Arora, A. J.

Girdhar, H. L.

Matta, R. P.

English

NOPR

Indian Journal of ChemistryVol. 14A, Auaust 1976, pp. 561-564Dissolution Potentials of Sodium & Potassium Chlorides, Bromides &Iodides & of Ammonium HalidesH. L. GIRDHAR, A. J. ARORA & R. P. MATTADepartment of Chemistry. University of Kashmir, SrinagarReceived 12 August 1974; revised 9 October 1975; accepted 27 October 1975Dissolution potentials of nine uni-univalent electrolytes, i.e. NaCI, NaBr, NaI, KCI, KBr, KI,NH.C:t,NH.•Br and NIl.I have been measured in water at 25°± 0·1"with the maximum inaccuracy of±2'5% using a vacuum tube voltmeter. The decay of the observed potentials as a function of timehas also been investigated. In order to find out the contribution of the diffusion potentials these weremeasured from separate experiments. True dissolution potentials of all these electrolytes havenegative sign with the exception of NH.CI and NH.Br which show positive signs. The magnitudeof true dissolution potentials has been interpreted in terms of heat of ion-water interaction andreal hydration energies of ions.REC..ENTL Y a good deal of interest has beenevinced in the study of electrical effectsduring phase transformations. Potentialdifferences have been observed to develop during(i) solidification and melting of dielectri cs=+, (ii)freezing of aqueous solutions of electrolytes'', (iii)precipitation from molten electrolytes+e, (iv) preci-pitation of electrolytes from saturated solutionsv-Pand (v) dissolution of el('ctrolyt('s9-12. In spite ofthese extensive studies the origin of such potentialsis not yet clear.Srivastava and coworkers'? measured the disso-lution potential of some uni-univalent electrolytes.Since they have not taken into account diffusionpotential, therefore, the measured dissolution poten-tial could not represent the true value. Rastogiet al.10 have computed the true dissolution potentialfrom their experimental data but the study is limitedto only three uni-univalent electrolytes. Therefore,no generalized relation of these potentials with pro-perties of the electrolytes could be made. With thisbackground, it was ~onsidere(l desirable to deter-mine the values of true dissolution potentials of somesimple systems. Such as the chlorides, bromidesand iodides of sodium, potassium and ammoniumcation the detailed data of which on ionic propertiesare available so that some relationship could be builtup.Materials and MethodsAnalytical reagent grade (BDH) chemicals wereused as such and LR quality chemicals were purifiedby crystallization from their saturated solutions.The deionized water prepared using Elgastat BIOSdeionizer was used throughout this investigation.Every time before use, the platinum electrodes(99·99% purity) were cleaned with boiling nitric; acid and finally washed with deionized water. These'cleaned electrodes were always kept in deionized; water when not in use. By adopting this procedurethe asymmetry potentials 'of electrodes were foundto be considerably reduced. The asymmetry correc-lion of various electrodes was in the range 0-12 mY.The potentials were measured using vacuum tubevoltmeter (Philips model GM 60001/90) having leastcount of 2 mY. Fluctuations in potentials due tostray external fields were avoided by earthing (i)the thermostat, (ii) the voltmeter, and (iii) coaxialcables connecting the experimental cell and volt-meter. All the potentials were measured at 2So +0'1 0.Measurement of dissolution potential- The re-ference electrode was kept dipping in deionized waterin the experimental cell. A thin layer of crystalsof electrolyte was deposited on other electrodes bycontrolled cooling of saturated solution preparedat about 85°. The smoothness of the crystalswas seen by observing under a microscope. Theelectrode loaded with crystals, immediat ely afterremoving from the crystallization bath was dippedin the cell and the maximum potential developedwas measured instantaneously. The experimentswere repeated several times to check the reproduci-bility of the results. The decay of observedpotential as a function of time was investigated.The above experiments were also performed withthe electrode on which the deposited crystals werecompletely dried but the results obtained were notreproducible.Measurement of diffusion potential- The referenceelectrode was kept dipping in deionized water inthe cell. The second electrode was immersed ina saturated solution maintained at 25°. Imme-diately after withdrawing the second electrode loadedwith a thin layer of saturated solution it was dippedinto the cell and the maximum value of potentialwas measured instantaneously. Since the potentialmeasured by adopting this technique