Hydrous thorium oxide: A study of its amphoteric behaviour

Hydrous thorium oxide: A study of its amphoteric behaviour

J. inorg, mwl. Chem.. 1972. Vol. 34, pp. 1053-1058. HYDROUS Pergamon Press. Printed in Great Britain THORIUM OXIDE: A STUDY AMPHOTERIC BEHAVIOUR ...

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J. inorg, mwl. Chem.. 1972. Vol. 34, pp. 1053-1058.

HYDROUS

Pergamon Press.

Printed in Great Britain

THORIUM OXIDE: A STUDY AMPHOTERIC BEHAVIOUR

OF

ITS

S. W. M. S U B U K T A G I N and R. PRASAD* Chemical Laboratories, Patna University, P a t n a - 5 India

(First received 8 April 1970; in revised form 25 August 1970) Abstract--The adsorption of anions and cations on hydrous thorium oxide has been studied. The method of least squares has been used to determine the adsorption isotherms. Hydroxide prepared with the deficient alkali adsorbs more anions, whereas samples prepared with excess of alkali adsorb more cations. The results have established the amphoteric character of hydrous thorium oxide. Heats of adsorption have also been calculated. INTRODUCTION

IN AN EARLIER communication [ 1], reference was made to the study of the adsorption of some dyes on the surface of hydrous thorium oxide. When thorium is precipitated with an alkali, it requires less alkali for complete precipitation than corresponds to Th(OH)4 precipitation. Hydrous thorium oxide, therefore, can be prepared under three different conditions. One with a lesser amount of the alkali, the second with an equivalent amount of alkali and the third with an excess of alkali. The precipitates obtained under these three conditions vary in their properties. It is the aim of this work to study the surface-nature of these three precipitates by the method of adsorption. Various useful expressions relating the amounts of adsorbent and adsorbate have been proposed [2]. For the present work, the Freundlich adsorption equation [3] has been adopted since our observations fit this equation well. In an adsorbateadsorbent system if x is the amount adsorbed by m grams of the adsorbent and c the concentration of the solution in equilibrium, the expression (x/m) = K c 1~" is obeyed, where K and n are constants for the given system and temperature. The Freundlich equation can be represented in the form[4] M Y - X + P = 0 where X represents log (x]m); Y represents log c; The constants P and M are equal to log K and (1 [n) respectively. The plots of log (x[m) against log c should yield a straight line but experimental errors give a confusing picture. The best way to fit experimental data to the equation is to use this method of the least squares. Thus, for n observations in the Freundlich equations, the squares of the nth error is given by M 2 Y , "2+ 2 M P Y , + p 2 _ 2 P . X , - 2 M X , Y, + X , 2. Thus, a summation of all the squares of errors gives M2E Y,f + 2 M P E I1, + qp2 _ 2 P Z X ~ - 2 M ~ X n Y,, + ZXn ~ where q represents the number of terms. *Present address: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-32, India. 1. 2. 3. 4.

R. Prasad and Arun K. Dey, J. inorg, nucl. Chem. 24, 1018 (1962). R. Prasad, Doctoral Thesis, Allahabad, (1959). H. Freundlich, Kapillarchemic 232 (1923). W.O. Milfigan, Private communication. 11)53

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S U B U K T A G I N and R. P R A S A D

T h e sum of the squares of the errors can be minimised by setting equal to zero the partial derivatives with respect to P and M respectively. Hence, O~An 2

OP

= 0 = MEY~+qP--'ZX~

or,

And OY.An 2 - -- 0 = M ~ Y ~ 2 + P'ZY~ -- Y.X,,Yn OM

or, M=

[~,XE Y -

q~LX Y]

[(~ y)2 _ qEy2] •

In the adsorbate and adsorbent system, equilibrium is established when the isothermal transfer of small quantity o f gas from the adsorbed to the gaseous phase brings about no change i n t h e free-energy of the system [5], i.e.

On/T

\ Oa/7"

where Fg is the free-energy o f the gas and is a function o f n and T. Fa is the freeenergy o f the adsorbate and is a function o f a and T. Fg and Fa can be measured in calories per ml while n and a can be measured in moles per ml. F r o m the second law o f thermodynamics, it can be shown that:

(01np

R T 2 \ a T ]a = q a + R T

On integrating we get RTaT~ LnP~ = Q isosteric.

