Padmini, P R K L

Rao, B Ramachandra

English

IACS Institutional Repository

ULTRASONIC VELOCITY IN SOME AQUEOUS SOLUTIONS OF ELECTROLYTEST>. It. K L. I*AT)MINI and B. RAMACHANBRA ItAO(U l t h a s o n ic  L A iio E A T o iiiE e , A n d h u a  U n i v e r s i t y , W a l t a i h )[Ueceiiml, July 22, I960)ABSTRACT. Tho variation of ultraHonio ’^-olocity, adiabatir conijDrosaibility, apparent molal compressibility and molar sound voloruty with ooiujcutiation is studied in oleven electro- lyto solutions. The results are intor]iroted in tho light ol Debye-Urnckel’s theory of oloctro- lytes. Tho liydration numbers uro ostiiuatod for all the electrolytes ami compared with tho data obtamod by other methods. Non-lmuar A’^ iiriationa of molar sound v’^ olocity with molar concentration observed in some cases is explamed m terms of molecular assooial.ion and ionio solvation.1. JNTl lO DUCT  TONUJtrafinnic velocity nieasumneutH in eleeirolytie Bolnlioiiw are of couRidcralilo importance as iliey enalile ns to test tlie validity of the several theories of eleiitro- lyte soJutions. Besides it has been possible to estimate tlie number of water mojeeules attached to the ions in solution known as hydration number. Following tho theory of Debyc-liuekel. Crucker (1933) has derived certain limiting Jaws for the apparent molal eoinpressihilitv and mtorpreted his experimental data in the light ol this theory. Huhsequently 'Bachem (193G), Scot, Ohellhaus and Wilson (1934). Krishnamurly (J950), llao and Uao (1958) sirudicd the apparent molal cmnpre.ssihilities in several solutions ol“ elec trolytes ol different valence types and rc^ported deviations from the limitmg laws While studying variations of iiltrasonic velocities in solutions of elee.trolytes eertain unusual features like dc- erease ol velocity with im reaso of emiceiitration liave also been j'eported by some investigatcu’s in a few electrolytes namely, pot-assium iodide, lead nitrate, lead acetate and nranyl acetate. Very recently, Marks (1960) has calculated the hydration inniibcrs for salts of alkali metals investigated by him.As very little work has been done particularly on the estimation of hydi'atioii numbers and molar sound vcloeities in the case of solutions of electrolytes, tho authojs have taken u]) this investigation by taking measurements for about eleven new electrolytes.6?2. R E S U L T SAqueous solutions of the following salts are studied.1. Litliiiim acetate,2. Sodium acetate.3. Potassium acetate.4. Cobaltous acetate,.5. Cadmium acetate6. Cadmium chloride.7. Cadmium Bromide8. Strontium Bromide.6659, Barium bromide.10. Cadmium Iodide.11. Zinc Iodide,566 P. R. K. L. Padmini and B. Ramachandra RaoTlie salts iiscil are ol eiti)ei' hi Merck oi* li. I), H. iSoJulioiis of different cou- ecutratioiiH are prepared and tlie ultrasonic velocities are deteriuined by a fixed path variable frerjiiciuy interferometer accurate to w/,see The densities {p) are rletermined by specific gravity bottle using a Bunge balaiiwi accnratt' to 1 ingm.The values of adiabatic compressibility /f and apparent inolal compressibility are calculaticd by using the relationsJ 0(/r,) _  /iA 1000 I_\000da/i_, ^ 1 6i __ JWhere /ij is the adiabatic compressibility of the solvent, f ' — Molar concentra- l/ioJi and M  -  the inoleeidar Aveiglit of the solute.TJie variation of ultrasonic velocity, adialiatie eoinpressibilily and apparent \ molal compressibility veith concentration for all the eleven salts investigated \ is shown graphically in Figs. 1 to 0.Fjq. 1. Variation of ultrasonic velocity with concentration of the electrolyte.Ultrasonic Velocity in same