Asociacion Iones Cericos y Sulfatos

7/29/2019 Asociacion Iones Cericos y Sulfatos

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ASSOCIATION O F CERHC ION S N'ITIQ SUL PHAT E(A SPECTR AL STUDY)'

Abst rac tCeric ion has been shown to associate with sulphate ion to form successivelyCe(SOJL+,Ce(SOd)p, and C e(S04)3 -. T he association constants relating thesespecies have been determined a t 25'C. Th e molar extinction coefficients of eachassociated complex have been found between 395 an d 430 mp. ,Migration experi-ments bear out the results qualitatively.

There is considerablc evidence of complex formation between ceric ions andsulphate ions in aqueous solution. Jones and Soper (2) have n~easured l-ans-ference in solutions of high sulphur ic concentrat ion and found migration ofcolor to the anode. Electronlotive force nleasurenlents by ICunz (3 ) , Noyesand Garner ( C i ) , Smith and Getz (7) indicate that a conlplex is certainlyformed, but its nature could not be explicitly determined. Moore and Anderson(4) found evidence for complexing from lcinctic studies on the reaction of cericions with arsen ite ions. Smith and Getz infer the existence of ceric sulpha tecomplexes from the formation of double salts such as (NH4)2Ce(S04)3.

Moore and Anderson (5) made a spectral s tu dy on the first conlplex of ccricion and sulphate in perchloric acid solutions. They analyzed their data by thelimiting logarithmic method and the method of cont inuous variations. The ydetermined the instability cons tant of the first conlplex and found it t o varywith tota l ceric ion plus sulphate concentrat ion. Evidence for higher conlplcxeswas obtained.

In view of the lack of accura te inforlnation on the conlplexes formed, itseemed advisable t o investigate the ceric ion - ulphate ion system thor ougl~l y.Th e most pronlising method appeared to be a spectral study supplemented byqualitative migration experiments.

Experirnen ta ln/faterials

Ceric perchlorate solutions: These were prepared by the method describedin the previous paper (1). ?'he solutions contained no detec table cl~loricle,sulphate, or nitrate.

Perchloric acid solutions were prepared from AjIerck's 70% perchloric acid.Sodium hydroxide solutions (carbonate-frcc) were prepared from Nichol'sReagent grade sodium hydroxide. Sodium sulphate solutions were preparedgravimetrically from Baker and Adams Reagent grade sodiunl sulphate. Tes t

1 J~an zrscript eceived Janua ry 6 , 1951.Conlribzl lion fr om fha Chenzislry Br an cl~ ,Alomic Energy Projccl, National ReseurclzCou?zci I, Chalk R i m , Onl . Issucd as 1V.R.C. No. 2516.


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H A l W J 4 ' I C K ALVD R O B E R T SO N : A S SO C I AT I O N O F C E R I C I O N S W I T H S U L P H A T E 829solutions were made by mixing appropriat e anlount s of these solutions to givethe desired ceric ion concentration, sulphate concentration, acidity, and ionicstrength.

Optical measurements were made using a Beckmann Quartz PhotoelectricSpectrophotometer. Th e temperature of the tes t solutions was kept a t 25.0f l0C. The procedure in malting measurements was identical with tha t out-lined in the previous paper (1).

In order t o characterize the system ceric ion - ulphate ion, the various com-plexes present must be ltnown, together with their extinction coefficients forparticular wave lengths and the association constants which'relate the variousconlplexes.

I t has been indicated t hat the co~ nplex C!(SO~)~=s present in high sulphateconcentrations. As will be shown later, the sulphate co~nplexescontain nohydrogen, or hydroxyl ion; hence it will be assumed that the species Ce4+(ilnassociated), CeS04++,Ce(SO4)" and Ce(S04)3=are present under the properconditions.

