BASE-EXCHANGE CAPACITY OF SILICA AKD SILIC.ITES
1417
Thanks are due Dr. R. N. Ghosh, DSc., F.N.I., Fellow of the Acoustical Society of America, for his valuable help and suggestions in the progress of these experiments. REFERESCES
PELLAM, J. R., A S D Gavr, J . K . : J . Chem. Phys. 14, 608 (1946): PRAKASH, S.: J . Phys. Chem. 36, 2483 (1932). PR.+KASH, S., A S U B r s w ~ s N. , S . :J . Indian Chem. SOC.8, 549 (1931). PRAKASH, S., A S U DUR. S . R . : J . Indian Chem. s o c . 6, 587 (19'29). PRAS.AD, G . : Kolloid-Z. 33, 279 (1923); see also J. Phys. Chem. 36, 2994 (1932). SRIVASTAVA, A. M.:Proc. Natl. .4cad. Sci. India 18, 65 (1949). ( i ) SRIYASTAVA, A . M :Conipt. rend. 231. 1223 (1950). (8) SRIVASTAVA, .4.31.: Iiolloid-Z. 118.146 (19600);D. Phil. Thesis, University of Allnhnbad. (9) SRIVASTAVA, A. 11.:J. Am. Chem. SOC.73, 489 (1951). (10) SRIVASTAVA, A . hf.: Proc. Natl. Acnd. Sci. India, in course of publication. (1) (2) (3) (4) (5) (6)
BASE-EXCHAXGE CAPACITY OF SILICA AND SILICATE MISERALS A. K. GANGULY 1-nit'ersity College of Science and Technology, Calculta, India
Received J u l y 17, 1960 INTRODUCTION
Base exchange as an important property of soils, clays, and clay minerals has long been recognized. An unequivocal definition of base-exchange capacity has, however, not been forthcoming; in fact, its ill-defined nature is responsible for the iiumerous methods now available for determining it. A large number of investigators have made comparative studies of the methods of measuring baseexchange capacity and have demonstrated the lack of agreement between the various methods. Mukherjee and Ganguly (9) and Mukherjee and Gupta (11) have made detailed investigation into the causes of these discrepancies, as applied to the soils and clays, and have formulated three important factors which determine base-exchange capacity. The three factors are the pH, the nature and concentration of the reacting cation, and the time of interaction. The concepts of the crystallinity and the layer lattice structure of the clay minerals which constitute the soil colloids have gradually become the pivot to which most of the properties of soil colloids have been linked. Mukherjee and Ganguly (10) have recently developed, on the bmis of published data, a systematic approach to the problem of ion exchange in silicate minerals related to soils and clays from the standpoint of their crystalline character. In accordance with these concepts, base exchange is primarily the result of isomorphous replacements in the crystal lattice, which are possible, so far as is known, in the case
1418
A. K. GANGULY
of the 2: 1 type of silicate minerals. The exchange capacity of kaolinite, which shows no isomorphous replacement, is due to the “broken bonds” a t the edges and exposed surfaces of the crystals. In this respect quartz or silica falls in line with kaolinite. The present paper deals with the significance of the exchange capacity of quartz, kaolinite, mica, and montmorillonite in relation to the crystal chemistry of each. EXPERIMENTAL
Materials Samples of pure quartz and mica were powdered for suitable periods in a McKenna ore grinder, which consists of an electrically driven agate mortar and pestle. Definite size fractions, where necessary, were separated by the sedimentation of suspensions saturated with sodium ion. Hydrogen systems were obtained by repeated treatment with dilute hydrochloric acid and then careful washing with water. The clay fractions of kaolinite and montmorillonite were obtained by the sedimentation procedure, and mere converted into the hydrogen systems by leaching with dilute hydrochloric acid. All the suspensions were finally made in double-distilled water and were preserved in Jena-glass or Pyrex-glass containers. Methods For a comparative study it was necessary to adhere to a single method, but for the purpose of handling a large number of measurements, the method should be fairly rapid without the sacrifice of the required accuracy. Of the large number of methods investigated by the author (9), those of Parker (12) and Schollenberger (15) have been found to give very satisfactory and reproducible values. Both the methods use high concentrations of leaching solutions, extreme care for the removal of which is essential for the desired accuracy. This is sometimes time consuming. In order, therefore, to expedite