and Laboratory Methods. 



1243 



Fig- 43- 



\VVv. "O.ol w\>n 



near the circumference of the test drop as exceedingly slender, colorless, trans- 

 parent needles, either singly, in sheaves, or in star-like clusters (Fig. 43). When 

 in tiny sheaves near the edge of the drop the 

 crystals have often a more or less brownish tint 

 when seen by transmitted light. Shortly after 

 the appearance of the bunches of needles at the 

 periphery, long, thin, slender and plate-like prisms 

 with obliquely truncated ends are formed through- 

 out the drop. These prisms are frequently 

 twinned, yielding so-called arrow-head or swallow- 

 tailed and X-like twins. These twin crystals are 

 the most characteristic of the forms assumed by 

 calcium sulphate of the formula CaSO^« 2H2O. 



Remarks. — The sulphuric acid employed 

 should be dilute and should not be added in 

 excess. Sulphate of sodium or of ammonium may also be employed, but less 

 advantageously. 



The best results seem to be obtained when the reagent is added to a dilute 

 neutral solution. If no crystals are visible after waiting a short time, the prepar- 

 ation may be cautiously concentrated. This procedure (evaporation) may, how- 

 ever, lead to the separation of such an amount of other salts as to render difficult 

 the detection of the crystals of calcium sulphate. A better plan is to hasten the 

 separation of the calcium salt by exposing the test drop to the vapor of alcohol. 

 This is conveniently performed as follows : place a small bit of filter paper on the 

 slide, a few millimeters from the test drop, invert a 25 mm. watch glass over the 



drop in such a way that part of the 



^ ,,,,^, ;;^ %„,x , &;;,,^,,,.^,,i ,,,,,,,,.,,,,,,^^^^^^^ filter paper is included under the 



glass (see diagram. Fig. 44), add 

 ^*^'''' sufficient alcohol to the part of the 



filter paper outside the glass to completely saturate it, no more. If the test 

 drop is situated at the corner of the slide, as is usually the case, place another 

 slip alongside to support the watch glass, as is shown in the diagram. Allow 

 the preparation to stand a few seconds, remove the glass and paper, and examine. 



Strong acids should be absent. In the event of their being present add 

 ammonium acetate, or, better, carefully evaporate the solution to dryness, if possi- 

 ble, and take up the residue with water. 



It must be ever borne in mind that in the presence of an excess of salts of 

 Group I, the solubility of calcium sulphate is usually so greatly increased that 

 the detection of calcium by this test is sometimes difficult. 



A more serious interference is that of the chlorides of the trivalent metals. 

 In the presence of these salts it is generally advisable to proceed as follows : 

 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 separated from the precipitate 

 by drawing off on the slide, filtration, or by means of the centrifuge, is concen- 

 trated if necessary, and tested for calcium with sulphuric acid. 



