STUDIES I N ADSORPTIOK. XIII. SCHULZE-HARDY LAW AND ADSORPTION BY 9. R. DHAR AND S. GHOSH
In a previous report1 of this series we made the following observations:-
“It is of interest to note that those substances, which can form complex salts with the adsorbent are likely to be adsorbed most. Thus Ishizaka2 has shown that potassium salicylate, potassium ferrocyanide and potassium oxalate are adsorbed most by Al(OH)3. Similar results are obtainable with Fe(OH)3. 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”. “It appears that hydrated manganese dioxide, which behaves like H2Mn03 and is also acid to litmus can adsorb large quantities of OH ions, because an acidic substance has naturally a great affinity for OH ions”. “NTehave observed that the basic portion adsorbed by hydrated manganese dioxide from substances like BaCL, CuS04,A1z(SO4)3,AgnT03, etc., cannot be removed by washing, whilst the basic portion adsorbed by hydrated manganese dioxide from KaC1, KC1, etc., can be removed slowly but completely by washing. Consequently the adsorption of the basic portion of these electrolytes is more or less permanent. Exactly similar results are obtained with hydrated silica, the basic portion adsorbed from NaC1, KC1, etc., can be removed slowly but completely by washing, whilst the basic portion adsorbed from CuS04, Alum, etc., cannot be removed by washing. Just as manganese dioxide is capable of adsorbing both acids and bases, similarly silica can 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 percent 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”. “Moreover it has been observed that the bigger the particles in a medium, the greater is its adsorption by an adsorbent”. “From our experiments on adsorption me find that substances like freshly precipitated Fe(0H) 3, hydrated MnOz, etc., have marked adsorptive power and are in certain respects comparable to charcoal, whilst substances like As2S3, Sb2S3,BaS04, etc., have very slight adsorptive power, though all these substances can adsorb appreciably electrolytes in the course of their formation. Consequently the uncharged substances like AF2S3, SbzSa,etc., cannot adsorb the precipitating ions to any appreciable amount. Hence the amount of different ions adsorbed by A s ~ S ~Sb2S3, , etc., in the process of J. Phys. Chem., 28, 457 (1924). Z. physik. Chem., 83, 97 (1913).
STUDIES I N ADSORPTION
629
coagulation is more or less equivalent, as was shown by Whitney and Oberl and Freundlich2. On the other hand, the neutralized substances like hydrated RTnOn, 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”. “We are of the opinion that charge reversal, amount of adsorption and complex formation go hand in hand and depend upon the chemical affinity existing between the adsorbent and the substance which is being adsorbed”. In the same paper it was also observed that “In Weii.er and Sherrick’s3 experiments 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”, More or less similar results were obtained by us4 in the adsorption of electrolytes by BaS04, obtained by the interaction of Ba(OH)2 and H,SO,. Very recently Beekley and Taylor5 have got similar results with AgI as the adsorbent. They have shown that less soluble salts are more strongly adsorbed and more soluble salts are weakly adsorbed by AgI. In the same paper from a survey of the experimental results on adsorption and coagulation we have given a new interpretation of the Schulze-Hardy law when applied to adsorption experiments. Recently our interpretation of the Schulze-Hardy law has been adversely criticized by Reiser6. Our interpretation of the Schulze-Hardy law is that in general an ion, which has a small coagulating power for a colloid. is most adsorbed by the colloid in question. Inversely the greater the coagulating power of an ion the less is its adsorption. Hence it follows that an ion of higher valency is less adsorbed than an ion of lower valency. We shall prove in the following pages that this view of the Schulze-Hardy law is both theoretically and experimentally sound. We shall first show that the interpretation of the Schulze-Hardy law hitherto held that the greater the valency of an ion the greater is its amount of adsorption is fallacious from the following arguments :(a) From experiments on coagulation of many sols with different electrolytes it is now established at least qualitatively that the greater the valency of the oppositely charged ion the greater is its coagulating power. This is really what is known as the Schulze-Hardy law, and this deduction is quite sound as it is based on experiments. (b) Several workers in this field notably Freundlich7, Bancroft8, Weiserg and others have tacitly assumed that the greater the adsorbability of an ion the greater would be its coagulating power. J. Bm. Chem. SOC.,23, 842 (1901). Kolloid-Z., 1 , 321 (1907). 3 J. Phys. Chem., 23, 2 0 5 (1919). Kolloid-Z., 35, 144 (1924). j J. Phys. Chem., 29, 942 (1925). 6 J. Phys. Chcm., 29, 955 (1925). i loc. cit. J. Phys. Chem., 19, 349 (191j). 9 !OC. cit.
