r

Indian Journal of ChemistryVol. 17A, January 1979, pp, 24-28

Some Common Features of the Action of UV Light & Gamma Rayson Hexacyanocobaltate(III) in Aqueous Solution

R. P. MITRA, B. K. SHARMA* & H.·C. P.ATTNAIKDepartment of Chemistry, University of Delhi, Delhi 110007

Received 28 March 1978; accepted 6 July 1978

Spectrophotometric-cum-polaro~raphic studies show that photolysis of [CoIII(CN)6P- with254 nm li~ht, which corresponds with a high energy d-d band of this ion and is at the tailof a CT band, causes photosubstitution as well as photoreduction, ~ivin~ [CoIII(CN).HzOP- and[CoIl(CN).13-. With 202 nm Ilght cor-responding the CT band, only photoreduction to [CoII(CN>s]3-takes place. Radiolysis of neutral and acidic solutions of K3Co(CN)6in the absence of dissolvedoxygen ~ives both [CoII(CN)1.3-and [CoIIJ(CN).H20)2- and the absorption curves of the radiolysedsolutions are similar to those of solutions photolysed with 254 nm light. In the case of air-saturated solutions, radiolysis gtves only [CoJIJ(CN).H2012- so Iong as dissolved oxygen ispresent, thereafter, both [CoJII(CN).H20)"- and [CoII(CN)6P-are formed. Radiolysis with oxygenbubbling through the solution ~ives only [CoIIJ(CN).H20p-. Fair agreement has been foundbetween observed G-values and those expected under the different conditions of radiolysis.

CERTAIN common features of the action ofUV light and gamma rays on hexacyano-cobaltate(III) ion in. aqueous solution, have

been observed by us, in the absorption spectra andpolarograms of solutions exposed to the two kindsof radiation. The absorption spectrum of theCo(CN)~- ion has two d-d bands at 260 and 313 nmand a CT band at 202 nm. Previous workers havedealt with the photolysis of the Co(CN)~- ion bythe action of UV light corresponding to d-d bandv+,We studied the photolysis of this ion by the CTband at 202 nm. The effect of photolysis by 254nm light was also studied. Though included in ad-d band with a 'peak at 260 nrn, it is near the shortwavelength end of this band and is situated at thetail of the CT band. With the 254 nm radiation,Moggi et aU found some complications in thealkaline region. Since photolysis of KaCo(CN)6would render the solution alkaline by the hydrolysisof one more CN- ions released during the photolysis,in the present investigation, the acid HaCo(CN)6'rather than KaCo(CN)6' has been used to avoid thesecomplications.

Materials and MethodsKaCo(CN)6 was prepared as described in litera-

ture", HaCo(CN)6 was prepared from KaCo(CN)6by ion exchange using the Hr-forrn of the resinAmberlite IR 120.

The cyanide ions released from the complex ionby the action of UV light and gamma rays wereestimated by the polarographic method: N2 gaswas bubbled through the solution undergoing photo-lysis and any CN- released as HCN was trappedas KCN in two successive receivers containing O·lNKOH. The test solution was then mixed withthe contents of the two receivers, and after adjustingthe pH of the mixture to 10·5 (by the addition ofB.R. buffer) and its ionic strength to 0·8 (by adding

I

solid KCI), its polarogram was recorded, using aRadelkis polarograph (Type OR-102, Metriplex,Hungary). The anodic diffusion current, whichwas due to the oxidation (at d.m.e.) of CN- ionsreleased during photolysis or radio lysis was noted,from which the amount of these ions was estimatedfrom standard curves.

A Bausch and Lomb monochromator with hydro-gen lamp was used as the source of UV light. Thelight intensity as measured by ferri-oxalate actiono-metry was 3·2 X 1014 photons/sec. Absorptionspectra were obtained in the wavelength range230 to 500 nm, using a Beckmann DU II modelspectrophotometer.

