Indian Journal of ChemistryVol. 28A, July 1989, pp. 565-569

Partial molar volumes and, viscosities of some transition metalsulphates in aqueous urea solutions

M L Parmar", Anita Khanna & V K GuptaDepartment of Chemistry, Himachal Pradesh University, Shirnla 171 005

Received 16 March 1988; revised 15 July 1988; accepted 19 August 1988

~artial molar volume and viscosity data for cobalt(ll), manganese(ll) and iron(ll) sulphates in urea-water mixtures (11.52, 20.31, 29.64 and 36.83 urea wt %) at different concentrations and temperaturesare presented. The density data have been analysed using the Masson's equation. The limiting apparentmolar volumes (~VO) and the experimental slopes (S,) have been interpreted in terms of solute-solvent andsolute-solute interactions, respectively. The ~vo values vary with temperature as a power series of temper-ature. Structure making/breaking capacities of the transition metal sulphates have been inferred from thesign of (ij2 ~vo / aT2 )p. All the electrolytes behave as structure makers/promotors. The viscosity data havebeen analysed by means of Jones-Dole equation. The activation parameters have also been obtained onthe basis of transition state theory in order to understand the mechanism of the viscous flow.

The partial molar volume and viscosity data areknown to give valuable information regarding so-lute-solvent, solute-solute and solvent-solvent inter-actions 1.2. Although studies on partial molar volumeand viscosity in binary systems are abundant, thoseon ternary systems are few. Moreover, physico-chemical studies on ternary systems in aqueous so-lutions are gaining importance because it is some-times difficult to arrive at a definite conclusion re-garding structure and properties of solutions fromthe studies on the binary systems alone.

A survey of literature showed that data of 2 : 2electrolytes, particularly transition metal sulphates,in non-electrolyte systems are non-existent. Sincethe structure of urea + water mixtures is of great im-portance in understanding protein denaturation+"in urea + water mixtures, the title investigation hasbeen carried out with a view to (i) discussing the par-tial molar volume and viscosity B eoefficient interms of solute-solvent interactions, (ii) understand-ing the effect of urea content on these interactions,(iii) investigating the structure making/breaking ca-pacity of transition metal sulphates from the tem-perature effect, and (iv) investigating the mechanismof viscous flow from activation parameters.

Materials and MethodsCoba\t(II) manganese(II) and iron (II) sulphates

and urea were of AR grade and used as such afterdrying over P20S' Fresh distilled conductivity water(sp. condo 10-0 Q-I cm-I) was used for preparingthe urea-water mixtures as well as a standard liquid.

The urea-water mixtures of varying compositions aswell as the solutions of electrolytes were made byweight and molalities were converted into molaritiesusing the standard expression described elsewhere".

Densities were measured with an apparatus simi-lar to the one reported by Ward and Millero" anddescribed elsewhere". The kinematic viscositieswere measured with the help of a capillary type vis-corneter" with a flow time 1474.5s for water at303K. Runs were repeated until three successivedeterminations were obtained within 0.1 S. Becauseall the flow times were greater than 100s, the kineticenergy correction was not necessary. The relativeviscosities of the solutions were calculated using therelation:

... (1)

where the symbols have the usual significance. Theapparent molar volumes (~v) were calculated fromthe density data using the expression:

... (2)

where C is the molarity of the electrolyte, d thedensity of the electrolytic solution, do the density ofsolvent (urea + water), and M2 the molecular weightof the electrolyte.

The viscosity measurements were carried out inan air thermostat while those of density in a waterbath, whose temperature was kept constnat within± 0.01 -c

565

INDIA N J CHEM. SEe. A. JULY 1989

• A ""_ •. _1_'-_1. 0 C' l/ v. r"L V :_ AA ..••.•IA~.A: •••.•." ,..f' •.•1", •••

INDIAN J CHEM. SEe. A. JULY 1989

Results and DiscussionThe apparent molar volume (¢J was calculated

from the density data using Eq. (2). It was found tovary linearly with the square root of the concentra-tion in conformity with the Masson's equation?(Eq.3),

¢, = ¢,o + 5, ie ... (3)

where ~~)is the limiting apparent ~olar volume andis equal to partial molar volume Vio at infinite dilu-tion, and 5, is the experimental slope. A sample plotfor cobalt(II) sulphate in different compositions ofurea-water at 303K is shown in Fig. 1.

