THE QUALITATIVE SEPARATION OF CADMIUM ION FROM CUPRIC ION1 MOSS VERNON DAVIS2AND FRED H. HEATH University of Florida, Gainsville, Florida
A METHOD is described for the qualitative separation of the cadmium ion from the cupric ion by the precipitation of cadmium cyanate with an alkali cyanate. The cupric ion forms a complex ion with the cyanate which does not interfere with the separation. A rapid, effective separation of the cupric ion from the cadmium ion is probably best achieved by the familiar alkali cyanide method. However, this method is usually objectionable to instructors who teach large classes of freshmen and sophomores qualitative analysis. The dangers of cyanides are too well known to be discussed here. The method described in this paper offersboth a safe and fairly rapid method of separation. This separation involves the precipitation of cadmium cyanate from a neutral or very slightly acid solution by means of an aqueous alkali cyanate, such as KCNO. If the cupric ion is present it will form a complex ion with the cyanate which will not interfere with the precipitation of t,he vhite cadmium cyanate. The color of the complex ion of copper formed by the cynnate is deep blue resembling the cupric-ammonia ion. The solution containing the suspected mixture of the two ions should not be more than very slightly acid since in acid solutions the alkali cyanates will react vigorously with the acid to liberate ammonia and carbon d i ~ x i d e . ~These gases are expelled violently on the addition of alkali cyanate to a more than slight,ly acid solution. If the cyanate-treated solution of the two ions is allowed to stand for a considerable period of time a pink precipitate may be noted to have formed above the white precipitate of cadmium cyanate, especially if the solution is slightly acid. The composition of this precipitate is unknown. The literature of the cyanates does not reveal any precipitate of this nature being formed under those conditions. However, the formation of the pink precipitate does not interfere wit,h the
subsequent identification of cadmium and its formation may be disregarded in this analysis. The composition of the pink precipitate as mentioned above is now under investigation by the authors. RECOMMENDED PROCEDURE
The following procedure may be applied to either semimicro or macro techniques of analysis. The filtrate from the bismuth separation, if of large volume, should be concentrated to a workable volume, a t least 20 ml., by evaporation on a water bath. The solution, on cooling, is then neutralized with dilute sulfnric acid so that the solution will he either neutral or very slightly acid. To this neutral or very slightly acid solution is added an excess of 3 M potassium cyanate. The curdy precipitate that first forms will be dissolved in an excess of this reagent resulting in the formation of a deep blue color, probably a cupri-cyanate complex ion. If cadmium is present a fine white precipitate will be observed in the presence of excess reagent. Cadmium present in amounts of 0.20 mg. per ml. may be precipitated and separated effectively from the cupric ion. Moderate amounts of the two ions require the addition of only 3 to 5 ml. of the alkali cyanate reagent. The precipitate of cadmium cyanate should settle in about 10 minutes if fairly large amounts are present. In the event that only small amounts are present, or for a rapid settling of the precipitate, the solution containing the precipitate may be centrifuged. The supernatant liquid is then decanted and the usual tests carried out on this portion to confirm the presence of the cupric ion. The presence or absence of the deep blue complex ion is nsually a good indication of the presence or absence of the cupric ion. However, this is not as sensitive as the usual tests for the cupric ion which, in any case, should he used to confirm the presence of the cupric ion. After the removalof tracesof the cupric ions by several .successive washings with distilled water the cadmium ' Taken from a thesis presented by Moss Vernon Davis to t.he Graduate Council of the University of Florida in partial fulfill- cyanate can be dissolved in dilute ammonium hydroxment of the requirements for the degree of Master of Science. ide and the cadmium ion can be detected by precipi%Presentaddress: Department of Chemistry, The Citadel. tation with hydrogen sulfide. The precipitate of Charleston, South Carolins. HEILBRON, I. M., and others, "Dictionary of Organic Com- cadmium cyanate may also be dissolved in concentrated pounds," Rev. ed., Oxford Univernit,y Press, h'm York, 1943, p. nitric acid followed by precipitation of the cadmium ion with potassium ferrocyanide. 584.
