m y , 1052
TITRATION CURVES ON C.TjAY bIINICnA1.S ATTAPULGITIC ANI)
NONTRONITE
638
TITliATION CURVES O F THE CLAY MINERALS hTTAPULGITE AND NONTRONITE BY R. P. MITRAAND H. B. MATHUR Department of Chemistry, Universitu of Delhi, Delhi, India Received June 4 , 1861
Hydrogen attapulgite and hydrogen nontronite show three inflections in their potentiometric and conductometric titration curves, indicating three stages of neutralization with the base. Both of them give a weak first inflection. Hydrogen attapulgite shows a sharp second inflection but with hydrogen nontronite, this inflection is also not so prominent. Hydrogen nontronite, however, gives a pronounced third inflection; only a prolonged interaction with the base reveals this inflection i n the case of hydrogen attapulgite. On continued treatment with a N BaClz solution of pH 9.0, hydrogen attapulgite takes up an amount of B a + + ions which is equal t80its base combining capacity given by the third or final inflection in it,s titration curve; the Ba++ions, under these c:oiiditions,replace all the H + ions on the surface, including those dissociated from t.he available OH groups. If the pH of the HuCI12solution is only 5.0 or 7.0, only the H + ions balancing the “isoniorplious charge” are replaced. In the case of hydrogen riont.t,oiiit,e,a complete replacement of 11+ions indicated I)y t’hefinal inflection in t8het,it,ration crirve takes place even wlren (,lie solritioii of BsCI2 has as low a p’FI as 5.0.
The titration curves of hydrogen clays, Le., acid of the titration curves have been discussed in relaforms of soil colloids, have been studied by several t8ion to the lattice structures of the minerals and investigators. 1--6 More recently, pure specimens factors which are known to play a ~ a r t ~in! ionic ’ ~ reof clay minerals have also been used for such stud- actions on surfaces. ies.e-s Mukherjee, Mitra and their co-worker~~-’~ The titration technique of Mukherjee and Mitral’ observed characteristic differences between the ti- was followed. “Continuous” as well as “bottle” tration curves of montmorillonite and kaolinite, titrations as explained below were done. In a con-, and they used these differences as criteria for iden- tinuous titration, changes of the pH were measured t,ifying the two minerals in soil colloids. Mitra following additions of increasing amounts of the and Rajagopalan” have reported the titration base to a given volume of the sol kept in an atmoscurve of hydrogen mica obtained by replacing the phere of pure hydrogen or nitrogen gas. A fresh exposed K + ions of ground muscovite by H+ ions. addition of the base was not made till the e.m.f. afTheir titration curve has a different form compared ter the previous addition remained constant to with that of hydrogen montmorillonite and hydro- within 1.0 millivolt for a period of at least 15 mingen kaolinite but hydrogen illite, as is to be ex- utes. In bottle titrations, increasing amounts of pected, gives an almost similar titration curve.12 the base were added to a fixed volume of the sol Not much work, however, appears t o have been contained in each of several Jena glass bottles, the done on the important series of clay minerals repre- mixtures kept overnight, and their pH’s separately sented by nontronite and attapulgite although Cald- determined on the following day. The “bottle” well and Marshall13have published some interesting titrations, by allowing a long time of interaction beobservations on them. However, as Marshall and tween the hydrogen mineral and the titrant base, C a l d ~ e l l ’themselves ~ point out, further work on made it certain that the resulting titration curve these minerals is desirable. The present investiga- truly depicted equilibrium conditions. Both the tion was undertaken with this object in view. continuous and bottle titration techniques gave t,iAqueous suspensions of hydrogen forms of the min- tration curves showing similar features. erals16 obtained on repeatedly treating their 2.0micron fractions with 0.03 N HCI, were titrated Inflection Points and Breaks in the Titration Curves potentiometrically and conductometrically with Figure 1 shows the titration curves of a 0.83% sol KOH and Ba(OH)2. Titrations were done in the of hydrogen attapulgite, obtained using the continupresence and absence of neutral salts. The features ous titration technique. Each potentiometric curve (1) R4. 