422 ANALYTICA CHIMICA ACTA

T H E E F F E C T OF H E A T ON S O L U T I O N S OF C E R I U M ( I V ) S U L P H A T E

I N S U L P H U R I C ACID A N D SOME A N A L Y T I C A L I M P L I C A T I O N S

DAVID GRANT AND DOUGLAS S. PAYNE

Department o[ Chemistry, The University, Glasgow (Great Britain)

(Received February 2oth, 1961)

INTRODUCTION

I t is commonly stated 1 that cerium(IV) sulphate solutions are stable when heated. This distinctive feature of cerium(IV) sulphate 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 oxidising agent subsequently being determined by titration with a suitable reagent.

The use of cerium(IV) solutions in analytical chemistry has been reviewed recently by YOUNG ~ and PETZOLD 3. PETZOLD has summarised the literature on the stability of cerium(IV) solutions and quotes the figures of WILLARD and YOUNG 4. According to the details given in their 1929 paper, these workers found that samples of o.I N cerium(IV) sulphate in 0. 9 to 1.8 M sulphuric acid showed no losses after boiling under reflux for up to 5 h. I t is not commonly recognised t h a t in a later paper (193o) WILLARD AND YOUNG, in reporting the reaction of various organic acids with cerium- (IV) sulphate in sulphuric acid solution under analytical conditions, mention the need for corrections of "0.03-0.05 ml" to allow for the loss of oxidising agent on boiling. The occurrence of this "b lank" in these experiments was attr ibuted to the oxidation being carried out in a beaker, whereas in the previous experiments, in which no blank was detected, a flask and reflux condenser were employed. In the experiments described, involving relatively unreactive organic acids such as formic or acetic acids, losses of cerium(IV) amounting to up to "o.21 ml" were encountered. BERRY 5, who in 1929 investigated the use of boiling cerium(IV) solutions for the quantitat ive oxidation of tartrates, reported that he had repeated WILLARD AND YOUNG'S first set of experiments and confirmed the stability of cerium(IV) sulphate. Solutions of cerium(IV) containing appreciable concentrations of nitrate or perchlorate ions were early shown to lack analytical stability on boiling 4. SMITH AND SULLIVAN 6 have observed only slight changes (~o.o3%) in an approximately o.i N solution of ceric ammonium nitrate in sulphuric acid, of concentration up to 0.5 M, after heating at ioo ° under a reflux condonser for periods of time as long as 3 h.

In marked contrast to these observations, WEISS AND SIEGER 7 have 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

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

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

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

2 C e + 4 + H20 ~ 2 Ce + s + 2 H + + ~02

This latter reaction is thermodynamically possible in acid solution, as is shown by the high value of the redox potential of Ce(IV)/Ce(III) in sulphuric acid (E0 = 1.44 V at 25 ° in I M sulphuric acid), compared with that for the oxidation of water in acid solution, (2 H20 ~ O~ + 4 H+ + 4 e; E0---- - - I . 229Va t 25°)S.The values for the redox potentials of the Ce(IV)/Ce(III) couples in nitric and perchloric acid are higher still, these solutions being appreciably less stable than the sulphuric acid system.

Although it is customary to refer to cerium(IV) solutions in terms of Ce ÷4, this ion is known not to be present in appreciable amounts, if at all, when ligand groups such as sulphate or nitrate are present; however, there are in these solutions which are only stable in the presence of acid, appreciable amounts of anionic species containing cerium(IV). Only in perchloric acid solution is the Ce ÷4 ion probably present to any great extent. The crystal structure of certain basic salts obtained from solution shows 9 them to contain poly cations such as ECesO4(OH)41 ÷12. Species of this kind containing several cerium(IV) atoms may be present in the cerium(IV) solutions as employed. Whatever the true nature of the species present, it is to be expected, in general, that the behaviour will be dependent on the acid concentration. Solutions with pH values up to 4 are of particular interest since this range is readily accessible; in the case 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 to note that ceric oxide reacts with concentrated sulphuric acid on heating with appreciable reduction of the cerium(IV), and that from time to time various ill-defined basic salts containing both cerium(IV) and cerium(III) have been reported to be obtained from sulphuric acid solution of cerium(IV) compounds, often merely on standing; further investigation of these observations is clearly desirable.

METHODS

Our attention was drawn to the inherent instability of acid ceric sulphate solutions at temperatures above 4 o° by difficulties encountered in the analysis of certain phosphites by the method of BERNHART 10 in which for the oxidation, an excess of ceric sulphate is used at the boiling point. RAO AND RAO 11 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 be carried out at slightly lower temper- atures and with a shorter heating time.