Radiolysis was done with gamma rays obtainedfrom a 60COsource (Gamma 220 cell, Canada AtomicEnergy Commission) in a cylindrical pyrex glassvessel having an inlet and an outlet. Through theinlet, nitrogen or oxygen could be bubbled as neces-sary and through the outlet any outgoing HCN gascould be trapped in a receiver containing KOHfor the estimation of CN- ions released during radio-lysis. Dose as measured by Fricks dosimetry was2·5 X 1017 eV/min/ml.

Results and DiscussionPhotodecomposition of hexacyanocobaltate(I I I) ion

- The spectral changes caused by the action of254 nm light (Fig. 1) are quite different from thosepreviously observed by Moggi et aU using 313 nmradiation (Fig. 1). We could reproduce their resultswith 313 nm light.

It is evident from Fig. 1 that as photolysis with254 nm radiation proceeds, absorbance increasesat all wavelengths in the range 230-400 nm. Theisosbestic points at 282 and 334 nm observed byMoggi et al.1 are nowhere in evidence. The initialpeak at 260 nm first changes to a shoulder and thenvanishes on further irradiation. The peak at 313

J

·MI!RA ei al.: IRRADIATION OF HEXACYANOCOBA:r-TATE(IIl)

25r------------------.

r

.8

7

60 80 )00

WAYE lENGTl-I -~= .5VIzUIo 4...•«u~ .3o

~lO 40 50 60 70 80 90 300 10 20 30 40 50 60 70 80 90 40010

W4VE LENGTH (n m )_

Fig. 1 - Spectral changes observed on photolysing whenO'OOlM H3Co(CN)6 solution with 254 nm radiation [Curves1. 2, 3, 4, 5, 6 and 7 correspond to irradiation for 15 min,30 min, 1 hr, 2 hr, 3 hr, 4 hi and 6 hr respectively. Theinset shows spectral changes when 0'001M solution of the acidis photolysed using 313 nm radiation. Curves 0, 1, 2, 3 and4 correspond to irradiation period of 0 min, 90 min, 130 min,

4 hr and 5 hr respectively]

nrn due to the Co(CN)~- ion is shifted towards theshort wavelength side and, finally, it appears asa broad band in the neighbourhood of 300 nrn.These features are not observed using 313 nm light.Absorbance at 380 nm increases from the beginningof the photolysis and, after 3 hr, an absorption peakat 380 nm slowly develops, whose height increaseson further irradiation. This peak indicates thespecies [COllI (CN)5HP]2- formed by the photolysis",However, this is not the only species which is formedfor, otherwise, the two isosbestic points at 282and 334 nm would have been observed.

The species, [CoII(CN)5P-, is known to be quitestable in solution. Its absorption spectrum showsone maximum at 280 nm (~ = 4030) and it has amuch higher extinction coefficient than [CoIII(CN)6]3-and [CoIII(CN)2HOP- in the range 200-300 nm(refs. 6-8). The broad maximum found on photo-lysis with254nm radiation suggests that [COlI (CN)5]3-is produced along with [CoIII(CN)5H202]-. Theincrease in absorbance in the 200-300 nm rangealso appears to be due to the formation of[CoII(CNh]3-.

Polarograms of the photolysed solution showedthree distinct waves (Fig. 2): (i) an anodic wavewith E i= 0·30 V (vs SCE) arising from the oxi-dation, at d.m.e. of CN- ions released by the photo-lysis; and (ii) two cathodic waves with half-wavepotentials -1·40 and -1·14 V(vs SCE) (with RCIas supporting electrolyte). The half-wave potential-1·40V is characteristic of the [COllI(CNhH20]2-ion", since [CoIII(CN)613- is not reducible at d.m.e.Hume and Kolthoff? reported a half-wave potential

,.

I

20

« 10~':::•... 5zUIa:~ 0u

5

1S~~~ __ ~ __ L- __ ~ __ ~~~~~ __ ~ __ ~o 1 2 1. e II 1.0 1.1 1.2 1.3 1.4 1.1 1.'

