422 ANALYTICA CHIMICA ACTA

THE EFFECT OF HEAT ON SOLUTIONS OF CERIUM(IV) SULPHATE

IN SULPHURIC ACID AND SOME ANALYTICAL IMPLICATIONS

IlI\vII‘) GRANT AND I)OUGI,AS S. PAYNE

Depuvlmorl of Chemislvy, The University, Glasgow (Gred Britain)

(Rcccivcd February moth, 1961)

It is commonly stated1 that cerium(IV) sulphate solutions are stable when heated. This distinctive feature of cerium(IV) sulphatc solutions has resulted in their appli- cation in numerous analytical methods involving the use of an excess of reagent at, or near, the boiling point, the residual oxiclising agent subscclucntly being determined by titration with a suitable reagent.

The use of cerium(IV) solutions in analytical chemistry has been reviewed recently by YOUNG” and I?E*rzox.M. PETZOLIJ has summarised the literature on the stability of cerium(IV) solutions and quotes the figures of WILLARD and YOUNG”. According to the details given in their xgzg paper, these workers found that samples of 0.1 N cerium(IV) sulphate in 0.9 to 1.8 M sulphuric acid showed no losses after boiling under rcflus for up to 5 11. It is not commonly rccognisecl that in a later paper (1930) WrLLAnu AND YOUNG, in reporting the reaction of various organic acicls with cerium- (IV) sulphatc in sulphuric acid solution unclcr analytical conclitions, mention the neecl for corrections of “0.03-0.05 ml” to allow for the loss of oxidising agent on boiling. The occurrence of this “blank” in these experiments was attributed to the oxidation being carried out in a beaker, whereas in the previous esperiments, in which no blank was clctected, a flask and reflux condenser were employed. In the experiments described, involving relatively unreactive organic acids such as formic or acetic acids, losses of ccrium(IV) amounting to up to “0.21 ml” were cncounterecl. BERRY5, who in 1929 investigated the use of boiling cerium(IV) solutions :for the quantitative oxidation of tartrates, reported that he had rcpeatecl WILLAXD ANI) YOUNG’S first set of experiments and confirmed the stability of cerium(IV) sulphatc. Solutions of ccrium(IV) containing appreciable concentrations of nitrate or perchlorate ions were early shown to lack analytical stability on boilingd. SMITH AND SULLIVANU have observed only slight changes (fo.030/~) in an approsimately 0.1 N solution of ceric ammonium nitrate in sulphuric acid, of concentration up to 0.5 M, after heating at IOOO under a rcflus condenser for periods of time as long as 3 h.

In marked contrast to these observations, WEISS AND SIEGER~~~~~ reported signif- icant losses, of 13 to 20% in O,I N cerium(IV) sulphate solutions on boiling, the loss being minimised by the addition of’ silver sulphate. Their experimental details are, however, meagre and it is difficult to appreciate fully the significance of their results. PETZOLD~ has commented that these results might well have arisen from the presence

nmlr. Ckit,r. nctn, 25 (rgG1) 422-428

EFFECT OF HEAT ON SOLUTIONS OF Ce(IV) SULPHATE 423

)f traces of material which could act catalytically in reactions involving the loss of :erium(IV) from solution, such as

2 (3+4 + l&O -+z Ce+3 + 2 H+ + 40,

This latter reaction is thermodynamically possible in acid solution, as is. shown by .he high value of the redox potential of Ce(IV)/Cc(III) in sulphuric acid (Eo = 1.44 V it 25” in I M sulphuric acid), compared with that for the oxidation of water in acid. ;olution,(zH2O~Os+4H++4c;E0= -1.229 V at 25”)s. The values for the redox Jotentials of the Ce(IV)/Ce(III) couples in nitric and perchloric acid are higher still, :hese solutions being appreciably less stable than the sulphuric acid system.

Although it is customary to refer to cerium(IV) solutions in terms of Ce-14, this ion s known not to be present in appreciable amounts, if at all, when ligand groups such ts sulphate or nitrate are present; however, there are in these solutions which are mly stable in the presence of acid, appreciable amounts of anionic species containing :erium(IV). Only in perchloric acid solution is the Ce +4 ion probably present to any great extent. The crystal structure of certain basic salts obtainecl from solution shows~ :hem to contain poly cations such as [Ces04(0H)4] +12. Species of this kind containing ;everal cerium(IV) atoms may be present in the cerium(IV) solutions as employed. UVhatever the true nature of the species present, it is to be expected, in general, hat the behaviour will be dependent on the acid concentration. Solutions with pH

ralues up to 4 are of particular interest since this range is readily accessible; in the :ase of solutions of pH approaching 4 a situation of incipient precipitation arises.

