Coulometric titration of +2 copper with electrogenerated +2 tin

227

‘I’hc use of clcctrogcncratccl 4-z tin as a coulomctric titrant - cslxxklly for “coulc~metric iodomctry” -- lins been dcscrilwcl in previous pillWrS from this laborntory~ . ‘1’1~~prcscnt study is conwrncd with tlw direct titriltion of +2 copper to the +I stntc in an acidic lxomitlc mcclium. Very l~robahl~* -t_2 copper could bc titrated by the coulomctric iodomctric tccllniqiic prc\*iously dcscrilxdl. Howcvcr tlic direct titration dcscribccl hcrcin has tlic advantage of grcatcr sclcctivit~*. In princil~lc, it is al$icablc to the clctcrmination of i-2 copper in tlw lxcscncc of otlwr osiclrtnts, lxovitlccl tlwir osiclntion potcnti:il is in few tenths of iL V ~rcatcr tlixn that of the cupric-cuprous couple and proviclccl their concentration is of the same ardor of magnituclc (or smaller) than tlw copper concentration.

The titration ccl1 wiis tlw same as that dcpictctl on p. 396, Vol. xc), (I()jS) of tliis journal, csccpt that a pl:~tinum ratlicr tllilll gold gcncrnting clcctrotlc was used. ‘I‘hc arw of the platinum gcncrntor clcctrodc was IO cm:! and the voliin~c of solution titratccl was 75 to go ml. The supporting clcctrolytc was 4 .1I soclium bromide-0.2 :\L stnnnic cl~loriclc-0.2 N liyclrocliloric acid, bccausc previous cspcricncc iixl sliown tliat. tliis composition was optimum for the gcncration of -{- 2 tin wit!) r00’;/~ current cfficicncy. IXssolvcd air was rcmovctl from the solution witli nitrogen. Complctc csclusion of ;iir is rcrluirctl because tlic bromiclc complcxcs of +I copper ant1 j-2 tin arc both rnpiclly osidizcd by dissolved osygen. ‘I’hc gcncrating current was controllccl automatically to ho.01 0/0by tlw ampcrostat previously clccribcd~. ‘I’lle gcneratinfi current clcnsit>~ CiLLl bc ;Ls large Its SO nl.l\/crll:?. this causes tlw current cffiIt must not bc smnllcr than ;lbout 5 m.4/crn”, lxxausc ciency for gcncration of +2 tin to fall lx2low IOO~/~. Generation of +2 tin at a platinum cathoclc in iLtI acidic bromitlc mcclium procwcls with practically rooO/Ocurrent cfficicncy onlv in virtlic of the fact tliat tlic clcctroclc acquires i1 film of hydrous stannic osiclc wl;ich incrcnscs the ovcrpotcntial rccluircd for reduction of hydrogen ion. Although this film is persistent, and is rcmovnblc only is milSima1 wlicn, by drastic solvents for hydrous stannic osidc, its cffcctivcncss freshly formccl. Consequently the platinum cathoclc should be freshly cathodixxl in the supporting clcctrolytc bcforc the sample is aclclctl. Ordinarily this is ;~ccomplisl~ccl by prc-titration to the c.p. potential. I
p.

232

J. J.

228

VOL. 21 (1959)

I.INGASlS

In tllc 4 111lxomitlc and Snlho-2, ilnd +T is less ccrtxin; hromitlc ion, midc solution x Ad bromitlc ion it swms

sollltiou usctl j-2 and +.f tin wry prdxlbly exist as Sn13rd-” c0plw.r is prcscnt chicfly iLS CuISrz-. ‘I’llc: state of tlw -4-z copper the fact that its color is rccltlisl~ brown sliows that it is complcsccl with llut whicll of tllc: scvcral broJn0 conlplcxcs prctlorninatcs in i1 1 ill brois not known. I
will iLlSO IX_!prcscnt.