In a previous paper it was shown that in perchloric acid solutions, ceric ionis most probably present as a mixture of Ce4+, CeOH3+, and (Ce-O-Ce)6f.The association constants relating these ions have been determined, togetherwith the molar extinction coefficients of each ion. Thus the relative amoun tsof t he ions, and the optical density of the solution can be deterinined for anyceric ion concentration in perchloric acid solution. In the remainder of thisreport Ce4+will be talten to mean ceric ion unassociated with sulphate, regard-less of its st at e of hydrolysis or dimerization. The term to, the extinction co-efficient of unassociated ceric ion, will be defined by

A s~na ll mount of cerous ion was invariably present in all samples, arisingfrom incomplete oxidation of stock ceriilnl solution and/or reduction of cericion during dilution. However, under the conditions used in these experiments,cerous ion does not absorb light. There is a possibility that sulphate concen-tration might be significantly reduced by the forination of a cerous- sulphatecomplex, but this has been neglected as no correction to equilibrium sulphateconceiitration from this source seems indicated i n the calculation of associationconstants.

I t was not certain whether the ceric sulpha te complexes contained hydrogenion or hydroxyl ion. This was resolved by determining the optical density ofceric ion solutions which had a fixed hydrogen ion to bisulphate ion ra tio overa range of acid concentrations. Under these conditions, as bisulphate ion is aweak acid, the sulphate ion concentration remains constant bu t the bisulphateion concentration varies. A change in optical density indicates either a changein the relat ive concentrations of the complexes, i.e. that bisulphate ion entered


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830 CAhTADIAN JOURNAL 01: CHEM I S TRY . VOL . 29

the complex as a unit or th at h j~d roxyl on is present in the complex. I t wasfound experimentally that negligible change occurred; hence it was concludedth at associated ceric sulphate does not contain hydrogen or hydroxyl ions.

Th e data obtained are required to establish the thermodynamic constantsof the following association equilibria:

Ce4+ + HSO4- + CeS04+++ H+

ICe(SO4)=I lHf1KO = -[ce SO 4)21 [I-lS04-1For convenience, the following symbols have been adopted:

Co, C1, Cz, and C3 are the equilibriutn corlcerltrations of Ce4+, CeS04++,Ce( S04 )2, nd Ce(S04)3" respectively and 0 , 1, 2, aiid 3 are the corresponcl-iilg molar extinction coefficients. At a particular \\rave length, the subscriptof the wave length in ~nillimicronswill be used, e.g.

CT is the total concentration of tetravalent ceri ~im n solution.S a n d ST re the equilibrium and total coricentrations of HSO4- respectively

in solution. At concentrations above 0.1 M HS04- the difference in valuesbetween these two quanti ties is negligible under the conditions of this study .

The equilibrium equations now become

Cz[H+]K 2 =---CISF i ~ s tComplex Ce(S04)

A series of solutions was rnade up in which the total sulphate concentrationwas maintained a t 1.00 X M and the total ceric ion concentration variedfrom 0.16 X to 5.2 X Ad. The rnolar ionic strength r was 2.00, andthe acidity 1.00 N in all cases. The optical density of these sol~ ltionswasllleasured from 395 nlp to 4.30 mp in steps of 5 mp. The optica l densi ty of thesolutions a t 400 mp is plotted against total ceric ion conce~ltrationn Fig. I.


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H A R D W I C K A N D R O B E R T S O N : A S S O C I A T I O N O F C E R I C I O N S W I T H S U L P H A T E 831

CERIC ION CONCENTRATIONFIG.1. Optical densi ty as a f irnction o f ceric ion cancentralion . Total s t~lp l t a te once~ztratio?t

= 1.00 X 10-3 moles/ l i ter . [El+] = 1.00 N. A = 400 mp.From Equation ( 2 a )

Direct solutioli of this equation is not simple. However, by successive ap-proximations values of 1C1 and are obtained which will fit the curve in Fig . 1.This value of K1 is used to find el values at other wave lengths. Optical den-


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832 C A N A D I A N J O U X N A L OF C H E M I S T R Y . T'OL. 29sities calculated fr o~ n hese t l values fitted the curves well tlirougliout thewhole range of ceric ion concentrations, which demonstra tes th at K l wasreasonably well chosen. The values of el are recorded in Table I. The valueof K, btained was 3500.