The partial molar volume at infinite dilution (¢~))and the slope (5,) data for all electrolytes in differentcompositions of urea-water and at 303K are givenin Table 1. It is evident from Table 1 (as well as fromFig. 1) that 5, is positive and large which indicatesthat ion-ion interactions are quite strong for all theelectrolytes. These interactions do not change ap-preciably with the change in composition in case ofmanganese (II) sulphate. These interactions, how-ever, decrease with the increase in urea content inthe cases of iron(lI) sulphate and cobalt (II) sulphate.

The values of ¢,o are positive for all the electro-lytes indicating thereby positive interactions be ..tween ions and solvent. The value of ¢,o increaseswith the increase in ur.ea content for cobalt (II)sulphate thereby suggesting that ion-solvent interac-tions increase with the increase in urea. The value of

128

126

124

12<1

120

'-0E 118

"'E~~ 116

114 _

112

11

0-----0 1P52.,. URE.A

~ 2(}ll~. UREA

~ 29·64-1. UREA

to

Q

•

o _4__ 8_rcl~lrjl (~~1 1'1) _20__ 24__ 28_ 32

Fig. 1- Plots of (A vs Jc for cobaltl ll} sulphate in differentcompositions of urea-water at 303K.

SOl)

Table 1-Partial molar volumes (¢,", cm' moll) and the experi-mental slope (5" em' I' , mol:' ') of some transition metal sul-phates in different compositions of urea-water (u;', by wt) and

at different Temp. (K)

Urea (%) 11.52 20.31 29,{)4 36.83

Cobalt sulphate at 303K

¢:' 109.4 112.0 113.6 11/l.4

S, 34.6 31./l 30.6 2/l.3

Manganese sulphate at 303K

~:I 10.5 41.5 19.0 - 14.5S, 313.5 29/l.4 321.0 250.0

Ferrous sulphate at 303K

¢;:' 107.0 J25.0 99.5 90.0s. 857.0 R31.0 799.0 725.0

Temp.(K) 298 303 308 313

Cobalt sulphate in J1.52 'J{, urea

¢~I 119.2 109.4 104.6 100.5S, 32.7 34.6 31.2 32.4

Manganese sulphate in 11.52 'Yo urea

¢~) 24.5 10.8 -0.5 ':"'9.5s, 30/l.0 313.5 305.0 319.0

Ferrous sulphate in 11.52 % urea

<p~1 161.5 107.5 70.5 52.55, 755.0 815.0 860.0 886.0

¢~)decreases with the increase in urea for manga-nese(II) sulphate and iron(lI) sulphate, having amaximum value in 20.31% urea-water. This sug-gests that solute-solvent inter.actions are highest atthis composition.

Since the behaviour of all the electrolytes in dif-ferent compositions of urea-water was found to beidentical at 303K, only 11.52% urea-water compo-sition was selected for studying the effect oftemper-ature. The linear plots of ¢, versus J C have beenobtaied at different temperatures (298, 303, 308and 313K) in 11.52% urea-water for all the trans-ition metal sulphates studied here. The values of thelimiting apparent molar volume (¢,O) and the experi-mental slope (5,) at different temperatures are alsorecorded in Table 1.

It is evident from Table 1 that the values of 5, forall the electrolytes at all the temperatures are posi-tive and large which indicate that ion-ion interac-tions are strong in 11.52% urea at all temperatures

PARMAR et 01.: PARTIAL MOLAR VOLUMES & VISCOSITIES OF TRANSITION METAL SULPHATES

----TEMPERATURE TI K --;,.

0r-__ ~2T98~ ~~ ~~ ~~ __

-2

-6

~~-8

-10

-12

-14

-16

Fig. 2-Variation of #. with temperature.

and these increase with the increase in temperaturein case of iron(II) sulphate whereas they do notchange appreciably with the change of temperaturefor cobalt(II) sulphate and manganese (II) sulphate.