iltlCl accordingly

l-IcllW

WC:

tllc titration

Sllilll ill~liCiltC tllc 4-Z coJljlc:r spccics

rl!actioIl

lll:ly IX: Written

;LS CulJr

tlic

tcI~7p!riLttlrc

n

ilS

and wit11 _c .\I USCC~in the: titriltiolls, illlC1 0.2 A? Ilyclrogcn ion, tllc fOrIlliLI l~otcntinl Of tllc St~lfltlic-Stilnll(~uS * .j. V 7)s. S.(:.K. (-1-0.201 V 71s.N.H.E.). Unth- tlic rc~uncl to Ix! -0.017 cuprous couple is -i-o,443 -& 7 tions tllc ff~IYllill ~X~t~!lltiill of the cupric(-l-0.660 V 7)s. N.I-[.I<.). ‘Hcllcc tllc fOrIllil1 c.m.f. of tllc titration rcilctioJ1 illltl its cquililxium COnStilIlt (at 55-00”) is At

(Z-n) +,

of 55-60”

Ixwmiclc

COUplc WEIS SilIllC! contliV 71s.S.C.lS.

is -1-0.459

. . . . . . . . . At room tcnipcraturc metric or ampcrornctric

pc1tcntinl Howcvcr

tlic xxaction is SOlll~\Vllilt slow, iln(L wit11 citlwr clc:tcction ill>Ollt IO nlin is r~:clliirctl for :ittninnicnt

or itltliciitor current iLt 55-60” cquililxiuni

V

(2)

potcntio-

of stcdy in tlli: iY~~JllcCliiltc!Gcinity of tllc ccluiVilk.nCX point. is rcacllccl ill R few sc:concls prior to tllu c.p. zinc1 in

ilbOllt 2 min right at tllc c.p. l;ig. T shows the: course of the potcntid chngc, Stl!iltly ]XltC!lltiillS ilftC1’ CilCll incrcmcnt Of gcncriltion, of j-2 copper in iL volume of 100 nil. A platinum

ELAPSED

iltlCl tllc tinlc rcclllirccl to rcacll in till! titriltioll ilt 57” Of I.59 Jllg illdici~tor

clcctroclc

was

used

in

TIME

Fig. I. Potcntirrl cllnn#c of pk~tirium illdiGltor clcctrodc during cuulumctric

of -1-a copper potcntiomctcr.

ion

titration of r.59 mg at 57O with a gcncratillg ct?rrcnt of G7.G mr\. l?ccorclctl automatic:~lly with rccorcling Total clnpsccl time on nbscissx. Nurnlwrs on c;~ll step arc tlw cumukrtivc gcncmtion tinic (see). l’lic tlrcorcticnl c.p. time is 35.6 see.

VOX,. 21 (1959)

COULOMETRY

229

OF CU+"

conjunction with a saturated calomel rcfcrcnce clcctrodc. TlLc wrvc was recorded with a recording potcntiomctcr and the cumulative gcncration time is indicated by the number on each incrcrnental step. ‘lk! theoretical c.1). time is 35.0 WC, :Lnd up to 3-j see it is seen that the potential bccomcs constant almost immcdi:Ltcly after each gcncration incrcmcnt. iIt 36 WC 3 min was rcquircd for the potcntinl to bcco~nc constant to -& 2 xnV. IxLt it was within 20 mV of its final value after I min. ‘I’hc crt-or in tlw c.p. time wo~ilcl have bcrcn only -+o.og see if tlic 30 see wading had been talwn nftcr only I min. ‘I’lic fact tliat the +z copper and + 2 tin react in a 2 : I mole rntio c;LLw:s the titration curve to bc nsymmctricnl. Conscqucntly the point of masimum slope occurs somewhat alwad of tlic true c.p. If no cxcws +4 tin wcrc present (i.f!., if the titration wcrc: pcrformccl in the classical ninnncr with a stantlartl solution of -+2 tin) tllc c.p. potential wo~ild lx! constant and indepcndcnt of the cluantity of +2 copper titrated. However, in tlw coulonictric titration tlic fact tllnt a large L’XCCSSof -1-4 tin ncccssnrily must lx: prcscnt caLiscs tile e.p. potcntinl to depend 011 tlw conccntrution of copper titrrLtcc1 (i.~. , on tlic concii. of -1-x copper prcscnt at tiw ~_p,). If no cswss of 1-q tin wcrc prcscnt it follows~froni cclLintion x tllat at tlic CclLliViL(%I. 4)c,p. I-Icncc from ccluntion 2 knee pilIt (cL1+2)c.p. (~~l-+~),.~. :intl (cl1 l.l)o.ll. = (SII’J),.,,. ((‘11 “)&g,. .-. .__ /,.‘/a .._. . ::_ ((‘II

(511

‘q&t,.

