Polarographic Procedure for Determination of Cobalt as Cobalt(ll1) Ammine .I. I. WATTENS'
AND 1. .\1.
KOLTHOFF
1 -nitvrsity of Minnesota, ldirrrrenpolis, Minn.
L new polarographic procedure for the determination of cobalt in the presence o f a large excess of nickel is described. The animino cobalt(I1) ion in amnioriiacal solution is quantitathely oxidized to a rose-colored ammino cobalt(II1) complex
ion with sodium perborate and the excess oxidizing agent is destro>ed b) boiling. The polarographic wave resul ting from the reduction of the amniino cobalt (TIT) ion to the & \ d e n t state is then obtained before the \t a\es of nickel o r zinc appear. The height of this wale is tlirectl? proportional t o the concentration of' cobalt.
T
HE polarngraphii* redu1.tiori wavr of cobalt(l1) ion in aqurous solution is usually obtainrvl a t a slightly more negatiw potential t,han the nickel( I1j ioii TWVR. Consequently, large itmounts of nickel and several other elements interfere in the usual Imlarographic procedures. ,+plications of the polarographic: \rave of trivalent (w1)alt for the determination of cobalt a n d the iise of sodium perborate to obtain quantita,tive oxidation of c o l d t \vith complete eliminntioii of the e x ( w s oxidizing agent art' described in this paper. The cohalt(II1) ammine method is especially applicable foi, t hi, tl[:teriiiinntioii of cobalt in electrolytically deposited mixturps of cobalt and iiickel, or in impure nickel salts. Sirkel a n d zinc do tioi inteifere, hut chromiuni, :is \vel1 :w iruri, manganese, anti coppw, ivheri presrnt in w c r s s , must br separ:ifed. Evans ( 5 )det clrrniiird cwbalt colorimrtricall>. as ammino rot)alt(III) ion aftt,r, wpitritting chromiuni, copprr, :rluminum, molyhdrnum, phosphorus, i.aritaluni, titanium, valiatliuln, tungsten, a n t i zirconium ;dong n.it h i i i i exctw of i i ~ ) i ihy the zinc oxide procedurr. Tho pol:irqy:ipliir procetlurt: should I F superior, its nickel dor.s not interfc'w, The method sliould be :ipplicshle after separation or i'oppt'i', I t , a t l . mcl tiii in copper-base alloys ti? t~lec~tt~nl!..+nt contlesorihed by 1,ing:tne (9). LI(~tz1( I O ) drtrrniined cohalt iotioinetrirally h). oxidizing i:ob:ilt I O t h e :ininiino r o h l t ( I I 1 ) ion \vitli Ii>.drogr,ii ptw';ide :in[l theii Iloililig in the presenve of potassium h ~ ~ t l r o x iuiitil t l ~ ~ no :immoni:t i~emiiined. The resulting hlnck cohaltic oxid(, w:ts trrated with an excess of potassium iodide and a&Iified. The lihwated iodine was ritrativl iyith sodium thiosulfate. Toniiwk and Frrihergrr ( I I ) ~Ir~trrrriinc~d col):ilt potrntiom[,tricxll?- I)?. tit rating a i l :iinniorti:ical
photonieter and glass electrode, provivl I hat hexammino c*ot)alr (111) ion is thermodynamically the more stalde. Howevcar, thc. honding of the complex is so predominantly covalent in n:lturt* that the rose-colored pentammino cobalt(III), which is formclil in ammoniacal solution, remains essentially unchanged for \vrcki .Ibrow-n hue was always observed in solutions in which thr ini(1:ition was incomplete!. APPARATUS
Tht: niaiiual apparatus, describrd in previous publicatioris (;) \vas used. An out,side saturat,ed calomel electrode or a i i i t ~ r c i i r , ~ pool iri the ( d 1 can be used as the anodr. i\ thermostatidlv controlled water bath \vas usnd to rnaintairi a ronstant, t c r n p ~ ~ i x t u r i ~ Irf2.i.00A n . i " c . REAGEZITS
-1stock solution of 0.1 -TI cobalt(l1) rahloride was preplt,i,il LI) ilissolving 23.80 grams of cobalt(I1) chloritlr hexahydrate. rtAageiir grade, i n water, adding 10 ml. of 1 hydrochloric acid, a,nd diluting to txactly 1 litcr. The molarity \vas verified by the nitxthc)dot Engle arid Gustavson (4). .immonium hydroxide, 6.3 .\', JY:\> prepared by mixing equal volumes of conccntratrd amnic~iiiu~\~ hydroxidc and water. .%mnioniuni chloride stock solutioii, 1 .I/ was preparcti. A \
PRELIMINARY SELECTION OF r w . i G E w r s ~ N D CONI)ITIOII5
Oxidation of the cobalt in animoniac:!l solutions bj- frw osygc'ii \vits inconipletc. Alfteroxidat,iorl ivith sodium peroxide or liy(ti.ogtLn pwi)ritic~,tho exccss perositlc ot twsily removed. 1't.t.,
Hrciic-k:i ( 2 ) ~ j ) o r t dpolarogtxms of a1rii:iiiio i ~ o b : ~ l t ( l l011I) taiiirci 11y dissolving solid ammino c~obalt(IT1 j silts in amiiioiiiad :inimonium rhloridr. H i > ot)se~vetlthe first polaroyratphic- \V:IVP starting :it -0.3 volt vcrhu' the h:tlurated calomel vlectrodt~(S.C.E.) for t h e reduction t o the divalent statt., follo\vetl l)y R srcorid \v:tve, twice as high, stwting a t - 1.3 volts (S.C.E.) for thct fir1:il roduc-tion to the free nictd. 1~'illis.Pric~ntl.and 1Iellor ( I d ) mr,:isiircd the half-\yaw pott.ritials of the t\vo ~ v a w s01)I xilied i v h m various solid nmniirio cok)alt(III) salts \vert' ilixw l w d i i i potsssiurn sulfatr: an the supporting elec:trol>.te, :ttrcl J,aitinen, Hailar, Holtzc~l:r\r,a n d Qu:tgliano ( 8 ).studied th(b rffect 11f the suppot,ting electrolyte on the hexammino cohalt(II1) ~ v a v e . T h e rose color of the complex ion obtained by quantit:rtivcL oxidatioil of c o l d t in :inimoniac~alsolutions indicates that it ('onsists I:irgeI>. of hydroxopelitammillo cobalt(II1j ion and not this ~ ~ l l ohexammino w robalt(II1). Bjerrum ( I ) , using t h r spprtro1
Prehent addre.. . Drimi:trni.nt of ('hPmi.try. Ohio.