is due to thediffusion of adhering solution layer on the elect~ode,therefore, it is being called as diffusion potential,ResultsThe observed dissolution pot ent ials and the diffu-sion potentials of uni-univalent electrolytes measuredwith respect to the reference electrode are given in561INDIAN J. CHEM., VOL. 14A, AUGUST 1976Table 1. These data have been corrected for asym-metry potential of the electrodes. The decay of theobserved potentials for these electrolytes as afunction of time were also plotted. For the sake ofeconomizing the space the decay curves for thechlorides of sodium, potassium and ammonium ionare shown in Fig. 1.DiscussionSince the dissolution potential is identified as aninstantaneous potential which is developed as soonas a tiny crystal starts dissolving in water, its valuewould be a sum of the potentials developed due tovarious parallel factors which lead to the initiationof dissolution process. Keeping in view the limita-tions of our present day knowledge regarding themechanism of dissolution process we should there--150o NaClII KCIo NH CI4>e -50-.;oQ.WI 00WIis1ft..0°+50+100~ ~ ~ ~ -L ~ __~o 50 100 150 200Time(sec I250Fig. 1 - Decay of observed potential with time of sodiumchloride, potassium chloride and ammonium chloridefore, probe into the mechanism for the developmentof this potential as a combined effect of severalparallel processes. Thus when the experiment iscarried out with completely dried crystals theinstantaneous observed potential will be the truevalue of dissolution potential. But the resultsobtained using dried crystals are not reproducible.This is probably due to some vital factors whichcould not be controlled during the process of dryingthe crystals, e.g. the rate of drying and the formationof cracks on crystals surface are found to have atremendous influence on dissolution potential. Thatis why, in the present investigation all the measure-ments were carried out with the freshly depositedwet crystals. This would imply that the electrodeis loaded not only with crystals but with theirsaturated solution also. Accordingly when theelectrode is immersed in water, firstly diffusion be-tween saturated solution and water occurs whichwill be followed by the dissolution of crystals andsimultaneous diffusion due to the creation ofconcentration gradient. Thus the maximum valueof observed potential would include both the truedissolution potential and the diffusion potential.Since diffusion potential arises when the mobilitiesof the diffusing ions of the electrolyte are different,therefore, one may expect some relationship of thediffusion potential with ionic mobilities. However,the experimental values of diffusion potential arefound to show no correlation with either 1+/1_, (1+-1_)or (1++1_)/(1+-1-) where 1+ and 1_ are mobilities ofcation and anion respectively.Rastogi and Shuklal" have qualitatively explainedthe mechanism of dissolution potential of four electro-lytes on the basis of unequal ionic mobilities of theions being detached from the crystal lattice. How-ever, the process of dissolution initially involvesbreaking of the crvstal lattice and then the inter-action of anions" and cations with the solvent.Depending- upon the magnitude of the ion-solventinteraction, the rate of removal of cation and anionfrom the crystal lattice will be different. This unequalrate of removal will thus lead to the formation of anTABLE1 - DISSOLUTIONANDDIFFUSIONPOTENTIALSOFUNI-UNIVALENTELECTROLYTESAT250Electrolyte Obs. Diffusion True Diffusion Ionic mobilities. flHcation- H,O Uheationdissolution potential dissolution potential (cmt/see volt) x 10' t.1Ianion- H,O Uhanionpotential (exp) potential (graphical)(mV) (mV) (mV) (mV) Cation Anion(ref. 14) (ref. 16)NaCI -120 -40 -80 -42 51·9235 79·0788 1·832 1·389NaBr -238 -70 -168 -76 51·9235 81·1516 2·035 1·513NaI -380 -112 -268 -109 51·9235 79·5970 2-446 1·717KCl -80 -48 -38 -42 76·1768 79·0788 1·523 1-140KBr -140 -52 -88 -49 76·1768 81-1516 1·693 1·242KI -230 -80 -158 -84 76·1768 79·5970 2·034 1·409NH,Cl +114 +24 +90 +24 76'3841 79·0788 1·340 0·905(ref. 15)·NH,Br +36 +20 +16 +22 76'3841 8i-t 516 1·489 0·896(ref. 15)NH,I -282 -200 -82 -200 76'3841 79'5970 1·790 1-119(ref. 15)·Ionic mobility calculated from limiting ionic conductance datal'.flH= heat of ion-water interaction; U/< = real hydration energy.562GIRDHAR et al.: DISSOLUTION POTENTIAL OF ELECTROLYTES-300.