Indian Journal of ChemistryVol. 26A. April 1987. pp.328-330

Thermodynamics of Adsorption ofAcetic Acid on Charcoal from

Dextrose Solution

ANIMESH K RAKSHIP & C E PRADEEPtDepartment of Chemistry. Faculty of Science. M.S. University of

Baroda. Baroda 390002

Receioed 26 May 1986; revised and accepted 19 August 1986

Adsorption of acetic acid on charcoal from dextrose solution hasbeen studied at dilTerent temperatures. The system obeys thewellknown Freundlich isotherm at all temperatures. The thicknessof the adsorbed layer on charcoal surface has been estimated and theadsorption equilibrium constants have been computed at differenttemperatures in various systems. The free energy and enthalpy ofadsorption have been also computed and discussed in the light of thesolvent structure.

Freundlich adsorption isotherm, which is anempirical equation and probably a special case ofLangmuir isotherm I -6. has been used quite often for .studying the adsorption phenomenon of fatty acidsand fatty alcohols on charcoal and other adsor-bents 7 - II. A literature survey shows that very fewquantitative results are reported on the adsorption ofacetic acid on activated charcoal from an aqueoussolution in presence of a second solute. This. though, isoffundamental importance in understanding the effectof solvent structure on the adsorption process. Hence,it was decided to study the adsorption of acetic acid onactivated charcoal in presence of various amounts ofdextrose at different temperatures.

Acetic acid (GR, SM) and dextrose (IP/BP, SM)were used without further purification. Activatedcharcoal (Merck, India) was heated in an oven at 100"Cfor six hours and then cooled in a desiccator before use.Acetic acid was estimated by titrating against standardNaOH solution. Doubly distilled water was used in thepresent study.

In p series of bottles. weighed amounts of charcoalwere taken. Acetic acid solution was made in aquo-dextrose solvent and the required volumes werepipetted into the bottles. Then various amounts ofsolvent (dextrose) were added to vary the initialconcentration <;>1' acetic acid. The concentration rangewas between 0.008 and 0.08 mol/litre. The bottles werecapped well and shaken in a wrist action shaker for - 3hr. before placing them in a thermostat (accuracy

tTaken from the disseration of CEP in partial fulfilment ofrequirement of M.Sc. degree of M.S. University of Baroda, Baroda.

328

± 0.1 C) at the required temperature for '" 2 hr withintermittent shaking. The solutions were thenimmediately centrifuged and titrated. The bottles werethen shaken again for 1.5 hr. placed in a thermostat foranother 30 min. the solutions were then centrifugedand titrated. The reproducibility of results indicatedthe establishment of equilibrium; in case of non-reproducibility, the procedure was repeated untilequilibrium was established (from a preliminaryinvestigation it was found that the time taken for theattainment of equilibrium was about 3 hr). The error intitration was less than 0.03 ~'()'As dextrose solution wassusceptible to microbial attack, a few experiments weredone under nitrogen atmosphere. 1M view of thereproducibility of the results, no further precautionwas taken.

The empirical Freundlich adsorption isotherm is:x/m = kx:'!", where k and n are constants, c is theequilibrium concentration and x/rn is the specificadsorption. A plot of log (x/m) against log cat 35"C isshown in Fig. 1 for some systems. This plot was drawnfor all the systems and values of k and n so determinedare given in Table I for all the temperatures, k is a roughmeasure of adsorbent capacity and n -I is that ofintensity of adsorption, where n is a number greaterthan unity1.3A. As can be seen from Table I, themagnitude of n I increases as a function ofconcentration of dextrose at a given temperature,tending to attain a constant value of I at - 5'/0

-1·14

••

+22

-2·38 -2,48-2,30 -2 ·32

~Ex01a

S·l.glucosf. 3·l.glucosf.

-1-42 -1·26

•

,/

+54 -1-38

-2·40-2'42 -2·32log C

-2·34

log C

Fig. 1- Variation of log (x/m] with log C for some representativesystems at 35"C

Table I-The k and n Values of Freundlich Isotherm atDifferent Temperatures in Various Solvents

Solvent kat n at

25°e 35°e 4O'e zs-c ss-c 40'e

Water 1.11 0.48 0.33 1.97 2.70 2.861% Dextrose 4.00 1.11 0.71 1.25 I.k5 2.132% Dextrose 6.40 1.46 1.21 1.10 1.56 1.823% Dextrose 8.00 3.01 1.43 1.00 1.28 1.674% Dextrose 9.40 5.16 2.42 1.00 1.14 1.435% Dextrose 10.70 7.58 5.17 1.00 1.00 1.08

dextrose. It is suggested that in aquo-dextrosesolution, we have a structured solventl2.

