368 ELEMENTARY CHEMICAL MICROSCOPY 



factory tests for mercury. At the moment the potassium iodide 

 strikes the drop a white or pinkish cloud appears, rapidly chang- 

 ing to yellow then to brilliant red. The mercuric iodide HgI 2 

 first formed is very soluble in excess of the reagent forming the 

 soluble compound HgI 2 - 2 KI. The precipitate therefore appears 

 as an ever-widening circle about the fragment of solid reagent 

 until the latter is completely dissolved. If the outer edge of the 

 brilliant red circle is now examined with a moderately high power 

 it will be seen to consist of tiny ruby red rhombs and rods to- 

 gether with more or less spherical masses and imperfect rosettes. 

 Precautions must be taken to avoid adding an excess of reagent; 

 otherwise no permanent separation will take place. In order to 

 avoid the possibility of error it is always well to add a fragment 

 of copper sulphate, which will take up the excess of iodide and 

 cause the separation of the mercuric salt. 



Precautions. 



A few very stable complex salts of mercury usually fail to yield 

 a test for mercury with potassium iodide. If therefore no test is 

 obtained for mercury, boil the unknown with strong nitric acid, 

 evaporate almost to dryness, dilute with water and test again. 



EXPERIMENTS. 



See under Lead, Method .4, page 371. 



D. Mercuric Salts can be Detected through the Formation of 

 Double Thiocyanates. 



This test is the reverse of that employed for the detection of 

 Zinc (Method A, page 353); of Copper (Method A, page 385); 

 or of Cobalt (Method A, page 412), to which the student is 

 referred for details. 



Add to a small test drop (which must not contain much free 

 mineral acid) a fragment of potassium thiocyanate about the 

 size of a pinhead. Stir until dissolved. Place next this drop a 

 tiny drop of water in which is dissolved a very little zinc sul- 

 phate. Cause the test drop to flow into the zinc solution. Char- 

 acteristic crystals of zinc-mercury thiocyanate will appear. 



Instead of zinc sulphate, copper sulphate or cobalt nitrate 

 may be employed. 



