ADSORPTION AND SCHULZE’S LAW BY HARRY B. WEISER
From an investigation of the coagulation by electrolytes of negative arsenious sulfide and antimony trisulfide sols, Schulze’ concluded that the coagulating power of electrolytes is greater the higher the valence of the ion having a charge opposite to t h a t on the colloidal particles. This conclusion has been supported by the later work of Prost’, Linder and Picton3, Hardy4, Freundlichj, and others: and has come t o be known as Schulze’s Law. illthough this so-called la\y is little more than a qualitative rule6, it cannot be doubted but that with most sols there is a tendency for the precipitation concentration of electrolytes t o decrease with increasing valence of the precipitating ion. As a rule also, the precipitating poiyer of electrolytes with niultivalent precipitating ions is very much greater than with univalent precipitating ions. The addition of electrolytes t o a sol which owes its charge t o preferential adsorption of ions, causes coagulation when the charge on the particles is reduced below a critical value by adsorption of ions of the electrolyte having a charge opposite t o that on the sol. Since adqorption of the ions having the same charge as the sol. the stabilizing ions, cannot be disregarded, particularly with electrolytes that precipitate only in high concentrations. the precipitation value of an electrolyte for a sol is that concentration which results in sufficient adsorption of the precipitating ion t o neutralize the combined adsorption of the original stabilizing ion and the stabilizing ion added with the electrolyte’. I n so far as influences other than the adsorbability of the precipitating ion may be disregarded, one should expect the precipitating power of an electrolyte t o be greater, the greater the adsorbability of the Precipitating ion. Furthermore, one should expect an ion of high valence to be adsorbed more strongly than an ion of low valence t o the extent that the precipitating power of an electrolyte increases with increasing valence of the precipitating ion in accord with Schulze’s Law. This interpretation of Schulze’s Law has recently been called in question by Dhar and his collaborators*. That adsorption of precipitating ions takes place during the coagulation of sols by electrolytes was demonstrated by Linder and Pictong in the course of their classic investigations on arsenious sulfide sol, thirty yeais ago. SubJ. prakt. Chem., ( 2 ) 2 5 , 431 (1882); 27. 32 (18831. Bull. .hxtd. roy. Belg., 13) 14, 312 (1887). J. Chem. Poc., 67, 63 (1895). Z. physik. Chem., 33, 385 (1900). j Z. p h y i k . Chcm., 73, 3 8 j ( ~ g r o ) . Banrroft: “.ipplied Colloid Chemistry,“ 213 f 1921‘1, IVeiscr: J. Phys. Chem., 28, 2 3 2 (1923). ’Dhar, Srn and Ghosh: J. P h p . Chem., 28, 45: (1924 Z. physik. Chem. 73, 385 (1910).
sequent investigations by Khitney and Ober’ on adsorption of various cations during the precipitation of arsenious sulfide sol led them to conclude that equivalent amounts of all precipitating ions are carried down by a precipitated sol. This result was confirmed by Freundlich? who deduced therefrom that the most readily adsorbed ion will precipitate in the lowest concentration and vice-versa. I n order to account for the marked influence of the valence of inorganic cations on the precipitation cf a negative sol, Freundlich makes the further assumption that cations of different valence are equally adsorbed from equimolar solutions which would explain the well-known fact that the precipitation values (which correspond in the first instance to equivalent amounts) are very different. Judging from the frequency with which the observations of Whitney and Ober and of Freundlich are referred to in text books of colloid chemistry, their conclusions appear to be generally recognized as quantitatively accurate. However, a closer survey of the experimental results throw3 some doubt on the accuracy of the deductions: the variation from equivalence in the adsoiption values found by Khitney and Ober is inore than 2 0 percent in certain cases; similar observations by Freundlich and Schucht3 on adsorption by colloidal mercuric sulfide show much wider variations than this, all of which are attiibuted to experimental errors; some investigations made in my laboratory on adsorption of anions by hydrous oxide sols4, disclose variations from equivalence that cannot be charged to experimental errors. Recently Freundlichj has returned to this problem again; but instead of studying adsorption directly, he investigated the effect of salts with precipitating ions of variable valence on the cataphoretic migration velocity of colloidal particles of arsenious sulfide and hydrous ferric oxide sols. There was shown to be a close relationship between coagulation by electrolytes and the effect of the latter on electrokinetic processes. With univalent to quadrivalent cations derived from complex cobalt salts, the adsorption of equivalent amounts seems to have a similar coagulating effect on colloidal arsenious sulfide and the cations of different valence appear to be adsorbed equally from equimolar solutions6. However, with colloidal ferric oxide there appears to be a wide variation from equivalence in the adsorption from equimolar solutions of salts containing complex cyanide anions. Thus the precipitating power of A U ( C K ) ~appears ’ to be twelve times as great as Au(CN)Z’ and Fe(CS)G”’ twice as great as Cu(CK)4”’. Moreover the precipitating action of bivalent Pt(ClrT)4”is but three times as great as univalent Au(CN)2’. J. Am. Chein. Soc.. 23. 842 i19c11. Kolloid-Z., 1, 321 (1907). 32. phgpik. Chcm., 73, 3 8 j (1910). Weiser and Middleton: J. Phys. Chem., 24, 30, 630 (1920) Freundlich and Zeh: Z. phgeik. Chem., 114, 65 (1924). 6Cf. Xatsuno: J. Coil. Sci., Tokyo, 41, S o . 1 1 (1921).