E. M.F. IVolts)_Fig. 2 - Polarogram of the photolysed solution USing254 nm

radiation

of -1·15V (vs SCE) for [CoII(CNhH20]3-. Adam-son et al." have shown that this species exists asthe [CoII(CNh13- ion in solution and it is this ionwhich is reduced at d.m.e. The cathodic wavefound in the polarogram of our photolysed solutionwith Ei=-l·14V vs SCE (KCl as supportingelectrolyte) thus appears to be due to the reductionof [CoII(CN)513- formed by the photoreduction ofthe [COIII(CN)6P-,

With increasing time of photolysis, the waveheights of all the three above mentioned wavesincrease, showing that more and more of [CoII(CN)5]3-and [CoIII(CN)5H20]2- ions as well as CN- ions areformed. Photolysis of [CoIII(CN)6]3-.with 254 nmradiation thus causes both photoaquation andphotoreduction of this ion.

The trend of photolysis with 202 nm radiation(Fig. 3) has some similarity with that observedwith 254 nm light. An important difference isthat, using 202 nm radiation, no absorption isfound at 380 nm even after 5 hr of photolysis, indi-cating that no [ColII(CN)5H20]2- is formed. Inagreement with this, the polarograms of the photo-lysed solutions indicated only two waves, a cathodicwave and an anodic wave. The cathodic wavewith E!=-1·15V (vs SCE) was due to the reduc-tion, at d.m.e., of [CoII(CNh]a- formed by thephotoreduction of [CoIII(CN)6]a-. The wave heightincreased with time of photoirradiation. The anodicwave with Ei= 0·30 V was, as before, due to CN-ions released during photolysis.

To show that [CoII(CN)5P- was not the result ofre.duction by h~dr~ted electron (e;q), photolysisWith 202 nm radiation was done with the additionof a small quantity of KNOa, and after differentperiods of irradiation, the photolysed solution wastested for NO; ion using sulphanilic acid and (X-

naphthylamine. The test showed the completeabsence of NO:!. NOs acts as a very good scavenger~or e~q <l:ndhad the latter been formed during photo-irradiation, some NO; would have been producedby the reduction of the NOs ion.

~s

INDIAN J. CHEM:, VOL. 17A, ] ANU-AR-Y 1979

8

WAVElEt~GHI Inml-

Fig. 3 - Spectral changes observed on photolysing O'OOlMH3Co(CN)6 solution with 202 nm light [Curves 1, 2. 3, 4. 5.6 and 7 correspond to irradiation period of 15 min. 30 min.

45 min. 90 min. 3 hr. 5 hr and 8 hr respectively]

Radiolysis of hexacyanocobaltate(II J)- So far,no systematic work has been done on the steady-state radiolysis of the [CO(CN)6]3- ion in aqueoussolution. Venerable et apo studied the pulse radio-

-lysis of some cyanide complexes of Co(III) including[CoIII(CN)6]3-, [CoIII(CN)sOH]3- and [CoIII(CN)5I]3-.They were mainly concerned with the reactivityof the hydrated electron. Effects of all reactivespecies have been considered in the present study.

(a) Radiolysis of neutral solutions in the absenceof dsssolued oxygen - A O·OOlM solution of K3CO(CN)6(PH 6·5) in triply distilled deaerated water wassubjected to radiolysis with continuous N2 gasbubbling through the solution to eliminate oxygen.An aliquot of the solution, tested polarographically,·gave no oxygen wave.

The spectral changes caused by radiolysis (Fig. 4)are very similar to those observed on photolysing[CO(CN)6)3- with 254 nm light (Fig. 1). Of parti-cular interest is the fact that on radiolysis for 30min or a longer period, the absorption- curves showmaximum at 380 nm, characteristic of the [COllI(CNlsH20P- ion. This maximum grows in heightas radiolysis proceeds. Based on this similarityin the absorption curves, it can be inferred thatthe three species [COIl(CN)5]3-, [CoIII(CN)5H20]2-and [CoIII(CN)6P- are present in the radiolysedsolution, as in the case of the solution subjectedto the action of 254 nm light. In agreement withthis, the polarograms of the radiolysed solutionsshowed, in addition to the anodic wave characteristicof CN- ions, the two above mentioned cathodic waveswith Et=-1·15 and -1·40V, indicating reduction,at d.m.e., of [COIl(CNls]3- and [CoIII(CN)5H20]2-respectively.