In commenting upon the stability of cerium(IV) sulphate solutions, it is relevant :o note that ceric oside reacts with concentrated sulphuric acid on heating with appreciable reduction of the cerium(IV), and that from time to time various ill-defined 2asic salts containing both cerium(IV) and cerium(II1) have been reported to be lbtainecl from sulphuric acid solution of cerium(IV) compounds, often merely on ;tanding; further investigation of these observations is clearly desirable.

METHODS

3ur attention was drawn to the inherent instability of acid ceric sulphate solutions at temperatures above 40” by difficulties encountered in the analysis of certain phosphites by the method of BERNHAW~O in which for the oxidation, an excess of :eric sulphate is used at the boiling point. RAO AND 13~011 have drawn attention to the inconsistency of the results by this method and have proposed the use of silver(I) ion as catalyst, thus enabling the reactions to bc carried out at slightly lower temper- atures and with a shorter heating time.

Investigation showecl that acidified ceric sulphate solutions underwent loss of oxidation equivalent on heating and that this loss ivas dependent upon a number of factors. The reaction taking place is reduction of cerium(IV) with resultant oxidation of water, as is shown by the restoration of the cerium(IV) by treatment with sodium bismuthate of the cerium(II1) produced. The extent of the loss varied considerably, in general being below about 2% for approximately 0.1 N cerium(IV) sulphate solu- tions in 1.5 M sulphuric acid, heated for up to I 11 under reflux; values as high as ZOO/~

may, however, be encountered for approximately 0.01 N cerium(IV) sulphate solu- tions. At temperatures below reflux the extent of the reaction is considerably less,

Altal. Clrina. Acla, 25 (IgGr) 422-428

424 D. GRANT, D. S. PAYNE

being in general below IO/. It is intended to publish an account of these and other experiments later, but it seems of importance at this stage to report such aspects of the work as are relevant to the analytical application of cerium(IV) sulphate solutions.

EXPERIMENTAL

Experiments were conducted by heating various volumes of cerium(IV) sulphate so- lutions f6r varying times under specified conditions and observing the decrease in ferrous ammonium sulphate titre. Solutions were, as far as possible, always titrated under the same conditions, ferroin being employed as indicator. It was established that loss of cerium(IV) occurred even at tcmperaturcs as low as 45” and that an appreciable difference in behaviour exists between solutions in which boiling actually occurred, and those in which it did not.

There is a distinction between solutions below and those at the reflux tcmperaturc, as rcgarcls the extent of the loss cluoted earlier, ancl also the depcndencc of this extent on the physical conditions of heating. In both, the loss of ccrium(IV) occurs in the course of an initial zero order reaction which comes to an abrupt stop after various periods of time up to 30 min. The rate of this initial reaction is much greater at the boiling point than at lower temperatures. The length of time for which it occurs and hence the extent, depends on physical factors, there being a dependence on the solu- tion-glass surface area ratio and type of licating at reflux temperature, At lower temperatures the reaction time depends on the reaction rate, which is probably controlled by the precise surface conditions, faster reactions proceeding for shorter times. The same inverse clependence is found for all temperatures (4,5-85°) studiecl below boiling point. As heating is continued, no further loss occurs until, for periods of heating longer than I h, some precipitation begins, catalysing further decomposition of the cerium(IV).

Below reflux temperature the rate of the initial reaction is probably dependent on the conditions of the surface of the vessel, the highest rate being achiev&l on treat- ment of the surface with boiling 3 M sulphuric acid for 25 rnin before use. Evidence suggests that repeated use of the vessels without this treatment leads to the extent of the loss becoming less. The clcactivation of the surface of the vcsscl, probably by the proclucts of the initial reaction, may lead to a situation where virtually no loss of cerium(IV) occurs on heating. This phenomena might well serve to explain the absence of loss found by other workers. As the vessel walls become catalytically inactive no visible deposit is apparent, although in certain experiments a white solid is somc- times seen. The variation in reaction rate with surface conditions results in wide variations in the extent of the reaction found after heating for a standard time, similar to that accepted as generally useful in quantitative cerium(IV) sulphatc osidations. For example, for a cerium(IV) solution of normality 0.145 N and sulphuric acid concentration of 1.5 M, after 30 min at G4O the loss varied from 0.03”/,, to 0.30/;, depending on the pretreatment of the flasks. Variations in acid concentration led to an increase in the loss as the sulphuric acid concentration increased, optimum low values for the loss being at about I M. Below 0.5 M precipitation begins on raising the temperature, the precise acid concentration depending on the cerium(IV) concentra- tion, and clccrcasing with decreasing ccrium(IV) concentration. With this precipita- tion losses of ccrium(IV) increase owing to the catalytic effect of the precipitate. It is possible to redissolve this precipitate by adcling sulphuric acid before titration.

