Effect of n-alcohols on the potentiometric titrations of rutile

Effect of n-Alcohols on the Potentiometric Titrations of Rutile JERZY SZCZYPA, LAURA W,~SOWSKA, AND MAREK KOSMULSKI Department of Radiochemistry and Colloid Chemistry, Maria Curie Sk#odowska University, 20031 Lublin, Poland Received September 2, 1987; accepted January 12, 1988 The effect of composition of mixed solvent (aqueous ethanol or l-butanol) on the potentiometric titrations o f rutile suspensions in 0.1, 0.01, and 0.001 M NaCI is studied. The value o f l i m e - o ( - d In ao/dc)j,Hj, where ao is the surface charge density, c is the alcohol concentration, and I is the ionic strength, does not depend on p H and slightly decreases when NaC1 concentration increases. The value of this coefficient equals 0.4 d m 3 mole -1 for EtOH and 1 d m 3 mole -l for BuOH. © 1988 AcademicPress,Inc.

Presence of ethanol causes a decrease of surface tension in a wide potential region and the Potentiometric titration is a method of potential of electrocapinary maximum instudies of surface charge density frequently creases. This effect is probably due to comused in order to characterize the surface prop- petition between ethanol adsorption and speerties of solids. Usually the titrations are car- cific adsorption of perchlorate ions. Lyklema (5) studied the curves Oo vs pAg ded out in aqueous solutions of 1-1 electrolytes. The studies of the system metal oxide- in the system AgI-0.1 M KNO3 in aqueous electrolyte solution led to the site-binding butanol. The slope of these curves decreases model which describes the equilibria in these when butanol concentration increases and all systems by a set of surface reactions. This the curves obtained for different butanol conmodel explains (at least qualitatively) the de- centrations have a common intersection point pendence of the surface charge density, zeta which corresponds to a negative surface potential, and adsorption density of mono- charge. Therefore the pzc shifts left on the pAg valent ions on the pH value and ionic strength axis when butanol concentration increases. The heterogeneous systems metal oxideof the solution (1). It is well known (2) that the replacement aqueous solution containing simultaneously of water by mixed solvent may change the ad- inorganic salt and organic compound(s) are sorption properties of particular ions. The so- of great practical importance. Such systems lutions in mixed solvents are widely applied occur in mineral processing, e.g., in flotation in the practical resolutions by means of ion- and in wastewater treatment. When the genexchange chromatography. Strelow (3) studied eral laws governing the surface charge forthe distribution coefficients of 45 cations be- mation in such systems are better known, optween resin AG MP-50 and HCI solutions in timization of the parameters of these processes aqueous methanol at different methanol and will be much easier. Systematic studies of the HC1 concentrations, Adsorption of alkali effect of the nature of solvent on the equilibria metal cations increased for increasing meth- in the system metal oxide-electrolyte solution anol content but adsorption of some multi- have not been carried out. Tschapek et al. (6) valent cations decreased. Morales et al. (4) have found that 1-heptanol does not adsorb studied the effect of ethanol on the electro- on 3,-alumina and aerosil from O.1 M KC1 socapillary curves in the system Hg-NaC104. lution and addition of this alcohol does not INTRODUCTION

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JournalofColloidand InterfaceScience,Vol. 126,No. 2, December1988

POTENTIOMETRY

change the pH value of aerosil suspensions. The above system is not a model one due to low solubility of 1-heptanol, porosity of the solid, and solubility of alumina. In order to study the effect of the nature of mixed solvent on the surface properties of the oxide-electrolyte systems, potentiometric titrations of the system rutile-NaC1 solution in dilute aqueous ethanol (or 1-butanol) were carried out. There are many reasons for such a choice as a model system. Titania used in the experiments is practically insoluble and nonporous and the surface properties of rutile are well known. Dilute ethanol and 1-butanol are resistant to atmospheric oxygen and the dilute solutions of NaOH and HC1 in these solvents cause the same display of pH-meter as the aqueous solutions of the same acid or base concentrations. Therefore, the values of surface charge density for different alcohol concentrations may be directly compared without taking the solvent effect into account. Moreover, adsorption of alcohols on ruffle from gaseous phase has been studied by several authors (7-9). The general conclusion from these studies is that the bound rutile-alcohol is stronger than the bound rutile-water. Water does not remove the preadsorbed alcohols from the rutile surface. Suda et al. (9) found that the amount of irreversibly adsorbed alcohol increased with the chain length. It follows from the analysis of IR spectra that the irreversible adsorption is due to esterification of surface hydroxyls. EXPERIMENTAL