measurements, without sacrificing either reproducibility or accuracy, the following procedure was adopted. To a known volume of the hydrogen systems an equal volume of a saturated solution of potassium chloride (analytical grade) was added. The mixture was allowed t o react overnight with occasional shaking and was then titrated with 0.05 N potassium hydroxide, using phenolphthalein as indicator. Usually, the pink color showing the end of titration disappears on standing for a little while. The titration is continued with small fresh additions of alkali for about 2 hr., until the pink color persists for nearly 3 min. The suspensions should be kept well stoppered during the titration. The results given in table 1 show that the values measured by the above procedure are fairly close to those obtained by the method of Parker. The last column gives the results of titration with potassium hydroxide alone, a method which has often been recommended (14)for measurements of base-exchange capacity. These last values are definitely lower. The results of base-exchange capacity presented in the following sections were obtained by titration with potassium
1419
B A S E - E X C H A N G E C A P A C I T Y O F S I L I C A AlCD S I L I C A T E S
hydroxide in the presence of half-saturated potassium chloride. This has been designated, for brevity, as the potassium chloride-potassium hydroxide method. RESULTS
Quurtz Quartz is structurally the simplest of the minerals studied. As a whole it has an electrically neutral lattice. Base-exchange property possibly develops as a result of hydrolytic cleavage: Si-0
i -Si=%-OH
+
OH-Si
H~OH TABLE 1 Base-ezchange capacities of hydrogen systems b y diflerenf methods RASE-EXCEANOE CAPACIIP
-i Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen
'
1
Parker'smethod
1
KCI-KOH method
1
millicquir./lW 8.
i
miliirquiv./100 g.
SAMPLE
montmorillonite. . . . . ' kaolinite . . . . . . . .. ' mica.. . . . . . . . . . . . illite.. . . . . . . . . . . quartz.. . . . . . .
.I
112.5 4.8 26.0 24.0 5.2
I I
115.0 4.4 26'o 26.9 5.3
I T2$&,",h miilicquiu./lW g. ~
~
98.0 3.8 18.0 17.9
TABLE 2 Particle size and ezchange capacity o j qziartz powder BASE-EXCEANGE CAPACITY SAYPLE
1
KCI-KOH method
N&CI-NH,OH method
---,
milliequit~.ll00g.
Clay-size quartz. . . . . . . . . . . . . . . . . . . . . .' Silt-size q u a r t z . .. . . . . . . . . . . . . . . . . .i Entire ground q u a r t z . . . . . . . . . . . . . . .
5.3 0.6 3.0
milliequit./100 g. ~
0.5
The dissociation of the OH groups so formed gives rise to the exchange spots. The hydrogen ion will have a very weak acid function, less than in silicic acid, to which quartz may ultimately be converted according to the above picture. Quartz will no doubt show an increase in exchange capacity with decrease in particle size, but according to Kelley and Jenny (4)the increase will be much less compared to kaolinite. The figures given in table 2 show the effect of particle size on the exchange capacity of quartz as measured by the potassium chloridepotassium hydroxide method. The clay- and silt-sized fractions were obtained by the sedimentation procedure; the entire ground quartz was predominantly a mixture of these fractions. I t will be observed from the single result given in
1420
A. IC. GANGULY
column 3 that the quartz suspension takes up only a small amount of ammonium hydroxide in the presence of half-saturated ammonium chloride. The lack of adsorption of ammonium ion is in agreement with the observations of Kelley and Jenny (4) and van der Meulen (17), who found little or no adsorption of ammonium ion by powdered quartz from ammonium salts. This is probably due to the very weakly acid function of the OH groups on the surface of quartz. The observed exchange capacity of clay-sized quartz ( < 2 p) may be compared with the values calculated from the numer of OH spots developed on the surface of quartz particles according to the mechanism of hydrolytic cleavage. Thus, particles of diameter 2 p and 0.2 p should possess exchange capacities of 0.89 and 8.9 milliequiv./100 g., respectively. The observed value lies within the two extremes and agrees closely with the mean, assuming a 50:50 mixture of the two sizes and a complete reactivity of the spots. TABLE 3 Base-ezchange capacities of treated silica gel SAMPLE
1
BASE-EXCHANGE CAPACLTY BY
KCI-KOH
Y~T~OD
m d l i q u i v . / l ~g.