EFFECT OF HEAT ON SOLUTIONS OF Ce(IV) SULPHATE 425

Under certain conditions the precipitated material redissolves simply on cooling, although this is in general a slow process. Data on the variation of loss with acid concentration is shown in Table I.

2o-ml lxJrti(JnS of 0.145 IV ccrium(lV). Time of heating. 65 min. Tcmpcraturc 87.0~ _+ 0. lo

Oncrnll odd cotrcetttmf imr “/” Ioss -- ._._ __...__.. -._--_. .__ . . . _. . .._.. -- __._. - ---- -----

10 I-. 70 4-5 0.60 2 .G 0.56 I.1 0.40 0.0 0.50 0.7 0.80

_ _ _ - ._ ..__ -.- ..__....- _

‘Tllc effect of temperature is not a simple one, although one can state that below 40~ the loss is in general negligible. Variation of loss with ccric solution normality below rcflus tempcraturc was not stucliecl.

Espcriments at the boiling point were carried out under reflus conditions, the tcmpcrnture varying mainiy with the acid concentration of the solutions studied. As the sulphuric acid concentration increases up to 5 M, the temperature gradually increases to I loo; with higher acid concentrations, considerably higher temperatures itre attained, cerium(IV) solutions in 50’14, sulphuric acid rcflusing at about 145~.

It became apparent that conditions of boiling (i.c. type of heating, extent of bubbling, even the shape of vessel) affected the results very markedly as is shown in ‘Table II, in which variations with vigour of boiling arc noted. The terms gentle, moclcratc and vigorous reflus refer to the rate of formation of bubbles.

VARIATION 01’ LOSS WITH CONDITIONS OF RRPLUX

IO ml of a 0.0113 N ccrium(IV) solution, ovcrnll acid concentration 1.3 M

-.-.-. -..-_____.____.--._._ .-_-. ̂..__I_

10 min untlcr vigorous rcflux 2.7 to min under moclcratc rcflux I .6 73 min under vigorous rcflux 7.9 73 min unclcr moclcrnte rcflux 5.8 73 min untlcr gcntlc rcflux 2.5 . _._.I .l_._.---l-_._--__ _--------

The estent of the loss of cerium(IV) varies widely from experiment to experiment unless care is taken carefully to stanclardise the conditions, such as the agitation of the solution and the extent of interface between solution, flask and air. The effect of variation in cerium(IV) and acid concentrations, as well as volume of solution is shown in Table III.

If the acid concentration and the esperimental conditions are constant the absolute loss of cerium(IV) equivalents tends to be independent of the cerium(IV) concentra- tion. Consequently larger percentage losses are found for more dilute solutions. This

Anal. Chim. Ada, 25 (1961) 422-428

426 D. GRANT, D. S. PAYNE

follows from the idea that the controlling factor is the catalytic activity of the glass surface which will bc indepenclent of the ccrium(IV) ~‘on~entration. The extent of the reaction can he increased by increasing the surface area of glass available, for

y” loss Ccriurn(I VJ lf*SO, volutnc of Crriutn(I VJ

comorlraf ion, N concortraf ion, ICI solut ion id duritlg itdial read iwr

___l_r.l _---_- __ __._ . ______-- ..._--_l--.--

0. I I ‘J 1 2.4 25 I.34 “‘0475 2.7 2s 1.52 0.0x35 1.5 4 * I.9 0.0305 2.G J; 2.26

o.07G.z 2.8; 35 O.G8

c3.0153 2.0 35 3.18 n.iq I .$ ‘2 . ‘2 35 I.2r) O.O‘~c$~ 1 ..I 25 1.05

_,_____.__ __.___.__-- . . . ._-_-.---- --we--

_-_ -_..--_-I__- 1_--.- --“-.-- -----

--_ . ..__. _..-_ -.------‘.- __-_._---.“-.-

example by ;&ling Pyres glass heliccs. The effect of acid. concentration is shown in Table IV.

