STUDIES I N ADSORPTION, PART VI
A New Interpretation of the Schulze-Hardy Law and the Importance of Adsorption in the Charge Reversal of Colloids. BY N. R. DHAR, K. C. SEN, AND S. GHOSH
Adsorption and Schulze-Hardy Law In some recent papers we had occasion to study the coagulating power of different electrolytes on manganese dioxide sol and also the adsorption of electrolytes by hydrated manganese dioxide both in course of precipitation and as well as by freshly precipitated and air dried solid free from all impurities.l It has been shown in these communications that in the coagulation of manganese dioxide sol, the Schulze-Hardy Law which establishes a relation between the coagulating power and the valency of the coagulating ion is but partially followed. It has also been definitely proved experimentally that in the adsorption of various electrolytes by precipitated manganese dioxide the ions most effective in coagulation are least adsorbed. Thus the coagulating power of silver ion is about I / S of that of mercuric ion, whereas its adsorption by precipitated manganese dioxide is about six times that of mercuric ion. Similar results have also been found with the air dried sample. From a survey of the literature on the subject we find that up till now the Schulze-Hardy Law has not been rightly interpreted in explaining the experimental results on coagulation and adsorption. In this paper we have given z1 new interpretation of the above Law and we are of opinion that our interpretation can consistently explain the experimental results on coagulation and adsorption. In this connection we have also discussed the question of charge reversal from general principles based on experiments on adsorption. The relation between the valency of ions and their coagulating powers have formed the subject of a large number of investigations. Experiments on these lines led to the generalisation known as Schulze-Hardy Law namely the higher the valency of an ion the greater is its precipitating action. The researches of Linder and Picton2 on colloidal arsenious sulphide lent powerful corroboration to this Law. The actual nature of coagulation of a sol by electrolytes is still obscure. Many workers in this field have sought t o explain the mechanism of the coagulation process on the assumption that the addition of electrolytes destroys the potential of the Helmholtz double layer, which exists between the suspended particles and the medium. The theory which is now generally Compare Gangrily and Dhar: J. Phys. Chem. 26, 701, 836 (1922): Chatterji and Dliar: Kolloid-Z. 33, 18 (1923.1 J. Chem. Sor. 67, 63 (1895).
N. R. DHAR, K . C. SEN AND S. GHOSH
458
accepted is that of Freundlichl who assumes that preferential adsorption of one ion of the electrolyte by the colloid particles neutralises the charge carried by them and thus destroys wholly or partially the electrical double layer. These neutral particles come in contact with each other due to surface forces and, since the surface energy of a system always tends to diminish, they form bigger particles and are thus coagulated. Experimental support to this theory was obtained from Linder and Picton's work on arsenious sulphide sol, who observed that when colloidal As2% is coagulated with a solution of barium chloride, the filtrate becomes acidic and an appreciable amount of barium is carried down by the coagulated mass. This selective adsorption of ions from an ndded electrolyte by sols has been observed by many workers. Whitney and Ober2 carried out some quantitative determinations of the amounts of certain ions adsorbed by colloidal Asz& in course of precipitation and concluded that the amount of an ion carried down by a colloid is independent both of the concentration of the colloid and electrolyte; and that equivalent amounts of different ions are carried down by the same weight of the precipitate. This simple relation was very suggestive and on the basis of these experiments by Whitney and Ober and on his own work on the adsorption of several organic ions, Freundlich3 assumes that, in the coagulation of a colloid, equivalent amounts of precipitating ions are carried down by the same weight of colloid. From this assumption he deduced that the most readily adsorbed ion precipitates in the lowest concentration and vice-versa. In order to test his views Freundlich made some experiments on the coagulative power of several electrolytes on colloidal arsenious sulphide and their adsorption by the colloid in the course of its precipitation. He used the following electrolytes:Aniline hydroahloride, morphine hydrochloride, para-chlor aniline hydrochloride, strychnine nitrate, new magenta, uranyl nitrate, ceric nitrate, etc. I n all cases the adsorption was coniparatively small and by comparing the adsorption isotherms for different salts with the coagulating powers of the same salts, considerable variation was observed. Thus the coagulating power of strychnine nitrate was far less than that of aniline hydrochloride, parachlor aniline hydrochloride and morphine hydrochloride; but the adsorption of strychnine nitrate by the same amount of coagulated As83 was considerably greater than that of any of them. Again the coagulating power of parachloraniline hydrochloride was 2 . 5 times greater than that of aniline hydrochloride; but the adsorption isotherms practically coincided throughout, showing that aniline hydrochloride which had a less coagulating power was also equally adsorbed. Further in the case of uranyl and cerium ions he found that the amount adsorbed in milli-equivalents was about thirty percent more for uranyl ion than for ceric ion, though the coagulating power of ceric ion was Kolloid-Z. 1, 321 (1907). J. Am. Chem. SOC.23, 842 (1901). 3 IColloid-Z. 1, 321 (1907).
2
'
459
STUDIES IN ADSORPTION
about nine times greater than that of uranyl ion. Freundlich's further experimental work on the adsorption and coagulating power of an ion on n, sol shows greater divergence between these two factors. Freundlich and Schucht' determined the precipitation value for colloidal mercuric sulphide and the amount of adsorption in the region of precipitating concentration. The adsorption was determined for metallic cations NH4,Ag, Ba, Cu and Ce, and for dye cations New Magenta, Brilliant Green, Auramine and Methylene Blue. The results are given in the following table:Cat ions.
Precipitation value.
Adsorption at precipitation value. Milliequivalents.