Ta - T2

P2

which can also be represented as RT1T2 L n cl = Q isosteric;

Ta - T2

C2

where cl and c2 are the concentrations for the same amount of ions adsorbed at temperatures T~ and T2. Thus, it will be possible to calculate the heat of adsorption. 5. R. Prasad and A. K. Dey, Kolloid. Z. 183, 153 (1962).

Hydrous thorium oxide

1055

EXPERIMENTAL Standard solutions of thorium chloride and sodium hydroxide were prepared. Three samples A, B and C of thorium hydroxide were prepared by adding calculated amounts of a standard solution of sodium hydroxide to 500 ml of each of 0.25 M thorium chloride solution. The following samples were obtained. (A) Precipitated with 10% deficiency of alkali. (B) precipitated with equivalent alkali. (C) precipitated with 10% excess alkali. The temperature was maintained at 25°C by keeping the bottles in a thermostat. The samples were thoroughly washed with carbon dioxide free water till the washings were free from thorium (where present), hydroxyl and chloride ions. The precipitates were mixed with water and the suspensions were then vigorously shaken with a microid flask shaker for two hours to ensure homogeneity of the suspension. The final volumes were adjusted by dilution with water. The adsorption of the different anions and the cations were measured by determining the initial and final concentrations of the adsorbates. To several 100 ml volumetric flasks, 5 ml of the suspension (after vigorous shaking before each pipetting) was added and to this varying amounts of the ions were added. The flasks were well shaken and the total volume was made up to the mark with carbon dioxide free water. The system was allowed to settle for 24 hours to attain equilibrium. Throughout the experiments, the temperature was maintained at 25°C. Similar experiments were repeated at 40°C and at 55°C to study the effect of temperature. A few representative results are given in Table I. The nine straight lines of table No. 1 are represented graphically in Fig. 1. At 25 °, 40 ° and 55°C, three equilibrium concentrations are observed which would correspond to the same value of x/m. From these equilibrium concentrations, heats of adsorption were calculated which are recorded in Table 2.

/O

0'5

--0"5

--I 'O

0"25

0'50 log

0'75

(x/m)(X)

Fig. l. Hydrous thorium oxide adsorption isotherms. Sample A, © at 25°C(I); 3) at 40°C(2); and • at 55°C(3). Sample B, z% at 25°C(4); • at 40°C(5); and A at 55°C(6). Sample C [] at 25°C(7); El at 40°C(8); and • at 55°C(9).

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SUBUKTAGIN

and R. P R A S A D

I

o~

o~ eq

? II II II

o

IIIIII

II II II

-r"

I I I I "K

o~

o ~D t~

.o 'r"

e.,

"t:l

< 09

o~ I

,O t9 ~9

¢$

I

I

I

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Hydrous thorium oxide Table 2. Heats of adsorption for varying Cu(II) ions by different samples of hydrous thorium oxide Sample No.