9. Anderson and H. G. Byers, U.S.D.A. Tech. Bull., 542 shows two inflections and each conductometric (1 936). curve, two breaks. The run of the curve and the (2) L. D. Baver, Soil Sci., 29, 291 (1930). pH a t the inflection depend, t o some extent, on the (3) R.Bradfield. Proc. fst Internall. Cong. Soil Sci., 4,858 (1927). base used for the titration, and it illustrates the (4) I. A. Denison, Bur. Standards J . Res., 10, 413 (1933). (5) R. P. Mitra, Indian J. Agric. Sci., 6,. 555 (1936). part played by the adsorption of the cation of the (6) C. E. Marshall, 8. Krist., 91,433 (1935). base in its interaction with H + ions on the sur(7) R. P. Mitra, S. N. Bagchi and S. P. Roy, THISJOURNAL, 47, 540 f a ~ e . ~ ,The ’ ~ pH’s at the inflections and the base (1943). exchange capacities (b.e.c.) calculated from them ( 8 ) J. N. Mukherjee, R. P. Mitra and D. K. Mitra, ibid., 47, 543 (1943). as also from the breaks of the conductometric (9) R. P. Mitra, Indian SOC.Soil Sci. Bul., 4,41 (1941-1942). curves are given in Table I. (10) J. N. Mukherjee and R. P. Mitra, J. Colloid Sci., 1, 141 (1946). Caldwell and Marshall13 did not use the conduc(11) R. P.Mitra and IC. S. Rajagopalan. N a l w e , 162, 105 (1948). tometric method of titration. They did not notice (12) R. P. Mitra and K. 8. Rajagopalan. Abstracts of Proo. 36th Indian Soi. Congress. the first inflection in the potentiometric curves. An (13) 0. G. Caldwell and C. E. Marshall, M i s s o u r i Agric. E z p . Sla. inspection of their curves shows that the first inflecBee. But., 354 (1942). (14) C. E. Marshall and 0. G. Caldwell, THISJOURNAL, 61, 311 (1947). (15) The minerals were obtained from Dr. 5. B. Hendricks through Dr. 5. P. Raichaudhuri of the Indian Agricultural Research Institute.
(16) J. N. Mukherjee, R. P. Mitra and S. Mukherjee, Trans. National Insl. of Sciences India, 1, 227 (1937). (17) J. N. Mnkherjee and R. P. Mitra. Indian J . Agric.Sci., 12,433 (1942).
cations, its features cannot be depended upon. However, above a pH of about 7.0, the reaction Conductometric curve with the base becomes extremely slow, and one is a t first at second break break left with no other alternative but to take recourse KOH 3.0(4.7)” 2 0 . 0 ( 7 . 6 ) 5.0 18.0 to the bottle titration technique. 5.5 18.0 Ba(OH)2 4.0 (4.7) 20.0(7.6) The inflection point a t pH 7.6 in the titration a The figures within brackets indicate the pH’s a t the incurves (see Fig. 1) of the hydrogen attapulgite, flection points. does not indicate all the H + ions on the surface. As tion was missed by them because of too large an stated above, a very slow reaction with base takes addition of the base at the very first installment. place above this pH. The course of the reaction in That this inflection is real and indicates H + ions on this range is revealed by the bottle titration curves the surface, will be seen from the fact that it is not given in Fig. 2. Each of these curves shows an indicated by the titration curve of the ultrafiltrate inflection at about pH 10.0 which marks the comof the sol given in Fig. 1. Caldwell and Marshall13 pletion of the slow reaction. A b.e.c. of about 55 also did not notice the inflection at about 20.0 meq. m.e. per 100 g. is obtained a t this inflection as also of the added base when the titration was done with observed by Caldwell and Mar~ha1l.l~The bottle KOH. Our titration curves with this base, po- titration curves do not show the first inflection due, tentiometric and conductometric, clearly bring out apparently, to too large an addition of the base at the very first installment. They give a somewhat this inflection. higher b.e.c. at the second inflection than the ‘(contirhous’J titration curves. TABLE I
B.e.c. (in meg. per 100.0 9.) Base used Potentiometric titration curve at 6rpt at second inflection inflection
12
1
r
2:
g
Y4..
8 E! Y
*&
Y2-
1’6
..