.

’ “) ,*.,,.

.

.

I

.

.

.

.

.

.

.

.

.

.

.

.

(3)

and since: (rLt 57”) /cc .,,. .--. I:‘&, ,‘L. cul * .-.
((‘11’ ‘)o-’ ‘8* (<‘II ’ “)&,‘.

illld

1:_,‘.I’.

I:‘,.,.

L.-

-=

f:;,‘.+2,

f&+2.

“:‘!_;‘.:!

c,‘tl

c,,‘l

-1.

)<‘K

21::,,+4,

1;

s,,t2

.

.

.

.

,

.

.

.

.

.

.

.

,

. . . . . * . . . . . * . .

3

(6)

ancl stannic-statixious coiiplcs From tlw formal potl2tltiillS of the cupric-cuprous cl\lOted :lhovC: tllc c.p. pOtclltiiL1 in tllis ciLS(’\Voultl bc $-o.I;)O V 7~s.S.C.E. Howcvcr with stannic tin prcscnt iLt IL conStiLnt concentration of 0.2 :\I it follows from equation 2 that at the c.p. (c’llf’):! Ii 1((:,,+“):: (S”.‘.“) I c.,‘. = o,;lJld

siiicc (Cl1 “)c.“.

:-: 2(S11+3)~,p,

\Vl! llil\*C: illStt!il(l

Of l!~~~liltiOll

((‘II “2.*. _j I\’ -‘--Tj-= .-.! ((.‘I1 ’ “) o.,‘*

J

. . . . . . . , . . , , . . . . . . .

(7)

froiii \vliicli 2

(cll+-)

l’lw conccntrntion Rcfcrerzccs

p.

233

o,p.

=z

-

5

‘/a

I\’ -‘b

((h

’

‘)‘/a

. . . . , . . . , , . . . . . .

of 4-r copper present at the c.p. is wry

(8)

nearly equal to tlw initial

concentration folknvs

of

-t-.2 Copper

as it Wry

the:

Of copper

approximation

good

‘I’llc:Sc litriltiOl1

lmttmtial

c.p.

ant1 if wc

titrated,

E&p. ‘I’l~us

VOL.

J. J. LIFiGANIS

230

=

dccr(:;Lscs

that

fO.lP7

instcnd

--

hy 24 mV

tlcnote

o.ozq

this

concentration

of CqUatiOn

log

c

for a tcm-foltl

. .

21 (IQ59) by

c’

it

6

. .

incrmsc

. .

.

in tllc

.

.

.

. .

.

.

(10)

concentration

titrated. char;tCtc~ristics curve

tllcorctical

for

iLT(:

tllc

coulomctric

in

d~l~1OllStIXtctl

CkLSSiCiLl titration titration

curve

of

Fig.

without S.IO-:’

2, in

which

csccss

;II

in

COlqX!r

I

is

-1-4 tin 0.2

111

the

thcorctical

tllc

prcscnt,

2 is

stannic

chloridc-

C’

0.4

FiK:. 2. ‘I’lworcticd titr;ition ciirvcs at 57”: (I) wit11 no cscoss of ._1-,) till: (2) with 0.1 IV stxnnic clllwitlc nncl 3z.C~ iiig of -I- 2 ccqqwr lwr 100 1111;(3) as 2, csccpt only I. 5~) 111~of -I- 2 ctq~lwr. WIC circles arc cS~l~~IGIllI!lltill points wit11 iL gcmtxtinl: cllrront of 67.63 Iii:\. ‘L’hc slim-t Ilc~rizr~rltal lmrs Oil

CXh

CIIrW!

IllWk

the

tllC(Jrd.iC;ll

~:~~lliv~iLk!IlCC

plillt

~1~~~eIlti~lk.

mkl, illld _j is t ilc! t huorcticul cOulonu!tric copper. ‘I’hc circles arc cSpcrilnc:ntitl points ol~tninccl wit11 tlic corrcsponcling cluantitic!s of coplwr, ilIlt tlicir ~c~ocl agrcxnwnt with t lie tlicorcticnl ci~rvcs is cviclcnt. In lxxcticc tlic IJoint of rnnsinial slop is t;lkcn as the cntl-point. Untlcr tllc cstnnt 4 116 sodium titration

conditions, prcccclcs

lx-omiclc-0.2

curve:

i1lld wit11 tllc

N

Of 2.5e~O-“’

true

I

to

c!cluivdmcC

Iiytlrocldoric

:I1

35 nig point

Of copper

per

x00 nil,

by 30 to 35 mV.