630
N . R . DHAR AND S. GHOSH
We shall show that this assumption has very little experimental support even in the adsorption of ions of the same valency not to speak of ions of different valencies. Combining this assumption (b) which is not yet experimentally proved, with the deduction (a) which is experimentally sound, Freundlich and other workers in colloid chemistry have come to the conclusion that an ion of higher valency, which has a high coagulating power, is more adsorbed than an ion of low valency, which has small coagulating power. We are of the opinion that this deduction is both experimentally and theoretically unsatisfactory. We believe that the Schulze-Hardy law that the greater the valency of an ion the greater is its coagulating power is generally applicable for the following reasons:Since the coagulation of an equal amount of a colloid by ions of different valencies, is a t first an electrical phenomenon, it will be clear that for the charge neutralisation of a fixed amount of any colloid, the absolute amount of ions, expressed in gram-moles, necessary in the case of monovalent ions will be greater than that of hi- or trivalent ions, simply because the net charge on a hi- or trivalent ion is greater than that on a monovalent ion. It is apparent, therefore, that only for neutralization of charge on a sol, the greater the coagulating power of an ion the less would be its amount of adsorption. Hence we are of the opinion that in the coagulation of a sol the valency factor of an ion is more important than its adsorbability. Previous workers in this field have given an undue importance to the question gf adsorbabilitp, which is no doubt of a great importance but is certainly not as great as that of valency. In this connection the following lines from an article by Freundlichl would be of interest :“It follows necessarily that the coagulative power of a given salt depends greatly both on the valency and on the adsorption coefficient, because with different (i6nic) values, either of valency or adsorption, very different concentrations are necessary in the solution in order that the equivalent quantity of the (active) ion shall be adsorbed”. In order to prove our case we have to give a summary of the results obtained in adsorption experiments hitherto carried on.
Arsenious Sulphide From the researches of Linder and Picton2, mhitney and Ober, Freundlich3, and Weiser on the coagulation of arsenious sulphide sol by different electrolytes, we find that the actual amount of adsorption expressed in millimoles is greater in the case of univalent ions like K, neufuchsin and aniline than in the case of bivalent ions like Ba, Sr, Ca, UOz or the trivalent ion Ce, which shows the least adsorption. It will be interesting t o observe that the adsorption of ions per gram of AszSa is small and varies from 0.021 to 0.082 Burton: “Physical Properties of Colloidal Solutions”, J. Chem. SOC.,67, 63 (1895). 3 Z. physik. Chem., 73, 385 (1910).
182
(1921).
STUDIES IN ADSORPTION
63 I
millimoles. This is certainly because as we have already emphasised that h2S3 is not a good adsorbent and the uncharged substance cannot adsorb the precipitating ions to a marked extent as will be shown from the direct experiments of Freundlich on the adsorption of electrolytes by a powder of As2S3. Hence the amount of different ions adsorbed by As2S3 in the process of coagulation is more or less equivalent as has been proved by different workers. In this connection, it will be interesting to note that Weiser has made the following observations in one of his papers1 on the adsorption of electrolytes by arsenious sulphide sol. “The observations recorded in Table X I show conclusively that concentration of Li below the precipitation value have a marked effect on the adsorption of Ba”. Thus at the precipitating concentration of a mixture, containing I/S value of precipitating concentration of LiCl alone, the adsorption of Ba ion is lowered more than 2 5 per cent; whilst from a mixture containing one half the precipitation value of LiCl alone the adsorption of Ba is decrease3 j3 per cent. , . , The high precipitation value of LiCl cannot be due to a very low adsorption of Li ion, which would displace but little Ba ion at concentrations below the precipitation value; but is due to a fairly marked adsorption of Li ion associated with appreciable adsorption of the stabilizing chlorine ion within the concentration limits investigated”. Several years ago Freundlich2 carried on some experiments on the adsorption of different electrolytes by powdered hszS3 and he found that the adsorption of ions in millimoles per gram of As2S3 from such electrolytes as iYH4C1, (rVH,),SO,, Ce(N03)3>uo2(NO3)2, para-chloraniline hydrochloride, etc. is very small and varies from 0.013 in the case of SHdCl to 0.023 in the case of parachloraniline chloride for almost equal concentrations of the above electrolytes; whilst with morphine chloride, strychnine nitrate, neufuchsin, etc., the adsorption is much greater and varies from 0.068 to 0.050. Hence it is evident from the above results that the chemical affinity of As2S3 is more pronounced in the case of morphine hydrochloride, strychnine nitrate, neufuchsin, etc., than in the cations of other electrolytes. It must be emphasised now that the adsorption of different ions by powdered As2S3 is essentially due to its chemical affinity for the ions in question. As these experiments were conducted with powdered AsZS3,the question of charge neutralisation by the adsorption of ions cannot enter in these cases This important point seems to have been missed by several workers in this field because they have attempted to correlate the coagulating power of an ion and its adsorption from experiments carried on with the powdered substance and not the sol, and this is entirely unsatisfactory. On comparing the results on adsorption obtained with the sol and with the powder of A s ~ S we ~ , find that in general the adsorption of the same ion is greater with the sol than with the powder. The difference is mainly due to J. Phys. Chem., 28, 232 (1924).