Titania from Ventron (Karlsruhe, West Germany) containing 90% of rutile and 10% of anatase was washed with water until the conductivity of water in equilibrium with titania (w/w ratio 1:1) was less than 2 ~tS, then it was dried at 380 K for 6 h and stored at room temperature for a month before the titrations. Specific area determined by low-temperature nitrogen adsorption was 4 m 2 g-l. NaC104.H20 from Ferak (West Berlin) and the other reagents from POCh (Gliwice,

593

OF RUTILE

Poland), all analytically pure, were used without further purification. The titrations were carried out in a specially designed Teflon vessel in the atmosphere of nitrogen saturated with the vapor of the studied solvent, at 293 + 0.5 K. In order to remove CO2 from the system, the initial solution was saturated with nitrogen before addition of titania. The commercial nitrogen was passed through washers with fresh 20% KOH solution and then with the studied solvent before passing it through the titration vessel. The pH value of the suspensions (5 g of titania + 50 ml of the solution) was measured by an OP-208/1 pH-meter (Radelkis, Budapest). The pH-meter was calibrated using the standard phosphate (pH = 6.88) and borate (9.22) buffers. The pH value in this paper is considered as the display of the pH-meter. The next portion of the titrant was added when the rate of change of the pH-meter display was less than 0.001 pH unit per minute but not sooner than 10 min after the last portion. Adsorption measurements were carried out using sodium-22 radiotracer (OPiDI, Swierk, Poland). The radioactivity of the solution was measured with a Gama Automat NRG 603 ( Tesla, Czechoslovakia). RESULTS

AND

DISCUSSION

The ~0 vs pH curves for the rutile and 0.1, 0.01, and 0.001 M NaC1 aqueous solution systems are presented in Fig. 1. The value ofpHpzc coincides with the literature data (10). A relatively low Esin-Markov coefficient, i.e., (d p H / d log CNaa) (11), is characteristic for the studied system. At low pH values the courses of potentiometric titration curves (volume of titrant vs pH) of the supporting electrolyte and of the suspension were only slightly different. This caused a great relative error in the determination of tr0. The effect of alcohol on the potentiometric titration curve is illustrated by Fig. 2 for the system 0.1 M NaCl-ethanol. Similar dependence is observed for the other NaC1 concentrations for ethanol and 1-butanol as well. The same tendency in alcohol Journal of Colloid and Interface Science, Vol. 126, No. 2, December 1988

594

SZCZYPA, WASOWSKA, AND KOSMULSKI

e •

---

0.1M 0~IM

0

p

-- 0.1M

@--ODIM

/

A

A

-- Q O O I M

--0.1 ¸ -OA

- 0.05 -

005

5

FIG. 3. Surfacecharge density in the systemrutile-NaC1 in aqueous ethanol for different NaCI concentrations at pH = 7.

-I- 0 0 5

FIG. 1. Surface charge density in the system rutileaqueous NaC1 for different NaC1concentrations.

effect on the potentiometric titrations was observed when NaCI was replaced by NaCIO4. Therefore the observed dependence is not unique for chlorides. Figure 3 presents the

J

(3-- 00/ow/w C2HBOH ( ) - - 1 o./o . . . . A--2 */0 . , &--t, o/o J . El - - 5 / , % ' "

nm - - 1 0 "/"*

'

/~ /

/ /

,

/''"~'/A

surface charge densities at p H = 7 taken from Fig. 2 and analogous figures corresponding to 0.01 and 0.001 M NaC1. The absolute value of ~0 decreases when the concentration of alcohol increases. At low alcohol concentrations, the decrease of ~o is almost proportional to the concentration of alcohol. The same trend is observed for different p H values for ethanol and 1-butanol as well. In order to compare the effect of alcohol on the run of potentiometric titrations, the following coefficient was introduced:

-0J

l i m ( - d In

tro/dC)pH,1,

c---~0 [] -a05

L

5

.

.

.

.