With

tlic

point

I rng of

Of mxsimnl

slopi!

coppcr per IOO ml --0.7’j&, illld with I ‘j/o. I-Iowcwr this

this cliscrcpnncy corrcsponcls to iL titration error of ;~pprosimntcly 35 nig of copper per 100 ml tlic corrcsponcling titration error is -0. ncgativc titration error is mow or less conipcnsatccl by tlw filCt that the current efficiency for gcncrntion of -t_2 tin is 0.2 to 0.3 “//o short of Ioo(l/o, which procluccs a corrcsponcling positive titration error. I~~~fo~crrc~r.s p. 232

VOL. 21 (qsg)

C0UI.O>lI:TI~Y

OF

231

Cu+”

Successive samples can lx titrated in the same batch of supporting elcctrolytc, but when this is clone two facts si~~ulcl bc kept in mind. Firstly. each sample after the first must bc freed from clissolvccl air bcforc it is xklccl, as othcrwisc some of the +I copper already prcscnt from the previous titration will be osiclizccl and ;L positive error will result. Sccontlly, \vhcn in cach cast the point of masimal slope is taken as tlw c.p., a negative titration error appears, which incrcnscs with CiLCll succeeding sample, due to the circumstance that tlic true e.p. potential shifts progrcssivcly to a less positix*c (more reducing) vduc as + I copper accumulntcs (ccl. 10). ‘fhc rntc of potential cliangc at the e.p., ancl licnce the precision, also tlccrcascs in the prcscncc of an incrcnsing concentration of $-.I copper. Conscqucntly, if masimnl ;iccurxy is nccdctl, no more than twQ or tlircc succcssivc samples should lx titrxtcxl in tlic SiUllC batch of supporting clcctrolytc. The end-point can dso lx: clctcctcxl am~,cronlctric~illy, 1,~ obsc!rx*ing tile cliangc in current bctwcen two Slllilll plut hum clcctrotlc!s wliicli arc subjwtccl to a sm;ill. COIlStilllt applied voltage (50 to 150 niV). ‘L’lic principle of two olcctrotlc nmlxromctry has lxcn discussed in tlctnil c:lscvAlcrc? . As cspcctccl, tlic arnpcroinc:tric titration cnrvc: is I’-Sliapccl and tllc point of minimum current occurs \*cry close to the ~!cl~li\filll~llCC point. Howcvcr, ;impcronictric clctcction is no more lx-ccisc: tllilll potontiomctric clctcction,

it is slows

ancl

Wit11 potcntionictric

data

conwnicnt

IxxauSc~ tit.ratcS

to

it tlw

rcqltircs known

a r!.p.

gr;q~lhxil

plot.

potential.

in

volume an ki\‘l:rilge

error

tllcsc

trials

of

&o.;{‘)<,,.

tlic

supporting

and then atljustctl

-

.._...._

._

Trial -_-__

less

one simply

‘l’itl>lc I tlomonstratc that tllc titration of 1.5 to -2.31116of copper in a of 75 to 90 nil, with potcntioriiutric c.1). dctcctic~n, can bc pcrforniccl wit11

‘1’1~

In

iltltl

dctcction

.._ ___.

._Cl4

,YfI. . . .._ _ -

clcctrolytc

(by c;ltllodic

...__ trt/wu ,,,I:

_.__ .._-.-

was

Kcncration)

._

_..__-

___

‘I’# #,I,’ 5 17.

.. ..__

frcctl from clissolvctl air with nitrogen, to an inrlici~tor clcctrodc potcntinl of

.._

._..._

cu

_ ,_..