"4
.o.i)
Ba CU (Cu(N0a)e) c u (CU SO,) Ag. Ce Auramine New Magenta Methylene Blue Brilliant Green
.044 * 03 ,22 ,@'
,012
,011
,008 ,007
.004
A perusal of the table shows that thc results disprove the assumption that equivalent amounts of ions are adsorbed at the precipitating concentration. Freundlich recognised this fact but he attributed the variation to the difficulties and errors of experiments. It should a t once be pointed out that assuming the correctness of the results obtained, the data show a remarkable fact that the ion having the greatest precipitation value is also adsorbed most and viceversa. This conclusion is exactly the opposite of that of,Freundlich. I n order t o further study the relationship between the coagulating power of ions and their adsorbability by a sol, Ishizalta in Freundlich's laboratory2 made some experiments on the adsorption and coagulation of colloidal aluminium hydroxide. Ry examining the isotherms in which the adsorption values are expressed in moles the order of adsorption of anions appears to be salicylate > ferrocyanide > oxalate > chromate > tartratc > sulphate > chloride > nitrate > thiocyanate > sulphanilatc, while the coagulating powers are in the order of ferrocyanide > sulphate > oxalate > tartrate > chromate > sdicylate > chloride > nitrate > thiocyanate > sulphani1at)e. These results indicate that ions of greater valency have high coagulating value and a t the same time they are highly adsorbed. Recent experiments, however, of Weiser and Middleton3 on the adsorption of ions by aluminium hydroxide sol are not in agreement with the above. These results will be discussed later on. Z. physik. Chem. 85, 641 (1913). Z. physik. Chrm. 83,97 (1913). J. Phys. Chem. 24, 630 (1920).
460
N. R. DHAR, K. C. SEN AND S. GHOSH
In a recent paper Gann’ determined a few adsorption values for the precipitation of aluminum hydroxide sol. Only five ions were examined and the object of the experiment was to test Freundlich’s theory that equivalent amounts of ions are adsorbed at precipitating concentration. The following table gives the results:Ion.
Salicylatr. Picrate. Oxalate., Ferricyanide. Ferrocyanide.
Precipitztion vnlue Millimole per litre.
8
Adsorption value a t precipitation concentration. In Millimoles. I n Milliequivalent
.30 . I8
.30
.36
.I8
I8 .36
.IO
.09
.27
.08
.073
.29
4
If we consider the results given in column 3, viz., amount of adsorption expressed in millimoles we immediately find that the greater is the precipitation value of an ion the greater is the absolute amount of adsorption. Moreover, another interesting fact is also observed that the greater is the valency of an ion the less the absolute amount of adsorption. Thus in this case the valencies of the ions varying from four to one shon a variation in the absolute amount of adsorption from .073 to .30 millimole. In recent years Weiser2 and his collaborators have studied the adsorption of various ions by precipitating barium sulphate, aluminium hydroxide, and ferric hydroxide in presence of excess of several salts. From the results of their investigations the order of adsorption of ions by precipitating BaS04 is ferrocyanide > nitrate > nitrite > chlorate > permanganate > ferricyanide > chloride > bromide > cyanide > sulphocyanate > iodide, the ferrocyanide ion being adsorbed the most and the iodide the least. If the adsorption values given above are expressed in gram anions instead of gram equivalent anions the order becomes nitrate > nitrite > chlorate > ferrocyanide > permanganate > chloride ferricyanide > bromide > cyanide > sulphocyanide > iodide, the nitrate ion being adsorbed most iodide the least. Weiser and Sherrick (Zoc. cit.) remark that there is nothing even to suggest the Schulze-Hardy Law in the case of barium sulphate as an absorbent. It will, however, be observed here that in the latter method of tabulation both the ferrocyanide and ferricyanide ions are considerably less adsorbed than many of the monovalent ions though their valencies are higher. In the second of the series of papers Weiser and Middleton (loc. cit.) determined the adsorption of several anions by precipitating a ferric hydroxide sol. This work is interesting from another point of view. The adsorption values have been determined at>the precipitating concentrations of electrolytes and 1
Kolloid-Chem. Beihefte. 8, 63 (1916). J. Phys. Chem. 23, 205 (1919);24,30, 630 (1920);25, 399 (1921).
46 1
STUDIES IN ADSORPTION
as such a comparison can be made between the coagulating power of the electrolyte and the amount actually adsorbed by the coagulating mass. Their results are given in the following table:Adsorpt,ion value
Anion.
Milljgram anions.
Phosphate. Citrat.e. Tartrate. Oxalate. Sulphate. Iodate. Dichromate.
*
572 I
.5018 .6232
.4364
Millieq,uivalen t anions. I . 7165 I .so46 I . 2464 .9128
.3804
.7609
.’is12
.75I*
Precipitation Value
,1559
In the discussion of their results the above authors reinark: “Froni the table it will be seen that all the ions are strongly adsorbed by hydrous ferric hydroxide ; the amounts varying in milliequivalents per gram from approximately .3 in the case of dichromate t o 1.7 in the case of phosphate ion. The average adsorption value in milliequivalents per gram of krsenic trisulphide was found by Whitney and Ober and by Freundlich to be approximately . 0 8 ; and the average value per gram of mercuric sulphide was but . 0 2 . The values for hydrous ferric oxide show clearly that %heamounts of ions carried down by a precipitated colloid are not equivalent. As a matter of fact the actual variation in the values is less than that noted by Freundlich with mercuric sulphide; but he attributed the variation from equivalents to the analytical difficulties connected with the determination of very small adsorption values. T h e relatively large adsorption values in the case of hydrous ferric hydroxide and the accuracy with which they may be determined indicate conclusively that the values are not even approximately the same. As befote explained other conditions remaining the same equivalent amounts must be adsorbed to neutralise the charge on the colloidal particles; but the adsorption does not stop with the neutralisation of the charge and the amounts actually carried down by the precipitate will vary with the adsorbability of the ions. “If the ions are arranged in the order of their adsorption values expressed in milliequivalent anions per gram of adsorbent the following series is obtained: “Phosphate > citrate > tartrate > oxalate > sulphate > iodate > dichromate, the phosphate being adsorbed the most and the dichromate the least. The precipitation values expressed in milliequivalent per litre would indicate the order of adsorption to be: “Dichromate > tartrate > sulphate > oxalate > citrate > iodate > phosphate. It is evident that there is a tendency for ions with the lowest precipitating values to be adsorbed the least and vice-versa, which is diametrically opposed to what one should expect. Since the ionisation constant for the third step in the ionisation of both citric and phosphoric acids is very small it might seem preferable to consider them as dibasic acids rather than tribasic.
