INFLGENCE OF ADSORPTION ON T H E COLOUR O F SOLS B E D OF PRECIPITATES BY N . R. DHAR
There is still considerable uncertainty concerning the colour of sols, The theoretical work on the optics of sols deals almost entirely with the light which is scattered from a given sample of a sol. In order to test the conclusions arrived at, spectrophotometric observations on the absorption of light by the same sample of the sol, have usually been undertaken. Steubingl has however, shown that a simple relation between the scattered and absorbed light need not necessarily exist. Colloid chemists have naturally laid special emphasis on the relation of colour to the size of the particles of a sol. Wo, Ostwald2 comes to the following conclusions :--“With increasing degree of dispersion (Le. decreasing size of particles) the absorption band of any colloidal solution moves to the shorter wave lengths” He also states that the absorption of a highly dispersed sol approximates to the absorption of the corresponding molecular solution. Regarding the first conclusion the following observations of Bancrofts will be of interest. “Colloidal gold solutions can be prepared which are red, violet, or blue by transmitted light. In general the blue gold solution contain coarser particles than the red ones, but this is not always true.” More or less similar conclusion is arrived at by Svedberg4and he observes that, as regards light absorption, there is no real difference between colloidal solutions made up of observable discrete particles and the corresponding molecular solutions. I n this paper I shall report certain results which will show that the colour of sols and of freshly coagulated substances depends a great deal on the nature of the material adsorbed by the sols or the coagulated mass. We shall first take the case of manganese dioxide sole5 When a sol of M n 0 2 is prepared by the interaction of KMnOa and H202 it is negatively charged and has a deep brown, almost black, colour. When such a negatively charged sol is treated with a few drops of FeCL, chargereversal takes place and the sol becomes positively charged. This positive sol can be dialysed for about a week and no trace of ion is available in the wash water. Now the colour of the positively charged sol is distinctly reddish in comparison with with the colour of the negatively charged sol. When the positive sol is coagulated by some electrolyte the supernatant clear li‘Ann. Physik, (4) 26, 329 (1008). Kolloidchem. Beihefte, 2, 409 (1910-11); “Licht and Farbe der Kolloide” (1924). 3 “Applied Colloid Chemistry”, p. 203 (1921). 4 Svedberg: “Die Existenz der Molecule” (1912). 5 Ganpuly and Dhar: J. Phys. Chem: 26, 701, 836 (1922); Sarkar and Dhar: Z. anorg. Chem. 121, 135 (1922); Chatterji and Dhar: Kolloid-2. 33, 18 (1923). 2
INFLUENCE O F ADSORPTION ON COLOUR
I395
quid is free from ferric iron whilst the coapulated mass when dissolved in HCl gives a slight indication of ferric ion. It appears probable, therefore, that the positively charped sol of MnOz is really a mixture of a large quantity of hydrated M n 0 2 and a very small quantity of Fe(OH)3and that is why the colour is distinctly more red than the negatively charged sol. When hydrated MnOz is precipitated by the interaction of KMn04 and MnSOe in presence of various electrolytes, we have observed different colours of the precipitated MnOz. We have proved that MnOz in the course of its formation adsorbs appreciable quantities of positive ions. When MnOz is precipita tedin presence aither ofApN03,Bi(N03)8,HgC12,Ni(?\r03) 2,or Pb(N03) the precipitate has a deep black colour; when precipitated in presence of Ba(N03)2, Mg(N03)2, etc., it is black, whilst when it is precipitated in presence of FeC13, Fe2(S04)3,etc., the colour of the hydroxide is distinctly reddish. When precipitated in presence of AuC13, CaC12, SrCl2, A1(X03)3, ammonium metavanadate, uranium nitrate, or platinic chloride the colour is reddish brown. In presence of cadmium sulphate the colour is brownish yellow, in presence of the thallous salts it is brownish grey, whilst in presence of stannous chloride it is distinctly grey, and with copper sulphate it is brownish black. When no electrolyte is added the colour of the precipitate is reddish brown. Hence it is quite clear that the colour of the manganese dioxide obtained as a precipitate depends a great deal on the nature of the ions adsorbed by it when in the