1. 24-hr. dry-ground* unignited silica gel.. . . . . . . . . . . . . . . . . . . . . 2. 900'C. ignited gel, followed by dry' grinding for 14 hr.. . . . . . . 3. Sample No. 1 ignited for 3 hr. a t 900°C.. . . . . . . . . . . . . . . . . . . . . 4. Sample No. 3 ignited for 6 hr. a t 900°C.. . . . . . . . . . . . . . . . . . . . . 5. Coarse silica gel approximately 1 cu. mm. s i z e . . . . . . . . . . . . . .
80.6 45.7 1.8 Trace 4.3
1421
BASE-EXCHANGE CAPACITY OF SILICA AND SILICATES
It is interesting to note that while a coa.rse amorphous silica gel (sample No. 5) shows appreciable exchange capacity, finely ground but ignited samples (Nos. 3 and 4) do not. Heating for considerable periods at high temperatures has been found (1) to induce crystallinity in amorphous silica gel. Prolonged heating, therefore, brings about a complete fusion of the OH groups, leading to loss of exchange capacity. Further results given in table 4 confirm these conclusions. The samples heated for 8 hr. or more gave evidence of the x-ray diffraction pattern of quartz (1). The base-exchange capacity was completely lost. The low exchange capacity of quartz is also understood in the light of these results. A point of difference must, however, be mentioned. Whereas the ignited sample of silica gel (sample S o . 2, tcble 3) on further grinding exposes fresh surface and s h o w considerable exchange capacity, this is not so with quartz samples, however finely divided the particles may be (compare, for instance, data in table 2). Possibly the crystallinity of the ignited gel is of a low order; hence grinding and rehydration cause appreciable hydrolytic cleavage but are unable to build up the original gel surface. TABLE 4 Variation i n the erchange capacity of powdered silica gel with time of heating
__..
~
B A S E - E X C R A S C E CAPACITY
___
BY
S.AKPLE
' 24-hr. dry-ground 24-hr. dry-ground 24-hr. dry-ground 24-hr. dry-ground
gel . . . . . . . . . . . . . . . . . . . . gel hezted a t 700°C. for 2 hr . . . . . . . . . . . gel heated at 700°C. for 6 hr.. . . . . . . . . . . . . . . gel heated at i00"C. for 8 hr. or more.. . . . . . . i ~
~
KCI-KOH
MEIHOD
__
mill;equii,,/fGG g.
86.8
24.8 0.7 0.0
Kaolinite Kaolinite has two types of OH groups: one on the exposed surface of the hydrargillite layer, and the other in the subsurface surrounded by 0-- ions. hlitra and coworkers (7, 8) from potentiometric and conductometric titrations of hydrogen kaolinite concluded that it behaves as a dibasic acid. The dibasicitp, according to them, arises from the differences in the dissociation energies of the two types of OH groups mentioned above. I t is, however, questionable if the subsurface OH groups are, in view of the compact structure of kaolinite, capable of reacting with the outside solution. Hendricks (2) believes that the exchange spots on the kaolinite surface are distributed laterally to the cleavage plane and are, therefore, developed a t the termination of valence bonds from Si-0 and AI-0 on the surface. Grinding of kaolinite obviously proceeds along both the cleavage and the lateral planes. Electron-micrograph studies by Shaw ( l G ) revealed that dry grinding of kaolinite hm a tendency to break the kaolinite particles laterally. Along the lateral planes two types of exchange spots are developed: one by cleavage along the shared edges of the aluminum octahedra, and the other by cleavage along the shared corners of the silicon tetrahedra, as in quartz. The OH groups linked to aluminum on the cleavage surface, as well as on the lateral planes (lo), and those linked to
1422
A.