I~I3’r%ofJ3~ suggcstecl that the erratic results of W~rss AND SIEGER~ were clue to cntalysis of the cerium(IV) --water reaction by impurities. We have esaminecl this point by using a solution of ceric sutphate prepared from spectroscopically pure cerium(IV) osiclc nnd AnalaR reagent sulphuric acid, nncl WC can find no difference in behaviour, as far as loss of osiclation equivalent on heating is concerned, between this solution and solutions prepared from commercial ccrin or commercial ceric sulphntc and commercial sulphuric acicl.

The effect of ndclecl silver(I), copper( ad mercury(II) ionson the loss of ceriurn- (IV) varies with the concentration of the ions; in general the estent of the loss on heating is increased. A clccreasc in the extent is, however, found for copper(I1) when the concentrntion of .copper(II) present is less than 4*x0-3 M. Larger concentrations increase the loss again. A careful study of reactions involving phosphite shows that here, althoLl~l1 the loss is not eliminated, it may bc sul~st~lltially reduced.

A trd. Chitrz. Actct, 25 (l@t) 422-428

EFFECT OP HEAT ON SOLUTIONS OF Ce(IV) SULPHATE 427

The loss of oxidation equivalent in cerium(IV) solution is of obvious importance in analytical reactions which recluire a temperature much above room temperature. For reactions at reflus temperature the method of heating is important in view of the variability of the loss svith the nature of the bubbling, etc. With this in mind the wide variations found by WEISS AND SIEGER~ can be largely attributed to their method of heating, apparently on sand baths. 13~0 AND HA0 * 1 found it necessary to employ ;I boiling water bath in order to control the errors encountered in the oxidation of the phosphi tc by ccrium (I V).

Two points emerge from this work, Firstly, the loss of osidation equivalent varies widely dcpcnding every much on the conditions of the reaction. Secondly, it is not possible to evaluate in advance the effect of the various factors and consequently, when the reagent is cmploycd in analytical reactions, care must be taken to investi- gate the Itl~~~t~ituc~~ of the blank carefully.

‘fhc eviclcncc so far collected is indicative of a reaction between the ccrium(IV) species and the water lending to the formation of hg’cirosyl radicals, which subsequent- 1). hrenk down probably to g;tscous oxygen. The reaction is apparently surface cata-

THK CISHIMI!THIC r\.Vr\LVSlS Olr I’HOSI’HITE 12 IIY OSlI7ATlON OP SrHa(Hl'O2)2

(‘l’hivx-ctical cqnivalcnt weight 62.3) ll..__-..--____. . _---- ..- _ _ __....._~-” L... -..+.- _..- -..---

Volumr of a sdu1tot, ml cJ.llOI 4~>par~~lt containing rd.59 g CC?i$it;W equirdmt

Srl~,(HlW,),tmo mi wei& ll_.-.-_-“______ -- -----.------

0.2 0.94 33.4 O-5 1.04 47.9 r .cJ 2.Na 55.7 .?.o 5.19 b..5 5.0 rr.57 61.4

to.0 2.~ .rg 64.7 -I_ . _. “_l_l..._l _ . . ..- .--_--.- _l_l _..__-. ---

19ach sotution had an initial cwium( I\‘) cont. of 0.0.~~7 ;V. l-i&K)~ cr~nc. 2 M and Ag(I) cont. (ct. ref.11) 6.r * xo-” hf.

On cxtrapnlation of a plot of the ml ccrium( l\‘) solution rcnctcd against the amount of phosphitc prcscnt to zero phosphitc, a gil~lank” loss of ccriuln (IV) of 0.47 ml solution is found. This may be used to work out a corrcctcd cquivnlcnt wcigllt, a value of 64 being obtained from the gradient of the plot. If the cquivalcnt weight (apparent) is plotted against the quantity of phosphitc taken it plateau value is ftwntd near the thcoreticnl rquivalcnt at 62.3.

lysccl and diffusio1~ to the surface of the glass and the nature of this surface are clear13 important factors in determining the rate of the reaction, although there is little &idcncc to suggest that the estcnt of the initial fast reaction is related in a simple faAion to the catalytic activity of the surface, The effect of the metal ions is to cata- lysc homogeneously the reaction of cerium(lV) and water, and thus increase the loss. Phosphite on the other hand will minimise the loss by reacting with the hydroxyl radicals almost as soon as they are formed at the surface of the vessel, thereb!, reducing the probability of the radicals reacting to give osygen. We have found that phosphite coniplcses with ccx5um(IV) in sulphuric acid solution and that the oxidation of phosphitc in these solutions, on heating, proceeds probably by way of reaction of phosphite ions with hydroxyl radicals. The incipiently formed radicals will thus be liable to interact with phosphite rather than to escape to the solution.

