A Simple Separation of the Group 11-A Sulfides
T. A. O'Donnell of Melbourne Melbourne, Australia
University
In 1958 Lehnnanl commented on the unreliability of the method described in most textbooks 011 qualitative analysis for separation of lead and bismuth. He poiuted out that when sulfuric acid is used some lead may stay in solution or some bismuth may precipitate, and he proposed the use of ammonium sulfate to separate these two cations. More recently Whitehead and Hatcher2 have reviewed the methods for norl-cyanide separation of copper and cadmium and have suggested a separation based on precipitation of copper as an acetylide from ammoniacal solution. These authors comment on the possible explosive nature of the copper(1) acetylide. This danger may not be great when the precipitate is wet, but it seems distinctly possible that the precipitate may he left by a student in a test tube aud that 011 drying it would present a great hazard. A group separation scheme, which uses simpler nlethods for the separation of lead from bismuth and of copper from cadmium than those reported above was set out in a textbook first published in 195.ia aud is giveu below. In an attempt to isolate a pure sample of %I0Bifrom Z'ol'b, the usual separatiou of lead and bismuth as sulfates was checked by the present author using tracer techniques. This separation was found to be very inefficient aud ma8 abandoned in favor of a new one
' LEBRMAK,L., J. CHEW.EDUC.,35,406(1958). T. H., A N D HATCHER, G. K., J. CHEM.EDIIC.. WHITEHEAD, 39,399 (1962). BROWN, R. I)., A N D O'I>ONNELL, T. A,, ''M&nuaIof Elementary Practical Chernmtrv." Melbourne University Press. 1st ed.,
based on the use of sodium hydroxide, which was found t o be very effective.' I n practice in qualitative analysis the lead-bismuth separation in the scheme below is easy to apply because sodium hydroxide solution is added to the ceutrifuged and washed mixed hydroxides of lead and bismuth. Under these conditions there are no difficulties associated with adjusting reagent concentrations. The basis of the copper-cadmium separation is that cadmium sulfide is completely soluble in 3 to 4 M hydrochloric acid, whereas copper(I1) sulfide is not. If the values of pK, and pK, for hydrogen sulfide are takeu as 7.2 and 14.2, the ionic product calculated for cadmium sulfide in a solution 0.1 M in cadmium, 4 Al in hydrochloric acid and saturated with hydrogen sulfide would be 3 X 10-2"a value greater than the generally accepted value of the solubility product, 1 X I t is shown in another communicationb that chloride ion concentratiou must be considered as a factor in addition to acidity in discussing the solubility of cadmium sulfide in hydrochloric acid solutions. Again, a practical advantage of this separation is that hydrochloric acid is added directly to the solid mixed sulfides and steps such as neutralization and coutrolled dilution are avoided. The reappearaucr of cadmium sulfide wheu ammonia is added to the acid solution after removal of copper(I1) sulfide is very easy to detect and normally resaturation with hydrogen sulfide is unnecessary. I t is suggested in the directions simply as a precaution.
J
O'~)OSNELII, T.A., Austmlian J . Chem., 8, 493 (1955). T. A,, J. CHEM.EDUC.,40, A313 (1963). O'DONNELL,
Separation of Group Il-A Cations
Group precipitate contains sulfides of Hg++,Bit++,'Cu++,Cd++, and Pb++. Heat with 6 drops water and 3 drops 14 M HNO.
Centri-
&."a
xld.
tientrrtuge.
I Precipitate:
B~(oH)?ancI P .~. . . ' ~ d d10 ' dl'011s z d , NaOH. ;Sti~ rwarm, A and centri ge. Solution: Residue: Mulion: HgS or Hg(SO&, 2 HgS. Dissolve in May rontajn Pbi+. Zontains Bi(OH)., aquaregia, dilute and verify Hgt+ plumbite with SnCL.
I 1
S ~ t y t e ~ o l u t i awith n H e Centrifuge. P ~ e c ~ ~ ~ r ~ t c t I~ o h t t o n : Centrifuge.
1
Contains Cd++. Make almost neutral and saturate with HY Pmipitate: CdS Volume 40, Number 8, August 1963
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