The temperature dependence of ~~ in 11.52%urea-water mixture for the different electrolytes canbe expressed by the following equations:~vo=6241.2-39.0T+0.06T2 ... (4)

(for cobalt sulphate)~vo= 4223.4 - 25.4 T+ 0.04 T2 ... (5)

(for manganese sulphate)~vo= 38745.5 - 245.7 T+ 0.39 T2 ... (6)

(for ferrous sulphate)The apparent molar expansibilities #. = (a~~)1

iJT) calculated from Eqs (4), (5) and (6) indicatethat tN. increases with the increase in temperature.The increase in magnitude per degree temperatureis positive indicating thereby that the behaviour ofall the three electrolytes is similar to that of symmet-rical tetraalkylammonium salts 10 and unlike that ofother common electrolytesl'-". The positive in-crease in tN. with increase in temperature for all theelectrolytes (Fig. 2) may be ascribed to "caging ef-feet"!", Further, it is also clear from Fig. 2 that theseplots are non-parallel which shows that the "cagingeffect" is different for different electrolytes studiedhere in 11.52% urea.

It is observed from Eqs (4) and (6) that (iJl ~~)1aT2)p for solutions of all the three electrolytes in11.52% urea-water is positive which means that allthe electrolytes studied here behave as structure

Table 2- Values of A (em312 mol-1I2) and B (cm ' mol-I) par-ameters of the Jones-Dole equation for some transition metal

sulphates in urea-water (Ok by wt.) mixtures

Urea (%) 11,52 20.31 29,64 36.83

AB

Cobalt sulphate at 303K

0,052 0,012 0.010 0.0040.636 0.784 0.867 0.894 .'

Manganese sulphate at 303K

0,103 0.043 0,135 0,1110.476 0,756 0,617 0.615

AB

Ferrous sulphate at 303K

A 0.028 0,003 0.054 0.013B 0.710 0.77! 0.766 0.761

Temp.(K) 298 303 308 313

Cobalt sulphate in 11.52 % urea

A -0.048 0,054 0,034 0.D38B 0,648 0,636 0,555 0.550

Manganese sulphate in 11.52 % urea

A 0.037 0.103 0.060 0,156B 0.486 0.476 0.463 0.450

Ferrous sulphate in 11.52 % urea

A 0.DI8 0.Q28 0.037 0.063B 0.715 0.710 0,700 0.674

makers/promotors keeping in VIewthe work of He-pler!'.

The relative viscosities of cobalt (II), manga-nese{II) and iron{II) sulphates in 11.52%, 20.31%,29.64% and 36.83% urea-water mixtures were de-termined at 303K. Since the viscosity behaviour ofall the electrolytes was found to be identical in dif-ferent compositions of urea-water, the relative vis-cosity for these electrolytes was determined only in11.52% urea-water at 298, 303, 308 and 313K toknow the temperature effect. The data were ana-lyzed by means of Jones-Dole 14 equation (Eq. 7),1]11]0= 1 +ACl12+BC ... (7)

where rJ and rJo are the viscosities of the solutionand solvent (urea + water) respectively, C the molarconcentration of the solute and A and B are con-stants which are specific for a given solute-solventsystem. Further, A is known as the Falkenhagan"coefficient that takes into account ionic interactionsand B is the Jones-Dole coefficient" that is related

567

INDIAN J CHEM. SEe. A. JULY 19R9

24

8

o 8 12 16 20 24 • 28____ JC.,rr(moIIW') -+

4

Fig.3-Plotsof(I]/I],,- I)OC vsJC forcohalt(lI) sulphateat different compositions of urea-water mixtures at 3()3K.

to the size of the ions and to the different ion-solventinteractions. The A and B coefficients, for all thetransition metal sulphates studied here, obtained asintercepts, and slopes of the linear plots of ('Y/ /'Y/o - 1/ J C) versus J C (a sample plot is shown inFig. 3 for cobalt sulphate at 303K), arc given inTable 2.