* Z. physik.
Chem., 73, 385 (1910).
N . R. DHAR AND S. GHOSH
632
the amount of the ion necessary to neutralize the charge on the sol. From the foregoing results on the coagulation and adsorption by As2S3 so! as well as by powdered As2S3we find that in general an ion of a higher valency is less adsorbed than an ion of a lower valency when the amount of adsorption is expressed in millimoles. In other words, when the adsorption is expressed in milliequivalents, the amount of adsorption is practically equal as has been first observed by Whitney and Ober. There is nothing in the experiments with As2S3 sol to show that the greater the valency of an ion the greater is the amount of its adsorption. Antimony Sulphide In a previous paper1 we have shown that the adsorption of Ba ion by SbzSa sol is decreased to the extent of 55 percent by the presence of K ions below the precipitating concentration of KC1. We have also observed that the adsorption of K ion by SbzS3 is also decreased by the presence of Ba ion. We have also proved that the adsorption of K (0.2571 millimole) ion expressed in millimoles is about 8 times that of Ba ion (0.0334millimole) under identical conditions. A very interesting fact has also been noticed that the sum of the adsorptions by Sb2S3of K and Ba ions expressed in equivalents is greater than the adsorption of Ba ions by Sb2S3,when coagulated by BaCL alone, but less than the adsorption of K ion when coagulated by only KC1. These results conclusively prove that univalent ions are more adsorbed than bivalent ones and are in support of our view that less the coagulating power of an ion the greater the adsorption. Mercuric Sulphide Freundlich and Schucht2 have obtained the results given in Table I with a sol of mercuric sulphide. TABLEI Cation 4"
Ag Keufuchsin Brilliant Green Auramin Methylene blue Ba Cu(Cu(NO3)z) c u (c u s0 4) Ce
* Coagulating power
Concentration of the sol grams per litre.
Coagulating power. *
Adsorption in millimole per gram of Hgs.
13.74 11.74
0.098 3.57 10.31 20.83 IO.64
>0.05
I3 ' 74
8.38 IO.03
14.96 8.29 8.26 14.43 10.45
10.31
1
Ghosh and Dhar: J. Phys. Chem., 29, 435 (1925).
Z.physik. Chem., 8 5 , 641
(19131.
0.008 0.004 0.011 0.0077
I .96
0.022
6.67 3.85 12.19
0.015
is the inverse of precipitation value in millimoles.
2
0.02
0.011
0.004
STUDIES I S ADSORPTION
633
Hence the order of the coagulating powers for the different electrolytes is Brilliant green > Ce > auramine > neufuchsin, methylene blue > Cu ( Cu(n'Oa)e) > Cu (CuSO4) > Ag > Ba > NH4, beginning with the electrolyte of the highest coagulating power; whilst the order of the adsorption > Ba > Ag > Cu ( C u ( N 0 3 ) ~) > Cu(CuS04), auramine > value is 4" neufuchsin > methylene blue > Ce > Brilliant green, beginning with the ion adsorbed most. From these results it is clear that the trivalent Ce ion, which has a very high coagulating power: is least adsorbed. whilst the monovalent ions like Ag, XH4,etc., with low coagulating powers show greater adsorption. Hence these results of Freundlich and Schucht are in direct opposition to the views hitherto held that the greater the valency the greater the amount of adsorption, but are in support of our generalization that the greater the amount of adsorption the lower the valency. Though the results obtained by Freundlich and Schucht are not exactly comparable because they used sols of slightly different concentrations with different electrolytes, yet the general accuracy of our conclusion is beyond doubt as shown by these results. I n the foregoing pages it was emphasised that several workers have tacitly assumed that the greater the adsorbability of an ion the greater would be its coagulating power. The experimental results on HgS do not support this assumption even in the case of ions of the same valency not to speak of ions of varying valencies. Thus amongst the two simple univalent cations Ag and XH4 the coagulating power of Ag ion is about thirty-five times greater than that of N H 4ion, but the amount of adsorption is more than double with KH4 than with Ag. Now, when we compare the