462
N. R. DHAR, K. C. SEN AND S. GHOSH
The only change that this would make in the series of adsorption values would be to put tartrate ion ahead of citrate and phosphate ions, the series becoming: Tartrate > phosphate > citrate > oxalate > sulphate > iodate > dichromate. Under these conditions the precipitation values would indicate the order to beDichromate > citrate > tartrate > sulphate > oxalate > phosphate > iodate.” In the third paper Weiser and Middleton (Zoc. cit.) have niade a quant,itative determination of the coagulating power as well as the adsorption of several ions a t the precipitating concentration by colloidal aluminium hydroxide. A few of the results are given below:Anion.
Ferrocyanide. Ferricyanide. Sulphat’e. Oxalate. . Phosphate.
Preripitation vrllue Millimolps per litre
,094 133 .2h9 .350 .346
‘
Adsorpt,ion per gram AI203 Milligram anion.
.3202 ,4046 .4984 .si10
.go88
It will be seen from this table that the order of adsorption is phosphate >
oxalate > sulphat’e > ferricyanide > ferrocyanide the phosphate ion being adsorbed the most and the ferrocyanide the least. Arranging the ions according to their coagulating power the series becomes, ferrocyanide > ferricyanide > sulphate > phosphate > oxalate, ferrocyanide having the greatest coagulative power and t,he oxalate the least. It is evident from these results that ions having the great,est coagulating power are the least absorbed; thus the tetravalent ion ferrocyanide has a high coagulating power and at the same time is the least absorbed. These results further show marked variation from those obtained by Freundlich and Ishizaka (loc. cit). Weiser and Middleton commenting on results of Freundlich and Ishizaka point out t’hat it is very unsafe t o draw conclusion from adsorption data where the amount of adsorption is very small as has been obtained by Ishizaka with “grown” alumina ( . 0 0 2 to .os5 millimole per gram). Further it may be remarked here that the adsorption values for colloidal aluminium hydroxide were determined above the precipitation concentration. Freundlich and Ishizaka themselves concluded that “smaller concentrations should have been used in the determination of these adsorption values such as were used in the determination of precipitation values”. It will be obvious from these considerations that the series obtained by Weiser and Middleton (loc. cit.) and by Freundlich and Ishizaka (Zoc. cit.) should differ materially as the adsorption values were determined under different concentration of the electrolytes and under different conditions. It has already been st,ated that in our own experiments on the coagulation of manganese dioxide sol and on the adsorption of ions by precipitated manga-
463
STUDIES IN ADSORPTION
nese dioxide we have noticed that, in general, ions of higher valencies having greater coagulating powers are adsorbed less than the ions of lower valencies. It is now desirable to analyse the actual experimental results obtained by us in details especially as the data obtained by Weiser and his collaborators confirm the conclusions we have arrived at from our own experiments. It may be noted here that, up t o this time, almost all researches on direct adsorption by a solid adsorbent have been performed with charcoal; but it is well known that even if charcoal be prepared from one source its composition cannot be made constant. Thus cocoanut charcoal, which has been very largely used for adsorption experiments differs in composition and impurities according to the manner and source from which it is prepared. Even the temperature at which carbonisation is carried out has very great influence on the adsorptive power of charcoal. Again it is extremely difficult to get the substance with particles of uniform size, which condition is of primary importance in the surface phenomenon. Charcoal, therefore, appears to be not at all suited for the investigation of the problem with a view to the formulation of the laws of adsorption. In the following table results of adsorption of some electrolytes by chemically pure and air-dried hydrated manganese dioxide are given. In all cases it has been observed that mainly the cations are adsorbed by the solid. Electrolyte.
Strength of the solution.