Investigation showed that acidified ceric sulphate solutions underwent loss of oxidation equivalent on heating and that this loss was 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(III) produced. The extent of the loss varied considerably, in general being below about 2 ~ for approximately o.I N cerium(IV) sulphate solu- tions in 1.5 M sulphuric acid, heated for up to I h under reflux; values as high as 2O~/o may, however, be encountered for approximately o.oI N cerium(IV) sulphate solu- tions. At temperatures below reflux the extent of the reaction is considerably less,

Anal. Chim. Acta, 25 (I961) 422-428

424 D. GRANT, D . S . PAYNE

being in general below 1%. I t 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 for varying times under specified conditions and observing the decrease in ferrous ammonium sulphate titre. Solutions were, as far as possible, always t i trated under the same conditions, ferroin being employed as indicator. I t was established that loss of cerium(IV) occurred even at temperatures as low as 45 ° and that an appreciable difference in behaviour exists between solutions in which boiling actually OCCUlTed, and those in which it did not.,

There is a distinction between solutions below and those at the reflux temperature, as regards the extent of the loss quoted earlier, and also the dependence of this extent on the physical conditions of heating. In both, the loss of cerium(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 heating 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 dependence is found for all temperatures (45-85 ° ) studied 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 achieved on treat- ment of the surface with boiling 3 M sulphuric acid for 25 min before use. Evidence suggests that repeated use of the vessels without this t reatment leads to the extent of the l~)ss becoming less. The deactivation of the surface of the vessel, probably by the products 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 some- 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 quanti tat ive cerium(IV) sulphate oxidations. For example, for a cerium(IV) solution of normality o.145 N and sulphuric acid concentration of 1.5 M, after 30 rain at 64 ° the loss varied from 0.03% to 0.3% depending on the pretreatment of the flasks. Variations in acid concentration led to an increase in the loss as the sulphuric acid concentration increased, opt imum 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 decreasing with decreasing cerium(IV) concentration. With this precipita- tion losses of cerium(IV) increase owing to the catalytic effect of the precipitate. I t is possible to redissolve this precipitate by adding sulphuric acid before titration.

Anal . Chim. Acta, 25 (I96I) 422-428

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

Under cer ta in condi t ions the p rec ip i t a t ed ma te r i a l redissolves s imply on cooling, a l though this is in general a slow process. D a t a on the va r ia t ion of loss wi th acid concent ra t ion is shown in Table I.

TABLE I

2o-ml portions of o.145 N cerium(IV). Time of heating, 65 min. Temperature 87.o ° ~ o.i °

OvetMl acid concentration % loss

io 1.7o 4.5 0.60 2.6 0.56 I. i o.4o 0.9 o.5 ° 0. 7 0.80

The effect of t e m p e r a t u r e is not a simple one, a l though one can s ta te t ha t below 4 °0 the loss is in general negligible. Var ia t ion of loss wi th ceric solut ion no rma l i t y below reflux t empe ra tu r e was not s tudied.

E x p e r i m e n t s a t the boil ing po in t were carr ied out under ref lux condit ions, the t empe ra tu r e va ry ing ma in ly wi th the acid concent ra t ion of the solut ions studied. As the sulphuric acid concent ra t ion increases up to 5 M, the t empe ra tu r e g radua l ly increases to IIO°; wi th higher acid concentra t ions , cons iderab ly higher t empera tu re s are a t t a ined , cer ium(IV) solut ions in 50% sulphuric acid ref luxing at abou t 145 °.

I t became a p p a r e n t t h a t condi t ions of boi l ing (i.e. t ype of heat ing, ex t en t of bubbl ing, even the shape of vessel) affected the resul ts ve ry m a r k e d l y as is shown in Table II, in which var ia t ions wi th v igour of boil ing are noted. The te rms gentle, modera t e and vigorous ref lux refer to the ra te of format ion of bubbles .

TABLE II

V A R I A T I O N O F L O S S W I T H C O N D I T I O N S O F R E F L U X

IO m], of a o.oi 13 N cerium(IV) solution, overall acid concentration 1. 3 M

% loss

IO rain under vigorous reflux 2. 7 IO rain under moderate reflux 1.6 73 min under vigorous reflux 7.9 73 rain under moderate reflux 5.8 73 min under gentle reflux 2.5

The ex ten t of the loss of cer ium(IV) var ies widely from exper iment to exper iment unless care is t aken careful ly to s t andard i se the condit ions, such as the ag i ta t ion of the solut ion and the ex t en t of interface be tween solution, f lask and air. The effect of va r ia t ion in cer ium(IV) and acid concentra t ions , as well as vo lume of solut ion is shown in Table I I I .