coagulating powers of the four cations from the organic dyes the following order is obtained :-Brilliant green > auramine > methylene blue > neufuchsin. Their adsorptions are in the following order auramine > neufuchsin > methylene blue > Brilliant green. I n the case of the divalent ions Ba and Cu, the coagulating power of Ba is less and the adsorption is greater than those of Cu obtained from either C U ( F O ~or ) ~CuS04. Hence it is clear that the assumption that the greater the adsorbability of an ion the greater is its coagulating power is not at all in accord with experimental resuIts. In this connection, it will be interesting to observe that the adsorption of the cations by a sol of HgS from the organic dyes is much less than that of other univalent cations. Their coagulating powers are much higher than those of other univalent ions; in other words, the univalent cations from dyes behave as polyvalent ions not only for the coagulation of HgS sol but also for a AsZS3 sol as has been shown by Freundlich. We are of the opinion that the explanation of this particular behavior is the following :It is well known that many of these dyes are of a colloidal nature though they conduct electricity. We also know that the charge on a colloid particle is much greater than that on an ion. Moreover, we know from the SchulzeHardy law that a polyvalent ion carrying several unit charges is a far better coagulant than an univalent ion; consequently the ionic micelles given out by an organic dye would behave as polyvalent ions, with a high coagulating power.
N. R. DHAR AND S. GHOSH
634
In a previous paper we have shown that the amount of adsorption by BaS04 is greater, the greater the size of the particles of the substance which is going to be adsorbed. Consequently the cations of the colloidal dyes are likely to be adsorbed in greater quantities than other simple univalent cations. Freundlich determined the coagulating powers of several cations from dye stuffs on a sol of arsenious sulphide. He also determined the adsorption of these cations by powdered As2S3 as has already been reported. He observed that from dilute solutions of alkaloids or dyestuffs a great deal of adsorption of the organic cations takes place even with powdered Ask33 in comparison with the adsorptions of the other univalent ions like KH4, Ag, etc. Moreover, it has been repeatedly observed that charge reversal can be readily effected by cations from alkaloids and dye stuffs. These results strongly support our view that amount of adsorption and charge reversal are essentially dependent on the chemical affinity of the adsorbent for the different cations. The order of the coagulating power of different cations towards a sol of As2SBis Ce > A1 > neufuchsin > crystal violet > quinine sulphate > morphine chloride > UOz > Ba > Mg > p-chloraniline chloride > aniline chloride > strychnine nitrate > H > K > Na > Li-beginning with the ion of the highest coagulating power. It will be seen that though Ce, Al, etc., are less adsorbed by powdered As2S3than any of the organic cations, the coagulating powers of trivalent ions Ce and A1 are greater than those of the organic cations. Moreover, the coagulating powers of the bivalent ions like Ba, Mg, UOz, etc., are greater than many organic cations like strychnine nitrate, aniline chloride, etc. Hence the contention that the greater the adsorbability the greater the coagulating power is not applicable to these cases. It should be clearly pointed out here that no strict comparison between the coagulating power of an ion and its adsorption could be made from the experiments of Freundlich on As2S3, because in the majority of the adsorption experiments he has used a powder and not a sol of AszSa. Ferric Hydroxide The adsorption and coagulating powers of different acids by a sol of Fe(0H) 3, have been determined by Weiser and Middletonl. Their results are given in Table 11. TABLE I1 Anion
Phosphate Citrate Tartrate Oxalate Sulphate Iodate Dichromate 1
J. Phys. Chem., 24, 30 (1920).
Coagulating power
Adsorption in millimoles per gram of FeaOs.