957
ADSORPTIOS A K D SCHULZE'b LAW
Experimental I n the light of Freundlich and Zeh's recent observations on colloidal arsenious sulfide, it might appear t h a t the variation from equivalent adsorption in TThitney and Ober's early experiments was due entirely to experimental errors. Accordingly they have been repeated with the following results : Adsorption of cations by arsenious suljide sols. Arsenious sulfide sols of different concentration were prepared by allowing a solution of arsenious oxide to drop slowly into water through which was bubbled a stream of carefully washed hydrogen sulfide. The several sols were washed with hydrogen t o remove excess hydrogen sulfide and were kept in an atmosphere of hydrogen until used. IOO cc portions of sol were precipitated in a mixing apparatus with
TABLE I Adsorption by Arsenious Sulfide Sols RIetal
Concn. of Sol Weight of ppt. obtained \fetal adsorbed grams per I from 180 cc of solution llillieq. per g I
Barium Strontium Calcium
6.42 6.42 6.42
0.0765 0.0606
Barium Strontium Calcium
11.80 I I .80
II .80
Barium Strontium Calcium
2I
Barium Strontium Calcium
19.00 29.00 29.00
Barium Strontium Calcium Potassium
2 1 .jo
.so 21 .so
o 116
0.0188
0.0762 0.0605 0.0187
0.0767 0.0606 0.0191
0.0768 0.0608 0.0188
o 060 o 056
0.0676 0.0541 0.0163
0.0679 o.oj41 0.0162
0.0702
0.0552
o.ojo3 o.ojj4
0.01~2
0.01 70
10.00
IO.00
I0 . 0 0 I0 . 0 0
Anilin Neufuchsin
Observer
2
o 107 0 093
0
043
0 072
o 069 0
073
0
049
o 046 o 050
Feiser Weiser Reiser Weiser Weiser Weiser We iser Weiser Weiser Weiser Weiser Weiser
Khitney Whitney o IOO Whitney o 082 Whitney o
I IO
o 092
o 074 o 076
and and and and
Oberl Ober Ober Ober
Fruendlich? Freundlich
Barium
3 '33 (approx)
o 086 Linder and Picton3
UO, Cerium
4 . I4 4 . I4
o 088 Freundlich4 o 069 Freundlich
842 (1902). (1907). J. Chem. Soc., 67, 64 (1895). Z.physik. Chem., 73, 408 (1910).
