NEUTRALIZATION O F ADSORBED IONS BY WILDER D. BANCROFT
Any substance may be brought into a state of colloidal solution provided we make the particles of that phase so small that the Brownian movements will keep the particles suspended, and provided we prevent agglomeration of the particles by a suitable surface film.' Coalescence may be prevented by a non-electrical film, by an electrical film (electrical charge), or by anj- combination of the two. An electrical suspension is due to the preferential adsorptiom of some ion. So long as the particles are all charged positively or all negatively they will repel each other and will not coalesce. S e u tralization of the charge causes precipitation through agglomeration. If we have a suspension which is stabilized by the preferential adsorption of hydrogen ion from hydrochloric acid solution, we have in the solution free hydrogen ions, free chlorine ions, and the adsorbed hydrogen ions which make the suspension behave like a cation though with a different migration velocity from that of hydrogen. If the suspension adsorbs an anion in an amount equivalent to the hydrogen ion adsorbed, the suspended particles will be electrically neutral. TVe get this by adding an electrolyte, preferably with a readilJ- adsorbed anion. Since we are dealing with selective adsorption, the concentration in the solution of the added anion necessary to cause an adsorption equivalent t o the hydrogen adsorption will vary with each anion. Since we are dealing with preferential adsorption, the nature of the cation must have an effect. To put the matter more generally, the amount of an electrolyte necessary to precipitate any given suspension will vary with the nature of the cation, the anion, and the disperse phase. This is not the usual way of stating the case. It is usually considered that only cations count in the case of Bancroft: Jour. Phys. Chem., 18,j j Z (1914).
364
Tl’ilder D . Baiicrojt
negatively charged sols, that only anions count in the case of positively charged sols, that the univalent ions all cause coagulation a t approximately the same concentration, the bivalent ions at a lower concentration which is practically the same for all bivalent ions, and the trivalent ions at a still lower c0ncentration.l The matter has been stated pretty clearly by Hober.? Some general rules apply, among them the law first formulated by Hardy3 that the precipitation of positive colloids depends chiefly on the anion and the precipitation of negative colloids chiefly on the cation; and also the law formulated a long time ago by Schulze.‘ that the precipitating power of the active ions is a function of its valence or of the number of electrical charges, which it carries. Of course there are other factors besides these two rules and these factors will now be discussed more carefully; the especial effect of organic ions has already been mentioned.” Everybody recognizes that hydrogen and hydroxyl ions are not to be classed with the other univalent ions because they are usually adsorbed much more strongly,j and everybody recognizes that there are other exceptions ; but I have found no clear recognition of the fact that Schulze’s law is merely a first approximation. In case of doubt it is generally safe to assume that an ion of higher valence will be adsorbed more than one of lower valence; but it is a mistake to consider this so-called law anything more than a guide. Since we are dealing with selective adsorption we shall expect to find that some univalent ions will be adsorbed by some substances more than some bivalent or trivalent ions. This is shown clearly in data by 0di.n on colloidal sulphur, given in Table I. I n the second column are given the liminal concentrations in gram atoms of the cations per liter necessary to coagulate Ii
1 Cf. Freundlich: Kapillarchemie, 350, 354 (1909) ; Zsigmondy: Kolloidchemie, j 4 (1912); Hatschek: -In Introduction to the Physics and Chemistry of Colloids, 33 (1913). 2 Physikalische Chemie der Zelle und Gewebe, 283 (1914). 3 Zeit. phys. Chem., 33, 3 8 j (1900). 4 Jour. prakt. Chem., ( 2 ) 25, 431 ( 1 S S 2 ) ; 27, 320 (1884). 5 Freundlich : Kapillarchemie, 354 (1909).
Setttralizatioiz
0.i
,Adsorbed Ions
365
the sulphur; in the third column are given the reciprocals of these \-slues, the atomic precipitating power so-called.