3.42
0.5721
5.99 4.21 3.81 4.12 1.67 10.00
0.5081 0.6232 0.4364 0.3804 0.7512
0.1559
63 5
STUDIES IN ADSORPTION
The above authors have concluded “It is evident that there is a tendency for the ions with the lowest precipitation values to be adsorbed the least and vice versa”. Consequently it is clear from the experimental results ofnWeiser and Middleton that our conclusion that the greater the coagulating power the less the adsorption is experimentally sound. Sen, in a thesis, has made a systematic study of the adsorption of different acids by freshly precipitated hydroxides of iron, aluminium, and chromium and he obtained the results given in Table I11 with freshly precipitated Fe (OH) 3. TABLE I11 Acid
Adsorption per gram adsorbent
Citric Oxalic Tartaric Sulphuric
0.8603 I ,0325
0.9585 0.8448
When the above acids are shaken with precipitated Fe(OHj3sols of positively or negatively charged Fe(OH)3 are obtainable. Therefore, in all the adsorption experiments Sen has used a normal solution of KC1 or SH4C1to coagulate the sols thus formed and in all cases the change in hydrogen ion concentration has only been estimated. Hence these results with precipitated ferric hydroxide are not comparable with those of Weiser and Middleton obtained with a sol of ferric hydroxide. But one fact comes out very prominently from these results that Fe(OH)3is a better adsorbent of acids and salts than AsZS3. Moreover, the large amount of adsorption of the acids is mainly due to the chemical affinity of ferric hydroxide for these acids. Though a large amount of adsorption of acids is observed with precipitated Fe(OH)3 it is useless to correlate this adsorption with the coagulating power of the electrolyte, because we are convinced that this high adsorption has nothing to do with the question of charge neutralization of the sol, but is essentially due to the chemical affinity of Fe(OH)3for the above acids.
Aluminium Hydroxide I n the adsorption of electrolytes by a sol of A1(OH)3 or by precipitated Al(OH)3, there is divergence amongst the results of different investigators. The following results are obtained by Gann’ for a sol of A1(OHj3, Table IV.
TABLE IV Anion
Salicylate Picrate Oxalate Ferricyanide Ferrocyanide Ko1loidchem.-Beihefte, 8, 63 (1916).
Coagulating power
Adsorption in millimoles per gram Ale03
0.125
0.30
0.25
0.
2.77
0 . I8
10.00
12.50
I8
0.09 0.073
N . R. DHAR BND S. GHOSH
636
It follows, therefore, from these results that the greater the coagulating power the less is the amount of adsorption and the greater the valency the less is the amount of adsorption. On the other hand, the results obtained by Weiser and Middleton’ are different from those of Gann. They are given in Table V.
TABLE V .Inion
Coagulating power
Ferrocynaide Thiosulphate Ferricyanide Sulphate Oxalate Chromate Dithionate Dichromate Phosphate
10.64 5.32 7.52 3.75 2.86 1.54 1.23 I . 13 2.89
Adsorption in millimoles per gram &03
0.3202 0.4096 0.4046 0.4184
0.5710 0.4352 0,3284 0.3185 0.8088
From the above results also it is evident that in the majority of cases the greater the coagulating power the less is the adsorption. It will be evident from the above results that Al(OH)3sol is a very good adsorbent for different ions, although Ishizaka has shown that ‘grown alumina’ can absorb ions to the extent only of 0 . 0 0 2 to .05 millimole per gram of the adsorbent. On the other hand Sen, has obtained the following results in the adsorption of acids by freshly precipitated Al(OH)3:Adsorption in millimo!es
Acid
Citric Oxalic Tartaric Sulphuric
0.783 0.952 0.933 0.896
In this case also the amount of adsorption due to chemical affinity is more pronounced than that due to charge neutralisation, because the precipitated L - ~ ~ ( Ocan H ) adsorb ~ large quantities of acids. From the above it will be seen that the results obtained by various investigators with a sol of A1(OH)3and different electrolytes are not concordant. In order to throw further light on the question of adsorption of different ions by Al(OH), experiments on adsorption of substances by a A1(OH)3sol are in progress in this laboratory. Chromium Hydroxide X o experiments are available on the adsorption of different electrolytes by a sol of CI-(OH)~.Sen has, however, determined the adsorption of different acids by precipitated Cr(0H)3 , and he obtained the following results :I
J. Phys. Chem., 24, 630 (1920).
63 7
STUDIES IN ADSORPTION
Acid
Amount adsorbed in millimoles
Citric Tartaric . Sulphuric Oxalic
2.196
3.170 3.919 4.106
These results show that acids are very highly adsorbed by Cr(OH)3and its adsorptive power is greater than that of either Fe(OH)3or A1(OH)3. In one of his recent papers Wejser has shown that as the concentration of K2C204is increased above the precipitation value the amount of adsorption of C z 0 4ion also increases. This only proves that freshly precipitated Cr(OH)3 has a great affinity for oxalate ions. Consequently from the experimental results of Sen and of Weiser we conclude that the adsorption of the oxalate ion by freshly precipitated Cr(OH)3 due to chemical affinity will be more prominent than that due only to charge neutralisation of a sol of Cr(OH)3. Hence it is absolutely useless to conclude that oxalate ion, because of its higher valency is more adsorbed than chloride ion as has been done by Weiser, as nobody as yet determined the adsorption of chloride ions by a sol and by freshly precipitated Cr(OH)3in order to show whether the adsorption of chloride ions by precipitated Cr(OH)3 due to chemical affinity is more pronounced than the adsorption necessary only for charge neutralization of the sol.