___.

/rrmf #UC

I:rrW

1.5s7

35..5

I ..iS 1

‘2

I ..j8i

35.0

I ..j’pt

.j-.O.oI

3

I .fjXi

3.i.7

I ..jSC)

-t

4

I .5x;

35.5

I ..jSI

‘J If’

I j.0 I

I

G..52

I 1

32.G3

1%

32.63

~~~~_._---___...___--_-__-__--I____ ftcfeYorm?s

p.

232

1.17.1 201

.7

730.4 730.0

-. .-...

_-

_.

I::,

_._-.

-0.000

fJ.5 5

“..i

I

-t -t.r,.

O.OOfl

--

t Cl.03 .-..C’.O

3 .?. *55 32.57

--0.d

__ . .._ --

I

.o..+

t-tr.5 ‘-. .C’. 2

.._.o.an

.__-.

0.7

0.002

I’.‘)‘)

._._ --

I:‘WW

,ttg

---0.3 .--0.2 __.-

---_

J,

232

VOL.

J. LINGI\NII

21 (qjg)

before the snmJ>le solution was ackkcf, TJlc ssmplc Solution _4 o.o:j s vs. S.C.E. was not frcccl from dissolved air before it WilS a&lcCl (except wlicn it was aclcled to a ~re~~iously t.itriItcCl SdUtkJn), but a few minutes WiLS allowed frzr the nitrogen stream to flus11 out the ctsygcn thus introclucccl hcforc the titration was hcgun. The point of ~~l~~xi~ltll~~SIOJX of tlic titration ct.frvCr was trtlten as the end-point. sanq-dcs (air-free) were ~rtlctecf to the previously Jn trials 15, 4, and 5 st~ccssive titratetl sdution. In trial 2 tllc: gC:ncrution Iiaci ken carrid bc~~oncl tJiC: C:.J>,to OlJtidn tlw tlaix ncccssary for Clctcrmining the pint of maximum slope of the titration curve. ‘l’hercforc th: solution was :djustt:rl hck to the c.p. J3rJtential (point of masimum A similar proccclurc evils SJOJW) by ;tnotJic gcnerution J>cforc: SamJh 3 was h_lccl. foJlowvC!d in tlae SIIc:cC:SSiVc trials 4 and _j* ~‘:itch SZ~TJJ~C soiuticm after tiic first wits frcccf frcJJT1 ~lissoIst!cl air Ixforc: it was ~tclrlcrl. h’otc that the titnitbtt error Ixxxt~nc~ progrcssively more ncgat ha “I’llis hrrJ~J~ncCJJ~~ausu as 4-x capJ>C:r~33rmuJA~Ci the tiiscropanq h!tWWtJ th! J”Jitlt Of mU,iJlJd SJO]‘” (IWCl :1S tJW U’lCl-J~~Jil3t) iltJCi th! tlTtC2 crpivrtlcncc JAnt incrcastd. Altf~uglt tlds cffoct fill bc smallcr dth tJlc larger amounts of copper, tltcsc trials Cfcmonstrak t JIGi~~~lCJ\~is~lJ~ility of titrating fnorc than two or tflrcc SUcccSSiVc sntn~dcs it1 tlw S;LJJIC: f>ittCIl of cJcctroJyte. h~ost of tJlc cJcmcnts CommonJy iISSoci;Ltd Wit11 CopJJcr, suds as zitx, tin, Icad, nickel, manganosc, ccshlt, aluminum, silver, iu1Cl mercury, Cl0 not intcrferc. Ferric iron Will intcx-fcrc J>cciluSc it iS rcCJrlccd coiicomitantJy wit11 tilr: -1-2 copper, but ferrous iron slioultl not interfere. Gold, wht:n prcscnt in tIrc -1-S state, is titratccl along with the coppc:r anti ot~ly il 0.25

sin&! sltodd

c*p.,

c~Jrr~!sJJ[JtJc~j~J~ acc~tratcly

hc pssibh

to

andyzc

Coppr-gdcl

to

tllc:

stuJ2

alloys

of by

the

tuw

first

clutcrrninin~;

nJctnls,

is ctlacrxwcl. tire

arid tilcn C‘tetcrtnining the gold by two t1JctiIlS hy tlic prcsc:nt titration, titration wit11 clcctrogenerntocl cl~JorocuJ~-011s ion by ;L mr:tJ1oCl previously

It

tllo cnulomctric clcscril~oCl*~. SWIJ of