Aqueous Solutions, etc. 567Fig 2CONC. MOLES/LITRE CC^Vanation of ultraaonig vt»lociiy with oonooniraiion of iho olocirolyto.V A R I A T I O N  O F  U L T R A S O N I C  V E L O C I T Y  A N D  A D T A  B A  T I C C O M P R E S S I  B J L f T Y  W I T H  C 0  N C E N ­T R A  T J O NIt van be Hceu from FigH. 1 and 2 lliat in almost all the salts studied, the velocity is found to increase with coinjentratioii except in the cases of zinc iodide, (iadniiuni bromide and iodide. Cadmium acietate solution showed a slight decrease in velocity initially and then showed gradual increase with concentration almost linearly. Although the anomalies of decrease of velocity with increase of concen­tration are noticed for those four salts, the adiabatic compressibility always showed the normal decrease with increase of concentration as can be seen in Figs, 3 and 4. The non-linear variation of velocity with concentration in the case of cobalt acetate is similar to the non-linear variation exhibited by some ferrous salts. A comparative study of the behaviour in the case of cadmium halides shows that the gradient of velocity-concentration graph decreases progressively from cadmium chloride to iodide. In a similar manner the velocities for barium bromide solutions are lower than those of barium chloride solutions studied by568 P . 7f, K* L, Padmini and B. Bamachandra RaoFig. 3. Variation ol adiabatic compressibility with concentration of the electrolyte.Baohem (1936). Tt appears from tlicso oliservatious that for a fixed positive radical iiu!rcase of atomic, weight of the negative railical in the halogen group decreases the ultrasonic velocity. This is also liorue out by data available for alkali halides studied by earlier works (Bachcm, 1936; Krishnamurty, 1950 and Wada et al., 1950).It. may also be noted that solutions of all 'the iodides studied so far show a decrease of velocity with concentration at first and later on show an increase in some cases.The coniprcBsibility is always found to decrease with increase of concentra­tion following closely the equation.,given by Giickor (1933).From a systematic study of a series of aqueous solutions of some sulphates Maiks (1944) has dra’wn the important conclusion that for a fixed anion the com-Ultrasonic Velocity in some Aquems Solutions, etc^  569Fia. 4. Variation of adialialic* compreaBibility with conoontratiori of tlu> olootrolyte.pi'essibiJit.y at a pavticular coiiceiitratioii doereaneK M’itli increasing ionit; radius of the cation. Here an allenqit is made to interpret tlie results of this investiga­tion in tile light of this rule, l^or this purpose the data available in literature are also utilised. Tbe conipressibility data lor different electrolyte solutions at a fixed ooncentration either 0.5 M to l.O M are presented in Table I.Comparing the sodium-llouride, chloride and nitrate solutions Arith tlu^  cor­responding potassium salt solutions for the same concentration it is seen that the compressibility of the potassium salt solutions is always less. This may be attri­buted to the higher radius of K < ion compared to Na+ ion. (Jompariiig the adia­batic compressibilities of IM solutions of MgCl ,^ CaCla, CdCJa seen that CdCla and CaClg obey the above rule while MgCla shows deviation from it This devia­tion may be due to the partial hydrolysis of magnesium chloride into niagnesium hydroxide. All the three bromide solutions studied follow this rule quite well. The two iodides of zinc and cadmium shoiv deviation from this rule. Similar deviations are noticed in the case of nitrates and sulphates of Fe, Co, Ni Avhich hydrolyse in aqueous