TABLE I -
Coagulation of Sulphur a t I 8 O - z o 0
-
~
Salt
~~
Liminal \ d u e gram-atoms Cations per liter
Atomic precipitating power of cation ~
HC1 LiCl XHIC1 ( X H $)?SO4 xH.&03 Sac1 h-a2S04 NaSO3 KC1 K2SO4 KS03 RbCl CSCl
hIgSO4 hIg(x03)~ CaClz Ca (1 03) 2 Sr(SO& BaC12 Ba(XO,)? ZnS04 Cd (Koa)2 -%IC13
cuso4 A h (N03)2 s1(S0 3 ) 2 U02(SO3)?
o 16
6 0 0
o o 0
o o
913 435 600 506 153 176 163
0 021 0 025 0 022
o 016 0
009
0 0093 0 0080
o 0041 0 0040 0 0025 0 0021
0 0022
o 0756 0 0493
I 1
2 3 1 7 2 0
6 1 5 7 6 1 47 5 39.7 45.5 63 I08 107 5 I25
245 247 385 475 46 I I3
0
0044
20 227
0
0098
I02
o 0096 0 0416 o 0137
2
3
10.5 22
4
73
Under the conditions of O d h ’ s experiments, sulphur is a negative colloid and the precipitation is therefore due to an adsorption of cations. The first thing to be noticed is that hydrogen ion is not adsorbed strongly by sulphur, the precipitating power of hydrochloric acid being much less than that of lithium, ammonium, sodium, potassium, rubidium, or caesium chloride. Instead of these univalent cations precipitating a t the same concentration, the required concen-
366
W i l d e r D . Bancroft
tration of lithium chloride is in round numbers one hundred times that of caesium chloride. The liminal values for barium and strontium are nearly equal, but calcium chloride requires a distinctly higher concentration. If we take the different bivalent ions the values range from 0.0756 for zinc t o o 0 0 2 2 for barium, a ratio of over thirty to one. The univalent ion caesium has a greater precipitating power than the bivalent zinc, cadmium, nickel, and uranyl; and about the same precipitating power as the bivalent copper, manganese, and magnesium. The trivalent ion aluminum has about the same precipitating power as the bivalent calcium and distinctly less precipitating power than bivalent strontium and barium. The specific nature of the adsorption comes out extraordinarily clearly with sulphur, practically the only orthodox thing being that nitrate, chloride, and sulphate behave practically alike, though even here Od6n considers that sulphate has a slight protecting action. This specific nature appears more clearly perhaps if we arrange the cations in order, the one with the greatest precipitating power coming first: Ba, S r > C a , Al>,lIg, Cs, Mn, C u > U O z > R b > K > K i , Cd, Zn > S a > XH4> Li > H. Sulphur is admittedly an extreme case, but Freundlich’ gives data for colloidal platinum from which I deduce the order: -41, P b > B a , UO:!>Ag>K, S a . Bivalent lead has practically the same precipitating power as trivalent aluminum. Univalent silver is nearer to bivalent uranyl and barium than to univalent potassium and sodium. If more cations had been studied we should very likely have got more distinct evidence t o specific action. As it is, it takes 130 millimols KaOH per liter to coagulate the platinum and only 2 5 millimols SaC1. The change from chloride to hydroxide has a more marked effect than the change from sodium to barium. It seems very probable that barium hydroxide would have no greater precipitating power than sodium chloride. From Pappada’s experiments with colloidal silver2 I deduce the folKapillarchemie, 3 5 2 (1909) Pappadd Gazz chim ita1 , 42 I, 2 6 3 (1912).
