and Laboratory Methods. 1455 



of this solution next to the drop to be tested. Tip the slide a very little and 

 cause the reagent to flow into the test drop. An amorphous precipitate of basic 

 zinc carbonate is generally at once produced. After a short time, if the reagent 

 is in excess, the double carbonate will appear at the edges of the test drop 

 nearest the reagent as small, colorless, triangular and tetrahedral crystals. (Fig. 

 70.) These crystals adhere strongly to the glass and are very characteristic of 

 zinc. 



Remarks. — It is essential that an excess of the 

 reagent be employed. Failure not infrequently results 

 from a neglect of this precaution. This is particularly V~y 

 true if the test drop is acid. Because of the neces- ^ A 



sity of adding large amounts of primary sodium car- ^ ^ ^ 

 bonate, the test drop must be of greater volume than >« ^._^ \l 

 is usual in micro-chemical testing and must be corre- ^^ 



spondingly dilute. 



The formation and separation of the double salt is 

 rather slow. ^P' 



Other carbonates, as for example, those of potas- ■^^S" ^^' 



slum and lithium, can be substituted for primary sodium carbonate, but the reac- 

 tions are not so satisfactory. 



Salts of ammonium must be absent. 



It is unfortunate that this, which is one of the most characteristic as well as 

 delicate of the micro-chemical tests for zine, should be open to many difficul- 

 ties. The chief of these lies in the fact that many elements are precipitated as 

 carbonates, and that these often bulky precipitates interfere with or mask the 

 zinc reaction. Among the interfering elements, those most frequently met with 

 are doubtless calcium, strontium, barium, magnesium, cadmium, lead, iron, man- 

 ganese, cobalt, nickel. Of this list, calcium, strontium, barium and lead will 

 probably have been removed by previous treatment with sulphuric acid. For 

 method for dealing with mixtures containing the remaining elements of the list, 

 see Separation of the Magnesium Group. 



If only a very small amount of cadmium is present, it is precipitated before 

 the zinc, and by avoiding the addition of an excess of the reagent, drawing off 

 the clear liquid and adding to the decanted liquid a fresh portion of the reagent 

 in sufficient quantity, the zinc can be precipitated as the double carbonate. 

 When considerable cadmium is present this method is not feasible. In such an 

 event recourse may be had to ammoniacal solutions, as suggested by Behrens.* 

 The test drop is made strongly ammoniacal and to it primary sodium carbonate 

 is added. Cadmium is immediately precipitated, while the zinc remains in solu- 

 tion. The clear solution is separated at once from the precipitate and allowed 

 to stand for a short time. Zinc separates from the decanted solution as the 

 double carbonate in the forms shown in Fig. 70. Some little skill and experi- 

 ence is generally necessary in order to obtain good results. 



* Anleitung, 2 Auf. p. 52. 