. H

+0.05

FIG. 2. The effectof ethanol on the potentiometric titration curves for the system rutile-0.1 M NaC1. Journal of Colloid and Interface Science, VoL 126, No. 2, D e c e m b e r 1988

where c is the alcohol concentration and I is ionic strength. This coefficient expresses the relative decrease of surface charge density per unit alcohol concentration. Taking limc-~0 follows from the difference a m o n g particular p H definitions which m a y be applied to nonaqueous solutions. The p H values defined in different ways converge when the solvent becomes pure water. Moreover, the p H values measured in pure water or in extremely dilute aqueous alcohols used as the solvents m a y be interpreted simply as the measure of activity of hydrogen ions contrary to the display of the

595

POTENTIOMETRY OF RUTILE TABLE I Relative Decrease of Surface Charge Density in the System NaC1-Rutile per Unit Concentration of Alcohols (dm 3 mole -l) l-Butanol

Ethanol pH

7

8

9

7

8

9

0.1 M NaC1 0.01 M NaC1 0.001 M NaC1

0.31 0.4 0.46

0.35 0.38 0.41

0.38 0.41 0.4

0.93 1.14 1.14

0.87 1 1

1 1 1.05

pH-meter in more concentrated aqueous alcohols. The value of the above coefficient is not affected by error in measurements of specific surface area and number of ionizable groups per unit area. It follows from Table I that the relative decrease of surface charge density is practically independent on the pH value of the solution. The ionic strength has only low influence on the phenomenon studied, namely,-the value of the above coefficient for 0.1 M NaC1 is less than that for 0.01 and 0.001 M solutions. It means that surface complexation (in terms of the site-binding model) prevents the decrease of surface charge density under the influence of alcohol. The measurements of adsorption show that the cations of the supporting electrolyte are adsorbed at a very low rate. Even for pH = 9 and a solid:liquid w/w ratio of 1:2, the fraction of sodium adsorbed does not exceed 5%. Therefore, the experimental error in these measurements was too great to find the effect of alcohol on sodium adsorption. Low sodium adsorption and the comparison of the surface charge densities measured in this paper with the total concentration of ionizable hydroxyls on the ruffle surface show that the majority of surface hydroxyls occur in the studied systems as neutral ( not ionized) ones. This comparison and independence of limc-~0( - d In a0 /d c) pH,1on the pH value and ionic strength of the solution suggest that alcohols interact mainly with the neutral surface hydroxyls. Esterification, according to the equation - T i - O H + ROH = - T i - O R + H20, is a probable mechanism of alcohol adsorption

on rutile, not only from gaseous phase but from aqueous solutions as well. The surface charge is formed by ionization of unesterified hydroxyls. Since some fraction of ionizable groups is blocked, the density of surface charge for both positively and negatively charged surfaces decreases when alcohol concentration increases. The effect of alcohol adsorption for low surface charge densities is similar to the effect of using a sample of lower surface area. The dependence observed in the present study is due to strong interaction of rutile-alcohol so the behavior of the other metal oxides in the presence of electrolyte solutions containing alcohols may be quite different. ACKNOWLEDGMENT The present work was financially supported by the Polish Academy of Sciences, Institute of Catalysis and Surface Chemistry, under Grant CPBR 3.20. REFERENCES 1. Davis, J. A., James, R. O., and Leckie, J. 0., J. Colloid Interface Sei. 63, 480 (1978). 2. Helfferich, F., "Ion Exchange," pp. 513-515. McGraw-Hill, New York, 1962. 3. Strelow, F. W. E., Anal Chem. Acta 160, 31 (1984). 4. Morales, J., Hernandez, A., and Arevalo, A., An. Quim. 79, 374 (1983). 5. Lyklema, J., PureAppl. Chem. 48, 449 (1976). 6. Tschapek, M., Wasowski, C., and Torres Sanchez, R. M., Z. Phys. Chem. (Leipzig) 260, 336 (1979). 7. Jackson, P., and Parfitt, G. D., J. Chem. Soc., Faraday Trans. 1 68, 1443 (1972). 8. Day, R. E., Parfitt, G. D., and Peacock, J., Discuss. Faraday Soe. 52, 215 (1971). 9. Suda, Y., Morimoto, T., and Nagao, M., Langmuir 3, 99 (1987). 10. Parfitt, G. D., Prog. Surf. Membr. Sci. 11, 181 (1976). 11. Lyklema, J., J. Electroanal. Chem. 37, 53 (1972). Journalof ColloidandInterfaceScience.Vol. 126,No. 2, December1988