K. GANGULY
silicon, should show a difference in their reactivity towards alkalis. The OH groups linked to aluminum will, owing to their weaker binding, be reactive a t a higher pH compared to those linked to silicon. The dibasic acid character of kaolinite observed by Mitra and coworkers (8) should, according to the author, be attributed to these two types of OH groups. Sufficient attention does not appear to have been paid to the complications arising from the effect of the breaking up of kaolinite or other mineral lattices along the lateral planes. The following experiments have been carried out to show some of these complications. The clay fraction of hydrogen kaolinite was ground for 70 hr. in the wet, as well as dry, condition. The potassium chloride-potassium hydroxide method was used for the measurement of the base-exchange capacity (column 3, table 5). The kaolinite suspension, after the titration, was leached with 250 cc. of 0.05 N hydrochloric acid solution, alumina was estimated in the leachate, and the baseexchange capacity (column 5) of the residue was determined by leaching with barium acetate solution as in Parker’s method, and replacing the adsorbed barium TABLE 5 Ezchange capacity and acid-soluble alumina of dry-ground and wet-ground kaolinite BASE-EXCHAIGE CAPACITY SAMPLE
~ Y method arker
Hydrogen kaolinite dry ground for’ i o hr. . . . . . . . . . . . . . . . . . . . . . , ’ Hydrogen kaolinite wet ground for: iOhr.. . . . . . . . . . . . . . . . .
,I
i
I 13.9
i
.41rOs
BASE-EXCEANOE CAPACITY OF ACID-LEACHED SAMPLE
ISHCI
KCI-KOH method
LE4CHAIE
per Len1
41.1
15.8
25.5
5.4
Parker method millicqwio.
/IO0 E .
I KCI-KOH ~
method
millirquio. I100 g.
by means of hydrochloric acid instead of ammonium chloride. The hydrogen system finally formed was then washed free from acid, and its base-exchange capacity was again determined by the potassium chloride-potassium hydroxide method (column 6). The results of these measurements are shown in table 5. The base-exchange capacity of the 70-hr. wet-ground sample as determined by Parker’s method is nearly three times the original (vide table 1, page 1415), and is nearly five times the original when measured by the potassium chloridepotassium hydroxide method. Such a discrepancy between these two methods has not been previously observed; hence it was interesting to look into the matter in greater detail. Acid leaching of kaolinite revealed the presence of considerable amounts of aluminum coming into solution. This is evidently due to the aluminum being exposed by lateral breaking. In this respect dry grinding is much more effective than wet grinding (Shaw (16)). The higher value obtained by the potassium chloride-potassium hydroxide method therefore appears to be due to the fact that the acid function of the exposed aluminum is more marked a t the higher pH which obtains in this method as compared with Parker’s. This is fully con-
1423
BASE-EXCHAXGE CAPACITY OF SILICA AND SILICATES
firmed by the results given in columns 5 and 6. The kaolinites, after removal of the released aluminum, now possess much smaller base-exchange capacities, but both the methods measure almost the same values. The final values, which are still greater than those of the original samples, perhaps represent the actual resultant effect of grinding on the base-exchange capacity of kaolinite. The amounts of aluminum dissolved by the salt and acid treatments are found t o be much more than can be accounted for by the difference in the base-exchange capacity measured by the potassium chloride-potassium hydroxide method before and after removal of aluminum. The original hydrogen kaolinite, which contained no free alumina, mas prepared by leaching with dilute hydrochloric acid. Assuming that the kaolinite structure is undisturbed, it may be shown that the replacement of aluminum through the lateral surface by hydrogen ions causes a separation of 2.37 A. between the oxygen-ion surface of the tetrahedral layer and the hydroxyl-ion surface of the sheet from which aluminum is displaced. Even the nonhydrated cations are larger in size than this distance of separation. It is, therefore, very likely that the exchange spots created as a result of the breaking process are not accessible to the reacting solution used for the measureTABLE 6 Efect of digestion with sodium carbonate on exchange capacity of kaolinite SAMPLE OF KAOLINITE
i oEIGINhL BASE- ! BASE-EXCEANGE CAPACITY AFTER TICEATMEAT*.I= NaCa i
i ~ 0 . 1 . .
!
..........
No. 2 . . . . . . . . . .
EXCHANGE
miiiirquiu. /IO0 p .
5.0 4.8
I
miliicquiu. /io0 g.
,
5.3 4.4
~
i
~
millicquiu. /IO0 8.