PO-.
-WC)
20
H, E .
’
a
’
6
60
40
OF
v
m s ~PEn 12
IS
IOOGMS.
te
of 21
H-ATTAPULGITE ad
27
Fig. 1.-Curve 1, KOH potentiometric; curve 2, Ba(OH)2 potentiometric; curve 3, KOH conductometric; curve 4, Ba(OH)2 conductometric; curve 5, ultrafiltrate.
Caldwell and Marshall,13 in effect, used the “bottle titration” technique. It is our experience-and the case in point further confirms it-that the continuous titration technique can be used with much greater effect for studying the fine features of the titration curves and, if the titration is not intended to be extended beyond a pH of about 7.0, it is certainly to be preferred to the bottle titration technique. I n a continuous titration, the electrode maintains a constant solution tension all the time. The absolute value of the pH at each stage of the titration may be slightly in error, but this error being constant throughout the titration, one gets a very smooth titration curve whose form and run are easily reproduced and are very dependable. Each point in the bottle titration curve, however, is the result of an independent determination of the pH on a separate quantity of the sol and is, therefore, subject to the uncertainties of a variable solution tension of the electrodel8 in the separate determinations. The titration curve is, therefore, difficult to reproduce and unless confirmed by repli(IS) J. N. Mukherjee, R. P. Mitra, S. Ganguli and Indian J . Agric. Sei., 6, 517 (1936).
B. Chatterjee,
41 31
10 20 30 40 50 60 70 80 90 Meq. of base per 100 g. of H-attapulgite. Fig. 2.-Curve 10, KOH potentiometric; curve 11, Ba(OH), potentiometric.
The titration curves of the hydrogen nontronite are shown in Fig. 3. Each curve, potentiometric and conductometric, shows three inflections, the first two of them, rather weak. The bottle titration curves, not shown here, also present similar features. Some of Caldwell and Marshall’s curvesthey carried out only potentiometric titrationsshow similar inflections but the curves are not very smooth and not much notice of the inflections, especially, the two earlier ones, appears to have been taken by these authors. We find that both the hydrogen attapulgite and the hydrogen noiitronite behave as a tribasic acid. However, differences between the two minerals certainly exist. Their b.e.c.’s calculated at the final (ie., third) inflection are materially different. Another point of dissimilarity which seems to be of more than passing interest is the fact that the reaction with the base beyond the second inflection is much faster in the case of the hydrogen nontronite compared with hydrogen attapulgite, which explains why even a continuous titration shows the third inflection of hydrogen
. .
May, 1052
TITRATION
63.5
CURVES ON CLAY MINERALS ATTAPULGITE AND NONTRONITE
nontronite quite clearly, though only a bottle titration brings out this inflection in the case of hydrogen at,tapulgite. Inflection Points and Breaks in the Titration Curves in Relation to the Lattice Structures of the Minerals Inflection points in the titration curves of dissolved acids indicate, as is well known, neutralization of H + ions having markedly different energies of dissociation. The three inflections in the titration curves of hydrogen nontronite and hydrogen attapulgite, therefore, appear to indicate the presence of H + ions in three distinct affinity levels on the surface. Mitra and Rajagopalanlg have discussed the structural factors which might be expected to give rise to different bonding energies of the H + ions on the surface. As emphasized by them and also by Marshall, 2o isomorphous replacements of cations within the lattice for others having smaller positive charges constitute an important source of the negative charge which could hold the H+ ions and other exchangeable cations on the surface. I n the case of attapulgite, Caldwell and Marshall13 believe that the b.e.c. of about 20 m.e. per 100 g. indicated by the second21inflection in the titration curves is due to this “isomorphous charge.” If this is really the case, a different type of isomorphous replacement might have to be assumed to explain the first inflection. Actually, replacements may occur in the tetrahedral as well as octahedral layers of the lattice. The separation of the negative charge from the surface will be greater in the latter case. The bonding energies of the H + ions on the surface will, therefore, be weaker and consequently the H+ ions will be neutralized at a lower pH. The first inflection may then indicate H + ions balancing the part of the