Arnikar, H. J.

Kalkar, C. D.

Sardesai, S. S.

English

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r "~--------I-"';-'Fig. 1 - Relaxations of the four type of ions around asubstitutional impurityr.2.3.4.300 • • 5.6.7."0E- 8.0u'" 9.' .•..J:<l 10.ll.12.13.Fig. 2 - Heats of formation of solid solutions of CSCl1-xBrasystem [Full circles show the experimental points (ref. 7)]and unpolarized. The change in the lattice energy,!:JE, due to the introduction of the single substitu-tional impurity consists of four parts, namelyrepulsive, electrostatic, electronic dipole self-energyand van der Waals energy. This is minimized,with respect to the displacement and dpole momentvariables to get tlEmin, employing the optimizationtechnique of Fletcher and Powells. Finally, tneheats of formation of th ~ solid solution at variouscompositions are obtained employing the statisticalmodel of Fancher and Barsch+, In the presentcalculations WJ have employed TKS polarizabili tiesv.The Born parameters, BCsCl = 3·0003 X 10-8 erg,PCsCl = 0·293 A, BCsEr = 2·7589 x l 0-8 erg and PCsEr= 0·3067 A were obtained by solving the Hilde-brand equations of state employing the lattice con-stants and cornpressil-ilities given by Tosi-''. Thevan der Waals constants were taken from Hajj!'.The results of our calculations on the heats offormation of solid solutions of CSClI_xBrx systemare shown in Fig. 2. Considering the simplicityof the model employed in the calculation, the resultsare in good agreement with the recently reportedexperimental values"; discrepancies occur only atNOTEShigh impurity ion concentrations. There is scopefor improvement in the theoretical values in thisrange of compositions by incorporating some orderparameter for the distribution of monovalentimpurities as attempted earlier by Wasastjerna-s.We found that the theoretical curve passes throughthe experimental points once we exclude the vander Waals contributions as was clone by Fancherand Barsch", This, however, results in the under-estimation of the lattice energies of pure components.The van der Waals contribution to the latticeenergies of these halides are important particularlyin view of the recent upward revision in the estimatedvalues of these coefficientsll,13.The authors are thankful to Prof. C. N. R. Raofor suggesting the problem and helpful guidanceand to the Air Force Office of Scientific Eesearch,US (71-Zl.J8), for support of this research.ReferencesDICK, B. G. & DAS, T. P., Phys. Rev., 127 (1962), 1053.T051, 1\I. P. & DOYAMA, M., Phys. Rev., 151 (1966), 642.PAl7L, S. & SENGUPTA, S, Phys, stat. sol., 68b (1975), 703.FANCHER, D. L. & BARSCH, G. R., j. phys. Chem. Solids,30 (1969), 2503.SHl7KLA, A. E., R\J\IDA5, S. & RAO, c. N. R., j. chc m,Soc. (Faraday Il), (1973), 207.SHUKLA, A. K., CHATTOPADHYAY, S. & RAO. C. N. R.,Crystal Lattice Defects, 6 (1976), 191.SHUKLA, A. K., AHLl7WALIA, J. C. & RAO, c. N. R, ].chem. Soc. (Faraday I), (1976), 1288.FLETCHER, R. & POWELL, J. D., Computer j., 6 (1963),163.TESSMAN, J. R., KAHN, A. & SHOCKLEY, 'V., Phys. Reu.,92 (1953), 890.TOSI, M.P., Solid State Phys., 16 (1964), i.HAJJ. F., ]. chein, Phys., 4 (1966), 4618.\VASASTJEIDIA, ]. A., Soc. su. Fenuica, Com m, Phys.Maths., 14 (1948), 1; 15 (1949), i.RAO, K. ]., I~AO, G. V. S. & RAO, C. N. R., j.Phys. C. 1 (1968), 1134.Rate of Dissolution of "I-Irradiated SodiumChloride Single Crystal Studied byAquoluminescenceH. J. ARNIKAR, C. D. KALKAR & S. S. SARDESAIDepartment of Chemistry, University of PoonaPoona 411007Received 19 November 1975; accepted 25 April 1976A new method has been developed for measurtng therate of dissolution of crystals by measur-ing changes inthe intensity of aquoluminescence dur-ing their disso-lution. The relatively high activation ener~y of 35·3kJ/mole observed for NaCI single crystal between 0_600suggests that its dissolution is not diffusion-controlled.SOME of the electrons trapped at colour centresin crystals, on receiving the necessary energy,recombine with holes giving rise to thermolumine-scence (TL). An alternative mode of releasing thestored energy is observed during their dissolution+e.When the crystal structure breaks clown duringdissolution the trapped electrons get hydrated andrecombine with the holes. In this last process, afaint bluish glow extending into UV