Manganese Dioxide In previous papers' we found that the adsorption of ions' by manganese dioxide in the course of its formation is in the following order:Ag > Na > Li > Cu > Cd > Ba > Ni, Zn > COZ > Th > Al. This shows that in general ions of lower valency are more adsorbed than ions of higher valency in the precipitation of manganese dioxide. We also measured the adsorption of cations by precipitated manganese dioxide and the order of adsorption is the following:Ag
> Cu > Cd > Zn, Mg > Ba > Sr, Ca > A1
Very recently we have again measured the adsorption of different cations by a sol of manganese dioxide as well as by precipitated manganese dioxide of a great purity. The sol was prepared by the interaction of K M n 0 4and HzOz and dialysed for a week and completely freed from alkali, and it contained 1.6 grams MnOz per litre. 2 0 C.C. of this sol were taken and I O C.C. of M / I O solutions of different electrolytes were added and the volume was always made up to 50 C.C. The following results are obtained :1
Ganguli and Dhar: J. Phys. Chem., 26,701, 836 (1922); Chatterji and Dhar: Kolloid-Z.,
33, 18 (1923).
N. R . DHAR AND S. GHOSH
638 Cations
K Ag Cu (CUSO4) Cu(CuCl2) Ba A1
Adsorption in millimole per gram of MnOl
5.514 5.85 3.90 5.095 I .98 0.03 (approx.)
The order of adsorption of cations by a pure sol of manganese dioxide is Ag > K > Cu > Ba > Al. From the foregoing results obtained with a pure sol of MnOa, we can conclude that the higher the valency of an ion the less is the amount of its adsorption. This result is in direct opposition to the view hitherto held that an ion of a higher valence would be more adsorbed than that of a low valence. From our recent experiments with freshly precipitated, well-washed and an air-dried sample of hydrated manganese dioxide the following results are obtained :-Cation
K Ag Cu(CuSO4) c u (CUC12) Ba A1
Amount adsorbed in mil!imo!e per gram of the adsorbent
1.159 I . 518 I . 131 I ,048 0.386 0.083
The order of aasorption of cations by pure hydrated manganese dioxide is Ag > K > Cu > Ba > Al. I n this case also we find ions of higher valency , are less adsorbed than ions of lower valency. These results as obtained now are in agreement with those already published. In this connection we have to emphasise the fact that the adsorption of ions is much greater by a definite weight of the adsorbent in the course of its formation than that obtained when the same weight of the absorbent prepared long ago is used. We have repeatedly observed that sols of Fe(0H) 3, Al(OH)3, Cr(OH)3, Mn02,etc., are much better adsorbent of ions than the precipitated substances. Moreover, the adsorbability of ions decreases with the ageing of the precipitates. When a sol is coagulated by an electrolyte, the neutralisation of the charge takes up a definite quantity of the oppositely charged ion, and if the neutral particles thus formed have a very slight chemical affinity for the coagulating ion we should always find that the adsorption for different ions expressed in equivalents should be practically equal just as we get in the case of As2S3 sol to a certain extent. When the adsorption is expressed in moles per gram of the adsorbent, as has been done in this paper we ought to find that the greater the valency of the ion the less is its adsorption. We have systematically expressed the amount of adsorption of ions in millimoles, and not in milliequivalents as has been done by other workers.