isomorphous charge which resides in the octahedral layer, the second inflection giving a measure of the total “isomorphous charge” located in both the octahedral and tetrahedral layers. The fact that the b.e.c. a t the first inflection is small shows, in the light of the above assumption, that only a limited replacement has taken place in the octahedral layer. One can think -of yet another explanation of the first inflection, assuming that all the isomorphous charge resides in the tetrahedral layer. It is possible that this inflection indicates only such of the H+ ions balancing the total charge as occur at the edges and corners of the crysta1s.l‘ Attapulgite, however, does not have a platy habit, which makes this explanation less plausible than in the case of the other clay minerals. Caldwell and Marshalll3 believe that the portion of the titration curve beyond the inflection point found a t about 20 m.e. of t’he added base, indicates neutralization of H + ions dissociated from the available OH groups of the crystals. An acid character of OH groups in clay minerals has often been assumedZ2without bringing any conclusive experi(19) R. P. Mitra and K. S. Rajagopalan, Indian J . P h y s . , 22, 129 (1948). (20) C. E. Marshall and W. E. Bergman, THISJOURNAL, 46, 52, 325,327 (1942). (21) It is the first inflection in Caldwell and Marshall’s curve*. (22) R. K. Schofied, Soils and Ferttlisers, Imp. Bur. Sod Scz., 2, 1 (1939).
1
0 1
:
15
3 .0
45 ,
60
75
90
I05
I20
135
Meq. of base per 100 g. of H-nontronite. Fig. 3.-Curve 1, ICOH conductometric; curve 2, Ba(OH)z conductometric; curve 3, KOH potentiometric; curve 4, Ba( OH), potentiometric; curve 5, ultrafiltrate,
KOH.
mental evidence to bear on such an assumption. Very strong evidence appears to have been obtained for t.he first time from Mitra and Rajagopalan’s work on hydrogen mica.” They have shown that the theoretically calculated amount of OH groups of the mica crystals reacts with the base after the H+ ions balancing the isomorphous charge of the surface have been neutralized. I n the light of this observation, there seems to be little doubt that the portion of the titration curve of the hydrogen attapulgite between the second and third inflection indicates the neutralization of the available OH groups of the lattice. One would be inclined, on a first thought, to interpret the three inflections in the titration curve of the hydrogen nontronite on the above lines. However, unlike hydrogen attapulgite, the second inflection in the case of hydrogen nontronite is not sharp and the titration curve, during its entire course, slowly changes its curvature till the final inflection is reached. No sharp distinction appears to exist between H + ions balancing isomorphous and hydroxylic charges so far as their energies of dissociation and neutralization by the base are concerned. Titration Curves in the Presence of Salts Figure 4 shows the titration curvesz3of the hydrogen attapulgite (HA) to which enough solid BaClz and KC1 were added so as to have normal concentrations of the salts in the sol salt mixture. The titration curve of the clear supernatant liquid above the coagulum of the sol KC1 mixture is also shown in the figure. An interpretation of such curves obtained with hydrogen clays and of the reactions they depict has been given by Mukherjee, Mitra and their co-workers.g*16 It will be sufficient for our purpose to recall their main conclusion, viz., that the quantity of H+ ions on the surface which is neutralized by the base is rather ill-defined and it depends, firstly, on the prevailing pH and, secondly, on the valency, radius (including water of hydration) and concentration of the cations present in the system. The higher the
+ +
(23) ObtRined wing the continuous titration technique.
R,, P. MITRAAND 1
AZO 11
features which would be expectecl if it contained Al+++ ions in addition to free H + ions, the first inflection in the titration curve indicat,ing the free H + ions. The Al+++ ions, in all probability, were obtained by a secondary dissolution process, being the resultt of the action of the free acid created by the excliniige of the H + ions, on thc hydrogen attapulgit e.25 Titration of the hydrogen nontronite in the presence of salts and of the clear supernatant liquid of the sol salt mixture revealed features similar to those found in .the case of the hydrogen attapulgite nntl they admit of a similar interpretation.
10
9
8 c
% 6
+
5
4 3 3
+
+
Fig. 4.--Curve 6, HA N liCI-IiOH; curve 7, HA N BaC12-Ba(OH)2; curve 8, HA N 1