is emitted towhich the name aquoluminescencc (AL) was1009INDIAN J. CHEM., VOL. 14A, DECEMBER 1976proposed by Arnikar". The intensity of the glowis greatly increased in the presence of fluorescein.It is suggested that the rate of dissolution of suchcrystals can be measured by following the variationof the intensity of this glow with time. We haveadopted this method for NaCI and the results arereported in this note.A polished single crystal of sodium chloride(about 100 mg) was irradiated by i-radiation froma 2 kCi 60CO source to a dose of 120 krad. After theroom temperature TL had wholly decayed? thecrystal was dissolved in water (15 ml) containinga trace of alkaline fluorescein. The vessel main-tained at the desired temperature was in immediatecontact with a photomultiplier coupled to a fastscaler. The intensity of AL was measured atintervals of 15 sec. The linear plot of log intensityvs time (Fig. 1) indicates that the dissolutionfollows a first order rate law.1=10 exp (-kt) ... (1)where I is the intensity of AL at time t; 10' theinitial value at t = 0; and k, the overall dissolutionrate constant.The rate of dissolution was studied at differenttemperatures and in aqueous solutions of sodiumchloride of different concentrations (Table 1). Theactivation energy obtained from the Arrhenius plotof log l~vs liT is found to be 35·3 kJ mole+ (Fig. 2).~~------.----------------------------,o Temperature 60°Co40oo3,02001---~---;0~.5-~--':':=0:---~---c,c';·5--~-2;;';'()"---.JTtME(minlFig. 1 - Variation of log aquolurninescence (log ALl withtime080'4-"",0.s-0·4-0829 3·1oo3·J ),!) 3·/to3K/ Tplot for calculating the energy ofactivationFig. 2 - Arrhenius1010TABLE 1 - RATE CONSTANT FOR TilE DISSOLUTION OFSODIUM CHLORIDE SINGLE CRYSTALS IN \VATER AND INSODIUM CHLORIDE SOLUTIONS OF VARYING CONCENTRATIONSAND TEMPERATURESCone. of aq. Rate Temp.NaCI solution constant (OK)(mg per ml) (min-I)0 3'22 3330 2·71 3250 2·3 3140 0·96 3050 0·74 2950 0·53 2910 0·43 27940 0·46 29180 0-41 291120 0·32 291160 0·18 291200 0·16 291Contrary views have been expressed in the pastregarding the role of diffusion in controlling thedissolution. Schneider et at.s consider that thedissolution of sucrose proceeds as a first orderreaction not controlled by diffusion. Zd anovskii'',however, finds the dissolution of alkali halides diffu-sion-controlled. Berner et at.l0 represented thedissolution of calcite in sea water by the equationR = ~1(Cs-C) ... (2)where R is the rate of dissolution, C and C, are con-centrations in the bulk and in the layer adjacentto the crystal surface of area A. D is the diffusioncoefficient and r the radius of the dissolving particle.In the present work the concentration of the bulksolution is zero initially and hence the value of therate constant is dependent only on Cs• However,the rate constant was found to decrease withincreasing concentration of the salt solution. Thisis to be expected from Eq. (2) as (Cs-C) no longerapproximates to Cs•Since the activation energy for diffusion-controlledprocess is known to be less than 20 kJ jmolell themuch higher value of 35·3 kJ mole+ observed byus for the dissolution of NaCl in water suggests thatthe process is not di ffusion-controlled. A similarconclusion had been deduced by Kulikov andMamaev12 for tb e dissolution of CclF2 with anactivation energy of 36·8 kJ mole-I.References1. ARNIKAR, H. J., DAMLE, P. S., CHAURE, B. D. & RAO,B. S. M., Naturunssenschaften, 57 (1970), 541.2. ARNIKAR, H. J., DAMLE, P. S. & CHAURE, B. D., Radio-chem, radioanalyt, Left., 5 (1970), 25.3. ARNIKAR, H. J., DAMLE, P. S. & CHAURE, B. D., J. chem,Phys., 55 (1971). 3668.4. VVESTERMARK, T. & GRAPENGIESSER, B., Nature, Lond.,188 (1960), 395.5. AHNSTRAM, G. & EHRENSTEIN, G. V., Acta chem. scand.,13 (1959), 855.6. ARNIKAR, H. J., DAMLE, P. S., CHAURE, B. D. & RAO,B. S. M., Nature, t.sea., 288 (1970), 357.7. ARNIKAR, H. J., RAO, B. S. M., GIJARE, M. A. & SAR-DESAI, S. S., J. Poona Univ., 46 (1974), 115.8. SCHNEIDER, F., EMMERICH, A. & JORN, K., Chem. Abstr.,54 (1960). 16147d.9. ZDANOVSKII, A. B., zs. fiz . Khim., 25 (1951), 170; Chem,A bstr. , 48 (1954), 4291c.10. BERNER, R. A. & MORSE, J. W., Am.]