and Laboratory Methods. 



1451 



MICRO-CHEMICAL ANALYSIS. 

 XVI. 



ZINC. 



Although zinc, from its position in the periodic system, closely resembles 

 magnesium in general, in its chemical behavior, the majority of the micro- 

 chemical reactions of the two elements are quite different. We have already 

 seen, however, that with uranyl acetate and sodium acetate, and with arsenic 

 acid, magnesium, zinc and cadmium give identical reactions. 



A number of reagents have been proposed for the detection of zinc, but of 

 these only the folio vving need to receive our attention : 



I. Double Sulphocyanate of Mercury and Ammonium. 

 II. Oxalic Acid. 

 III. Primary Sodium Carbonate. 

 Of these, the third is the most sensitive and most characteristic, but is not 

 so simple, convenient nor so easily applied as is the first. The second reagent- 

 oxalic acid — is unsatisfactory and of comparatively little value. 



/. Ammonintfi Mercuric Sulphocyanate added to neutral or slightly acid solu- 

 tions containing Zinc, precipitates a Double Sulphocyanate of Zinc and Mercury. 



ZnSO^ + [^(NH^CNS) 



Hg(CNS)2]= [Zn(CNS)2 

 (NH,),SO, 



Hg(CNS)2] + 



Method. — The reagent is prepared by adding to an almost saturated solution 

 of mercuric chloride a saturated solution of ammonium sulphocyanate in slight 

 excess of the amount required by theory to form the double salt of the formula 

 given above. The solution thus prepared is employed as the reagent. It suffers 

 no deterioration on keeping. 



Next, to a small drop of the solution to be tested, place a tiny drop of the 

 reagent and cause the latter to flow into the test 

 drop by means of a glass rod, at the same time in- 

 clining the slide. Almost immediately, pure white 

 feathery crosses and branching feathery aggregates 

 separate (Fig. 68). These skeleton crystals, when 

 thick, appear black by transmitted light, snow 

 white by reflected light. The normal crystal of 

 the double sulphocyanate of zinc and mercury is 

 said to be a right-angled prism of the orthorhombic 

 system, but under the conditions which obtain in 

 ordinary practice, only skeleton and dentritic forms 

 will be seen. 



Remarks. — Employ dilute solutions only. Mitigate the action of free min- 

 eral acids by the addition of ammonium or sodium acetate. 



Fig. OS. 