Setttralizatiolz o j ddsorbed Ioizs
367
lowing order of adsorption : A1 > Ba, Sr, Ca > H > Cs > Rb > K > K a > Li. From these data Pappadh concludes that the migration velocity is the determining factor with the univalent cations; but this cannot be true. The difference between aluminum and hydrogen is not very great, one drop of IO HC1 producing a coagulation and one drop M 2 0 A1C13. In tenth-normal solutions potassium iodide, nitrate, and sulphate produce no coagulation. The reason given by the author is that these anions react with the colloidal silver. In normal solutions the iodides, nitrates, and sulphates are said t o precipitate a t the same concentrations as the corresponding chlorides and bromides. The effect of concentration is a little obscure in other respects since 5 or 6 drops of normal KC1 precipitate 2 cc o 0 6 Ag, ~ whereas ~ it takes only 30 drops I O KCl to produce precipitation. The essential thing from my point of view is that the different univalent cations have different liminal values ; the difference between hydrogen and lithium is greater than that between hydrogen and aluminum. From experiments on mastic1 we get the data given in Table 11. TABLE I1 Coagulation of Mastic Liminal value, gram atoms
Atomic precipitating power of cation
I .0
I
0.123
8
0 . 00I 2 , j
800
0.010
IO0
0.02 Hg, > H > Ba, Ca > Zn > Ag > Ka. Only three anions are given in the table, so it is impossible to tell what effect the anions have. A good many experiments have been made on mastic with different acids but the degree of electrolytic dissociation varies so as t o make these results inconclusil-e. II-ith Prussian blue PappadA? found the order of the cations to be: Fe, Al, C r > Ba, Cd > Sr, Ca > H > Cs > Rb > K > Ka > Li. Sulphates, nitrates, chlorides, bromides, and iodides all behaved alike. Practically the same order of cations was obtained for copper ferrocyanide.’ In the cases studied by PappadA the specific adsorption appears to play a very small part. The data for arsenic sulphide, however, give us variety enough. The order of cations is Ce, In, benzidine, Bl>new fuchsine, crystal violet > quinine > morphine, U 0 2 Sr, Ca > Be, Zn, Ba > Alg > p-chloraniline, toiuidine > aniline > strychnine > guanidine > H > E;> S a > Li. The organic cations come in where they please and play hal-oc with any rule as to valency. The chlorides and nitrates gi\-e practically the same \-slues and the sulphates are not far out of line, though it seems probable that the restraining power of sulphate is rather greater than that of chloride or nitrate. The liminal 1-alues in gram atoms of the cation per liter are o 0056, o 0066, o 0086, 0 , I I O and > o 240 for potassium nitrate, sulfate, formate, acetate, and citrate, from which one can deduce that the order of adsorption of anion is: citrate>acetate> formate > sulphate > nitrate, chloride. It is a great pity Ogg: Zeit. phys. Chem., 27, Z S j (iS98J. Zeit. Kolloidchemie. 6, S3 (1910). PappadB: Ibid., 9, 136 (1911). Freundlich: Kapillarchemie, 351 (1909j ; Freundlich and Schucht: Zeit. phys. Chem., 80, j64 (1912).