J. Am. Chem. SOC.,23,
* Kolloid-Z.,
1,
322
958
HARRY B. TVEISER
40 cc of S 5 0 solutions of the chlorides of the respective metals, diluted t o IOO cc. After standing two hours, the mixtures were centrifuged and 180 cc of the supernatant liquid were analyzed. Particular precautions were taken in all the procedures t o make the observations as nearly accuiate as possible. Barium and strontium were precipitated and weighed as sulfate; and calcium was precipitated as oxalate and weighed as oxide. The experiments were all done in duplicate and the adsorption values were calculated from the average of the two results. The observations are recorded in Table I together with similar observations by other investigators. It will be noted that the variation from equivalence in the adsorption of calcium, strontium and barium ions for each sol used by the author is somewhat less than in the oftquoted observations of Whitney and Ober. While it may be that this is due to experimental errors inherent in the analytical procedures, it is more likely, in the light of observations t o be recorded later on, that there is a real difference in the amounts of the several ions carried down, as the results seem t o show. Thus barium seems t o be adsorbed a bit more strongly than strontium in every case. The observations with calcium are more erratic; but the determination of small amounts of this element are relatively less accurate because of the relatively low molecular weight of calcium oxide and the high factor for calcium in this compound. If, for the moment, the amounts of the several ion carried down by a given sol are admitted t o be equivalent, it would seem hardly necessary to point out that the adsorption value of a trivalent ion expressed 211 viols will be only onethird and that of a bivalent ion one-half the adsorption value of a univalent ion. But the recognition of this obriour fact furnishes the basis of Dhar’s novel interpretation of Schulze’s Law t o which we will return in a subsequent paragraph. Contrary to Freundlich’s belief1 the adsorption expressed in equivalents per gram of arsenious sulfide varies widely with different sols. This results from the difference in purity, stability, and size of particles that obtain with different preparations. -4s an illustration of the influence of the purity of sol on the adsorption of precipitating ions, experiments were carried out on two sols, each containing 2 7 . j g per liter. The results are given in Table 11. Sol KO. I , had the odor of hydrogen sulfide; while sol KO.2 was freed from excess hydrogen sulfide by thorough washing with hydrogen. Xs might be expected the adsorption of barium ion from bariuin chloride is less the smaller the excess of hydrogen sulfide in the sol.
TABLE I1 Sol so
S jo BaC12 to eKect
coagulation Total volume 200 rc
I
35
2
35
Iiolloid-Z., 1, 321 (1907).
BaSOa iemaining in 180 c c
Ua adsorbed lIillieq
I
2
per gram
o 0527 o 0568
o oj29 o 0569
o 069
o 056
959
ADSORPTIOS ASD SCHULZE‘S LATI-
Adsorption o j anions by hydrous oxade sols. Although the amounts of barium, strontium, and calcium adsorbed by a given arsenious sulfide sol are not far from equivalent, this is by no means true in other cases. I n Table I11 are recorded some earlier observations on adsorption of anions during the precipitation of colloidal alumina’. I n addition t o the anions listed, experiments with thiosulfate and phosphate were carried out but the values for these ions are omitted, the first because of contaniination of the precipitating solution by sulfate; and the second because of the improbability that the precipitating anion is PO,”’ rather than a mixture cf H,PO,’ and HPO?”. The adsorption values as recorded in the second column, are not eren approsi-
TABLE I11 .Inion
Ferrocyanide Ferricyanide Sulfate Oxalate Chromate Dit hionate Dichromate
.Idsorption 1Iillirquivalents per pram
Precipitation value potassium salts 1Iilliequivalents pc’r liter
I . 21‘4
0.375 0.900
0.997
0,538
I .142
0,700
0.870
I
0.657
1.62j
0.629
1
I .280
,300
775