---------------------------------~>E.: -1500a,c:0 -tOO•.. o NoX~b. K X0111III - 50 n NH~Xi541 X=CI.Br or 1:J•..00I- 1'0 2'0 3'0+50b.Hco'tion-H20/AHanion_H2o+100Fig. 2 - Plots of true dissolution potential versus ratio of heat of ion-water interaction-300-250~-200>E.,; -150oe,o NaXb. K X[J NH~XX: CI,Br or Ic:0 -100•..E0'"111 -50041:J 00•..t- I'Z 1-4 1'5 ('6 ('7Fig. 3 - Plots of true dissolution potential versus ra.tio of real hydration energieselectrical double layer in the vicinity of solidelectrolyte. Therefore, true dissolution potentialshould be related to the heats of hydration and realhydration energies of various ions because theseproperties are the ture measure of 'i,m-water inter-actions. The plots of true dissoh\ion potentialagainst ratio of (i) heats of ion-wateb, interactions,and (ii) real hydration energies of cation and anion('8are shown in Figs. 2 and 3 respectively. There isa linear relationship of true dissolution potential foreach set of halides with both these ionic properties.It is of interest to mention that when the ratio ofreal hydration energies of cation and anion is lessthan unity the sign of dissolution potential ispositive such as in the case of NH,Cl and NH,Br.However, when this ratio is greater than unity the563INDIAN J. CHEM .• VOL. 14A. AUGUST 1976sign of potential is negative. This is explainableon the unequal rate of removal of cation and anionfrom the crystal lattice.Decay of observed potential - Recently Rastogiand Shukla'? have reported that the observeddissolution potential of KCI. NaCl, NH4CI and BaCl2decays exponentially. However, the present dataof decay of observed potential show a pair ofintersecting straight lines (Fig. 1). Unless a quanti-tative theory of the phenomenon is available, it willbe difficult to say that the curves have some discon-tinuity or are exponential. When the electrodeloaded with crystals and the saturated solution isdipped in water the following three processes arelikely to occur: (i) the diffusion between saturatedsolution on crystals and water, (ii) the dissolutionof the crystals and simultaneous diffusion ofsaturated solution and (iii) only diffusion when allthe crystals on the electrode are dissolved. Sincethe first process is of very short duration, the diffu-sion potential would immediately be superimposed bythe build up of dissolution potential. Though wehad some indications of such a rise in potential butit could not be recorded accurately as a function oftime. Furthermore, the potential due to first processmay also include asymmetry potential due to theelectrodes. Now the second process will givepotential both due to dissolution and simultaneousdiffusion. When all the crystals have dissolved,the potential will be due to only the third processand the decay curve should have a trend to approachzero potential with passage of time. This is whatone should expect when the system achievesequilibrium. If we ignore the built-up stage ofpotential the decay curve will consist of two564straight lines instead of three and the potential atthe point of intersection will correspond to themaximum value of diffusion potential. The averagevalues of diffusion potential calculated from anumber of decay curves (or each electrolyte are inaccord w.th the experimental values as shown inTable 1.AcknowledgementOne of us (R.P.M.) is ,,-rateful to the CSIR, NewDelhi, for the award of a junior research fellowship.References1. COSTA, R c., Chem. Abslr., 41 (941), 1511.2. GUTMAN:-I, F., Reo st.«. Phys., 20 (1948), 457.3. HO[)R[NGUES, E., Chrm . .1/ bstr., 49 (J 955), 8653.4. l\:RALSE, Ji. 8: HENNINGEf', M., Chon. Al.str., 47 (1953),10939.5. \VORKMAN, E. J. 8: HEYKOLDS, S. E., Phv>. Re1!., 74(1948), 709; 78 (1950), 254. .6 GILL, E. W. B., J. a/>i'>l. Pltys . (S"PPi.), 2 (1953), 16.7. PAl<l<EllWA, )i. C. (,: EV[)T, 1\. J., Nature, Lond., 33(19(,5), 208.8. HASTOGI, l{. 1'. & SHUKLA, R. D., J. scient. indo Res., 29(1970), 317.9. RASTO(;I, H. P., BATRA, n. P. & ])AS, H. K., Nature,Loud., 191 (1%1), 7(,';'.10. R.ASTO(il, K 1'. 8: SHUKLA, R 1)., J. appl. 1'lIys., 41(1970), 2787.11. Sr<lVASTAVA, R. C. 8: 1\!-tURAN,\, O. P., J. Lndia n chem.soc., 48 (1971), 1151.12. RASTOGI. R. 1'., SlILKLA, R. n. 8: DHAGAT, S., J. electro-chcmirul Soc., 121 (1974), 1563.13. ANTROPOV, L., Theort:l-iCll1 electrochemistry (lvIir Pu1Jli .her,Mo,;cow), 1970, 113.14. BOCK1<lS, J. O. M. & REnllY, 1\. K. N, Modern electro:chemistry, Vol. ., (1'1"'''1111 1',.".;, )!cw YOI'k), 1970, 6X.15. BOCKRIS, J. o. M., Modern. aspects of rlcctrochemisiry(B. S. I'llblic.vt io ns. London)' 1954, 52.16. 1\NTIWPOV, L., Theoretical electrochemistry (Mir Pu hlisherMoscow), 1970, 87.

Dissolution Potentials of Sodium & Potassium Chlorides, Bromides & Iodides & of Ammonium Halides

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Dissolution Potentials of Sodium & Potassium Chlorides, Bromides & Iodides & of Ammonium Halides

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