TABLE IMOI.AR EXTINCTION COEFFICIENTS

* Ta ke n fro?iz a values fo r ceric perchlorale solulions a t 1@3M Celv. Tf~i .7 ncludes Cc'+,CeOHSC, a n d ( C C - O - C ~ ) ~ + s rep or te d i n t1ze previous paper ( 1 ) .In order to check these vaIues, solutions containing s r~lpha teon ( 5X 10-41M)

and excess ceric ion were niade up. The measured optical densities were coin-pared with those calculated from the nlolar extinction coefficients in Table I.The agreement was within O.GC%.The Eigher Complexes Ce(S0J2 and Ce(S04);

The extinction coefficients of the highest complex c3 , ma)- be found bymeasuring the light absorption of solutions which are low in ceric ion and highin sulphate ion. A series of solutions wl ie ~e T= 1.00 X lop314, [Hf]= 1.00 A f ,I? = 2.00, and sulphate concentration varied from 0.12 to 0.7 M , were testedin this manner. A plot of optical density against to tal su lphate ion (Fig. 2)shows tha t the optical density approaches an asymptote, the value of whichis e3CT. The values are recorded in Table I.

The values for 2 and R 2 may be determined by assunling that fro111about0.12 rlJ sulpliate to 0.7 M sulphate the only complexes pl-ese~lt re Ce(S04)and Ce(SO4);. Deviations of the optical density from the value e3C indica tethe presence of the second complex. In thi s region one can assume that S ST.From Ecluation (4n )


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IIARDWICK AND ROBER TSON: ASSOCIATION OF CERIC IONS WI TH SULPHATE 833

M O L E S / L I T E RTOTAL SULFATE CONCENTRATION

FIG.2. Optical density a s a function of szrlphate i on concentration. Total ceric io n concen-tration = 1.00 X llT3 noles/ l i ter . [Ht] = 1.00 N. = 400 nzp.By successive approximations, values of K3 and 2 may be found which will

satisfy Equation (10). The value of K3 is 20; e? vallles are recorded in Table I.To determine Kz , the optical densities of solutions of the folloiving com-

position were measured: C T = 1.00 X 10-3M [H+] = 1.00 A4 r = 2.00, andtotal sulphate was varied from .004 M to 0.12 211. Th e results are plotted inFig. 3.

From equationC 3 - R3SC2 [H+l

and C2 - KzS- --C , [H+lone obtains Ca - R2I


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831 CANADIAN JOURNAL OF CHEMISTRY. VOL 29

M O L E S / L I T E RTOTA L SULF ATE CONCENTRA-UON

FIG. . Optica l hn s i t y a s a f z~nc t ionof sulphate ion concentration. Total ceric ion concen-tration = 1.00 X 1 C 3 ntoles/liter. [H+]= 1.00 N. = 400 nzp.If a value of l i z is assumed, then one can calculate C1, Cz, and Ca, and hence

the optical density of the solution. A comparison with the measured opticaldensity indicates the accuracy of the KZ value chosen. By successive approxi-mation one can obtain a value of K z which will give good agreement betweenmeasured and calculated optical densities. The value of Kz determined in thisway was 200.

At the sulphate concentrations used for the evaluation of K z, the amountof sulphate associated with the ceric ion is an appreciable fraction of the'total. In Fig. 3 the total sulphate is plotted against optical density. To findthe correct optical density, an approximate value of Kz must be known inorder to determine the equilibrium concentration of su lphate. This correction,,however, is simply applied as par t of the method of successive approximations.

The association constants were calculated from da ta obtained a t 400 and395 mp. Fig. 4 shows a comparison of the measured with calculated opticaldensities as a function of total sulphate concentration. T he agreement is goodover the whole range. The values of the association constants and extinctioncoefficients are therefore quite satisfactory.


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I i A R D W I C K A N D R O B E R T S O N : A S S O C IA T I O N O F C E R I C I O N S W I T H S U L P H A T E 835In Fig. 5 the fraction of the ce r i ~ ~ mn each stage of association is plotted

as a function of total s ~il pha te oncentrati o~l. t is not possible to obtain onetyp e of association complex free from the oth ers, except at high sulphateconcentrations.

LOG TO TAL SULFATE CONCENTRATIONFIG. . Co ~np ari son f calczrlatsd and m easured optical density a s a functio n of total sulphateco?zcentration. Tota l ceric io n co7~centration= 1.00 X 1 c 3 moles/liter. [HC ]= 1.00 N. =415 mp. Straight line represents calculated valz~es; ircles, experinzenlnl points.