A

B

C

log c

Heats of adsorption (cal) 25°/40°C 40°/55°C 25/55°C

Log x/m

25°C

40°C

55°C

0'200 0"400 0'500 0"600 0"700 0'800

1'1025 0'2515 0"3265 0'4005 0"4250 0'4995

0"2320 0"3810 0"4550 0"5290 0'5545 0'6325

0"4085 0.4790 0"5145 0"5490 0"5350 0'5695

3667 5500 3667 3054 3638 1855 3638 623'3 3667 --607'7 3766 --1963

4540 3376 2790 2204 1632 1039

0'500 0"600 0"650 0-700 0'800 0"850

0'0150 0"2350 0"3450 0-4058 0'6300 0-7365

0'2740 0'4450 0"5525 0"6650 0"8825 0"9950

0.3985 0'5050 0-5825 0.6600 0-8150 0'8825

7334 5945 5875 7335 7150 7320

5691 4007 3524 3772 2745 2167

0"400 0"500 0"600 0"650 0'700 0'800

0'2000 0'3850 0"6150 0"7310 0"8480 1"0800

0'4075 0"5565 0"7100 0'7850 0"8600 1"0120

0"5050 5877 0"6475 4855 0'7910 2690 0"8625 1529 0"9350 339"8 1'0810 - 1 9 2 6

3880 1871 935 - 156 --2104 -3506 3039 2836 2524 2415 2338 2120

4526 3895 2612 1952 1291 14"84

RESULTS

The Freundlich equation obtained from the experimental results for Cu 2+ using the least squares method is plotted in Fig. 1. The experimental values (log x/m vs. log c) have also been recorded. They are in good agreement with the calculated lines. As is obvious, the plot of log x/m against log c should be a straight line, hence any disagreement of the experimental points is attributed to the experimental errors. The experiments were repeated with Ag ÷, C2042- and [Fe(CN)6] 4- as adsorbates (results not shown here). A summary of the comparative adsorption of these ions on the various surfaces of the hydrous thorium oxide is given below: SampleA Sample B Sample C

[Fe(CN)G] a- > CeO4~- > Ag+ > Cu e+ Cu 2+ > Ag ÷ > C2Oa2- > [Fe(CN)6] 4Cu 2+ > Ag + > C2042- > [Fe(CN)6] 4-.

It may be observed that in sample A anions are adsorbed in preference to cations while in the case of sample B and C the adsorption of the cations is more important than that of anions. These observations clearly support the hypothesis on the amphoteric character of thorium hydroxide. In order to explain the behaviour of the precipitated thorium hydroxide, the following mechanism may be postulated:Th(OH)4.

:

Th(OH)3 + . H + + OH-

31NC V o k 3 4 N o . 3 - 1

.

" Th(OH)3 + + O H " ThOz + H20 + H + " H20.

(I) (2) (3)

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S U B U K T A G I N and R. P R A S A D

Equation (1) shows the basic properties of the hydroxide and explains the association of acidic group with the hydroxide. Evidently this reaction will be prominent in an acidic medium where the removal of the hydroxyl group will be favoured. Equation (2) explains the behaviour in an alkaline medium where the conditions will be favourable for the removal of hydrogen ion. In this case, the adsorption of a basic group with the hydroxide will be favoured because of its acidic properties. Equation (3) shows the liberation of water as a result of the mutual neutralization of the acidic and basic properties of the hydroxide leading to the formation of an inert type of product represented empirically by ThO2. The hydrous thorium oxides were prepared under two different conditions, i.e. one with deficient alkali where the medium was still acidic and the other with excess of alkali where the medium was strongly alkaline. An alternate interpretation of the behaviour of these two samples is Th(OH)4[medium acidic] . ' Th(OH)~-+ O H Th(OH)4[medium alkaline] . " ThO(OH)3- + H + H ++OH-. "H20

(4) (5) (6)

In equation (4), the hydroxide appears basic and adsorption of anion should be predominant. In Equation (5) the hydroxide is acidic in character and adsorption of cations on the surface should be predominant. These postulates are in agreement with the results reported above for the adsorption of Cu z+, Ag ÷, C2042and [Fe(CN)6] 4- on the surfaces of samples A, B and C. It is known that an excess of alkali or increase in temperature or ageing leads to a decrease of the surface reactivity which also results in a decrease in the adsorptive capacity of the hydroxide and forms an inert type of compound. Treatment with excess alkali also results in dehydration of the hydroxide. Thus thorium hydroxide, if it dehydrates, should loose the water molecules as shown below HO\ /

HO

Oi.-. . .. .. .. . . .I-I(~, /D \ Th

\

,- . . . . . . . .

_/

O~_____H__OJ

/ OH Th

HO\ --~

\

OH

/

HO

/ \O Th

\

/

O

/ OH Th

\

OH

Freshly precipitated thorium hydroxide takes 9 months at room temperature to become completely crystalline and the particles are definitely bigger in size [6] which suggests that dehydration might have a role in forming bigger particles. The values of the heats of adsorption give a quantitative measure of the affinity of the hydroxide for the ions. These results also confirm the amphoteric character of the hydroxide. Acknowledgements-The authors are grateful to Professor J. N. Chatterjea, Head of the Chemistry Department, Patna University for providing the laboratory facilities and to Dr. Arun K. Dey of the University of Allahabad who had initiated this subject with the senior author (R.P.). 6. R. Prasad, M. L. Beasley and W. O. Milligan, J. Electron Microscopy 16, 101 (1967).