The solution radiolysed in the absence of dis-solved oxygen would contain the reactive primary

26

I

>-...'"~ .2o..J

«~...Q.

o

o --'.--'- ...J----L-L._t.~L.-.l_'___.l...~-..J.--'--'--'----l,LO 60 eo 300 20 LO 1;0 80 LOO 2()

W.WE LENGTH In rn) _

Fig. 4 - Spectral changes during radiolysis of a O'OOlMK3Co(CN)6 solution (PH = 6'5) in the absence of dissolvedoxygen [Curves 1. 2, 3. 4, 5. 6 and 7 correspond to irradiation

time of 5, 10. 20. 30. 45, 60 and 120 min respectively]

species R-, e;q, OR-, H202. Since both oxidizingand reducing species are present in the radiolysedsolution, the net oxidation or reduction would dependon the G-values of these species. We comparedexperimentally determined yields of products, G(exp),with their theoretical yields, G(theor.). For thispurpose, concentrations of [CoII(CN)5]3- and [COllI(CN)5H20j2- at any stage of radiolysis were cal-culated from the spectral data.

The optical density of the irradiated solutionat 380 nm was due to [CoII(CNlsJ3- and [CoIII(CN)sH20P- ions, as the [CoIII(CN)6]3- ion would notabsorb at this wavelength. From the measuredvalue of the O.D. and the known valuesl" of E3S0for [CoII(CNlsJ3- and [CoIII(CN)5H20]2-, the indi-vidual contributions of the [COl!(CN)5J3- and [COllI(CN)5H20]2- ions to the O.D. and, hence, theirconcentrations, were calculated for different periodsof irradiation, from which G values for their productwere calculated. The plot of concentration of[CoII(CNlsP- vs time of irradiation was linear till55 rnin of radiolysis. From the slope of this linearplot, G[Col! (CN)5)3- was found to 0·51. The con-centration of [CoIII(CN)5RP]2- when plotted againsttime, also gave a linear plot, from which a value5·2 for G[CoIII(CN)sHP]2- was found.

Since both R· and eaq can reduce [CoIII(CN)6J3-

to [Coll(CN)sJ3- or to the transient [Co(CN)6J4-,

G (1) reduction = GR' + Geaq = 0·55+2'76 = 3·31.In as much as radio lysis was done in de-oxygenatedwater, OR· and H202 are the possible oxidizingspecies. Assuming that only the OR· radical iseffective, Goxidation = GaR = 2·74. Thus the theo-retical G values for the production of [CoII(CN)5J3-is =GR' + Ge;;:q-GaR' = 3·31-2·74 == 0·57, whichis in fair agreement with the value 0·51 obtained

MITRA et al.: IRRADtATION OF HEXACYANOCOBALTATE(IIt)

experimentally. This agreement shows that H202is not taking any part in the reaction, which is alsoexpected, remembering that in the presence of ascavenger {here, the [CoII(CN)5]3- species}, dimeri-zation of OH· radicals to give H202 would notoccur.

The production of [CollI(CN)5H20]2- can be theresult of the collision of a reactive water moleculewith [Co (CN)6J3-.

Radiolysis in 0'8N H2S04 solution in the absence ofdissolved oxygen - A O'OOlM solution of K3CO(CN)6in 0'8N H2S04 was radiolysed with continuousbubbling of N2 through the solution. The absorp-tion spectrum of the solution radio lysed for 30min or a longer period was found to be identicalwith that obtained on the radiolysis of a neutralsolution of K3CO(CN)6' showing that the speciespresent in the irradiated solution are again [COllI(CN)5H20]2-, [CoII(CNh]3- and [CoIII(CN)6]3-. Thepresence of [COllI(CN)5H20]2-, [COIl(CNh]3- andCN- ions was confirmed by polarographic studies.