4.6
1 ~
i
Ah03
rLnl
1 I
1.0
1
CCtlf
0.6 1.8
ment of base-exchange capacity. I t should, however, be noted that on a statisti$al basis, always only a certain fraction of exchange spots of the total number can be sufficiently reactive to contribute to the base-exchange capacity. The removal of reactive aluminum by acids or any other treatment appears to be necessary for a proper assessment of the increase in the base-exchange capacity of kaolinite as a result of grinding. The following experiments illustrate the apparent inactivity of subsurface OH groups as well as the great stability of the kaolinite particles to drastic treatments. Four samples of kaolinite, two from each of two different sources, were taken in well-stoppered Jena-glass bottles. One sample of each of the two kaolinites was (a)digested with an excess of 2 per cent sodium carbonate solution in a steam oven for one month, and ( b ) the other two samples were leached with a boiling solution of 2 per cent sodium carbonate for several days. The leachates were analyzed for soluble alumina, and the residues were treated first with water to remove the free sodium carbonate and then with dilute acetic acid and finally with water to remove the acid. The base-exchange capacities of the four samples were measured by Parker's method. The results indicate that, al-
1424
A. K. GANGULY
though small quantities of aluminum are dissolved, the baseexchange capacity remains almost unaltered (see table 6).
Mica For the purpose of a better understanding of the relative functions of the exchange spots located on the cleavage and lateral surfaces, mica affords a very suitable system. Micas belong to the 2: 1 lattice type of minerals (13), and can break on grinding along both the cleavage and the lateral planes. The hydrogen ions of hydrogen mica formed by the replacement of potassium ions on the cleavage surface are in a different position in the lattice from those in hydrogen kaolinite or hydrogen quartz. In the case of micas, the sheets are not electrically neutral and the neutrality is maintained by the incorporation of potassium, calcium, etc. ions, which take positions between the sheets and are located within the hexagonal cavity formed by oxygen atoms. TABLE 7 Effect of acid leaching on ezchange capacity of dry-ground and wet-ground mica
1
BASE-EXCHANGE CAPACITY pea 0. OF UNTREATED SAMPLE
I
KCI-KOH
~
1
1w
MICA
I ~
I
Parker
IN TEE ACID LEACHATE BAINY 'A ::tc LEACRArE
___ K millirquir.
8ASE.EXCBANGE CAPACITY OF ACID. LEACHED RKSIDUES
AI+&
per
CL"l
i 7.6'
4.0
4.8
1
4.8
3.4
7.8
1
7.7
15.01
17-hr. wet-ground mica. . , . , . . , . . . .
18.0t
*Volume of leachate = 250 cc. t Volume of leachate = 500 cc.
Finely divided mica was prepared by the grinding of small bits of it in the McKenna ore grinder. The base-exchange capacity of the powdered sample without any pretreatment was determined by the potassium chloride-potassium hydroxide method and the method of Parker. The potassium content of the barium acetate leachate obtained by the latter method was also determined. The removal of adsorbed barium was effected by leaching with dilute hydrochloric acid. The acid leachate was analyzed for potassium and aluminum as well as barium. The acid-treated residue was again used for the measurement of baseexchange capacity by the potassium chloride-potassium hydroxide and the Parker methods. The results of these measurements carried out with a dryground and a wet-ground sample of mica are given in table 7. The discrepancy between the base-exchange capacities of the untreated dryground sample as determined by the potassium chloride-potassium hydroxide method (column 2) and Parker's method (column 3) is due to the fact that under the leaching conditions obtaining in Parker's method the exchange spots become reactive, partly as a result of the removal of potassium ions from the cleavage
BASE-EXCHANGE CAPACITY OF SILICA AND SILICATES
1425
surface and partly owing to the removal of exposed aluminum from the lateral surface, whereas this is not possible in the case of the potassium chloride-potassium hydroxide method. It is, however, clear that the amount of potassium ion released is considerably less than the observed exchange capacity. Acid treatment of the residues after leaching with barium acetate solution brings out a much larger quantity of potassium ion, which is increased when the volume of the leaching solution is doubled (column 5). The acid leachate contains an appreciable quantity of alumina also. However, the base-exchange capacities of the arid-leached residue measured according to the potassium chloride-potassium hydroxide method and Parker’s method are not correlated with either the potassium or the aluminum content of the leachate. But the base-exchange capacities measured by the two methods are now in good agreement. The base-exchange capacity, as determined by the potassium chloride-potassium hydroxide method, of the untreated sample (column 2), which was already slightly alkaline in aqueous suspension, demonstrates that addition of potassium chloride even in the alkaline region can bring into reaction some hydrogen ions which under normal conditions do not react. Dry grinding was shown by Shaw (16) to, break the kaolinite particles more along the lateral than the cleavage planes, whereas the reverse takes place on wet grinding. If this is so in the case of mica also, the dry-ground samples on acid leaching would show a higher content of aluminum and less of potassium, whereas their contents would be the reverse if the wet-ground sample were similarly treated. The figures given in column 5 of table 7, in fact, indicate that in the process of wet grinding cleavage is easier than lateral breaking; moreover, the wet-ground sample shows an appreciably higher exchange capacity than the dry-ground sample.