63 9
STUDIES IN ADSORPTION
It is easy to conceive that in the coagulation and adsorption by sols, ions take part as a whole and not in parts, and consequently the expression of the results of adsorption and coagulation in millimoles and not in milliequivalents seems more sound and the results of coagulation and adsorption when expressed in millimoles seem more readily conceivable. We have already emphasised that many authors notably Freundlich, Weiser and others have assumed that the greater the adsorbability of an ion the greater will be its coagulating power. This assumption seems apparently reasonable specially with ions of the same valency, because along with the electrical attraction of the oppositely charged ion towards the sol particles, the chemical affinity between the sol and the coagulating ion may help in the coagulation of the sol because we know that the greater the chemical affinity of an ion towards an adsorbent the greater is its adsorbability. But unfortunately the experimental results available in this field do not support this assumption and it seems likely that there is some unknown factor, which counteracts the effect of chemical affinity of an ion for a sol in facilitating its charge neutralization and coagulation. The assumption that greater the adsorbability the greater the coagulating power will be disproved from the following experimental results even in the case of ions of the same valency:Sol
Fe(OH)3 1,
A1(OH)3
>,
Anion
so,
ildsorption in millimole per gram of the adsorbent
Coagulating power
Czo4
0.3804 0.4364
4 . I2 3.81
Picrate Salicylate
0.18 0.30
0.125
0.25
Author Weiser and Middleton 7)
,l
Gann tl
It is well known that a solution of KzCz04 is more hydrolysed and contains more OH’ ions than a solution of K 8 0 4 of the same concentration. Consequently in presence of K2Cz04the positively charged Cr(OH)3 sol will be more unstable than in presence of the same concentration of KzS04. Hence if we take into account the hydrolysis and the hydrogen ion concentration in solutions of KzCz04 and K2SO4a solution of KzCz04should coagulate a sol of Cr(OH)3in a smaller quantity than KZSO4. Hence the suggestion of Weiser based on hydrolysis to explain the difference in the behavior of KzCz04 and KZSO4is of no avail. Ishizaka has determined the coagulating power of various anions towards a sol of A1(OH)3and he has also determined the adsorption of the same anions by “grown alumina”. He has observed that potassium salicylate is about nine times more effective coagulant than KC1, the coagulating power of salicylate ion is 187.7 whilst that of chloride ion is 21.9.
640
N. R . DHAR AND S . GHOSH
From the adsorption curves obtained by Ishizaka of various ions with ‘grown alumina’ we find that the adsorption of 0 . 0 2 millimole is reached a t concentrations 0.046M in the case of chloride and o . o o ~ Min the case of salicylate. Now, if the view that the greater the adsorbability of an ion the greater would be its coagulating power be correct, the coagulating power of the salicylate ion should be 46 times greater than that of the chloride ion, but in actual experiment we find that the ratio of the two coagulating powers is nine, instead of the calculated value 46. In the case of substances like Fe(OH)3, Cr(OH),, Al(OH)3, Mn02 etc., the chemical affinity of these substances for the coagulating ions is much more pronounced than in the cases of As2S3, Sb2S3etc. Consequently the correlation between the coagulating powers or the valencies of ions with their amount of adsorptionin the cases of sols like Fe(OH)3,hl(OH)3, @r(OH)3,MnOz, etc., is less satisfactory than in the cases of AsnS3, HgS and SbzSa sols because the precipitates of the latter class of substances have a small chemical affinity for their coagulating ions. If by any means we can find out the amounts of adsorption of an electrolyte by a definite weight of the sol and by its freshly obtained precipitate as soon as it is formed, we shall find that this difference of the two adsorption values expressed in moles would be greater in the case of univalent ions than polyvalent ions, because this difference arises from the adsorption due only to charge neutralisation of a sol and to this difference only the Schulze-Hardy law should be rigidly applicable.
Barium Sulphate From our experiments on the adsorption of different anions by BaS04 obtained by the interaction of equivalent amount, of Ba(0H)z and H2S04, we obtain the following order of adsorption:-Cr207” > Sz03” > Br03’ > As03’” > Cz04” > IO4’ > 10,’ > Mn04’ > NO%’> Fe(CN)G’”’ > Fe (CX)G”’ > C1’ > CNS’ > Br’ > I’ whilst Weiser and Sherrick working with different barium salts and KzS04obtained the following order :NO,’ > NO%’ > CI03’ > Fe(CN)E”” > Mn04’ > C1’ > Fe(CT\T)G”’ > Br’ > CN’ > CXS’ > 1’, the results are expressed in millimoles. It will be observed from the above results that in general monovalent and bivalent anions are more adsorbed than tri- or tetravalent anions. The adsorption series as obtained by us or by Weiser and Sherrick are not in complete agreement with our interpretation of the Schulze-Hardy law, because the positively charged BaS04 is capable of adsorbing positive ions as well, and the neutral molecule of Bas04 can also adsorb the negative ion, the positive ion or the neutral molecule of the electrolyte. Our experiments on the precipitation of BaS04in the presence of KCl and &c204, show that the adsorption of C&4” ion is greater than that of C1’ ion, though according to our view of the Schulze-Hardy law it should be otherwise. We have found that along with the adsorption of Cz04”ion by BaS04appreci-
64 I
STUDIES IN ADSORPTION
