a solu¬ 
ble sulphate instead of sulphuric acid is not to be recommended. 
EXPERIMENTS. 
a. Try reaction, in the manner given above, on salts of calcium in neutral solution. 
b. Try the effect of precipitating in the presence of free HCl; then in the presence 
of free HNO3. 