Amount adsorbed in milliequivalent p ~ r gram MnOp
.618 MnClz . 5N ZnSO4 .5 & .854 .5 K 1.463 CUSOl . 4 8 pu’ 963 CUClZ .5 i Y .381 Ni(N03)Z Potash alum .48 N ,272 .46N .272 Fez(SO4)3 T h (NOa)r .48 N .581 Arranging the ions in order of their adsorbability and beginning with the one most absorbed, the following series is obtained :Cu > Zn > Mn > Th > Ni > Fe, Al. In another set of experiments the following series is obtained :Ag > C u > Cd > Zn, Mg > Ba > Sr, Ca > Al. In the following table results obtained by the adsorption of electrolytes by hydrated MnOz in course of precipitation are given:9
Electrolyte Amount adsorbed in Conc = N / I Milliequivalents per gram MnOz
AgNOa NaCl LiCl CdSOc CUSOI BaClz
6.107 2
.876
I .382
.goo 1.150
.594
Electrolyte. Conc = N / I
Amount adsorbed in Milliequivalents per gram MnO2
ZnSOl Ni (NO3)2 FeCL Potash Alum UOz(N03)z Th (NO4 ) 4
a431 *
503
1.234 .082
.293 .og6
464
N. R. DHAR, K. C. SEN AND S. GHOSH
Examining the above table the order of the ions according to their adsorption becomes Ag > Na > Li > Cu > Cd > Ba > Ni, Zn > U 0 2 > Th > Al. It will be observed from these quantitative results that many ions of lower valency are more adsorbed than ions of higher valency. Thus the monovalent silver, sodium, and lithium ions are more adsorbed than any of the bivalent, trivalent, or tetravalent ions. Again the trivalent ion aluminium is far less adsorbed than most of the bivalent ions. These facts show that the ions of higher valency, which in general have greater coagulating powers are adsorbed the least. This conclusion arrived at from a careful scrutiny of the experimental results of different investigators as well as of our own is in direct opposition to the view hither to held, notably by Freundlich and Weiser. Bancroft in his article on precipitation and peptisation’ has interpreted the Schulze-Hardy Law in a peculiar way. “While it is generally true that an ion of higher valence will be adsorbed more strongly than that of lower valence, this so-called law of Schulze-Hardy is only a first approximation, and should be considered only as a guide.” Again on page 11 he says “Whilst there is unquestionably a tendency for ions of a higher valence to be adsorbed more strongly than ions of a lower valence, the experiments which have been cited show that there are many exceptions and that the fundamental rules is that adsorption is specific both as regards the adsorbing substance and the ion The above view has been accepted by Lewis2. There are adsorbed”. two statements in the above quotation which require some comment. From the experimental results already cited it is clear that there is absolutely no justification in holding the view that ions of higher valence are adsorbed to a greater extent than those of lower valence. On the contrary the results show that ions of lower valence are adsorbed more than those of higher valence, and it seems to us that the real explanation of the Schulze-Hardy Law rests in the recognition of this fact. Since the coagulation of an equal amount of a colloid by ions of different valencies, is at first an electrical phenomenon, it will be clear that for the electrical neutralisation of a fixed amount of any colloid the absolute amount of ions, expressed in gram molecules, neceesary in the case of monovalent ions will be greater than that of di- or tri-valent ions, simply because the net charge on a di- or tri-valent ion is greater than that of a monovalent ion. It will be apparent, therefore, that for one purely electrical neutralisation by adsorption, the greater the coagulating power of an ion, the less will be its adsorption. In our opinion, therefore, up till now, the real significance of the Schulze-Hardy Law has not been clearly perceived. The few apparent deviations from this rule can be easily explained. I n t h e coagulation of a colloid there are two distinct steps in which adsorption occurs. The first step is the electrical neutralisation of the charge on the colloida1 particles through adsorption of an ion carrying a charge opposite to that on the sol and only here the Schulze-Hardy Law is applicable. The adsorption, however, does not stop there, but the coagulated particles further act as an
...
1 2
Second B. A. Report on Colloid Chemistry, p.8. (1918). System of Physical Chemistry 1, 348 (1921).
STUDIES IN ADSORPTION
465
adsorbent, taking up an additional amount of the electrolyte or ion. The amount of this second adsorption will depend on the adsorbability of the electrolytes or ions and the nature of the coagulated mass concerned and hence the final amount of adsorption may have any value depending on the above factors. The Schulze-Hardy Law cannot be rigidly applied to these cases. If the adsorption by the neutral particles is not appreciable, then the Schulze-Hardy Law is likely to be followed; but if the neutral particles can adsorb the ion or the electrolyte appreciably, complications will arise and the Schulze-Hardy Law may not be applicable. The existence of these two steps in adsorption has been recognised by Weiser and his collaborators (Zoc. cit.) who have emphasised that in the determination of adsorption by a coagulating colloid the amount of adsorption by the agglomerated particles must be taken account of. It will thus be observed that the interpretation of the SchulzeHardy Law advanced in this paper is in agreement with experimental results, and it explains in a consistent manner the fact that in majority of cases an ion with the greatest coagulating power is least adsorbed. Bancroft (Zoc. cit.) has stated that ions of higher valency are adsorbed “more strongly” than ions of lower valency. We are not aware of any direct experiment in which the attractive force by which an adsorbed ion is retained by the coagulated mass has been determined. Moreover the adsorption process is practically instantaneous in the majority of cases investigated. Linder and Picton* have shown that when arsenious sulphide is precipitated by barium chloride or strontium chloride, only a small portion of the metallic radical is adsorbed by the coagulated sulphide. If the precipitate containing the adsorbed salt is shaken with potassium or sodium chloride, the adsorbed metal is displaced by sodium or potassium and comes out in the solution which can be tested. This shows that the bivalent barium is not adsorbed more strongly than the univalent potassium or sodium and this is contrary to Bancroft’s view. We have found that hydrated manganese dioside containing some adsorbed copper or any other metal loses the metal when shaken with potassium or sodium chloride or with the aqueous solution of any other electrolyte. This phenomenon is of general occurrence. These results indicate that in the present state of our knowledge of the phenomenon of adsorption, we cannot say with precision whether one ion is adsorbed “more strongly” than another, when the adsorption is of the same type.
