1965, Vol. 3, pp. 227-300.
Pergamon Press Ltd.
AREA OF CARBON
Printed in Great Britain
(Receiwed 9 May
Abstract-Adsorption of iodine by carbon blacks from aqueous, chloroform and benzene solutions was studied and conditions for estimating surface areas of carbon blacks were standardized. It was found that the adsorption of iodine by carbon black from 0.3 N aqueous solution of iodine in potassium iodide (14 moles per mole of iodine) after 75 hr of contact as well as that from 0.3 N solution of iodine in chloroform or benzene after 15 or 20 days of contact yieids uhimate value which is not exceeded on raising the concentration or prolonging the time of contact and can serve as index of surface area, irrespective of the nature of the carbon black. Adsorption was found to be mostly physical in nature.
methods which may combine dependability of the B.E.T. technique with simplicity of a procedure requiring only one experimental value of adsorption of a solute from a suitable solvent.
THE B.E.T. technique involving low-temperature gaseous (nitrogen) adsorption is generally regarded as a fairly reliable method for estimating surface areas of many non-porous solids including carbon blacks. The adsorption of iodine at the solid-water interface, under standardized conditions, with respect to concentration, time of contact and the ratio I-/Iz has also been recommended for measuring surface areas of carbon blacks(‘). However, the values obtained have been found to be in agreement with the nitrogen (B.E.T.) values only if the carbon blacks are basic in character and if the magnitude of adsorption does not exceed 90-100 mg. The reasons for the discrepancies have not been elucidated and no alterations in the conditions of estimation have been suggested. The method has also been critic&d by WATSON and [email protected]
) but these workers used very low ratios of I-/I2 in the aqueous solution and also did not allow sufficient time of contact for the attainment of the end-point in aqueous or non-aqueous solutions. The importance of surface area measurements in the study of physico-chemical properties and reinforcing functioning of carbon blacks is quite obvious. It is also necessary sometimes to follow changes in surface area accompanying certain treatments during manufacturing or other operations. There is a need, therefore, to standardize A
2. EXPERIMENTAL Fifteen samples of commercial carbon blacks, representing furnace, channel and colour blacks, furnished by M. L. STUDEBAKERwere selected. Solutions of iodine in water as well as in benzene and chloroform were used. The solvents were of high-grade purity and were stored in tightly corked bottles in a dark room and were redistilled before use each time. Carbon black (1 g) was mixed with 100 ml of the given solution in a stoppered Pyrex-glass bottle and the suspension shaken for a few minutes to effect proper wetting and then allowed to stand in an incubator maintained at 25°C for a required interval of time with occasional shaking by hand, after which an aliquot of the clear supernatant liquid was titrated against standard sodium thiosulphate. In order to assess the reversibility of adsorption, the remainder of the solution, in a few cases, was filtered over glass-wool and the residue washed first with distilled water and then with benzene. The amount of iodine in each solution was determined by titrating against sodium thiosulphate. The recovery, when corrected for the blank, varied from 96 to 100 per cent of the amount 227
RAI PURI and R. C. BANSAL
adsorbed; and therefore the adsorption of iodine by carbon blacks under the conditions specified in this paper may be taken as largely physical in nature. It may be mentioned that in the case of three samples of carbon blacks, namely Mogul, Mogul-A and ELF-O, a small amount of iodine (-3 mg/g) was found to be converted into hydriodic acid. This was determined separately and wss not included in the amount of iodine removed from the solution. 3. RFStJL’lS AND DISCUSSION
The amounts of iodine adsorbed from O-2 N aqueous solutions by eight different samples of carbon blacks after 24 hr of contact are plotted graphically in Fig. 1 against the various KI/I, ratios (I-/I, ratios) used in the preparation of these solutions. It is seen that the magnitude of adsorption decreases with increase in the ratio but tends to acquire a constant value when the ratio is 14:l or more. It appears that, since at relatively lower concentrations of I- the interaction of iodine with the solvent is less, that with the surface must be excessively high so that ‘cooperative adsorption’ referred to by KIPLING et LIZ. sets in. Consequently adsorption is in excess of “first degree saturation” of the surface. With gradual increase in the relative concentration of I-, the interaction of iodine with the solvent increases and that with the surface decreases. Ultimately a proper balance is struck between the two and adsorption acquires a constant value. The effect of concentration of iodine in aqueous solution with I-/I, ratio of 14:l with three samples of carbon blacks (one representative of each type) is shown in Fig. 2. The isotherms are of the Langmuir type indicating gradual increase in surface coverage with increase in concentration approaching saturation (i.e. completion of the monolayer) when the concentra~on is ~0.3 N. The effect of time is shown in Fig. 3. It is seen that the time required for the attainment of the end-point varies with the capacity of the carbon black to adsorb iodine. This is only 10 hr in the case of Philblack-A which has the lowest capacity and as much as 72 hr in the case of ELF-O which has the highest capacity. One hour of contact, suggested by SNOW(r) for all cases is not justifiable. This may explain the failure of SNOW’Smethod in the case of neutral and acidic carbon blacks.
