Maurice M. Vick Louisiana State University
Baton Rouge and E. Lundol Harris McMurry College Abilene, Texas
Ion Exchange Separation of Cadmium
The procedure that is generally employed by the various schemes of qualitative analysis for the identification of cadmium ion consists of the precipitation of yellow cadmium sulfide a t the end of the analysis for the paxticular group or sub-group containing the radmium ion. Successful performance of the t,est thus depends upon the quantitative removal of all other ions prior to the test, and it is not uncommon for students to ohtain a black precipitate as a result. The following ion exchange separation is an alternative to the usual advice of "go back and repeat the analysis." I t has hren used successfully with both a hydrogen sulfide .scheme1and a non-sulfide scheme2 of analysis. The ion exchange resin t,o be used is a strong-hase type anion exchanger (e.g., Dowex-1) which has been conditioned to the bromide ion salt form. With the hydrogen sulfide scheme, t,he solution which is used for the test is the solution obtained from treating the copper anb-group precipitate with nit,ric acid. With the nonsulfide scheme, it is convenient to use a solution containing the entire group.
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STUBRS, MOR~IS F., A N D JONES,W. NORTON,JR., "Lahoretory Exercises in General Chemistr.~," W'm. C. Brown Co., Ih~buque,Iowa, 1954, p. 235. WEST, PHILIPW., AND VICE, MAURICEM., "Qualitative -4nah.sis and Analvtical Chemical Seoarations." 2nd ed.. The ~ a e h i l l n nCo., N& York, 1959, p. 52.
414
/ Journal of Chemical Education
Ten drops of the appropriate uuknown solution are added to about one gram of the air-dried resin contained in a test tube, then tn-enty drops of 0.1 M potassium bromide solution are added. The mixture is agitated vigorously in the test tube with a stirring rod for thirty seconds, after which the resin particles are allowed to settle. The supernatant liquid is drawn off with a small pipet or medicine dropper and discarded. Another twenty drops of t,he potassium bromide solution are added and the procedure is repeated. The resin should he washed in this manner with 0.1 M potassium bromide for a total of five or six times. After the last wash portion has been withdrawn and discarded, the resin is washed thoroughly but only once with concentrated ammonium hydroxide. A few drops of the ammoniacal supernatant. liquid may he withdrawn and tested for the presence of cadmium ion by the addition of a drop or two of sodium sulfide solution. The separation is simple aud requires only a few minutes to perform. Furthermore, it serves as an introduction to a modern technique a t a time when it will be most appreciated. .kft,er a brief explanation of t.he fundamental principles of anion exchangers, t,he elucidation of the chemistry involved in the above separation m.akes an excdlent exercise for the student.