Seutralizatio?z o j ,-ldsorbed I O H S
369
that Freundlich did not try other combinations, such as barium acetate for instance. From the experiments on hydrous ferric oxide,l the order of adsorption of the precipitating anions appears to be Cr207> SO4> OH > salicylate > benzoate > formate > C1> X03> Br > I, while the order for the cations is: H > B a > M g > T I , S a , K. The univalent ions do not all behave alike and neither do the bivalent ones; but the upholders of Schulze’s law can comfort themselves with the fact that the two sets do not overlap except in the case of hydrogen. There is no such comfort in the case of albumin. I have shown? that the probable order of adsorption of anions, so far as known, is. sulphocyanate. iodide > chlorate > nitrate > chloride > acetate > phosphate > sulphate > tartrate, the sulphocyanate ion being adsorbed the most and the tartrate ion the least. Here there is nothing even to suggest Schulze’s law and the firm belief which most people have in Schulze’s Ian- is probably one reason for the marked failure to account satisfactorily for the phenomena with aluminum. IT-ith the cations albumin appears t o be fairlp orthodox for the order of adsorption appears to be Th, UO?> Cu, Zn > Ca > N g > Li > K , S a > SH,, though even here the lithium stands higher in the series than it has been found with other substances. n‘hile there is unquestionably a tendencv for ions of a higher valence t o 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 rule is that the adsorption is specific both as regards the adsorbing substance and the ion adsorbed. Since the important thing in the neutralization of an adsorbed ion is the adsorption of an ion of the opposite charge, we may get neutralization when we have a colloid with the opposite charge. In other words, we may neutralize an adsorbed ion with another adsorbed ion instead of by a free ion. ~
l Freundlich Kapillarchemie, 3 j z , 358 (1909 , Zsigmondq chernie, 181 I 1912), Pappad& Zeit Kolloidchemie, 9, 233 (191 I ) Bancroft Jour Phys Chem , 19, 352 (191j)
Kolloid-
W i l d e r D . BancroJt
37 0
The usual statement is that sols having the same charge do not affect each other perceptibly, while sols having opposite charges precipitate each other. Neither of these statements is as accurate as i t should be. I shall take up first the case of sols having opposite charges. Positive and negative colloids will precipitate each other when in proper proportions and provided adsorption takes place.2 I see no theoretical reason why we should not have a positively charged and a negatively charged sol, neither of which adsorbed the other to any appreciable extent. I n that case these two sols would not precipitate each other. Since complete neutralization takes place only when one sol has adsorbed the amount of the sol carrying an equivalent amount of the ion having the opposite charge, it follows that the amount of one sol necessary to precipitate a given amount of another sol will vary with the degree of adsorption; it will therefore be a specific property and not an additive one. This can be tested experimentally on data by Biltz given in Table I I L 3 I .1mg
TABLE I11 gold completely precipitated by ~~
-
-
~
-
--
_
1 Freundlich : Kapillarchemic, +++ (1909) ; Zsigmondy : Kolloidchemie, 56 (1912); Hober: Physikalischc Chemie der Zelle und Genebe, 291 (191+). 2 Bancroft: Jour. Phys. Chem., 18, 5 j j (1914). 3 Freundlich : Kapillarchemie, 41j ( I 909).
Xeutralization o j Adsorbed Ions
371
Alumina is more effective than chromic oxide in precipitating antimony sulphide and much less effective in precipitating arsenic sulphide. The alumina must therefore be adsorbed more by antimony sulphide than chromic oxide while the reverse must be true for arsenic sulphide. Cerium oxide is less effective than ferric oxide and thorium oxide in precipitating gold, but is more effective than either of these in precipitating the sulphides of antimony and arsenic. The phenomenon is thus specific, varying with the nature of the two colloids. This seems not to have been realized before. I n fact Freundlich’ says definitely that L‘oneseems to find approximately the same order regardless of what sol is to be precipitated.” This statement is true, but it misses the important thing in the experiments which was that the order was not always the same. ITe can now take up the case of sols having the same charge. The statement that neither has any perceptible effect on the other is based solely on the fact that no precipitation occurs. lye know, however, that cases of adsorption are not limited to colloids or electrolytes having opposite signs. Charcoal adsorbs both bases and acids. Silver bromide adsorbs silver ions or bromine ions as the case may be. There is therefore no theoretical reason why precipitated hydrous ferric oxide might not adsorb chromic oxide and vice versa. If the precipitated substance will do this there is no reason why the peptonized substance should not. Nagel? has recently shown that this does occur and that it accounts for the behavior of mixtures of chromic and ferric salts with excess of alkali. Hydrous chromic oxide is peptonized by caustic potash while hydrous ferric oxide is not. If the chromium salt is present in large amount relatively to the iron salt, the ferric oxide will adsorb the peptonized chromic oxide and be peptonized by it, going apparently into solution. If the ferric salt is present in excess, it will adsorb the peptonized chromic oxide carrying it out of the liquid phase. ~
~~~
Kapillarchemie, 445 (1909). Jour Phys Chem., 19,331 (1915).