It may be of interest at this stage to consider tlie question of minimal concentration of an electrolyte necessary to coagulate a sol. I t is well known that a certain minimum concentration of electrolyte is necessary to coagulate a Tiven amount of a colloid, though a small percentage of the added electrolyte at the minimal concentration is actually used up in coagulating the sol, the ma.ior portion of the electrolyte remains iinadsorbed. No satisfactory explanation of this fact has yet been given. It is certainly interes‘ting that n niinimum concentration of an electrolytc would be necessary for the precipitation of a colloid, when the actual amount required for the coagulation is far less
N . R . DHAR, K. C. SEN AND S. GHOSH
466
than the so-called minimal concentration. In order to understand this process it is necessary to take recourse to certain kinetic consideration. It is well known that in explaining the finite velocity of ordinary chemical reactions in homogeneous medium it is assumed that all the molecules of the reacting substances are not in the same state of reactivity. At any instant, only a very small pcrtion of them are reactive, and these active niolecules or ions determine the velocity of the particular reaction at a certain temperature' We can conceive therefore, that all the ions from an added electrolyte and carrying a charge opposite to that on the sol are not in the same state of activation, and that only the active ions can be adsorbed by the sol and are capable of precipitating it. Hence a minimal concentration of an electrolyte mcans that concentration where the amount of active and consequently of adsorbable ions is just sufficient to precipitate the sol. In order, therefore, to explain the experimental results on adsorption we have to assume that only a few percents of the ions carrying a charge opposite to that of the sol are active as far as precipitation of the sol is concerned and that the active portion of the ions is adsorbed by the solin course of its precipitation. It is an experimental fact that with the same weight of a solid adsorbent, the greater the concentration of the solute, the greater is the adsorption. On the addition of a small amount of an electrolyte to a sol, only a certain percentage of the ion carrying a charge opposite to that of the sol will be adsorbed by the colloidal particles depending on the concentration of the added electrolyte, If the particles of the sol are of uniform size, fractional precipitation of the sol is not likely to occur, because the uncharged colloidal particles carrying the adsorbed material may come in contact with the neighbouring charged particles and are likely to form bigger particles in combination with the charged particles. Because of their charge and of their Brownian movement, these bigger particles, though now more unstable, 'will still remain in suspension. Hence the presence of charged particles is likely to exert a peptising influence on the neighbouring uncharged ones. Gradually the concentration of the added electrolyte is increased and with it the amount of adsorption of the precipitating ion is also increased. At the minimal concentration, that much of the precipitating ion is adsorbed which is required for the complete neutralieat;on of the electric charge on t)he colloid particles. There are some apparent deviations from the simple case discussed where the particles of the sol are assumed to be of uniform size. In the case of colloidal sulphur, mastic, etc., fractional Precipitation by electrolytes is possible, because all the particles in the sols are not of the same size and consequently of the same degree of stability. Thus Oden2 obtained particles of different size on fractional precipitation of a sulphur sol. Moreover, Murray3 has obtained particles of mastic of different sizes by fractional precipitation of a sol by means of hydrochloric acid. These facts show that in the colloidal soluCf. Dhar: J. Chem. SOC.111, 745 (1917). Kolloid-Z. 8, 186 (191I ) . 3 Phil Mag. August, 401 (1922). 1
STUDIES IN ADSORPTION
46 7
tions of these substances, particles of different sizes are present. From our experiments on the adsorption of sols with precipitated BaSOe, we have observed that the larger the particle the less is its stability and sols containing large particles are readily adsorbed and precipitated by Ba SO,. Hence fractional precipitation of a sol on the addition of an electrolyte is likely to take place in those cases where the particles of the sol are not of uniform size and consequently of different degrees of stability. From a critical survey of our own experiments as well as of the existing data on the effect of concentration of a sol on its coagulation by electrolytes, we have shown in the foregoing paper of this series that the greater the concentration of a colloid, the greater would be the amount of an electrolyte necessary to coagulate it, irrespective of the valency of the precipitating ion. This generalisation is also supported by the experimental work of Weiser and Nicholas1. It is also well known that emulsions behave like sols in most respects. Hence it is very likely that this simple rule regarding the effect of concentration of a sol on the coagulating power of an electrolyte should also hold in the case of emulsions.* Charge Reversal of Colloids In the foregoing paper3 of this series it has been shown that freshly precipitated ferric hydroxide when shaken with the solution of arsenious acid paesev into a negatively charged colloid. In the same paper it has also been observed that in presence of protecting substances like glycerol, cane sugar, grape sugar, etc., ferric hydroxide, cobalt hydroxide, cupric hydroxide etc., can be peptised and can be made to take up negative charge. By t8he gradual addition of an alkali to a mixture of ferric chloride and glycerol or sugar there are three definite stages through which the colloid passes-first it becomes positively charged, then coagulation occurs on neutralisation c f t k p change and finally it passes into a negatively charged colloid. Thie order can be changed by the addition of suitable electrolytes. In another part of this series of papers it has been observed t h a t freshly precipitated ferric hydroxide passes into a negatively charged sol when shaken with aqueous solutions of sodium arsenite, sodium oxalate, sodium tartrate, sodium citrate etc. Recently we have observed that when ferric hydroxide is precipitated in the cold by mixing ferric chloride and ammonium hydroxide and washed with distilled water till the filtrate is free from chloride, the ferric hydroxide can pass as a negatively charged sol due to the adsorption of OH’ ions from ammonium hydroxide. In other words ferric hydroxide can very readily pass into a negatively charged colloid. Moreover, it is well known that in the ordinary methods of preparing the sol we get a positively charged colloid. These facts immediately point to the influence of the medium on the nature of