course of its coagulation. Negatively charged stannic hydroxide sol can be prepared very readily and the sol is optically clear, whilst it is difficult to prepare a positively charged stannic hydroxide sol. M7hen a few drops of ferric chloride are added to stannic chloride and the mixture is allowed to dialyse for a week, an unstable positively charged sol of stannic hydroxide is obtained. This positively charged sol has a distinctly yellowish colour possibly due to the adsorption of ferric ions. The sol cannot be freed from its yellow colour by further dialysis. We1 have prepared negatively charged Fe(0H) sol either by shaking freshly precipitated Fe(OH)3 with arsenious acid or by the interaction of FeC13 and KOH in presence of free arsenious acid. The latter method is more satisfactory. I n either of these two methods the sol can never be freed from the arsenious icid and the question a t once arises whether the sol of Fe(OH)3 also contains ferric arsenate. The colour of the negatively charged dilute sol is yellowish brown, whilst in a concentrzted condition it is dark brown. Consequently Lhere is not much difference in the colour of the positively charged and negatively charged Fe (OH)3sols. The negatively charged sol can be prepared by the interaction of FeC13and KOH in presence of such substances as glycerine, sugars etc. By Lhe gradual addition of KOH to a misture of FeC13 and glycerine or sugars there are three definite stages throuph which the colloid passes-first it becomes positively charged and Sen and Dhar: Kolloid-2.33, 193 (1923); J. Phys. Chem. 27,376 (1923); Sen, Ganguly and Dhar: J. Phys. Chem. 28, 313 (1924).
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N. R. DHAR
then coagulation occurs and on charge-neutralisation it finally passes into a negatively charged colloid. We1 have prepared negatively charged chromium hydroxide sol by the interaction of chromium nitrate and caustic alkali in presence of arsenious acid; but there is not much difference in the colour of the positive or negative chromium hydroxide sol and it was not expected. I n presence of protective substances like glycerol, cane sugar, grape sugar, etc., the hydroxides of copper, cobalt, and nickel can be obtained either as positively charged or as negatively charged colloid. When a dilute solution of n’i(N03)2is added to glycerol and carefully mixed and a few drops of dilute NaOH is added a green positively charged Ni(OH)2 sol is obtained. No change of colour takes place on the addition of an excess of alkali. It appears, therefore, that the colour of the positively and negatively charged colloid of Ni(0H)Z is the same and is practically identical with the colour of Ni(N03)2.We have shown that in ammoniacal nickel salt solution part of the nickel exists as a negatively charged Ni(OH)z sol. On the other hand marked colour changes are observed with cobalt nitrate. When a little cobalt nitrate is mixed with an excess of glycerol and a few drops of KOH is added, the colour remains practically the same as that of cobalt nitrate but when large excess of alkali is added a deep blue sol is obtained. In presence of cane sugar and a large excess of alkali a violet coloured negatively charged cobalt hydroxide sol is obtained. Grimaux2 long ago suggested that in ammonical copper salt solution a part of the copper is complex and a part is suspended as copper hydroxide. When a copper salt is mixed with an excess of glycerol and alkali is added drop by drop, at first we observe practically no change of colour but when an excess of alkali is added a deep blue colour is obtained. This observation can also be repeated with several sugars. It appears, therefore, that the colour of the positively charged cupric hydroxide which is first formed when there is an excess of cupric ions is different from that of the negatively charged colloid of copper hydroxide, which comes into existence when the alkali is in excess. Masson and Steele8have shown that when caustic alkali is added to water containing cupric tartrate in suspension and the whole thing is vigorously shaken a complex tartrate is formed according to the following chemical change :jNaOH qCuC4Hdh = hTazC4H406 Na3Cl2H7Cu4OI8 gH20. They have observed that the solution is neutral, Kahlenberg4 however maintained that the blue salt is produced by the interaction in the proportion CuC4H406: KOH and represented by the formula K2Cu2C8013H8.