LOG EQUILIBRIUM SULF ATE CONCENTRATIONFIG.. Per cent of ceric ion present i n each complex a s a fzrnclion of sulphate con cent ratio?^.Total ceric ion concentration = 1.00 X 1 c 3 moles/liter.


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836 C A N A D I A N JOURNAL 01; H E M I S T R Y . VOL. 29

Qualitative migration experiments were made in order t o obtain the chargeon th e ions at various sulphate concentrations. A three compar tnlent cell withfine sintered g-lass partitions was used (Fig. 6) . I n the center cornpal-tment aceric solution was placed [H+] = 1.00M, J? = 2.00, [GIV]= 1.00 X 10-311f audcontaining various am oi~ nts f sulphate. In the outer con~ part ment s n iden-tical solution was placed, but containing no ceric ion. -411 liquid levels wereequal. i\/Iigration was determined by the appearance of color in the outer com-partments. A potential of 67 1 / 2 volts was applied a t electrodes placed in theouter cells. Th e results are found in Table 11.

FIG. . Three compartnzent rni,gation cell.

TABLE I TRESULTS F MIGRATION EXPERI~IENTS

All solutions [H+] = 1.00Af, I = 2.00; [Cerv] = 1.00 X 10-3fiI(center cornpartmelit only)Tota l colic. Natu ral~o i r i . of s~~ip i i a te ~ i~ ig ra t io~imolcs/liter 24 hr.

Ind uced migration of color,direction --ostly towards cathode, stuall tracetowards anodeI 0 5 1 None I Migration both directionsC 1 . 5 1 None I Towardsanale only

These migration experinlents are in complete agreement with the spectralresults as recorded in Fig. 5. Solution A contains mostly &(SO4)++ andCe(S04)2bu t with a small amount of Ce(S04);. Solution B contains similaramo unt s of all three forms. Solution C contains mostly Ce(~04);, a littleCe(S04)2, but practically no Ce(S04)*.


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H A R D U ' I C R A N D R O B E R T S O N : A S SO C IA T IO P ? O F C E R I C I O N S W I T H S U L P H A T E 837Discussion

I t has been shown th at ceric ion associates successively with one, two, andthree sulphate ions to form complexes. As might be expected, th e tendency t oassociate becomes slnallcr as the number of sulphate ions in the complex in-creases. There is no indication that more than three sulphates are ever assoc-iated with a ceric ion.

I n concentrated ceric sulphate solution, practically all the cerium will bepresent as Ce(S04)3=.Thi s correspo~lds o the tendency for ceric ion to formdouble sulphate salts such as ammonium ceric sulphate (NH4)2Ce(S04)3.Th e data presented in this paper do not agree with th at reported by Mooreand Anderson (5). We can offer no explanation for this. However, it can bepointed o ut th at those worliers used limiting logarithmic and c on ti n~ ~o usari-atiorls neth hods of analyzirlg their data, which we find cannot be used rvith our

da ta , as the associated species overlap one anot her t o such an extent. Further-more, Moore and Anderson found th at solutions of ceric perchlorate obeyedBeer's Law up t o 0.01 41, whereas the deviations from Beer's Law fornled th ebasis of a complete stu dy in a previous paper (1). It is therefore difficult toreconcile th e results from t he two studies.

References1. HARDWICIC,. J. and ROBERTSON, . C an . J. Chem. 29: 818. 1951.2. TONES. E. M. 2nd SOPER.F. M. T. Chem. Soc. 802. 1935.3. K U N , 'A. H. -J. Am . ~ l i e m . oc. 23: 98. 1931.4. ~ ~ O O R E ,. L. and ANDERSON. . C . J. Am. Chem. Soc. 66: 1476. 1914.5. MOORE,R. L. and ANDERSON,R. . J. A m . Chcrn. Soc. 67: 167. 1915.6. N o r m , A. A. an d G A R N E R , . J. Am. Chem. Soc. 58: 1265. 1936.7. SMITH,. J. an d GETZ, C. A. In d Eng. Chem. , Anal . Ed. 10: 191. 1938.

Asociacion Iones Cericos y Sulfatos

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