It follows from the resulta presented above that in order to get ultimate and definite values of iodine adsorption at 25°C from aqueous phase the concentration of the solution should be $5~0-3N, the I-/I, ratio shouid be 314 and the time of cmtact shoufd be 372 hr. Similarly from plots of adsorption isotherms of iodine from chloroform solution on a few typical carbon blacks (Figs. 4 and 51, it follows that in order to obtain the ultimate (maximum) value, the concentration of the solutionshould be O*J N and the time of contact chap be 20 days. The rate of adsorption from chloroform solution is seen to be extremely slow (Fig. 4). The process does not become even perceptible during the first two days or so, It then becomes faster and approaches the equilibrium value after about 15 days. It appears that as carbon black is more readily wetted by chloroform, iodine encounters greater competition for the surface from chloroform than from water. The rate of adsorption from solutions in benzene is seen to be even slower (Fig. 6). This is because benzene, being a compact molecule, can wet carbon black even more readily than chloroform. However, it is interesting to note that if sufficient time is allowed for the proper attainment of equilibrium, the magnitude of adsorption for each carbon black is nearly the same irrespective of the nature of the solvent (Figs. 3-5). WATSON and PARKINSON(~)compared adsorption from aqueous and organic solvents (cyclohexane and benzene) after allowing only one hour of contact. Under these conditions, equilibrium cannot be obtained. The amounts of iodine adsorbed by 15 samples of carbon blacks from aqueous and from chloroform solutions, under the conditions specified above (in italics) for each solvent, were determined. Taking 1 mg of adsorbed iodine to cover 1 m2 of the surface(‘) specific surface of each sample was calculated. The results are given in Table 1. The nitrogen surface using B.E.T. technique and iodine surface using the procedure suggested by SNOW, both determined by STVJDEBAKERt4),are also reproduced. It is seen that in the case of carbon blacks with basic reaction (pH >7*22), the iodine surface whether obtained by measuring adsorption from chloroform solution or from aqueous solution by the procedure suggested by us in this paper or by SNOW(‘) is in fair agreement with the nitrogen surface. But in the case of
AREA OF CARBON
FIG. 1. Adsorption of iodine by carbon blacks from aqueous solutions with different I-/Is ratio.
0.2 Normality of solution
FIG. 2. Effect of concentration on adsorption of iodine by carbon blacks from aqueous solution. 200 P F
FIG. 3. Effect of time of contact on adsorption
of iodine by carbon blacks from aqueous solution.
by carbon blacks from chloroform
FIG. 5. Effect of concentration
FIG. 4. Effect of time of contact on adsorption of iodine
on adsorption of iodine by carbon blacks from chloroform solution.
FIG. 6. Effect of time of contact on adsorption of iodine by carbon blacks from benzene solution.
and R. C. BANSAL
TABLE 1. pH VALUE8ANII SURFACEAREASOF CARBONBLACKS Iodine surface (adsorption from solution phase) Carbon black
Nitrogen surface (B.E.T.*) (ma/g)
Aqueous solution Chloroform C.W.S. method*
*See reference 4.
carbon blacks with acidic reaction, iodine surface obtained by SNOW’S procedure falls considerably short of the nitrogen surface. However, the value obtained by the method suggested in this paper from aqueous as well as from chloroform solution is seen to be fairly close to the nitrogen surface in the case of these carbon blacks as well. Thus measurement of adsorption of iodine from aqueous or chloroform solution under the conditions specified above should provide a suitable method for estimating surface area of carbon black. Since only a single determination is required and a number of samples can be examined at the same time, the longer time of contact (75 hr for aqueous solution) may not be a serious handicap. 4. SUMMARY
Adsorption of iodine by carbon black from O-3 N aqueous solution of iodine in potassium iodide (14 moles per mole of iodine) after 75 hr of contact
as well as that from 0.3 N solution of iodine in chloroform or benzene after 15 or 20 days of contact yields ultimate values which are not exceeded on raising the concentration or prolonging the time of contact and can serve as index of specific surface area irrespective of the nature of the carbon black. Acknowledgements-The authors are grateful to M. L. STUDEBAKER,for the gift of carbon blacks. One of the authors (R.C.B.) is thankful to the Council of Scientific and Industrial Research for the grant of Senior Fellowship. REFERENCES quoted by M. L. STUDEBAKERin Rubber Chem. Technol. 30, 1400 (1957).
1. SNOW C.
2. WATSON J. W. and PARKINSOND., Ind. Eng. Chem.
47, 1053 (1955). 3. KIPLING J. J., SHERWOODJ. N. and SHOOTW P. V. Trans. Faraday Sot., 60,401 (1964). 4. STIJDBBAKERM. L., Rubber Chem. Technol. 30, 1400 (1957).