It is evident from Table 2 that the A coefficient ispositive in the entire composition range of urea-wa-ter mixtures at 303K, indicating thereby positiveionic interactions. In the case of MnS04 the A va-lues do not change appreciably with the change incomposition of urea + water mixture whereas thesedecrease with the increase in urea content for cobaltsulphate. Further, the magnitude of A increases withincrease in temperature for ferrous sulphate mean-ing thereby that ionic interactions improve with theincrease in temperature. The magnitude of A doesnot show a systematic change with temperature forcobalt sulphate and manganese sulphate.

The B coefficients for all the electrolytes are posi-tive and large in the entire concentration range ofurea-water mixtures reported here indicating there-by the existence of strong ion-solvent interactions.The magnitude of B increases with the increase inurea content fot cobalt sulphate indicating therebythat ion-solvent interactions increase with the in-

568

Table 3- Values of ""II ;'. and ""II~" in urea-water at 303K

Urea "" 11,"' "" II'II,(0;', hy wt ) (k.I mol r t ) (kJ mol -')

CoSO. MnSO. FeSO.11.52 IIA 53.5 37.9 57.020.31 11.6 62.0 50.7 02.329.64 ILl) ill).t 47.:1 st.o36.R3 12.2 70.0 -l5.5 oOA

32

crease in urea. The magnitude of B has maximumvalues in 20.31% urea-water for both manganesesulphate and ferrous sulphate suggesting therebythat ion-solvent interactions are highest at this com-position. These conclusions from viscosity data arein excellent agreement with those drawn from dens-ity data.

Further, linear plots have also been obtained fromall the electrolytes at all temperatures(298, 303, 308and 313) in conformity with Jones-Dole equation.The A and B parameters obtained from the inter-cepts and slopes respectively at different tempera-tures are listed in Table 2. The magnitude of B ispositive and large for all the three electrolytes in theentire range of temperatures and decreases with theincrease in temperature indicating thereby that ion-solvent interactions decrease with increase in tem-perature. Further, the negative temperature coeffi-cient (dB/ dT) of B in the case of 11.52% urea-wa-ter suggests '6 that all the electrolyte'S behave asstructure makers/promoters.

Viscosity data have also been analysed on the ba-sis o~ a transition state treatment as suggested byFeakins et al." The B-parameter in terms of this the-ory is given by Eq. (8),

v.o - ~ j7,0 [~ Ol - A Ill]B =' 2 + _,_ #2 /J. #11000 1000 RT

... (8)

where 11) and Vi) are the partial molar volumes ofsolvent and solute (at infinite dilution) respectively,~ fiilt is the contribution per mole of solute to thefree energy of activation for viscous flow of the solu-tion and ~ filOl the free energy of activation per moleof the pure solvent and is given by" Eq. (9).

~filol=~GII=RTln('Y/ol1)/hN) ... (9)

where h is the Planck constant, N the Avogadro'sconstant and 'Y/o the viscosity of the solvent.

Now, Eq. (8) can also be written as

PARMAR et al.: PARTIAL MOLAR VOLUMES & VISCOSITIES OF TRANSITION METAL SULPHATES

To estimate A,uPI, it is necessary to know L1,u/H.This parameter was calculated from relation (9).The values of /).,ujlf are recorded in Table 3. Thepartial molar volu~e ( J11)of urea at infinite dilutionwas found to be 42.22 ern mol- I at 303K. The va-lues obtained from Eq. (10) for all the three electro-lytes in different urea-water compositions at 303Kare also given in Table 3. It is evident from Table 3that /).,uIOf values are practically constant at all sol-vent compositions. This implies that /).,ujit may bedetermined by B-coefficient and (J11- VII) term.The positive value of /).,ujlt, which increases in caseof cobalt sulphate and first increases and then de-creases in case of manganese sulphate and ferroussulphate, suggests that flow rate is not favoured byurea content in the solvent. This effect is more pro-nounced in water rich region.

AcknowledgementOne of the authors (A.K.) is highly grateful to Hi-

machal Pradesh University, Shimla for the award ofa Post-doctoral Fellowship.

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