In the acidic solution, since e~q is converted intoan equivalent amount of H·, Gtotal (reduced) hasthe same value (0'57) as in the neutral solution.The experimental G-value for the production of[CoII(CN)5P- was found to be 0'54, in fair agree-ment with Gtotal (reduced).

G [CoIII(CNhH20]2- calculated by the methoddescribed earlier was 5·2, as in the case of theneutral solution. The polarographically determinedanodic diffusion current due to the oxidation ofCN- ions at d.m.e., when plotted against time ofirradiation, gave a linear plot up to 60 min of radio-lysis and G(CN-) calculated from the slope oflinear plot was 5·8. The sum of G[CoII(CN)5]3-and G[COIII (CNhHP]2- was found to be 5:72. Theclose agreement between these two values is con-sistent with the fact that CN- ions are released intwo ways: (i) by the dissociation of the transient[COIl(CN)6J4- to give [COIl(CN)5J3- and CN- and(ii) by aquosubstitution in [CollI(CN)6]3- to give[CoIII(CN)5H20]2- and CN-.

Radiolysis of air-saturated neutral solution - Up to2 hr of radiolysis of a O'OOlM solution of KaCo(CN)6containing dissolved oxygen, the spectral changes(Fig. 5) were identical with those found on photo-lysis of [COI!I(CN)6J3- on irradiation with 313 nmlight (Fig. 1).

In th latter case, even after very prolongedphotoirradiation, the absorption curves pass throughthe two isosbestic points at 282 and 334 nm andthe only photoproduct which is formed is [COIII(CN)5H20]2-, besides a CN- ion. These two isos-bestic points are also indicated by the absorptionspectra of the radio lysed solutions up to 2 hr ofradio lysis. It follows, therefore, that up to 2 hr ofradiolysis, only [COIII(CN)5H20]2- is formed from[Co III(CN)6]3-, besides a CN- ion.

In the presence of oxygen, HO; radicals andH202(HO;+HO~ == H202+02) would be formed bythe radio lysis of water. Both these species canoxidize [COIl(CN)5J3- to [CoIII(CN)6]3-. We foundseparately that [COU(CN)5J3- in aqueous solutioncan be oxidized to [CoIII(CN)6J3- by the additionof a very small amount of HP2' Since the oxidizingspecies are now HO', HP2 and OH', the total G

I

-'«'-'•..0-o

..,•..;;; 2zwo

so 60 300 20 LO 60

WAvE LENGTH I n m I _

Fig. 5 - Spectral Changes during radiolysis of O'OOlMK3Co(CN). solution (PH = 6'5) in the presence of dissolvedO2 [Curves 1,2, 3,4, 5 and 6 correspond to irradiation time of1hr, 2hr, 135 min, 145 min, 160 min and 180 min respectively]

value (G~~~I) will be GR,o, + GOR' + GRO; = 0'72+2·74+0·55 == 4·01. The only reducing speciesin the presence of dissolved oxygen is e~q andGtotal = G- = 2'76 Hence Gtotal is greaterred eaq . 'OXthan G~~daland, consequently, whatever [COl[(CN)5J3-is formed by the reduction of [CO(CN)6J3-, is oxidizedto [Co (CN)6]3-, no stable complex of Co in a higherstate of oxidation being known.

After 2 hr of radio lysis , all the dissolved oxygenwhich the solution originally had was convertedinto H0:i and H202 and the latter were fully uti-lized in bringing about the oxidation of [CoIl(CN)5J3-to [CoIII(CN)6P-, The polarogram of radiolysedsolution showed no trace of oxygen-wave at thisstage. Hence, when the radio lysis was continuedfurther, the solution contained both [CoIlI(CN)sH20J2- and [COlI(CN)5]3-, i.e. the radiolysis patternwas now similar to that found on radio lysis in theabsence of oxygen. This will be seen on comparingthe absorption curves (Fig. 5) corresponding toradiolysis periods of 2 hr or more with the absorp-tion spectra shown in Fig. 4.