able quantities of K ions are also adsorbed. The positively charged BaSOe can adsorb K ions from KzCz04 and hence more of the oxalate ion would be necessary for the charge neutralisation and precipitation, whilst in the case of KC1 no K ion is adsorbed. Hence a small amount of C1 ion is enough for charge neutralisation. From the following experimental results it will be seen that Na ions from Xa3As03and K ions from KzCzO4 are adsorbed by BaS04:Electrolyte
Original concentration
Percentage of adsorption
0.08833 M 0.06894 k f
3.7 2.0
It is evident, therefore, that a colloid can actually adsorb an ion carrying the same charge as the sol and this adsorption stabilises the sol. Consequently if the amount of this adsorption is large, deviations from Schulze-Hardy law will take place. Freundlich has repeatedly emphasised that when there is no complication, the values of a and n in his exponential formula x = acn, should be the same for all electrolytes. In other words, according to Freundlich, the adsorption curve of ions with a certain sol should be the same irrespective of the valency of the coagulating ion. If we accept this view of Freundlich, which has been questioned by KO.Ostwald', it follows that the amount of adsorption of a trivalent ion and an univalent ion should be equal a t the same original concentration; consequently, it has not been proved at all that an ion of higher valency is adsorbed more than an ion of lower valency. Curiously enough the majority of experiments on sdsorption and coagulation carried on by different investigators are strongly in support of our view that the smaller the coagulating power of an ion the greater is its amount of adsorption. Thus the experiments of Freundlich and Schucht on the coagulation and adsorption by a sol of mercuric sulphide our experiments on SbZS3, the coagulation and adsorption experiments of Gann on A1(OH)3sol and on Fe(OH)3 sol by Weiser and Middleton, and our experiments on a sol of manganese dioxide conclusively prove that the greater the valency of an ion the less is its amount of adsorption and the smaller the coagulating power of an ion the greater is its amount of adsorption. Another interesting fact which is observed from a survey of the experimental results is this:Our experiments on the adsorption of cations by precipitated hydrated MnOz conclusively prove that even with the precipitate, an ion of higher valency is adsorbed much less than an ion of lower valency. When comparisons are made in the adsorption of ions like NH4, Ba, UOZ, Ce, etc.,by powdered Ask33 we also find that there is a general tendency of the ions of lower valency to be adsorbed more than ions of higher valency a t equal concentrations of the electrolytes. It is evident that in these cases the question of charge is unimportant, because the adsorbents are neutral subKolloid-Z., 26, 28, 69 (1920).
N. R. DHAR AND S. GHOSH
642
stances. It is very difficult to explain these facts in the present state of our knowledge on the mechanism of adsorption, but we are throwing out the following tentative suggestion :If we consider that the chemical affinity of K and Ba ions for hydrated manganese dioxide to be practically equal and if we assume that two K ions are adsorbed by an unit surface of hydrated manganese dioxide, the force of repulsion between these two ions is e2/r2,where e is the charge on an univalent ion r, the distance between the ions. Hence the force of repulsjon between two bivalent ions will be 4e2/r2on the assumption that two barium ions could be taken up by an unit surface of R!h02. Since the force of repulsion for a bivalent ion is 4 times greater than that for a monovalent ion, it is reasonable to expect that less of Ba ions would be adsorbed than K ions on an unit surface of the hydrated manganese dioxide under otherwise identical conditions. Many authors have deduced the adsorbability of different ions from their coagulating powers towards a certain sol. It is well known that in alkaline medium albumin is negatively charged and cations are effective in coagulating albumin, whilst in the presence of acids the albumin is positivelty charged and the anions are effective. Bancroftl has arranged cations and anions in series of gradually increasing or decreasing adsorptive power from the coagulating powers of the anions on a positively charged sol, and from those of the cations on a negatively charged sol. In view of the facts brought forward in this paper these adsorption series obtained from the coagulating powers seem untrustworthy.
Summary I. The generally accepted view that an ion, which is highly adsorbed also possesses a high coagulating power is not supported by experiments on adsorption. There is no justification for the assumption that the greater the 2. valency and hence the coagulating power of an ion the greater is its adsorption. 3 . On the other hand, experiments on the coagulation and adsorption of different ions by sols of HgS, SbzSa, Al(OH)3, Fe(OH)3,MnOz etc., strongly support our view that the greater the valency of an ion the less is its amount of adsorption and smaller the coagulating power of an ion the greater is its amount of adsorption. 4. It has been shown that the large amount of adsorption of ions associated with the coagulation of the sols like MnOz, Fe(OH)3,Al(OH)3, Cr(OH)3 etc., is mainly due t o a marked chemical affinity of these substances for different ions. 5 . It seems very likely that the valency of an ion is a more important factor in coagulation than its adsorbability. Chemical Laboratory, University of Allahabad, Allahabad, I n d i a September 25, 1926. 1
J. Phys. Chem., 19, 349 (1915).