Montmorillonite The base-exchange capacity of montmorillonite is mainly the result of the partial substitution, i.e., isomorphous replacement, of aluminum by magnesium in the octahedral layer of pyrophyllite. The exchangeable cations, which lie chiefly between the silicate layers, neutralize the negative charge of the lattice developed in this way. Ross and Hendricks (14) obtained a fair agreement between the base-exchange capacity of montmorillonites measured by titration with sodium hydroxide alone, using phenolphthalein as indicator, and that calculated on the basis of isomorphous replacement. I t has been mentioned earlier that potassium hydroxide alone titrates a much smaller quantity of acid than it does in the presence of a high concentration of potassium chloride. In the case of hydrogen montmorillonite, where most of the exchange spots will be located between two sheets, the hydrogen ions mill probably have a stronger acid function and will therefore be entirely neutralizable by alkali alone. However, the increase in base-exchange capacity of montmorillonite with decreasing particle size observed by several investigators (3, 4,5 , 7) suggests the existence of hydrogen ions other than those arising out of isomorphous replacement.
1426
A.
K. GANCIULY
The exchange capacity of montmorillonite due t o isomorphous replacement may be calculated from the chemical analyses (6). In the octahedral layer of montmorillonites the main replacement is of aluminum by magnesium. If one magnesium ion replaces isomorphously one aluminum ion, one unit of negative charge is developed in the lattice. This charge is balanced by cat,ionswhich become exchangeable. A consideration of the analysis of a number of hydrogen montmorillonites (listed in table 8) shows that the total number of bivalent atoms slightly exTABLE 8 Formulas of some montmorillonite samples SAXPLE NO.
T E T U H E D P A L LAYER
OCTAXEDIAL LAYEP
3. ............................ 6.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.. . . . . . . . . . . . . . . . . . . . . . . . . . . 11.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.. . . . . . . . . . . . . . . . . . . . . . . . . . . 19. . . . . . . . . . . . . . . . . . . . . . . . . . . .
24.. . . . . . . . . . . . . . . . . . . . . . . . . . . 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32... . . . . . . . . . . . . . . . . . . . . . . 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Author’s . . . . . . . .
* From Ross and Hendricks
(14).
ceeds the deficit of trivalent cations in the octahedral layer from the ideal value of 2.0. If this excess of bivalent cations is considered to be exchangeable, the total unsaturation deireloped in the lattices is given in column 3 of table 9. Then the calculated value of the eschange capacity mill greatly esceed the experimental value in every case. Since hydrogen montmorillonites were used for the determination of exchange capacity by means of alkali titration, it may be assumed that all the exchangeable cations are constituted of hydrogen ions. The excess of magnesium ion or any other bivalent cation should therefore
1427
BASE-EXCHANGE CAPACITY OF SILICA AXD SILICATES
be considered to be present in the lattice in a nonexchangeable form. The ions may be assumed to accommodate themselves in the unoccupied places of the hydrargillite layer, giving rise to the formation of a partial brucite lattice. In this way part of the negative charge is neutralized by these bivalent cations rendering themselves nonexchangeable. The calculated lattice unsaturation after neutralization by bivalent cations and the corresponding base-exchange capacity are shown in columns 4 and 5, respectively, of table 9. It will be seen that, except in the case of samples KO.3, 13, and 29, the calculated values are higher than those estimated by Ross and Hendricks from titration with sodium hydroxide. TABLE 9 Calculaied and ezperimental values of the base-exchange capacitu of montmorillonite samples given i n table 8 I SAMPLE NO.
