THE ADPORPTIOS O F CHROMATE I O S S BY COLLOIDAL ALUMISUM HYDROXIDE R T BE?; H . PETERSOS AND KEITH H. STORKS
A great deal of research has been done on the adsorption and coagulating properties of electrolytes on colloidal suspensions of metallic hydroxides.’ Many attempts have been made to formulate an equation which would express the relation between the amount of the coagulating ion adsorbed and the amount of the colloid of which the well known Freundlich isotherm is the most familiar.* The coagulation or precipitation values of different electrolytes on such colloidal systems have also been intensively studied in an attempt to find a relation between the valence of the coagulating ion, the mass of the colloid, and the effect of the associated ions. The actual mechanics of coagulation, that is, what actually happens to so reduce the stability of the system that the particles coalesce is rather difficult to picture. I t has been shown,3 that if the potential or charge on the colloidal particle is reduced below a certain value, the particles will adhere and coagulation take place. It hardly seems reasonable that this neutralization of the charge by adsorption of oppositely charged ions should be the same type of adsorption as that occurring ujter the coagulation of the system has taken place. If the mechanism of the adsorption which actually causes the coagulation is different from that which occurs on the surface of the coagulated particle, the adsorption isotherms as usually obtained may not have any significance in any theory attempting to explain the coagulating power of electrolytes. .1 search of t,he literature did not reveal any data on adsorption at concentrations below that required for coagulation, although in many cases the isotherms are extrapolated to zero concentration of tht. electrolyte. If the above hypothesis be true, it follows that thP adsorption isotherms obtained at concentrations less than and greater than that required for complete coagulation will show a break at that point where one type of adsorption ceases and the other begins. It was t o test this hypothesis that the following experiments were performed. Experimental Aluminium hydroxide was precipitated from a purified sample of the chloride with ammonia. This was washed by decantation with distilled water until dispersion began. The partially purified precipitate was then Weiser. J. Phys. Chem., 28, 232 (1924); Gann: Kolloidchem. Beihefte, 8, 125 (1916); Weiser and Middleton: J. Phys. Chem., 24, 639 (1920); Sen: J. Ppys. Chem., 31, 419, 525 (1927); Weiser: “The Hydrous Oxides” (1926); Ghosh and Dhar: Studies on Adsorption,” A series of researches puhlished in J. Phys. Chem. during 1927-1gzg. Freundlich: “Kapillarchemie” (1922); Swain and Urquhart: J. Phys. Chem., 31, 231-76 (1927).
Pouis: Z. physik. Chem., 89, 91, 186 (1915).
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BEN H. PETERSON AND KEITH H. STORKS
dialyzed electrically using a potential drop of 80 volts until a moderately sensitive galvanometer placed in series gave only a slight deflection. This treatment yielded a product very free from adsorbed ions of any sort. The purified hydroxide was then dispersed in conductivity water and peptized with a few drops of hydrochloric acid. The positive suspension so prepared remained evenly dispersed during the three weeks it was allowed to stand before any samples were taken. Enough of the suspension for a complete run was transfered to a separate flask and I j o cc. samples transferred to each of twelve glass stoppered bottles. An equal volume of conductivity water was placed in an equal number of similar flasks to serve as blanks, and the suspensoid and water flasks paired. To each sample and corresponding blank was then added 50 cc. of a solution of potassium chromate of varying concentrations, each pair, suspensoid and blank, containing the same amount of electrolyte. The concentrations of the potassium chromate was so arranged that coagulation took place at about Flask No. j. The entire set-up was allowed to stand three weeks to insure equilibrium. The residual chromate was determined by electrometric titration according to the method of Eppley and Vosburg‘ using a standard solution of ferrous sulphate. An aliquot part of the blank was titrated first and then an equal volume of the supernatant liquid in the cases where the colloid settled readily. Where COagulation was incomplete, the colloid was removed by ultra filtration using a special Berkfeld filter cone. The first few cc. of the filtrate so obtained was discarded in order to avoid any error on account of adsorption by the cone itself. Preliminary experiments showed no adsorption after the first few CC. had filtered through. The concentration of the chromate in the blank being known, the concentration of the residual solution from which the colloid had been removed could be calculated as: cc.FeSO1 for residual soln, cc. FeSO, for blank
x
Conc. blank = Conc. Res. Soln.
and from this value the amount of chromate adsorbed could be determined by difference. The mass of the colloid was determined gravimetrically from samples taken at the beginning of the run.