If the acid concent ra t ion and the exper imen ta l condi t ions are cons tan t the absolute loss of cer ium(IV) equiva len ts tends to be independen t of the cerium(IV) concentra- tion. Consequent ly larger percentage losses are found for more di lute solutions. This

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

426 D. GRANT, D.S. PAYNE

follows from the idea t ha t the control l ing factor is the ca t a ly t i c ac t i v i t y of the glass surface which will be independent of the cer ium(IV) concentra t ion. The ex ten t of the react ion can be increased b y increasing the surface area of glass avai lable , for

TABLE III

L O S S O F C E R I U M ( I V ) I N P Y R E X G L A S S V E S S E L

% toss Cerium(IV) HtS04 Volume of Cerium(IV)

concentration, N concentration, M solution ml during initial reaction

o.1191 2. 4 25 1.34 0.0475 2.7 25 2.52 0.0835 1. 5 io 4.i9 o.o3o~ 2.6 35 2.26

0.0762 z.o 35 0.68 O.O153 2.6 35 3 "18 O.O414 2.2 35 1"24 0.0494 1 "4 25 I.O 5

TABLE IV

V A R I A T I O N O F L O S S W I T H A C I D C O N C E N T R A T I O N

(a) Samples o /zo ml 0.0056 N cerium(IV) sulphate heated under reflux/or 4 ° rain

(b) Samples o/50 ml 0.050 N cerium(IV) sulphate heated under reflux ]or 56 rain

Concentration of Concentration of % loss HtSO, M % loss HIS04 M

o.13 lO.76 0.8 2.Ol 0.58 7.9I 1.5 1-41 t.o 7.74 2.6 1.77 2.8 7.94 4-4 1.89 4.6 8.42 6.2 2.57 9.1 13.53 9.8 4.58

example b y adding Pyrex glass helices. The effect of acid concent ra t ion is shown in Tab le ' IV .

PETZOLD 8 suggested t ha t the er ra t ic resul ts of WEISS AND SIEGER 7 were due to cata lys is of the c e r i u m ( I V ) - w a t e r reac t ion b y impuri t ies . We have examined this point b y using a solution of ceric su lpha te p repared from spect roscopica l ly pure cerium(IV) oxide and A n a l a R reagent sulphuric acid, and we can find no difference in behaviour , as far as loss of ox ida t ion equiva len t on hea t ing is concerned, between this solut ion and solutions p repared from commercia l ceria or commercia l ceric su lphate and commercia l sulphuric acid.

The effect of added silver(I), copper( I I ) , and mercury( I I ) ions on the loss of cerium- (IV) varies with the concent ra t ion of the ions; in general the ex ten t of the loss on hea t ing is increased. A decrease in the ex ten t is, however, found for copper ( I I ) when the concent ra t ion of copper( I I ) present is less than 4" lO-8 M. Larger concentra t ions increase the loss again. A careful s t u d y of react ions involving phosphi te shows tha t here, a l though the loss is not e l iminated , it m a y be subs tan t i a l ly reduced.

Anal. Chin*. Acta, 25 (196I) 422-428

EFFECT OF HEAT ON SOLUTIONS OF Ce(IV) SULPHATE 4 2 7

The loss of oxidation equivalent in cerium(IV) solution is of obvious importance in analytical reactions which require a temperature much above room temperature. For reactions at reflux temperature the method of heating is important in view of the variability of the loss with the nature of the bubbling, etc. With this in mind the wide variations found by WEISS AND SIEGER 7 can be largely attributed to their method of heating, apparently on sand baths. RAO AND RAO 11 found it necessary to employ a boiling water bath in order to control the errors encountered in the oxidation of the phosphite by cerium(IV).

Two points emerge from this work. Firstly, the loss of oxidation equivalent varies widely depending very 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 employed in analytical reactions, care must be taken to investi- gate the magnitude of the blank carefully.

The evidence so far collected is indicative of a reaction between the cerium(IV) species and the water leading to the formation of hydroxyl radicals, which subsequent- ly break down probably to gaseous oxygen. The reaction is apparently surface cata-

T A B L E V

T H E CERIMETRIC A N A L Y S I S OF P I i O S P H I T E l0 BY O X I D A T I O N OF S r H 2 ( I ~ P O 3 ) 2

(Theoret ical equ iva len t weight 62.3)

Volume of a solution ml o.H9x ,tpparera containing L859 g Cerium(IV) equioalent

SrHI(HPO,}I/xoo ml reacted wright

0.2 0.94 33-4 0.5 1.64 47-9 i .o 2.82 55.7 2.0 5 . i9 60. 5 5.0 12.57 62. 4

io .o 24.25 64.7

E a c h so lu t ion had an init ial cer ium(IV) conc. of 0.0487 N, H2SO4 conc. 2 M and Ag(I) conc. (cf. ref. n) 6. I" IO -$ M.