solutions. It is obvious that the offec.t of water ofTABLE I570 P. It. K, L. Padmiiii and B. lirmachandra i?aoSaltIonicCone. ^  X 1012 radiusFormula Moles/ Cm^/dyne of thelitre A catioiSodium fluoride Potassium fluoride Sodium ehlond(! Potassium chloride Magnesium c-hloj ido Calcium chloride Cadmium chloride Sodium iid-rate PoiasHuini ni Irate Strontium nitrati  ^IJni ium niti'iile Nickel nitrate Harmm bromide Strontnun bromide Carlimum bromide Zmo Iodide Cadmium iodide MagncHUim Hulphate ZiiiC! sul^ihate Iron Huljdiate bithium acetate Sodium acetate Potassnuii acetate Cadmium acetate Cobaltous acetateNaFKFNnClKOIMrCLCaCl.CdClaNaNOaKNOsSr(NOa),Pa(NOa),Ni(NO.,)oBaProSrBi-2CdBi'2ZnlaCdXoMgSO,ZnS04FeSO.iLi/CHaCOO) 2HoO Na(CH3C0C))3IT2O K(f!H3COO) (M(CH'3COO)22H20 Co(CHaCOO)24H2()(1-1) 0 5 42.14 0.95(1-1) 0.5 39.43 1.33(1-1) 1 .0 38.02 0.95(1-1) 1.0 30.46 1.33(2-1) 1.0 35 70 0.05(2-1) J.O 30.50 0.99(2-1) 1.0 39.37 0 97(1-1) O.fi 41.73 0.95(1-1) 0 5 41 03 1 33(2-1) 0.3 41 01 1 13(2-1) 0.3 40 52 I 35(2-1) 0 3 41 80 0 09(2-1) 1 0 .‘l.-i. 90 1 35(2-1) l.O 38.59 1.13(2-1) 1 0 39.88 0.97(2-1) 1.0 37 27 0 74(2-1) 1.0 39.40 *0 97(2-2) 0.5 36 77 0.05(2-2) 0 5 30 34 0 74(2-2) 0 .5 38 80 0.75(1-1) 1 0 39 15 0.00(1-1) 1.0 38.54 0.95(1-1) 1.0 39.77 1.33(2-1) 1.0 38.32 0.97(2-1) 1.0 35 52 0 72hydfatioii is to decroaso llie compreHsibiJity of the solution wc howeverc(mi])are the throe .salhs lithium, sodium and cadmiuin acetates havintr nearly the same nimilier of molectiJes ot uater of c» ystallization, it will bo seen that the coin- [iressihility dee-rcase from lithmin to eadmiuui where as the ionic radius increases from Li+ to progressively, 'the deviation from this rule observed in the ease of cobaltous acetate may be partly duo to liydrolysis and partly due to the largeUltrasonic Velocity in some Aqueous Solutions, etc. 571Fia 5. KoliiiLion butwoon ajipatejil; luolal coiiiprossibility ami Hquaria-oot of uonconiiaiiion.numbor of nioleoules of water of oi-yKtallisation, the high value of comprcBsibility for Kolulions of ])otassimii aeetate eoinpared to others mspite of iis large louie ladius of 1.33 may b(^  atij“ibuted to the lack ol water (d‘ crystallisation.A P P A l l E N T  M O L A L  C O M P K E S S l B l h l T I E S  A N D  E S T r M A  T I  O N O F  It Y  D K A T I O N N U M U E 11 SIt can be seen from Figs. 5 and 6 that the apparent molal compressibility varies almost linearly with scpiare root of concentration in ftll cases except cadmium salts for which deviations ai e noticed particularly at lower comsentrations. The values of <p{lc2) for most of the salt solutions are generally negative and there are very few cases for which positive values are refiorted. In this investigation jiositivc values of are observed for ihe case of cadmium acetate, bromide and Iodide and strontium bromide solutions. The values of the gradientsare determined and presented in 'fable II along with the theoj-etically computed values. . . .572 P , P . K. L. Fadmini and B. RamacJiandra RaoFio. 6. Kolaiion bui-woon apj)avt)n.t luolal coniprosBibility and Bipiaro root/ of roncoiitration.TABLE II'rypoSalto±Eloctro-JytolO'o C.Q.S. 'J’hoorotjoalJO* Expl.