. Sci., 274 (1974),108.11. Chemical kinetics, Vol. 1, edited by C. H. Bamford &C. F. H. Tripper (Elsevier, Xcw York), 1969, 252.12. KULIKOV, B. A. & MAMAEV, K A., Zh. jiz. Khim., 47(1973), 91; Chem . Abstr., 79 (1973), 43933e.Verification of the Rate Law of Oxidation ofChemisorbed Organic Residues onPlatinized-Platinum Electrodes inAcid ChlorideP. SIDHESW ARAN*Department of Chemistry, Indian Institute of TechnologyPowai, Bombay 400076Received 23 February 1976; accepted 26 A pril 1976The proposed rate law for the anodic oxidation ofchemisorbed carbonaceous residues on platinized-platinum electrodes has been verified by the establish-ment of the absence of any influence of chloride ionconcentration on the energy of oxidation of the residues.IN the. electro-oxidatio.ll .of car1J<!naceous residueschemisorbed on platinized-platinum 1-4 electrodein acid sulphate solutions, the following rate lawwas experimentally obsclved1,4,5:i = nFkOorg(I-0org) exp ~<p,F/RT ... (1)where i is the current passing, n is the number ofelectrons involved in the oxidation of the residue,F is the Faraday, ~ is the charge transfer coefficient,<Pris the potential measured versus reversible hydro-gen electrode (RHE) in the same experimentalelectrolyte at PH, = 1 atmosphere. This kineticrate law was developed on the basis of the followingmechanism 3-6:H20 ~ OHads+H++e- ... (2)Rads+CHads s~~ ROHads ~a~ CO2 ••. (3)In the case of acid chloride medium, however,the experimental rate law was different and it wasinfluenced by pH and chloride ion activity as givenbelow? :i = nFk'Oorg(1-00rg)aCl~aJ-I'!' exp ~'4)rFIRT ... (4)where aH+ and aCl- are the activities of proton andchloride ions in solution, rx and rx' are fractions (0·2and 0·5 respectively) and W ~ 0·5.Further during our studies with chloride ions thefollowing dependence was also noticed+-":[_2ir_-J = 52 mV2 log aCl- Oorg,TIn other words, the plateau potential of film oxida-tion was a function of log [Cl].<Pr= f (log aCl-)Substituting k', Arrhenius equationbe rewritten as. ( Ea)J = nFA exp - RT Oorg(1-0org)-a -a' ~<PrFX aCl- a H+ exp -RT... (5)... (6)(Eq. 4) could*Present address: Surface Chemistry Section, HindustanLever Limited Research Centre, Chakala, Bombay 400093.NOTES~'" •'0E25'0OJ'" •'"w20•OL-~~~~~~~~~_~~~~~~~o o.a 0.4 0.6 0.8'Pr(V)Fig. 1 - Ea vs .pr plots for the anodic oxidation of chemi-sorbed formaldehyde in 0'1M HCI+O-4SM H,SO. (-0-)and l'OM HCI (-.-) at Oorg=O'S... (7)(where A is the Arrhenius factor and E« the energyof activation for the oxidation of the residue).The energy of activation could be obtained bydifferentiating Eg. (7).[2 log iJ _ -Ea01/1' OorgpH ..pr - Z-:303R ... (8)If our experimental rate equation is accurate, weshould get E; independent of [Cl-J as [Cl-J #- O.This is understandable because the potential con-straint of Eq, (8) takes care of [CI-J as in Eq. (6).The present note is an effort in this direction.The details of the experiments were describedelsewhere'':". The chemisorbed films of formalde-hyde were formed in acid sulphate solutions withsteady state polarization in the potential span of0·4 to 0·5 V. The film was thoroughly washed withdeoxygenated experimental electrolyte and oxidizedin (i) O·IM HCl+0'45M H2S04 or (ii) 1M HCl, atthe required current density (in the range 2·5 X 10-5to 4x 10-4 A/geom. cm-) and at the required tem-perature (in the range 4-80").The Tafel plots for the chemisorbed formaldehydefilms obtained in 1M HCl and at an organic coverageof 0·5 and at various temperatures are similar tothose obtaineds-" at O'lM HCl+0-45M H2S04,From these plots we could determine the E; atvarious <Pr, and Cl (as in Eq, 8). The log i vs liTrelations obtained at various <Prwere used to deter-mine Ea. The Ea-<Pr dependence of these filmspresented in Fig. 1 clearly demonstrates that thevariation noticed in the E; is insignificant, despitethe fact that [Cl] was changed by one order.This is in complete agreement with our expectationand supports the rate law and consequently themechanism proposed in acid chloride medium.The author is thankful to Prof. Hira Lal, Depart-ment of Chemistry, Indian Institute of Technology,1011

Rate of Dissolution of γ-Irradiated Sodium Chloride Single Crystal Studied by Aquoluminescence

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Rate of Dissolution of γ-Irradiated Sodium Chloride Single Crystal Studied by Aquoluminescence

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