1
BASE-
I
MOLECULAR WEIGET
EXCAANGE CAPACITY CALCULATED PROM PRECEDING coLum
IUNSATURATION
TOTAL
~
UNS*TUPATION,
,
AFTER NEU-
;?;;{;Uph; CATIONS ~
~~
3. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.. . . . . . . . . . . . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12... . . . . . . . . . . . . . . . . . . . . . . . 13. . . . . . . . . . . . . . . . . . . . . . . . .
,
365 360 365 368 365 365 365 366 360 360 370 363 360
I
i
1
480 430 430 800 480 450 480 620 840 620
1 5 0 0
320 330 330 320 320 330 340 340 330 320 340 340
360
1
87 7 90 6 904 87 0 87 7 90 4 93 1 92 9
1
~
~
1
91 9 93 6 100 0
BASEEXCHANGE CAPACITY' F O U N D BY I TITRATION WITA I L L 1 ALONE
1 'i
100 81 70 85 66 97 84 85 70 87 80 98
* For samples 3 to 36, the values are from Ross and Hendricks (14). Sodium hydroxide thus fails to neutralize completely even those hydrogen ions which owe their origin to isomorphous replacement. The agreement is somewhat closer with samples 1 1 and 32. However, with the sample used by the author (shown a t the bottom of tables 8 and 9) the base-exchange capacity calculated from the isomorphous replacement comes very close to that obtained by titration with sodium hydroxide or potassium hydroxide alone. This value (98 milliequiv.) may be compared with the much higher base-exchange capacity (115.0 milliequiv.) measured by the potassium chloridepotassium hydroxide method. The difference clearly indicates that sodium hydroxide or potassium hydroxide alone does not always titrate the hydrogen ions arising out of isomorphous replacement; and even if it does so, it does not measure the total exchange capacity. In montmorillonites, like the micas, there are therefore exchange spots other than those caused by isomorphous replacement. As shown earlier, they are
1428
A. K, QAhWULY
possibly due to the weakly acidic hydrogen ions of OH groups linked to aluminum or silicon atoms on the broken surfaces, or those entrapped within lattice sheets but near enough t o the lateral edges t o be brought into a neutralizable condition in the presence of a high concentration of a neutral salt. Treatment of montmorillonite with potassium chloride-potassium hydroxide, it must be noted, does not bring any aluminum into solution, like the acid-leached kaolinite or mica samples mentioned earlier. SUMbMRY AND CONCLUSIONS
The above results of the determination of the base-exchange capacity of quartz, kaolinite, mica, and montmorillonite illustrate some of the complicating factors which present themselves in such measurements. I t appears that sufficient attention has not been given to eliminate these factors in the interpretation of the observed increase in the baseexchange capacity of certain clay minerals on grinding. Moreover, it can be inferred from the results that the exchange spots on the lattice surface of clay minerals are not of equal energy values, there being definite levels of energy of interaction. The bonding energies are determined more or less by the nature, as well as the location, of atoms concerned inside the lattice. Hydrogen ions of hydroxy groups bonded to silicon, aluminum, and magnesium will show decreasing acid function in the order mentioned. Again, within the frame&-ork of the lattice the relative position of these atoms will decide the accessibility of the hydrogen ions associated with them. The thanks of the author are due Dr. S. K. Mukherjee for help and interest in this investigation. REFERENCES (1) BHATTACHARYA, S. B . : Science and Culture 13, 469 (1918). (2) HESDRICKS, S. B . : Ind. Eng. Chem. 37, 625 (1945). (3) JACKSON, M. L., A N D TRCOG, E.: Proc. Soil Sci. SOC. Am. 1939, 136. (4) KELLEY, W . P . , ASD J E N N YH.: , Soil Sci. 41, 367 (1936). (5) >f.4RSHhLIr, E.: Icrist. 9OA, 433 (1935). (6) ~ I A R S H A L LC, . E.: Colloid Chernistryof Silicate Minerals. Academic Press, New York (1949). (7) MITRA,R. P . : Indian SOC. Soil Sci. 4, 41 (1942). (8) MITRA, R . P . , A N D RAJAGOPALAX, S.: Proc. Indian Sci. Congress, 35th Congr. l-, 15. (9) MCKHERJEE, S . I