mately equivalent. Since the variation noted cannot be due t o experimental errors, it was necessary t c inquire further into the mechanism of the precipitation process. IT hile IT hitney and Ober and Freundlich are doubtless right in concluding that adsorption cf equivalent anicunts of various ions will lower the charge below the critical value necessary for agglomeration, the actual amount of a given ion carried down is determined ( a ) by adsorption of ions by the electrically charged pal ticles during neutralization and ( b ) by adsorption cf salt by the electrically neutral particles during the piocess of agglonieration and settling. The amounts of (a) will be approximately equivalent but the amounts of (1;) will vary with the nature and concentration of the electrolyte?. The marked effect of salt adsorption on the amount of an ion carried down by a sol is shown in a striking fashion by a study of adsorption of oxalate icn during the precipitation of a hydrous chromic oxide sol. The sol was prepared by prolonged dialysis in the hot of a sol formed by adding ammonia t o a solution of chromium nitrate until the particles of precipitated oside just failed to redissolve. The concentration of the purified sol was adjusted t o two grams C r z 0 3per liter. Because of the instability of the preparation it was stored in a pyres vessel and pyres apparatus was used in all experiments. I O O cc portions of the sol were precipitated with the precipitaticn value of potassium oxalate and with several concentraticns above the pyecipitation Weispr and JIiddleton Lac cit \Yeisrr and Niddleton J P h > s Chrm , 24, 3”. 631 (1920)
5160
HARRY B. WEISER
value. After standing one hour, the mixture was centrifuged and a portion of the supernatant liquid withdrawn and titrated with standard permanganate. The results are given in Table IT’ and shown graphically in Figure I . The lowest concentration was the precipitation value, at which all the electeolyte was adsorbed. The increased adsorption above the precipitation value was all due to adsorption by electrically neutral particles and it is
FIG.I .%dqorption of Oxslate by Hydrous Chromic Oxidc
TABLE IV. hdsorption of Oxalate by Hydrous Chromic Oxide Volume of K / j O Iferricyanide >oxalate >sulfate > chromate > dithimate >dichromate. Considering the precipitation value of tke several plotasqiuni salts, we find the order of precipitating pori-er beginning at the greatest t o be: feirocyanide >ferricyanide >sulfate >oxalate > chroniate > dithionate > dichramate. The order of adsorption determined directly is the same as the order deduced f r m i precipitation data with the exception of oxalate and sulfate which are reversed. The cause of this exccption iq not known: Ixit in this connection, attention may be called to smie unpublished work of Everett E. Porter which disclosed that the older of precipitating FDwer of oxalate and sulfate for chromic oxide sol is determined by the hydrogen ion concentration of the precipitating solution. If the adsorption value is evpreqsed in equivalents, as seems logical, since neutralization is determined by the number of adsorbed charges, the results given in Table 111 ale in accord with the usual interpretatim of Schulze’s Law, that the ion of highest valence is most readily adsorbed. A l tthe same time the data furnish evidence of the qualitative nature of this so-called law since a number cf ions of the saiiie valence show a wide variation ir. adsorption. In a recent cominunicatioii by Dhar, Sen and Ghoshl t o which attention has already been called, the conclusion was reached that an ion which has a high precipitation value for a colloid is iiiost adsorbed by the colloid and viceversa. I n support of this conclusion are cited some observations of the author and of Freuridlich; hut more especially the results of Dhar and his collaborators on adsorption during the precipitation of manganese dioxide. “Thus,” it is pointed out. rage 464, “the monovalent ions silver, sodium, lithium are more adsorbed ihj- manganese dioxide) than any of the bivalent, trivalent or tetravalent ions. These facts show that the ions of higher valence which in general have greater coagulating powers are adsorbed the least.” Dhar’s observations were not made during the precipitation of a purified sol but on manganese dioxide formed by mixing potassium permanganate and manganese sulfate in the presence of various electrolytes. I n the solution from which the oxide separated there were the two reacting electrolytes; the salt whose adsorption was measured; together with the soluble products of J. Phya. Chem 2 8 . 4 j F 11924).