+
Sen and Dhar: Kolloid-Z. 34, 262 (1924). *Compt. rend. 89, 1434 (1884). J. Chem. SOC., 75, 725 (1899). 42.physik. Chem. 17, 590 (1895).
+
+
INFLUENCE O F ADSORPTION ON COLOUR
I397
It appears, therefore, that there is considerable difference of opinion among different workers with regard to the constitution of Fehling-.’ssolution. We can assume that Fehling’s solution consists mainly of negatively charged C U ( O H ) ~stabilised by the adsorption of tartrate and hydroxyl ions. In this connection the following lines of Masson and Steele’s paper would be interesting. “Sulphuretted hydrogen solution decomposes all the cuprotartrates precipitating cupric sulphide or a mixture of this with the sul’phide of the other metal. The ease with which this occurs is a little surprising as truly negative metallic radicals do not generally break up in this way.” It is well known that when caustic alkalis are added to the copper salt solutjon a deep blue negatively charged sol of Cu(0H)Z is obtained which can be readily retained when filtered through an ordinary filter paper. With regard to these substances we made the following statement in a previous paperl:--“It is very difficult to explain the production of the same colour in all these cases on the view of complex formation as the same colour is developed by so many different reagents. We have to assume that the posiin ammonical copper solutions, the negative ion Cutive ion C U ( X H ~‘‘) ~ (OH)4’’ in alkaline copper hydroxide solutions, the negative ion C12H7Cu40 1 3 ”’ in tartrate solutions. The complex copper salt solutions containing alkali and glycerol or sugars, having such different compositions, all have the same blue colour. The real explanation of the existence of the same blue colour in all these different substances seems to lie in the fact that we are probably considering the same substance in all cases, namely negatively charged colloidal C U ( O H ) ~due to the adsorption of hydroxyl ions, which are present in all cases and this negatively charged colloidal Cu(O€I)2 has the blue colour.” . Consequently the conclusion of Wo. Ostwald and Svedberg, which states that as regards light absorption there is no real difference b e h e e n colloidal sohi tions and the corresponding molecular solutions, is not applicable to these cases. In this connection the following remarks of Bancroft2would be of interest. “If we measure a copper electrode in a solution of alkaline copper tartrate we find that the concentration of copper ion is extremely low. That is all the measurement tells us. If the copper is in true solution, it must be present as a complex salt, as is unquestionably the case with potassium silver cyanide. If we have peptised copper oxide or hydroxide present, there is no need to assume the existence of a complex copper salt at all. It seems practically certain that the effect of sugars in preventing the precipitation of the heavy metal hydroxides by the alkalies is due to the formation of colloidal solutions.” When Fe(OH)3 is precipitated in presence of KzCrzOi,KMn04, NaZSzO3, etc., the colour of t8hehydroxide is different from the colour of the freshly precipitated Fe(0H) obtained in absence of the above electrolytes.
2
Sen and Dhar: J. Phys. Chem. 27, 376 (1923). “Applied Colloid Chemistry” 2 1 2 (1921).