13 andincrease in the concentration of dextrose or decrease intemperature makes the solvent relatively morestructured. Acetic acid, water and dextrose, all threecomponents, take part in structure formation. It is seenfrom Table I that the decrease in temperature orincrease in dextrose concentration increases both kand I/n. Increase in n -I decreases c ':", and k and c' n

work in opposition to each other in determining theamount of specific adsorption. Where cln is moredominating-particularly in low concentrationregion-the amount of adsorption decreases. That iswhy at low temperatures or in presence of moredextrose (i.e. relatively a more structured solvent)adsorption is less. Qualitatively speaking, morestructured the solvent, lesser amount of acid isavailable for getting adsorbed and hence lesser is theamount of adsorption.

It is possible to look at the adsorption process in thefollowing way:

Acid (in solution phase) ¢ Acid (in adsorbed phase)

As the concentration of acid in solution has beendetermined experimentally, if concentration of acidadsorbed can be computed, then the adsorptionequilibrium constant (KadJ can be calculated for theabove process!". The free energy of adsorption canthen be determined by the relation: i\G~ds= - RT InKads'To compute Kads' the surface area of adsorbent(activated charcoal) was determined by the use ofwellknown BET method (surface area = 816.94 m'),Monolayer adsorption was assumed add 20.5 A 2 wastaken as the cross-sectional area of - COOH group I 5 .

The thickness of adsorbed layer of acetic acid wascalculated to be 4.7 A assuming that the molar volumeof acetic acid in pure liquid state and on adsorbedmonolayer was same'". Moreover, temperaturevariation affects molar volume and hence the thicknessof monolayer on charcoal surface, but the effect hasbeen neglected in these calculations; the thickness ofadsorbed acetic acid monolayer was taken to be thesame at all temperatures (4.7 A). Hence, the

NOTES

Table2-The CalculatedValuesof Adsorption EquilibriumConstant K'd5 in Various Media at DifferentTemperatures

and Associated ThermodynamicQuantities

Media K.d• at AG.:d. Alf'ad.(kcal/mol) (kcal/mol)

25°e 35°e 400e35°C 35°C

80.0 78.8 78.4 -2.66 -0.5456.3 54.1 55.7 -2.43 -0.7046.8 44.8 44.2 -2.32 -0.7240.5 38.3 37.6 -2.20 -0.7237.8 36.2 35.5 -2.18 -0.7336.6 35.0 34.S -2.17 -0.77

Water1% Dextrose2% Dextrose3% Dextrose4% Dextrose5% Dextrose

concentration of adsorbed acetic acid was computedby determining the amount of acetic acid adsorbed fromthe knowledge of its initial concentration andequilibrium concentration. Surface area of activatedcharcoal multiplied by monolayer thickness gave thetotal volume in which the acid was adsorbed. Hence,the concentration of adsorbed acetic acid in mol perlitre could be computed and used to calculate Kadsat alltemperatures.

The free energy of adsorption, calculated from Kads'was found to be a function of concentration at aparticular temperature. A plot of i\Gads against c islinear above ~O.03 mol/litre, but below thisconcentration, i\Gads shows a sharp decrease. Theestimation of i\G at c =0 became difficult as the curvecould not be extrapolated with confidence. Hence, weextrapolated the linear part of the curve to c=O andtermed it as i\Gads'We recognise the weakness of thisprocedure and hence the approximate nature ofabsolute magnitudes of Kadsgiven in Table 2, but wehope that at least qualitatively, it gives a relative ideaabout the different systems.

The i\Gads SO obtained varied linearly withtemperature, and hence, (oi\G.%T) was computed.The well known relation,

i\H~ds= i\G~ds- T(oi\G~ds/oT)

was used to estimate the heat of adsorption, i\F.ds' Thevalues of free energy and heat of adsorption are givenin Table 2. It is quite clear from these values that theexothermicity (the amount of heat change) in theseprocesses is very low, and more structured the solventis, the higher is the heat of adsorption, as expected. Therelative spontaneity of adsorption decreases as thesolvent becomes more structured on addition of moreand more amounts of dextrose and also becauserelatively lesser amount of acetic acid becomesavailable for adsorption. Hence, we conclude that asthe amount of dextrose increases, the process of

329

INDIAN J CHEM., VOL. 26A, APRIL 1987

adsorption becomes relatively less spontaneous,though relatively more exothermic.

Thanks are due to the authorities of IPCL, Barodafor surface area measurements and to Prof. P.K.Bhattacharya for laboratory facilities.

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