11-ilder D . Baiicrojt
372
It is to be noticed that the chromic oxide, when in excess, acts as a so-called protecting colloid to the iron oxide. Everybody is familiar with the fact that gelatine is adsorbed by colloidal gold, for instance; but that is usually treated under the heading of protective colloids rather than under the heading of mutual action of two colloids. The case of chromic and ferric oxides is merely another illustration of the fact that the distinction between a suspension colloid and an emulsion colloid is now arbitrary and unsatisfactory. Coming back for a moment to the behavior of two oppositely charged colloids, there is a special hypothetical case which is perhaps worth mentioning. Suppose we have two sets of finely divided particles neither of which adsorbs the other appreciably, and let us also suppose that one set of particles adsorbs a given cation very strongly, while the other set of particles adsorbs a given anion very strongly. If we take a mixture of these two sets of particles and add a small amount of the salt of the given base and the given anion, we shall have a colloidal solution which will conduct electricity very well but which will contain no free ions to speak of because, by definition, the cations have been practically completely adsorbed by one set of particles and the anions by the other set of particles. This particular case has not been realized, but an intermediate one seems to have been found by AlcBain and Martin2 in sodium palmitate solutions. " Most authors since Kahlenberg and Schreiner3 have, as a matter of course, ascribed the conductivity exhibited by soap solutions largely to free alkali hydroxide. I n previous papers from this laboratory the same tentative suggestion was made, but it was each time clearly stated that it was only a working hypothesis until these experimental data should be ascertained. Kow it is certain that the conductivity of soap solutions is, only to a very minor extent, due to hydroxyl ions. Further, on account of the fact that the rise of boiling ~~
1 2
Bancroft: Jour. Phys. Chem., 18, 556 (1914). Jour. Chem. SOC.,105,965 (1914). Zeit. phys. Chem., 27, 5 5 2 (1898).
point in certain soap solutions is practically all required to account for the sodium ions alone,I the conductivity cannot be wholly ascribed to simple palmitate i,ons. The suggestion we made is that we ha\-e here a new type of aggregate or micelle, the mobility of which, owing to the reasons given in the paper cited. is comparable with that of a true anion. Of course, further investigations are proceeding in this laboratory in order to bring this to the test of direct experiment. Incidentally. the above shows, further, that undissociated soap is present chiefly or entirely in colloidal form.” -1s I see the matter the sodium palmitate is hydrolyzed and the hydroxyl ions are adsorbed to a great extent by the undissociated palmitate and possibly by the insoluble palmitate acid also, though this seems less probable The adsorbing substance thus becomes the anion. owing to the adsorbed hydroxyl. Because of electrometric measurements, McBain? considers that there is practically no hydrolysis. Electrometric measurements only show the concentration of hydroxyl ions in solution. I do not believe for a moment that an adsorbed hydrogen ion or hydroxyl ion behax-es electrometrically like a free hydrogen or hydrosq-1 ion. An adsorbed chlorine ion. for instance. would not gi\-e a test with silver nitrate Under these circumstances the electrometric measurements are satisfactory for showing the concentration of hj-droxyl ions in the solution, but they are utterly worthless for showing the degree of hydrolysis of sodium palmitate. For the same reason I am very sceptical as t o any conclusion in regard to albumin solutions which is based on electrometric measurements. The adsorption of an ion by a colloid gives us an electrically charged colloid with a migration velocity of its own. This migration velocity, so far as studied, is of the general AIcBain: Trans. Faraday SOC., 9, 9 9 : Zeit. Kolloidchemie, (1913).
McBain and AIartin: Jour. Chem. SOC.,105,95; 11914). Bancroit: Jour. Phys. Chem., 19,349 (1915).