Experimental Results The results are recorded in the Table as, millequivalents chromate left unadsorbed, “C”, millequivalents adsorbed, “X”, in columns I and 11. I n column I11 are given the values of X/M, and in column IV the values for log. “C”. I n column V are given the values of log. X/M. The star marks the sample containing the lowest concentration of chromate that coagulated completely. M, is the mass of the colloid. ‘Eppley and Vosburg: J. Am. Chem. SOC., 44,2148 (1922).
651
ADSORPTION OF CHROMATE IONS RY COLLOIDAL ALUMINA
TABLE NO. I 2
3 4
5# 6 7
8 9 IO I1
I2
X1203per sample, “M”, equals o ,09988gms. “C” “X” X/M log. “C” 0.23538 0.16407 1.64267 9.37157-10 0.31472 0.16522 1.65419 9,49729-10 0,39783 0.16210 1.62295 9.59970-10 0.64844 0.15146 1.51642 9.81187-10 1.50661 9.86297-10 0.72941 0.15048 1.71656 9.89676-10 0,78843 O.IjI45 1,84581 9.93223-10 0.85551 0.18436 0.93005 0.18981 1.90038 9.96851-10 I .oog16 0.19469 1.94924 0 . 0 0 2 2 5 1.18422 0.21561 2 , I j869 0.07343 I ,46059 0.23920 2.39847 0.16453 2.a5663 o.23411 1.71443 0.28j32
log. X/M
0.21556 0 . 2I859 1032 0.18081
0 . 2
0 .I7800
0.23467 o ,26618 0.27885 0.28986 0,33419 0,27929 0.45585
These results are plotted as Xt’Magainst C in Fig. I and as log. X/M against log. C X I O in Fig. 2.
x
IO
2.€
P.e
1.8
I.. 1
FIG.I Adsorption of Chromate Ions by Aluminum Hydroxide
Discussion of the Results The curves shown in Figs. I and 2 show three distinct parts and have been labeled “a”, “b” and “c”. ‘‘a ” represents the data obtained a t concentrations below the coagulation value of the added electrolyte, terminating a t point “0” which was the last sample to be coagulated. The section marked “b” was unexpected. The supernatant liquid was clear to the Tyndall beam, showing complete coagulation. Section “c” marks the isotherm as usually obtained. Two additional sets of data on suspensions similarly prepared,
BEN H. PETERSOS AND KEITH H. STORKS
652
respectively three months and three years old showed the bame type curve, except the secticn “b” was: more extended ‘These data are not recorded a5 the resultb ‘tre similar uggestd in the introduction ,ivms amply borne out by experiments ‘it least on colloid suspensions which are charged If the mechanics of orption Mere the wnic in the uncoagulated and coagulated regions the curveb would be continuous, i e , there Rould be no sudden break Thc break shown, so eutirme as to even reverie the slope, indicates that the type of adsnrprion k n thc coagulated syatem is entirely different frorn the type of adsorption n f wncentrations below the coagulation values of the added electroll te I n explmation for the section “b” is suggested I t is well known that the coagulation of a charged suspensoid takes place before the charge is completely neutialized “ b ’ may represent a combination of the neutralization adsorption repreirntcd by “a” and the surface adsorption as represented by “e”.
I
!
I
7I
7
9
LOG fG.
‘0)
FIG 2 LoKrrrithrn of the Adsorption Isotherms
Summary The adsorption of potassium chromate by colloidal aluminum hydroxide hasibeen determined at concentrations above and below that required for complete coagulation. 2. The results show three separate and distinct types of adsorption for which the explanation is offered : “a” represents a neutralization adsorption, that is, the neutralization of the positive charge on the suspensoid particle. “c” represents surface adsorption, not electrical in type and “b” represents a combination of these two types, “a” and “c”. 3. The extrapolation of adsorption data to zero concentration of the adsorbed substance is not always justified. I.
Department of Chewristru Coe College Cedar Rapids, I o u n .