On ex t r apo la t ion of a plot of t he ml cer ium(IV) so lu t ion reac ted aga ins t t he a m o u n t of phosph i t e p re sen t to zero phosph i te , a " b l a n k " loss of cer ium(IV) of 0.47 ml solut ion is found. Th i s m a y be used to work o u t a corrected equ iva l en t weight , a va lue of 64 be ing ob ta ined f rom the g rad ien t of t he plot. If t he equ i va l en t we igh t (apparent ) is p lo t ted aga ins t t he q u a n t i t y of phosph i t e t a k e n a p l a t eau va lue is found nea r t he theoret ica l equ iva l en t a t 62. 3.

lysed and diffusion to the surface of the glass and the nature of this surface are clearly important factors in determining the rate of the reaction, although there is little evidence to suggest that the extent of the initial fast reaction is related in a simple fashion to the catalytic activity of the surface. The effect of the metal ions is to cata- lyse homogeneously the reaction of cerium(IV) 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, thereby reducing the probability of the radicals reacting to give oxygen. We have found that phosphite complexes with cerium(IV) in sulphuric acid solution and that the oxidation of phosphite 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 n a l . Ch im. Ac ta , 25 (1961) 422-428

428 D. 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 sulphate solution in sulphuric acid and the resulting data is then plotted to enable the loss of cerium(IV) 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 described above, whilst revealing the possibilities of serious errors in determinations based on the use of ceric 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(IV) sulphate solutions by other workers, now can be recognised as arising from the losses described above.

ACKNOWLEDGEMENT

We thank Messrs. ALBRIGHT AND WILSON for a grant to D.G.

S U M M A R Y

Ceric su lpha te solut ions m a y undergo appreciable losses of cer ium(IV) on hea t ing . The overall loss is acid dependen t , be ing a t a m i n i m u m in a b o u t i M su lphur ic acid, and is a lmos t i n d e p e n d e n t of the cer ium(IV) concent ra t ion . Consequen t ly as t he concen t ra t ion falls t he ac tua l percentage loss increases. The loss, resu l t ing f rom the reac t ion of cer ium(IV) w i th water , is ca t a lysed hetero- geneous ly a t t he glass sur face and also on t he surface of a n y prec ip i ta ted mater ia l . The e x t e n t of the loss var ies cons iderably wi th the condi t ions of t he expe r imen t . Solut ions of ceric su lpha t e p repared f rom commerc ia l grade cer ium (IV) su l pha t e or ceria and f rom spectroscopical ly pure ceria, behave in t he s ame fashion. T he effect of added silver(I), copper(II ) , mercury( I I ) and phosph i t e ions in the c e r i u m ( I V ) - w a t e r react ion is repor ted.

RI~SUMt~

Les a u t e u r s on t effectu6 une 6tude d r m o n t r a n t que les per tes en c r r ium(IV) , pa r chauf fage de so lu t ions de sul fa te de c r r i um, d r p e n d e n t p r i nc i pa l emen t de la t e n e u r en acide. La per te es t mini - m u m dans l 'acide su l fur ique I M e t ne d r p e n d p r a t i q u e m e n t pas de la concen t r a t i on en cr r ium(IV) .

Z U S A M M E N F A S S U N G

Es wird nachgewiesen, dass der Ver lus t an Ce(IV) be im Erh i t zen einer Ce(IV)-sul fa t l6sung v o m SAuregehalt der L r s u n g abhi tngt . Der Ver lus t is t a m ger ings ten in einer I M Schwefels l iure l rsung und is t unabhi ing ig yon der Ce-Konzen t ra t ion .

R E F E R E N C E S

x I. M. KOLTHOFF AND R. BELCHER, Volumetric Analysis, Vol. III, Interscience, New York , 1957, P. 129. P. YOUNG, Anal. Chem., 24 (1952) 152.

3 W. PETZOLD, Die Cerimetrie, Verlag Chemie, W e i n h e i m Bergst r . , 1955. a H. H. WILLARD AND P. YOUNG, J. Am. Chem. Soc., 51 (1929) 149; 52 (193 o) 133. 6 A. j . BERRY, Analyst, 54 (1929) 461. 6 G. F. SMITH, V. R. SULLIVAN AND G. FRANK, Ind. Eng. Chem., Anal. Ed., 8 (1936) 449. 7 L. WEISS AND H. SIEGER, Z. anal. Chem., 113 (1938) 3o5 • s W. M. LATIMER, Oxidation Potentials, Prent ice Hall , New York, 1952. 9 G. LUNDGREN, Arhiv Kemi, io (1956) 183.

10 D. N. BERNHART, Anal. Chem., 26 (1954) 1798. tl K. B. RAO AND G. G. RAO, Z. anal. Chem., 147 (1955) 274.

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