12,
256
Wilder
374
D.Bancrojt
order of magnitude of free ions.‘ Consequently, the presence of a colloid may increase or decrease the conductivity of a solution. Raffo and Rossi? found that colloidal sulphur cuts down the conductivity of sulphuric acid and sodium sulphate solutions very much. Paternb and Cingola3 also found a marked decrease when tannin was added to a potassium chloride solution. In many cases, however, there was no apparent effect. Of course, if the colloid adsorbs both ions or the undissociated salt, the conductivity will necessarily decrease4and we do not know whether the marked change with tannin, potassium chloride and water is due to a change of migration velocity or to a wholesale removal of potassium chloride from the solution. Patten once described what always seemed to me a very interesting experiment. He placed a coarse powder in a dilute solution of an electrolyte, allowed the powder to settle, and measured the conductivity of the supernatant liquid. He then stirred the solution and measured the conductivity again while the powder was suspended between the electrodes, finding an increase in conductivity. The powder adsorbed one ion and of course carried the other down with it when it settled. So far as I know, this experiment has never been published and I cannot give numerical data. My impression is that the paper was presented at the Toronto meeting of the American Chemical Society. The general results of this paper are: I . The neutralization of an adsorbed ion is due to specific adsorption. The concentration of a given electrolyte necessary to neutralize the charge on a given colloid will therefore depend on the nature of the cation, the anion, and the colloid. 2 . It is inaccurate to say that the cation is negligible in the precipitation of a positive colloid and the anion in the 1 2
4
(19 10).
Zsigmondy : Kolloidchemie, 46 (19 I 2 ) . Gazz. chim. ital., 42 11, 326 (1912). Ibid., 44, I , 36 (1914). Wolfgang Ostwald : “Gedenkboek aangeboden aan Van Bemmelen,” 269
.l-eutralizatiovi o j Adsorbed Ioizs
375
precipitation of a negative colloid, though this statement is often approximately true. 3. Univalent ions are not all adsorbed alike; nor are bivalent ions or trivalent ions. The order of adsorption is specific with each colloid and is not determined exclusivelyand perhaps not a t all-by the migration velocity or solution pressure of the ion. 4. Since the adsorption is specific, Schulze’s law is only an approximation. Certain univalent ions are adsorbed by certain colloids more than certain bivalent or trivalent ions. In many cases there is, however, a marked tendency to increased adsorption with increasing valence. 5 . Mixtures of two sols will not precipitate each other unless adsorption takes place. 6. Since adsorption is specific, the order of precipitation of a negative sol by a number of positive sols will not necessarily be the same for any two negative sols. 7 . It is not accurate to say that two sols having the same sign have no effect one upon the other. Adsorption may, and often does, take place. 8. Hydrous chromic oxide, which is peptonized by caustic potash, may act as a protecting colloid for hydrous ferric oxide, which is not peptonized by caustic potash. 9. Since an adsorbed ion does not necessarily give the reactions of a free ion, electrometric measurements may, and do, lead to false conclusions when applied to colloidal solutions. IO. It is probable that sodium palmitate solutions are hydrolyzed to a very much greater extent than appears from electrometric measurements. I I . If two colloidal sols did not adsorb each other appreciably and if one adsorbed the cation of a given electrolyte very markedly, while the other adsorbed the anion of the same electrolyte very markedly, the addition of a small amount of the electrolyte to a mixture of the two colloids would produce a solution which would conduct electricity without there being any appreciable amount of substance in true solution.
1%-ilder D . Bancrojt
376
1 2 . In sodium palmitate solutions, it seems probable that the hydroxyl ions set free by hydrolysis are adsorbed practically completely by the colloidal soap. 13. Addition of a colloid to a solution will increase or decrease the conductivity if the adsorbed ion has a greater or lesser migration velocity than the ion adsorbed. In so far as both ions are adsorbed the conductivity will decrease. 14.When adsorption by a colloid causes hydrolysis, the conductivity will depend also on the nature of the products of hydrolysis.
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