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N. R. DHAR
Schenck’ has shown that in presence of A1(OH)3,CU(OH)~ does not turn black readily. We have observed2 that all those salts, which produced hydroxides soluble in caustic alkali namely salts of Zn, Al, Sn, Pb, etc., retard markedly the change of b2th blue copper hydroxide to the black form as well as that of the blue variety of cobaltous hydroxide to the pink form. I t seems probable that the presence of Al(OH)S, Sn(OH)4,Pb(OH)2 etc., in the colloidal state tends to peptise the cobalt hydroxide or cupric hydroxide and hence dehydration of these hydroxides become difficult and that is why retardation in colour change takes place. When iodine is adsorbed by starch, saponarin, basic lanthanun acetate, cholalic acid, etc., blue substances are obtained. Barger and Miss Fielda who have done same good work on these compounds have classified these blue products in the following way according to their degree of dispersion. “I. The first group contains highly cdloidal substances, such as starch, basic lanthanum acetate, the product of the action of 70% sulphuric acid on cellulose, thallonic acid, isolichinin, and various other ill-defined plant substances. I n this group the blue compound is never obtained other than amorphous. 11. The members of this group are crystalline semi-colloids with molecular weight of something like 500; the blue compound is generally obtainable both in amorphous and crystalline conditions, Examples are : cholalic acid and the glucoside saponarin. 111. I n the third group the blue iodine compounds are only known in the crystalline state, as in the case of the alkaloid narceine. Another example of this class is the product of the action of iodine or ergoxantheine. Possibly the second and the third of these groups are not sharply differentiated.” Just as the colour of a gold sol depends upon the size of the particles of the sol, Harrison4 has directed attention to the similarity of the colour of the adsorption complexes of iodine with starch and dextrines, where similar variations of colour are produced by varying the size of the particles of carbohydrate. Berczeller5 shows that similar variations in the colour of the adsorption compounds of lanthanum hydroxide can be produced by varying the size of the particles of the hydroxide. The above author has also drawn attention to the colour variation in certain copper compound, in certain reactions of bile pigments and some furfural aldehyde reactions. In a previous paper6 I have shown that on the addition of an alcoholic solution of iodine to starch the conductivity of the mixture is much greater than the sum of the conductivity of the individual substances. The substance obtained by the adsorption of iodine by starch is appreciably conducting and J. Phys. Chem. 23, 284 (1919). Chatterji and Dhar: Trans. Faraday. SOC.Discussion Oct. 3 J. Chem. SOC.101, 1394 (1912). 4Kolloid-Z. 10, 45 (1912). 6Biochem. Z.84, 160 (1917). 6 Dhar: J. Phys. Chem. 28, 1 2 5 (1924). 1
2
25
(1920)
INFLUENCE O F ADSORPTION ON COLOUR
I399
behaves like an unstable iodide. It seems probable that micellar ions are given out. It will be noted that the same blue colour is obtained when iodine is adsorbed by such different substances as starch, dextrine, cholalic acid, basic lanthanum acetate, etc. It seems likely, therefore, that this blue colour is due to the existence of iodine'as the dispersed phase in all these adsorption compounds, where starch, dextrine, etc., serve as the dispersing media. It would be interesting to determine what colour iodine would have in the colloidal state. When small quantities of AgN03 are added to dilute K2Cr04containing gelatin the colour of the silver chromate obtained is yellow, just like that of K2Cr04. When an excess of AgN03 is added, the yellow sol of AgzCr04becomes red possibly by the adsorption silver ions. The following results were obtained in a previous paper.* No. Concentration of the Gelatin = 5% I N/1250 Yellow N/63
4
N/62j N/416 N/312.6
5
N/250
6
N/208 N/2oo
2
3
((
(L (I
N/108.2 N/I95.6 N/291.6
L( N/500 Just red 7 Red We have also observed that yellow AgzCr04sol carries a negative charge whilst the red sol is positively charged. Consequently Ag2Cr04peptised by gelatin exists as a yellow coloured negatively-charged sol by the adsorption of chromate ions and as a positively charged red sol by the adsorption of silver ions. Hence the behaviour of Ag2Cr04peptised by gelatin is similar to those of the halide of silver, though no appreciable difference in colour is noticeable with negatively and positively charged sol of the same silver halide. Summary and Conclusion I. In this paper several cases have been reported, where the colour of the positively charged sol is different from that of the negatively charged sol of the same substance or from that of the electrolyte from which the sol is prepared. 2. The colour of freshly precipitated MnOz, Fe(OH)3 etc., depends a great deal on the nature of the substance adsorbed by the precipitates in the course of their formation. 3. It seems probable that the blue colour of adsorption compounds of iodine with starch, dextrine, cholalic acid, basic lanthanum acetate, etc., is due to the existence of iodine in the colloidal condition in such compounds.
Chenzzcal Laboratory, Unzverszty of Allahabad, Allahabad, A p r i l SO, 19%5.
Sen and Dhar: Kolloid-Z. 34, 270 (1924).