3
J. Phys. Chem. 25, 742 (1921). Compare, however, Rhatnagar: J. Phy?. Chem. 25, 735 (1921). Kolloid-Z. (1923).
468
N . R . DHAR, K. C . SEN AND S. GHOSH
the electrical charge carried by the colloidal particles. The presence or absence of certain kinds of ions in the medium accounts for the nature of the electric charge on the particles of the sol. In a foregoing paper' we have emphasised the view that the amount of adsorption of an ion by an adsorbent is a fundamental factor in charge reversal. Valency of the ion in question is only important when it implies the amount of charge on an ion, because in order to have reversal of charge the colloid needs adsorb only a small amount of ions of higher valency in comparison with ions of lower valency. Hence if the amount of adsorption is great, a monovalent ion can reverse the charge on a colloid. Thus hydrochloric acid has been found to reverse the charge on antimony sulphide sol, similarly sodium hydroxide or potassium hydroxide can reverse the charge of ferric hydroxide peptised by sugar or glycerol.
It has already been stated that freshly precipitated hydrated manganese dioxide can adsorb a large amount of silver ions from silver nitrate. It has now been observed that monovalent silver ions can reverse the charge on colloidal manganese dioxide, consequently it is apparent that those sols which are likely to adsorb markedly monovalent ions like Ag', Hg' (ous), should undergo charge reversal in the presence of these ions. We have repeatedly observed that freshly precipitated ferric hydroxide is a very good adsorbent and it has been found out that it adsorbs both acids and alkalies. From equivalent concentrations the amount of adsorption for an acid is much greater than that for an alkali, because Fe(OH)3is more basic than acidic. It has also been found that it markedly adsorbs mainly the acidic portion from the solutions of salts like sodium oxalate, sodium arsenite, sodium phosphate, sodium citrate, sodium tartrate, etc., and that it is impossible to free ferric hydroxide from these substances even by repeated washing and it has been conclusively shown that these electrolytes charge freshly precipitated ferric hydroxide negatively. It is also well known that these salts form complexes with ferric ion. It is certain, therefore, that a neutral substance like ferric hydroxide can pass into a positively or negatively charged sol according to the amount of adsorption of a positive or a negative ion. Recently Weiser2 has shown that freshly precipitated cupric hydroxide when shaken with solutions of KC1, NaC1, adsorbs the acid portion leaving the filtrates alkaline. In this connection it is of interest to note that those substances which can form complex salts with the adsorbent are likely to be adsorbed most. Thus Ishizaka (Zoc. cit.) has shown that potassium salicylate, potassium ferrocyanide and potassium oxalate are adsorbed most by a sol of aluminium hydroxide. Similar results are obtainable with ferric, hydroxide. Evidently the phenomenon of adsorption is most marked when there is some sort of chemical affinity between the adsorbent and the substance which is being adsorbed. J. Phys. Chem. 27, 376 (1923). J. Phys. Chem. 27, 501 (1923).
469
STUDIES IN ADSORPTION
We have already proved‘ that hydrated manganese dioxide is a good adsorbent and it has been found that it adsorbs mainly the basic portion from salt solutions and very small portion of the negative part is adsorbed. It also adsorbs both hydrogen and hydroxyl ions and from the following results it will be seen that for equivalent concentrations the hydroxyl ions are more adsorbed than hydrogen ions. Solution used.
Sulphuric acid. Acetic acid. Sodium hydroxide. Sodium hydroxide. Potassium hydroxide. Potassium hydroxide.
Amount of H. or OH’ present originally ,0094 gr. of H’
of H’ ,017 gr. of OH’ .os4 gr. of OH’ ,017 gr. of OH’ .034 gr. of OH’ ,0162 gr.
Amount of ,H, or OH’ adsorbed.
of H’ of H‘ ,008 gr. of OH’ .OII , gr. of OH’ .oo8 gr. of OH’ , 0 1 1 gr. of OH’ .0003 gr. ,0003 gr.
It appears, therefore, that hydrated manganese dioxide which behaves like manganic acid H2Mn03,and is also acid to litmus, can adsorb large quantities of OH’ions because an acidic substance naturally has a great affinityfor OH’ ions. Incidentally we should like to mention that our experimental results carried on with chemically pure absorbents are in entire disagreement with the views expressed by Oden and Andersson2 on the nature of the decomposition of an electrolyte by adsorption. It is evident that a neutral substance having a large surface will adsorb gases and liquids and that is why substances are copiously adsorbed by charcoal. When solutions of electrolytes are shaken with charcoal, both the acid and basic portions are adsorbed and consequently no question of change of electric condition of the adsorbent arises. If there is some sort of chemical affinity between charcoal and the substance which is being adsorbed, the result will be more marked. I n Weiser and Sherrick’s experiments (Zoc. cit.) on the adsorption of substances by BaS04 in the course of precipitation it is found that those ions which form sparingly soluble barium salts are adsorbed most, because sparingly soluble salts are more allied to barium sulphate. We have observed that pure silica, when carefully washed with hydrochloric acid and freed from the acid by repeated washing, is shaken with salt solutions like CuSo4, NiC12, alum, etc., can adsorb the basic portion from these salt solutions. The adsorbed substance cannot be removed by washing. We have also observed that the basic portion adsorbed by hydrated manganese dioxide from substances like Bac12, CuSO4, NiCl,, alum, AgN03, etc., cannot be removed by washing, whilst the basic portion adsorbed by hydrated MnOz fromgNaC1, KC1, etc., can be slowly but completely removed by washing. Consequently the adsorption of basic portion of substances like CuS04, NiC12, etc., is more or less permanent, and hence many authors call these Chatterji and Dhar: Eoc. cit. J. Phys. Chcm. 25, 322 (1921).