A,prsl. Chim. Ada, 25 (1961). 4227428

428 I). GRANT, D. S. PAYNE

In the analysis of phosphites it has been found necessary to adopt a procedure whereby the oxidation equivalent is determined for different amounts of phosphite using a constant amount of ceric sulphilte solution in sulphuric acid and the resulting cl&a is then plotted to enable the loss of cerium(fV) to be evaluated (Table V). The appearance of solid material in the solution may make the loss considerably larger and the accurate assessment of the correction difficult.

CONCLUSION

It must be admitted that the work &scribed above, whilst revealing the possibilities of serious errors in determinations based on the use of ccric sulphate solutions, does not offer any single solution to the problem, which must be treated and solved inde- pendently in every situation, Previous difficulties reported in certain applications of cerium(IVf sulphate solutions by other workers, now can be recognised as arising from the lbsses*dcscribecl above. -

ACI~NOWI~EI~GDblEN’1

We thank Messrs. Al.nlUc,H-r ANI:, WILSON for a grant

SUMRIAltY

to D.G.

CIcric sulphatc solutions may unclcrgo npprcciablc losses of corium( LV) on heating. ‘The uvcr:LII loss is acid clopcnclcnt, being at a minimum in about 1 M sulphuric acid. and is almost intlcpenrlent of the corium(iV) conccntmtion. Conscqucntly as the concentration falls the actual perccntajic loss increases. The loss, resulting from bhe reaction of ccrium(lV) with water, is catalyscd hctcro- ~cneausly at the &tss nurfacc ilncl nlso on tlic surface of imy prccipitzttccl mntcrinl. The cxtcnt of the loss varies C~~nsicI~r~L~ly with the c~}n~li~i~~ns of the cxpcrimcnt. Solutions of curie sulphatc prcparcd from ccm~mrxc i.il ~‘r,icI~’ carium( tV) suiptiatc er ccria and from spcctrosco~~icnlly pure ccria, behave in the same :r~-l~~~.~~t ‘1 IS* effect of nciclccl silver(f), copper( f I), mcrcury(1 If and ptlosphitc kJnS in the ccrium( 1 V) - water reaction is rclx~rtccl.

I,cw nutcurs ont cffcctud unc! dtudc cldmontmnt clue lus purtes cn cdrium(IV), par CllaUCfi~gC clc solutions dc sulfate clc cdrium, tlbpcndcnt principalcmcnt de la tcncur cn aciclc. La pcrtc cst mini- mum clans l’acidc sulfuriquc! r M ct nc dbpcnd pratiqucmcnt p;LH clc la concentration cncdrium(IV).

1:s wird nachgcwicson, dass dcr Verlust an Cc( IV) bcitn Erhitzcn uincr C:~(IV)-sulf;rtlbuung vcjtn S~turc~~l~~lt clcr I..&ung alSin@. 13cr Vcriust ist am gcririgsten in cincr I AZ SchwcfeluartrelastlnR und ist ~In~~~i~~~I~~i~ von clcr Cc-Konzcntrtltion.

* 1. M. i<o~~lioPP AND It. ~lzLclti3I, ~~o/rrmelvic .drtdysis, v(J). f II, Intcrscicncc, New York, 1957, 1’. 129.

u P. YOUNC., Anut. C/tom., ah (1g52) 152. :I W. l%czoLD, Die Ceviwchic, Vcrlsg Chcmic, \Vcinheim I?lcrgstr+, 1955 J H. l-1. WILLAK~ AND 1'. YOUNG, J. Am. Ckem. Sot., 51 (~9W 140: 52 1930) $33. fJ A. J. BJmizY, nltalyst, 54 (1j)q) d&61. * G. 17. SMIT~I, V. R, SULLIVAN AND G. FRANK, Id. Rug, CJtcrrr., Ami. IX., 8 (1g30) 449. 7 L. Waxss AND ?-I. SIISG~K, b. ancti. C/tern., I 13 (1938) 305. 6 W, M. LATIMISR, Osidaiiott Polcafids. 13rcnticc. I+all, New York, zgs2. 0 G. LUNDGREN. Avkiv KLTUJ~, IO (rg56) 153.

fta D. N. BERNIIART, A~tnt, Clrctiz,, 26 (1954) 17g8. Ii I<. B. RAO AN13 G. G. RAO, %. citarif. CI~ctn., ‘47 (t’)55) 274.

s‘f?t& ChI’Iit. ,fGttt, 25 (1961) 422-428