1456 



Journal of Applied Microscopy 



Exercises for Practice. 



Try precipitating Zn in acid, neutral and ammoniacal solutions. 



Test mixtures of Zn and Cd, first in neutral, then in ammoniacal solutions. 



Experiment with Zn in the presence of the interfering elements noted above. 



CADMIUM. 



Cadmium, in the absence of zinc, can be very easily and satisfactorily 

 detected by either : 



I. Ammonium Mercuric Sulphocyanate, or 

 II. Oxalic Acid. 



But if zinc is also present, great care must be exercised to avoid being led 

 into error, for these two elements are very much alike in their chemical 

 behavior. 



Several other reagents have been suggested for the detection of cadmium, 

 but it can be said of all of them that the results are not satisfactory, even when 

 working with pure salts of cadmium, and that they fail completely when dealing 

 with complex mixtures. 



/. Cadmium forms, ivith Ammonimn Mercuric Sulphocyanate, a Double Sulpho- 

 cyanate of Cathniutn and Mercury. 



CdSO^ + [2 (NH^CNS) • Hg (CNS)2] = [Cd (CNS), • Hg(CNS)o] + 



(NHjio,. 



Method. — Proceed exactly as directed under Zinc, Method I, avoiding an 

 excess of the reagent. Long, highly refractive prisms separate. (Fig. 71.) 



/"^X The appearance of these prisms varies with 



/ / \\ \\ the conditions which obtain at the time of their 



^r Jf^ \ \ n \x formation, as, for example, the concentration, depth 

 of the test drop, amount of reagent added, acidity, 

 etc. These variations are, however, not of a kind 

 to render the test doubtful ; long prisms, either 

 singly or in groups, being the rule. 



Remarks. — The remarks made under zinc are 

 applicable to cadmium in every case. 



The double sulphocyanate of cadmium and 

 >P''"°-°"^n> mercury is more soluble than that of zinc, hence 

 Fig. 71. the reaction is slower and more concentrated solu- 



tions should be employed. 



If a small amount of zinc is also present, mixed crystals containing zinc 

 and cadmium first separate whose crystal form can be described as non-feathery 

 skeletons ; soon after this the cadmium double salt separates in its usual form. 

 In order that this sequence shall be brought about, it is best to employ a solu- 

 tion somewhat more dilute than when zinc is absent. Much zinc usually pre- 

 vents the formation of any of the prismatic crystals of the cadmium salt, only 

 mixed crystals resulting. 