Radiolysis of air-saturated acidic solutions' - Theabsorption curves of a radiolysed solution of O'OOlMKaCo(CN)6 in 0'8N H2S04 containing dissolvedoxygen were similar to those obtained on irradiationof neutral aqueous solutions in the presence ofoxygen. Up to 2 hr of irradiation, the absorptionspectra were exactly similar to those found on thephotolysis of [CoIII(CN)6]3- with 313 nm light, afterwhich differences appeared. The continuous in-crease in absorbance in the range 230-330 nm, foundafter 2 hr of radiolysis, and the shift of the 313 nmpeak towards short-wavelength side, confirmed thepresence of [COIl (CN)5J3- species in the radiolysedsolution at this stage. The concentration of [COIl(CN)5J3- was calculated by the method describedearlier and this concentration, when plotted againstthe time of irradiation, gave a linear plot, from

27

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tNDtAN j. CHEM., VOL. 17A. JANUARY 1979

obtained on the radiolysis of K3CO(CN)6 solutionhaving an initial pH of 6·5.

A O'OOlM K3CO(CN)6 solution (PH 6'5) wasfinally radiolysed with continuous bubbling of O2through the solution. The absorption curves ofthe radio lysed solution (Fig. 6) are exactly similarto those found on the photolysis of [ColII(CN)6]3-with 313 nm light. The absorbance maximum at313 nm slowly decreases in height and that at 380nm, characteristic of [COllI(CN)5H20]2-, becomesmore and more prominent. The isosbestic pointsat 282 and 334 nm persist till 6 hr of radiolysis.These spectral features confirm that, as long asdissolved oxygen is present, radiolysis of the [COllI(CN)a]3- ion produces only [ColII(CN)5H20]2- though,in the absence of oxygen, [CoII(CNh]3- is formedalong with [CoIII(CN)5H20]2-.

""c(

U0-n.o

>0-

inzw '2c

oL-L-L-L-L-L-~L-L-~~~~~J-~~~210 40 so 80 lOO 20 40 60 80 400

WAVElENGTH In m 1_

Fig. 6 - Spectral changes during radiolysis of a 0·001MK3Co(CN). solution (PH = 6'5) with continuous bubbling ofO2 during irradiation [Curves 1, 2, 3, 4, 5 and 6 correspond

to irradiation time of 1, 2, 3,4, 5 and 6 hr respectively]

which GCO(CN)3-Was calculated and found to havethe value 0'54, agreeing with the value obtainedon radiolysis of [COllI(CN)6]3- in 0·8N H2S04 inthe absence of dissolved oxygen:

Optical density due to [ColII(CNhH20P- at 380nm was calculated as before and plotted againsttime of irradiation. From the slope of the linearplot obtained, G[colII(CN)H,0]2- was calculated andit was found to have a'value agreeing with that

28

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References

1. MOGGI, L., BOLLETA, F., BALZANI, V. & SCANDOLA, F.,J. inorg. nucl. cu«, 28 (1966), 2589.

2. VIAENE, L., D'OLIESLAYER, J. & DEYAEGENE, S.,}. inorg.nucl, cu«: 37 (1975), 2435.

3. WRIGHT ON MARK & BREDESEN, DALE, Inorg. Chem., 12(1973), 1707.

4. ADAMSON, A. W., CHIANG, A. & ZINATO, E., }. Am.chem, s«: 91 (1969), 5467.

5. Inorganic synthesis, Vol. II (McGraw-Hili, New York),1950.

6. KING, N. K. & WINFIELD, M. L., ]. Am. chem. Soc., 83(1951), 5710.

7. ADAMSON, A. W., I. Am. chem, Soc., 73 (1951), 5710.8. ALEXANDER, J. J. & GRAY, H. B., I. Am. chem, Soc.,

89 (1967), 3356.9. DAVID N. HUME & KOLTHOFF, 1. M., }. Am. chem, Soc.,

71 (1949), 867.10. VENERABLE (II) GRANT, D., HART, E. J. & HALPERN,

JACK. 0., }. Am. chem, Soc., 87 (1965), 2514.