962
HARRY B. TTEISER
the reaction p9tassium acid sulfate and sulfuric acid. This niake, an aliiiost hopelessly complicated system ; and it seems unsafe t o chaw any conclusions whatsoever frcni the oliservations until n e know more ahout the effect of the foreign electrolytes on the rate of precipitation and phy.icd character of the precipitate and until soinething IS knom-n of the influence of the other salts in the system on the adsorption (jf the salt investigated. To cite but one example : aluminuin nitrate is atlmrhed about eight times as strongly as aluniiiiuiii sulfate whereas the *uliates cf cobalt, copper and cadmium are each adsorbed somewhat inore than their respective nitrates. hluniinuin is not “far less adsorbed” than strontium, nickel. cobalt. zinc, barium or cadmium ions if the values for the nitrates are compared. Dhar, Sen and Ghosh are in entire agrecnient with the a u thor‘s Tiew that adsorption takes place in two steps on adding an e!ectrolyte t o a qol. To quote their own words: “ I n the coagulation of a colloid there are t x o distinct steps in which adsorption occurs. The fiist step ii the electrical neutralisation of the charge on the colloidal paiticleb through adsorption of an ion carrying a charge opposite t o that on the sol and only here the Pchulze-Hardy Law is applicable. The adsorption, however, does riot stop there, hut the coagulated particles fuither act as an adsorbent, taking up x i additional amount of the electrolyte or ion. The amount of this second adwrption will depend on the adsorbability cf the electrolytes or ions and the riatuie of the coagulated mass concerned and hence the final amount of adsorption niay have any value depending on the above factors. The Schulze-Hardy Law cannot be rightly applied t o these cases. If the adsorption by the neutral particles is not appreciable, then the Schulze-Hardy Law is likely t o 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 he applicable.” Dhar. Sen and Ghosh merely say that the Schulze Law applies only t o the neutralization process which is accomplished hy adsorption of equivalent amounts of precipitating ions. Froin this it is deduced, that if the.e equivalent values are expressed as gram inols. the adsorption value will he less the greater the valence of the ion and vice versa. Granting the premiseq, one can find no quarrel with the obvious deduction. However, few people will agree t h a t Schulze’s Law means only that neutralization of sols js accomplished by adsorption of equivalent amounts of ions of different valence. TThile adsorption of equivalent aillourits of precipitating ions will effect neutralization in case the adsorption of the stabilizing ion niay he neglected, the important question is: why is the neceiiaiy adswption ohtained with zery lou. concentrations of ceitain electro1y:es and only with i e r y high concentrations of others? Other things being cqual one would expect neutralization by adsorption t o be accomplished with the lowest concentration of electrolyte containing the most stmngly adsorbed precipitating ion. I n so far as Schulze’s Law holds, the higher the valence of the ion the greater should be the adsorbability.
963
ADSORPTION ASD SCHULZE’S LAW
Investigations of the relationship between the precipitation value of electrolytes and the adsorption of the precipitating ions during the precipitation process have been carried out with multivalent ions chiefly. This is unfortunate, since with a series of such ions both the precipitation values and the adsorption values are likely t o be SO close together that it is hazardous t o draw conclusions, particularly when the differences may be cf the same order of magnitude as the errors inherent in the experimental procedure. It ~vould be much better to study the relationship between precipitating power and adsorbability with univalent precipitating ions : but this has seemed impracticable heretofore, since the precipitation concentrations are usually so high that the change in concentration resulting from adsorption is too small to measure directly with an>- degree of accuracy. I t i, possible however to determine the relative adsorbability of univalent ions during the precipitation of sols by an indirect method that consists essentially in determining the effect of the presence of univalent precipitatinp ions on the adsorption of an easily estimated multivalent ion. Some experiments illustrating this method will be given in the subsequent paragraphs.
Adsorption by arsenzoiis sziljide f r o m mzxtrtres of electrolytes. The precipitation values of barium chloride and of sevcral chlorides containing univalent cations were determined foi an arsenious sulfide sol containing 2 7 . 5 grams per liter. This was done by finding the smallest amount of the several solutions in I O cc that will just cause complete coagulation of I O cc of sol within two hours. The results expressed in milliequivalents per liter are given in Table T’.
