A SEPARATION METHOD AND QUALITATIVE TEST FOR CADMIUM ION INVOLVING ALKALINE TARTRATE COMPLEXES JAMES P. McREYNOLDS Lehigh University, Bethlehem, Pennsylvania
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F THE common cations only cadmidm, ferric, magnesium, and the alkaline earth ions, are dissolved in cold alkaline tartrate solutions but reprecipitatedupon heating. All the other common cations either are insoluble or form solutions stable to heat. Advantage has been taken of this property of cadmium to devise a method of separating i t from the other ions of the copper sub-group in the hydrogen sulfide procedure. Of these ions, mercuric is insoluble in the reagent while lead, bismuth, and copper form solutions stable to heat. Two dternative procedures have been snccessfulIy applied in a student qualitative analysis c ~ u r s e . ~ The first of these is to divide the solutiqn, after the removal of lead as the sulfate, into two portions. One of these is tested for bismuth and copper in the usual manner, the other for cadminm. Sample directions are given for a semimicro scale. To one ml. of sample add one ml. of 1 N Rochelle salts solution, neutralize excess H2S01and make about 0.5 N with excess NaOH.3 The resulting solution should be clear. Upon boiling a white precipitate of Cd(OH)z results. For further confirmation the copper ion may be removed by washing and the precipitate converted directly to the sulfide. The most serious objection to this procedure is that the rather large amounts of Has04 usually present after the removal of lead may cause so much heat to be generated
in the process of neutralization that a clear solntion will not be obtained. This will not seriously affect the test unless the removal of the earlier appearing ions has been incomplete. It is recommended that the neutralization be camed out with cooling. The alternative method is applied after the removal of bismuth byammonium hy&oxide. To the ammoniacal solution add one or two ml. of Rochelle salts solution and excess NaOH. Upon boiling to remove ammonia Cd(OH)%is precipitated. Further confnmation may be made as in the iirst procedure. Extremely large excesses of. copper ion affect the sensitivity. Not less than one part in 10,000 of cadmium ion must be present in solutions containing 20 parts in 10,000 of copper ion for the test to work snccessfully. The practical limit under ordinary conditions is about one part in 100,000 of cadmium ion. The test seems to be more sensitive-than this but the problem of visibility of the precipitate as i t forms and of centrifuging to carry out the c o n h a t i o n test sets this as a working limit. The test is certainly not as sensitive as the sulfide in very dilute solutions. The chief advantage of the test is that i t is more free from interference than any of the common procedures. In the absence of femc, magnesium, and the alkaline earth ions only cadmium will give the first test outlined regardless of the point in the procedure a t which it is introduced. The use of the sulfide as further confirma1 Present address: Ohio State University. Columbus, Ohio. 'The author wishes to thank Professor T. H. Hazlehurst and tion makes the test applicable in the presence of any his students for their co6peration in the testing of the procedures. common ion4 except ferric.
The concentration suggested for NaOH is not given as an exact requirement for the test but as the best lower limit for the excess added. Hi~herconcentrations in excess and somewhat lower limits seem io make very little difference in the success of the test.
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'The term common ion is taken to cover: Ag+ Hg2+' H g + ¶ pbfa, Bi+3, Cu+l, Cd+2 Sn+P Sn+4 As+a sh;' Fe+{ ~ e + s ' Co+', Ni+5 Mn+', Alf\' ~r",' ~ n + % ~a : + * :~ r + ~ , ' C a +Ml ,~ + P : Na+, K+.