Ihiriihiis.
Ohio S t a t ? ITnirrriity,Crl-
wniahly the dif5culty in removal i xitlr is due to its p r e r t w ~ ~ i n this complix. It, was finally thst' sodium perhimiti,. SaHO:, -LI120, rapidly and quantitativctly osidizcs cobalt t o thix t rivalr~i~t~ complex amminr in an aninioritai;al solution at hoiliiig rc.mpwaturc1. One gram of sodium perborate t8etrahydratc. I heorc~rirallysufficicnt to oxidize 760 nig. of cobalt (11) ion, w a i ris:rd in all t he determinations. The, t + r c r s $ pcrhorate was decomp o n d 1)y boiling the solution 5 ti, 10 minut,cxs. In order to avoitl tssccwivtb loss of ammonia, a small 15-CIll. (6-inch) Liebig reflu\ roiitI~~iist.r was connectpd t o thr Erl(wiieyer oxidation flask. Thit iipper orificc, of tho condenser \\-as connecttd with a small n-alcr trap cxiiiraining about 5 ml. of wate irhich was finally ad the> sanipltl. The nitrogen used to recp air from sampl passed through two wash hott.lrs ~ 0 1 1 iniug similar conccriti of animonia. For routine analysis, a 100-ml. Pyres voluinrtric fiask f i t t c d \vi1 h a glass tube to scrvr as an air condrnwr vittl h(. riseti during thc heating. The nitrogt,ti uscd to remow osygi'n rlc?t.d not lw p a w d through iwsh lrott It** containing aniiiioniuni
tna\irunm cwmpkltely. PKOCEDUHE
The sample to be determined may contain from 1 to 120 ing. ut' cobalt as the nitrate, sulfate, chloride, or pwrhlorate in about 2.5 1466
1467
V O L U M E 2 1 , NO. 12, D E C E M B E R 1 9 4 9
f
\\&veof thr cot):ilt(III) c~oiiiple\ i z a r cmdingly of prarticd importarice and \\as investigated. The concentrations of ammoni:a and ammonium ion also inHuenee the qolubility of other ammino nietiil ions piesent in the sample. Thc vffect o f the ammonium hydroxidr ant1 wpporting tslecti olyte concentrations on the diffuqion current is qhonn in Table I . From i h r datn in Table I it i i evident that the diffuiion cuiient is ronstant \I ithin e.cprrinierit:tl error when the conwntration of :tmnioriia is varied from 0.31 to 3 1 lr. FICINever, N hen the concentration of ammonium hydroxide was -1.3 -1.5 -1.7 -1.9 -0.1 -0.3 -0 5 -0.7 -0.9 -1.1 increased tu 7.5 JI the diffusion current deere:asd 11)- 8%. The oxidation was V O L T S VS. S.C.E quantitati\c iri :ill the solution4 eontainFigure 1. Polarogranls of Cohalt(II1) and CobaIt(l1)in Ammoniacal Solutiou ing 10 ml.oi moir of 6.25 41 ammonium 1 . Cobalt(II1) ammine, S millimolur 2. Like curve 1 but cohalt not oxidized hldroyide pei 100 ml. of final solution. 2.5 M a m m o n i u m hydroxidr 3. Like r i i r v e 1 h u t no cobalt prerrnt 0.1 M ammoniiim rhlnrid,. Within experimriitiil error, the diffusion 0.02 yo gelatin currents measured at. -0.9 or - 1.6 volts (H.C.I.:.) ~vercnot altered by increasing 1111. of aqueous solut,ioir. Tratiaf'er the sain le quantitativtsly lo n the imiiioiiium chloride ronccmtratioii from 0.1 21 t,o 0.R Jf or by 250-nil. E:rlenmeyer flask :+rid add 10 m r of 1 J f ammonium replacing the ammonium hydrtrxide with the indifferent electroc.hloritlt.. 40 nil. of 6.25 .I/ ammonium hydroxide, and about. 1 lyte, potassium chloride, in it aonc:rritr:ition range of 0.1 to 1 '14. gram o f sodium perborai ( ' o r i n t ~ ~at small rcHux condenser and alloiv the solutiori 10 l)oil The third, fourth, and fift 11 s:tnipIcs, ivhic.11 were triplicates, were gtxitly in a water bath for 10 niinut,es to decompose t,hc excess :inalyeed 1)y the rec~onini~ntledprorr:drirtx :ir i d yielded the snme Iwrboratt.. After the solution has cooled, transfer the samplc. result xvithin * 1 quantitat.ively, along nith a little water used to rinse the conIn samples rontniriirig 0.6 gram of nickel i o i i , 10 nil. of 6.25 ,M