TABLE TPrecipitation of Colloidal *drsenious Sulfide. ElertiolL t c
Barium chloride Lithium chloride Sodium chloride Potassium chloride Hydrochloric acid
I’Icclpltntlon vd1ir l i ~ l l ~ e ncl ~m ut s ~per ~ liter. 2
74
88 7 73 5 63 7 52
5
The adsorption of barium ion during precipitation of the same sol wa. next detei mined froin a solutions of harium chloride and f r x n several niixturcs ef barium chloride with a chloride containing a univalent cation. as shown in the first column of Table 1-1. The adsorption values were determined on I O O cc portions of sol in the qanie manner as described in an eerlier paragraph. From the ohservations it will be noted t h a t univalent ions cut down the adsorption of barium in the order: lithiuni < sodium < potassium < hydrogen. Since, under otherwise constant conditions, one should expect the adsorption of a given cation to be cut down by the presence of a second cation in proportion t o the adsorbability of the latter, it follows that the order of adsorbability of the univalent ions is: hydrogen >potassium >sodium >lithium. This is
964
HARRY B. WEISER
TABLET’I -Idsorption of Barium 1011from Mixtures. I3n S O wmair,ing ill
18.:
I
30 30 30 30 30 30
cc cc cc cc cc cc
X 50 BaCI2 S 50 BaCI2+3o K 50 BaClo+30 5j o BaC12+30 N 50 BaC12+30 K j o BaC12+30
cc cc cc cc cc
Barium adsorlird Grams llillieq. per pram .4s2RB 0.0109 0 . 0 5 8 0.010j 0 . 0 j 6
C? 7
0.0463 0.0466
0,0464
0.0609
0.0609
S 5 0 LiCl S 2 LiCl 0 . 0 5 j 2 0.0;74 O . O O 3 j 0.019 S 2 Sa(‘] 0.0590 0 , o j 9 2 0.002; 0.014 S z KC1 0 .0603 0.0602 0.0018 0.009 S
2
HC1
0.0013
0.OOj
exactly the same as the order deduced from the precipitation values of salts. Table 1-,assuming that the salt containing the most readily adsorbed cation precipitates in lowest concentration. Further, the results in Table T-I furni>h almost conclusive proof that the univalent ions are adsorbed more strongly than bivalent barium. For example the adsorption of barium is cut down but very little by the presence of an equivalent amount of lithium and 25 times the concentration of lithium cuts it down but two-thirds of the value in the abyence of lithium. Similar results were obtained from a study of the relative effect of chloride and sulfate on the adsorption of oxalate by hydrous chromic oxide. Adsorptzon by c h r o m i c oxide from ?iiixticres. Precipitation and adsorption experiments Jt-ere carried out on the hydrous chromic oxide sol used in earlier experiments. The results of the observations are given in Table 1-11which is self-explanatory. The slight effect of chloride a$ compared with sulfate on
TABLE \’I1 Adsorption of Oxalate Ion from Mixtures Clectrolg t e adder1 t o I C O cc sol ‘l’otnl voliimc 2 0 0 cc
50 cc X j o Oxalate
S
50 Oxalate j o cc S 50 Oxalate 50 cc K 50 Oxalate j o cc
O\nlntc. ndsoilicd l\IilllCC~ pel g
+ 5 0 cc S j o K C 1 + 50 cc S KCl + 5 0 cc r\‘ jo iX2SOa 2
I49 I42 1 I18 o 990
Precipitntinn vnlur4 AIillieq pw 1
1
K+?201
I
IIC1
o 6jo 8 75
&SO4
o 620
the adsorption of oxalate leaves no rooin t o doubt but that the univalent ion is adsorbed much less than the divalent one. I n the light of these observations, the conclusions of Dhar and his collaborators appear t o be both theoretically and experimentally unwund.
Summary Echulze’s Lam, that the precipitating power of an electrolyte is greater the higher the valence of the precipitating ion, is but little more than a qualitative rule. 2. I n so far as Schulze’s Law holds, the adsorbability of an ion is greater, the higher the valence. I.
ADSORPTIOS ASD SCHL-LZE’S LAW
965
3. The conclusion of Dhar and his collaborators that ions with the lowest precipitating power are adsorbed the most and vice-versa, i. both theoretically and experimentally unsound. 4. Xn indirect method has been described for determining the relative adsorbability of weakly adsorbed univalent ions. j. With strong electrolytes containing weakly adsorbed precipitating ions and the $ame stabilizing ion, there is a direct relationship between the relative adsorbability of the precipitating ions and the coagulating p3wer of the electrolytes in the sense that the electrolyte containing the iiiost readily adsorbed Precipitating ion, coagulates a sol in lowest concentration. 6. The amounts of various precipitating ions carried down on precipitating a sol are determined by ( a ) adsorption by the electrically charged particles during neutralization and (b) adsorption by the electrically neutral particles during the process of agglomeration. The amounts of (a) will be equivalent in case the adsorption of the stabilizing ions of the sereral electrolytes is constant or is negligibly small: hut the amounts of ib) will vary with the nature and concentration of the electrolyte. 7. From 6 , one should expect the adsorption values 3f various ions t o approach equivalence more nearly, the less the adsorption capacity of the precipitated particles. This probably accounts for the values being more nearly equivalent with arsenious sulfide so1 than with hydrous oxide sols having many times the adsorption capacity. D e i ariincrct of Cheinzsiry, Thc R7cc Instiircfc, IIou9toii. Texas.