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substances manganites of the corresponding metals1. Exactly similar results are obtained with hydrated silica, the basic portion adsorbed from NaCl, KCl, etc., can be slowly but completely removed by washing, whilst the basic portion adsorbed from CuSor, alum, etc., cannot be removed by washing. Just as manganese dioxide is capable of adsorbing both acids and bases, similarly silica can also adsorb both acids and bases and a base is more adsorbed than an acid, because silica is acidic in nature. The amount of adsorption of acids by silica is very small and is only a few per cent of the total quantity of the acid taken. This adsorption is more or less due to the surface effect and is allied to the adsorption of substances by charcoal, but the adsorption of bases by silica is connected with chemical affinity and is more permanent. It is well known that silicic acid can remain peptised in presence of both H' and OH' ions. In presence of H' ions, it remains positively charged, whilst in presence of OH' ions it is negatively charged. The uncharged substances like silicic acid, hydrated MnOz,Fe(OH)3, etc., can pass into a positive or a negatively charged sol due to the adsorption of H' ions or OH' ions according to circumstances. Prom the foregoing results it is apparent that charge reversal will depend a great deal on the amount of adsorption as well as on the permanency of adsorption. Those substances which are markedly adsorbed and which cannot, be removed by washing are more active in charge reversal than those which are not adsorbed in large quantities and are removable by washing. From our own experiments we have observed that negatively charged ferric hydroxide sol can be obtained by arsenite ion, oxalate ion, tartrate ion, etc., which are markedly and permanently adsorbed by ferric hydroxide. Similarly charge reversal has been observed with hydrated manganese dioxide by even the monovalent Ag' ion which is markedly and adsorbed by hydrated manganese dioxide. From our experiments we find that hydrogen ions and hydroxyl ions are appreciably adsorbed by freshly precipitated ferric hydroxide, a t the same time charge reversal can be readily effected by these two univalent substances. It is very likely that H' ions will be adsorbed appreciably by sulphides of arsenic, antimony, etc., and that is why charge reversal of these substances is possible with the help of H' ions. Recently Mukherjiz has stated that silica can adsorb acetic acid, citric acid, salicylic acid, hydrochloric acid, etc., This fact has been contradicted by Joseph and Hancock3. We have observed that hydrated MnOt can adsorb' acetic acid and other acids; similarly we have found that silica adsorbs acetic acid and other acids to a slight extent. This kind of adsorption of substances by uncharged adsorbents like charcoal, silica, hydrated MnOz, etc., is mainly a surface effect and is insignificant in comparison with the adsorption due to the chemical affinity between the adsorbent and the adsorbing substance, as 1 Cf. Sarknr and Dhar: Z. anorg. Chcm. 121, 135 (1922). *Phil. Mag. 6, 44,343 (1922); Nature 110, 732 (1922). 3 J. Chem. SOP. 123, 2022 fr92.31.
STUDIES I N ADSORPTION
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has already been said that silica will adsorb bases in greater amount than acids. Very recently Glixellil has observed an augmentation of the acidity of silica gels under the influence of a neutral salt. The effect can be readily explained by the adsorption of NaOH by the silica particles, setting free hydrochloric acid. In view of the above facts the phenomenon of soil acidity can be very readily explained. We know that soil contains silica as well as humic acid. These two substances will certainly adsorb the basic portion from neutral salts present in the soil setting free acid and thus making the soil acid. The adsorbed basic portion might form unstable adsorption compounds of the type of the so-called manganites which are sparingly soluble. It is well known that sodium salts of pyroantimonic acid, dihydroxy tartraric acid, complex silicic acid, etc., are very sparingly soluble. It is very likely that the adsorption compounds obtained from humic and silicic acids would be sparingly soluble. Moreover, it has been noted in the foregoing pages that the basic portion adsorbed by Mn02, SiOz, etc., from such salts as NaC1, KCl, etc., are very slowly removed by repeated washing. Hence from the foregoing experimental result8sit appears that soil acidity is most likely due to the adsorption of basic portions of salts by silica, humic acid, etc., present in the soil. We are, therefore, unable to support the view on soil acidity expressed by Oden2 which rests on the assumption that humic acid adsorbs the inorganic and organic acids already present in the soil. It must be emphasised that substances like Fe(OH)3, A1(OH)3, Cr(OH)3, AszSa,Hgs, CuS, SnO2, S, SiOz, etc., which are neither strongly acidic nor basic can adsorb either H’ or OH’ ions and pass into positively or negatively charged sol and hence the formation of colloids and charge reversal with these substances are comparatively easy. It is now easy to explain the charge reversal of a colloid simply from the adsorption point of view. The first step in the forrriation of a colloid is the preferential adsorption of one ion which peptises the substance and gives the necessary charge to the otherwise uncharged substance, for the stability of the sol. If we now add an electrolyte, the charge on the colloidal particles will be neutralised by the oppositely charged ions. Two things might happen a t this stage. If the particles of the neutral substance cannot adsorb more of the precipitating ion it will coalesce and finally coagulate. If, on the other hand, the neutral substance is capable of adsorbing more of the precipitating ion immediately, it will be converted into the oppositely charged sol and consequently charge reversal will take place. Evidently charge reversal will depend a great deal on the amount and permanency of adsorption of the precipitating ion by the neutral substance obtained by the coagulation of the colloid. Linder and Picton (loc. cit.) have proved that when arsenious sulphide is precipitated by BaClz or SrClz the metallic radical is adsorbed by the coagulated sulphide. The above aut’hors have observed that the adsorbed ions lCompt. rend. 176, 1 7 1 4(1923). Trans. Faraday. Sot. 17,292 (1922).