-  (K ,)V  1 ( ) »c,- 0Hydra­tion numb orIjitluimi acotato 20:*0 ].I 2 82S 0.3 20..70 - 2 .9 0 3 GOHodiuru aootato 3,11^0 1-1 2 828 0.3 23.00 - 3 .5 5 4,40rolaasiuni aooliiXo M 2 S2S 0.3 12 50 - 3 ,3 0 4.20("adiumiii iinota^i liHiO 2-1 14 70(1 32.0 40.00 -1-3.50 4.40(,'abaliouH aiiotato 4HjO 2-1 14 700 32.0 110 00 -2 2 .9 0 28,50Barmiii brnmido 2-1 14.700 32.0 07.44 -1 2 .2 0 15.40Strontium bromido 2-1 14.770 32.0 15.80 —3.66 4.50Cadmiiiiu broimdo 2-1 14.700 32.0 43.00 - 0 .8 0 —Cadmiuiii iodido 2-1 14.700 32.6 34.00 - 0 .3 8 -Oarlmium uhlorido 2-1 14.700 32.0 20.06 - 5 .0 0 6.20Zinc lodido 2-1 14.700 32.0 62.10 - 9 ,1 8 1.40In the (1 -1 )  type eleotroJytoR sluclicd the slopes of almost all the solutions sh(>w deviations from the theoretieal values but it is found that the (‘xperimontal values arc always higher. There is a fairly good agieeincnt between experimental and theoi-etical values in the ease of the (2 — 1) type salts cadmium acetate, bromide and iodide although these solutions showed unusual behaviouj- in other physical properties. Although strontium bromide ajid cadmium chloride belong to (2—1) valence type, their gradients are oven smaller than the theoretical values of the (1 —1) typo electrolyte solutions. In general, it may be concluded from the above study of the apjiarent molal compressibilities that Gucker’s limiting law is only in qualitative agreement with the experinienlal value>s.It IS well known that the water molecules in the immediate neighbourhood of ions in an electrolyte solution are ini imately bound to the ions. This process is known as hydration. The first layer of water molecules attacJicd to the ion is known as primary water of hydration Estimation of jirimary hydration numbers of ions is now possible by several methods based on diffusion, ionic mobilities, activity eoetficients etc. Hydration numbers can also be estimated from a study ol apparent, molal compressibility on the assumption that the ion as wtII as the primary water of hydration are incompressible compared to the free solvent molecules. Wada and Shimbo (1950) have given Ihe following relation from which the hydration number can be calculatedLin«->()Ultrasonic Velocity in som£ Aqueous Solutions, etc. 573Vh is the volume of ])riiuary whaler of hydration for more of the eh'ctrolyte. Thelimiting value (^/c^ ) can be obt ained hy cxtrajiolatmg ^{k») versus y'c graph to zero c->0concentration. Using the experimental values of {^k )^ and /y, thi^  values of Vbc—>0are estimated. By making the further assumption that the molar volume of the solvent molei’ulos in the primary hydration sheath is the same as that of the pure solvent, the combined primary hydration number for the electrolyte is obtained by dividing V h  by the molar volume of water. The experimental values thus obtained for all the electrolytes investigated are presented in the last column in Table II. The values of hydration numbers of lithium, sodium and potassium acetates are nearly of the same order of magnitude as the values'4, 5 and 6. for the ions Le+, Na+ and respectively available in Utetature (Bell, 1953). This leads to the conclusion that the contribution due to the acetate radical is very loW. The low values of 4.4 obtained for cadmium acetate shows that the hydration number of €^ 1+“*’ ion is of the same order fbs Na+ or K+ ions although it is dohhiy*' ionised. Assuming the contribution of Cl~ ion to combined hydration number as unity as obtained from activity coefficients method, it will be semi that Hie data for CdCJj leads to the same value of about 4 for the primary hydrationof C(l '-+ ion. The abnormally low values of hydration number obtained for CVlBrj and Cdlg may be attributed to the anomalous variation of ultrasonic velocity with concentration and the non-linear nature of {^k )^ versus C“ plot. Cobaltons acetate gives an abnormally high value of 28.5 for the hydration number part of which may be due to hydrolysis. Taking the value for bromine as one, the hydration number for Sr comes out to be approximately 4 which is of the same order as Na+ or K+ ions. Similarly, the values obtained for Ba ‘ ' and Zn ”^* ions are 13 and 9 respectively.574 P. R. K, L. Padmini and B, Ramachandra RaoM O L A R  S O U N D  V E L O C I T Y  A N D  A S S O C I A T I O NMKao (1940) has shown that the molar sound velociiy K givfiii by --(k')^^®isindependent of temperature for non-associated liquids. For associated liquids like water and alcohol, R varies appreciably with temperature and Wcissler has utilised this variation for studying the change of relative association with temx)era- turc. Tt is well known that in the case of ideal solutions where there is no asso­ciation or complex formation, the R value for the solution estimated by using the J'elaiionML(F)i/3where+ C^ =1^+varies linearly with molar concentration of the solute from the value of R for the solvent to that of the solute. The theoretically computed values of R from the known molar sound velocity inci-emonts for the atoms may be faken to be equal to the values for the ideal solution. The R values for the various solutions investigated are computed and it is found that the R variation with molar fraction is nearly linear in all cases ex(;ept cadmium halides, Tt is also found that the computed values of R for the solutions arc always higher than the theoretically experimental values. Taking the ratio K  of computed to experi­mental value of R as an index of the degree of association and solvation, these values are plotted against molar fraction (7^  ^ as shown in Fig. 7. The value of K  — 1.253 obtained for zero concentration gives an idea of the "degree of asso­ciation in pure water which is responsible for the lowering of R. In the presence of an electrolyte the water molecules will be dissociated, consequently there will be a decrease in the value of R. Simultaneously there is also the phenomenon of solvation which increases the vahie of K. In all the acetates studied K  decreases with increase of concentration showing a decrease of association of water mole­cules with concentration. Tn the case of aqueous solutions of cadmium chlorideUltrasonic Velocity in som,e Aqueous Solutions, etc- 575M OLAR F R A C T IO N  (C rO  F j g , 7 . Vai'Jiiijon of “K ” fuctor witli molar fracLion “ Cm”bromide and iodide peculiar v^ariations of the factor K  are observed. Tlie curves show maxima and minima indicating tliat the ])roccBS involved is quite complex. This effect is particularly predominant in the case of cadmium iodide for which the K factor continuous to increase with ct)ncentration, a feature which is quite unusual.A C K N O W L E D G M E N TThe authors are indebted to (council of Scientific and Industrial Research, New DcUii, for financial assistance given to this research project.576 P. R. K. L. Padmini and B. Ramachandra BooREFERENCES13nll, R, r  , IDJiR. Endfavour, 17, HI.Bufthnm, Ch., 1930. Z, F. Phya k^, 101, 541.Dutta, A. K. and Bhoeh, 13. B.» 1043. Ind. Jour. Phys., 17, 19,Guckor, R. T,, 1933. Jour. Ampr. Chem. Soc., 55, 2709.KriHlmamtiriy, BU. 1950. Jour. hid. Pes., 9B, 215, lOB 140 (1951). MarkH, G. \V,, 19.51, Jour. Acous. Soc. Ameri>ca^  31, 936,Marks, G. W., 1900, Jour. Acova. Soc. America, 32,Boo, M. R., 1940. Ind. Jour. Phys, 14, 109.Bao, K, iS, and Rao, B. R., 1958. Jour. Sci. Ind. Pcs., 17B, 444.fcicoit, A. r., ObonbauB, and Wilson. R. W., 1934. Jour. Phys. Chrni, 88, 937,Wada, fcjbiiubo, Oda., 1950. Jo^ r^. Acous Sor. America, 22, 880.

Ultrasonic Velocity in Some Aqueous Solutions of Electrolytes

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Ultrasonic Velocity in Some Aqueous Solutions of Electrolytes

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