N . R. DHAR, K. C. S E N AND S. GHOSH
472
cannot be removed by washing. We repeated the above experiments with sols of As2 S3and Sb2 S3 and we have obtained results contrary to the observations of the foregoing authors. We have been successful in completely removing Baor Sr from the coagulated sulphides by repeated washing with water. 1,inder and Picton have also shown that the coagulated sulphides containing Ba or Sr when shaken with excess of K or Na salts give out the Ba or Sr Salt in solution and this can be tested. This has been recently contradicted by Charrioul; but we have found that the observation of Linder and Picton is quite correct and that excess of sodium or potassium salt, can drive out Ba or Sr ions from their adsorbed condition. This happens because the adsorption of these substances is practically of the same order, and since, the univalent ions are used in large excess, the effect of mass will predominate. Charriou has also shown that chromic acid adsorbed by Al(OH)3 cannot be driven out by chlorides, nitrates or acetates of alkali metals but it can be displaced by oxalates, sulphates, phosphates, etc. These results can be very readily explained from the work of Weiser and Middleton (Zoc. cit.) on the adsorption of the foregoing ions by Al(OH)3. Weiser and Middleton have shown that dichromate ion is highly adsorbed and hence the displacement will depend on the amount of adsorption and roncentration of the displacing electrolyte. If the displacing electrolyte is only slightly adsorbed by Al(OH),, then chromic acid will not be displaced even by concentrated solutions of the displacing electrolyte.
It should be eniphasised that the amount of adsorption by a definite weight of adsorbent would be directly proportional to the molecular weight of the substance in question under otherwise similar conditions. It has already been shown that the amount of adsorption of electrolytes by ,freshly precipitated manganese dioxide increases with the increase in their atomic weights when the elements occur in the same periodic table.2 Similar results have been obtained by Oden and Andeisson3 in the adsorption of alkali and alkaline earth metals by charcoal. In the adsorption of anions, Oden and Langelius (Zoc. cit. p. 385) have shown that, in general, amongst ions having the same valency, the greater the molecular weight the greater is the adsorption. Moreover, it has been observed that the bigger the particles in a medium, the greater is the adsorption by an adsorbent. From our experiments on adsorption we find that substances like freshly precipitated Fe(OH)3, hydrated Mn02, etc., have marked adsorptive power and are in certain respects comparable to charcoal, whilst substances like As2S3, Sb2S3, BaS04, etc., have very slipht adsorptive power, though all these substances can adsorb appreciably electrolytes in the course of their formation. Consequently the uncharged substances like AszS~,Sb2S3, etc., cannot adsorb the precipitating ions to any appreciable amount. Hence the amount of different ions adsorbed by SbzS3, etc., in the process of 'Compt. rend., 176 1890 (1923).
* Ganguly and Dhar: J. Phys. Chem. 26, 836 3
J. Phys. Chem. 25,311(1921).
(1922).
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coagulation is more or less equivalent as was shown by Whitney and Ober (Zoc. cit.) and Freundlich (Zoc. cit.). On the other hand, the neutralised substances like hydrated MnOz, Fe(OH)3,etc., are capable of adsorbing appreciably the precipitating ions; hence in these cases the amount of ions adsorbed by a coagulating sol is bound to be different and are not in equivalent proportion. From the discussions and our researches in this line we are of the opinion that charge reversal, amount of adsorption and complex iormation go hand in hand and depend upon the chemical affinity existing between the adsorbent and the substance which is being adsorbed. Experiments on adsorption and reversal of charge in various directions are in progress in these laboratories.
Summary ( I ) From a survey of the experimental results on coagulation and adsorption it is found that an ion which has a high precipitation value (a small ~oagulat~ing power) for a colloid is most adsorbed by the colloid. Inversely the smaller the precipitation value (that is, the greater the coagulatiiip power), the less is the adsorption. This is the proper explanation of the Eo-called Schulze-Hardy Law. The above generalisation is supported by the actual experimental results of various workers. (2) On the addition of an electrolyte to a sol the first step is the neutralisation of the electric charge on the colloid particles, by the adsorption of ions carrying a charge oppusite to that of the sol. The second step is the further adsorption of the electrolyte by the neutral particles. It is only for the first step that the Schulze-Hardy Law is applicable.
(3) A suggestion based on kinetic and adsorption points of view, explaining the existence of a minimal concentration of electrolytes for the coagulation of colloids, has been advanced. (4) Charge reversal depends essentially on the amount as well as on the permanency of adsorption. Ions such as hydrogen, hydroxyl, etc., which are more or less permanently adsorbed in large quantites are most active in charge reversal. ( 5 ) Substances which are neither strongly acidic nor strongly basic, are capable of adsorbing positive or negative ions and pass into the colloidal state. Charge reversal with these cases are also very frequent.
(6) It appears that charge reversal is possible in those colloids where the neutralised particles of the sol can immediately adsorb more of the precipitat-
ing ion
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N . R. DHAR, K. C. SEN AND S . GHOSH
(7) We are of opinion that charge reversal, amount of adsorption and complex formation go hand in hand and depend on the chemical affinity existing between the adsorbent and the substance which is being adsorbed. (8) Recent experimental work on the adsorption of the basic portion from salt solutions by silica, cupric hydroxide, manganese dioxide, etc., proves that soil acidity is very likely due to the adsorption of basic portion from salts by silica, humic acid, etc., presrnt in the soil. Chemical Laboratories, Allahahad University Alldinbad. Sept. $6. 1967.
