THE E F F E C T O F HYDROGEN 10s CONCENTRATIOS ON T H E T’ISCOSITY O F HYDROSOLS O F ALUMINUM, CHROMIC, AND F E R R I C OXIDES* BY J O H S H. YOE A S D EGBERT B. FREYER
Introduction One of the most significant properties of colloidal solutions is the viscosity. The importance of this property was recognized by Graham almost threequarters of a century ago when he referred t o the viscometer as a “colloidoscope.” I n I913 a general discussion’ on colloids and their viscosity took place at a meeting of the Faraday Society and papers were presented by WO Oetwald, Henri, Pauli, Freundlich and Ishizaka, and Hatschek. Here, Ost~ a l dagain pointed out “the importance of viscosity for the study of the colloidal state” and Freundlich presented results “on the rate of coagulation of Al(OH)3 sols as measured by the viscosity change.” Freundlich had been interested in comparing the precipitation of A%1(OH)3 sol by various anions with the adsorption of these ions by A203 [hydrous aluminum oxide] and found that the precipitation of the sol by electrolytes could best be studied by the change in viscosity and that likewise the rate of coagulation could thus be determined. Several years previous, Kawamura? had made use of the change in viscosity in his study of the precipitation of sols by electrolytes. Freuncllich prepared his Al(0H) sols by Crum’s method, by gradual hy-drolysi~of aluminum acetate and expulsion of the acetic acid by continued heating. The colloidal solutions thus obtained were perfectly colorless, almost like pure water, and were only slightly turbid. Their viscosity was only slightly greater than that of pure water. The effiux time of the sol in an Ostwald viscometer was 5 2 seconds as compared with 49 seconds for water. The hydrosol was very stable and Freundlich says ‘‘we did not notice any change in the properties after several months, except that the precipitation vslue had decreased by a few per cent.” After adding to the hydrosol a sufficient amount of an electrolyte t o effect complete precipitation jt was put in an Ostwald viscometer and the rate of effius determined from time to time. It was observed that these rates increased in a regular manner and finally attained a maximum value which corresponded t o complete precipitation. In the above viscosity measurements various electrolytes (KK03, KCKS, SH4C1, KSSOI, K3Fe(Ch’)G, K-salicylate) were employed as precipitating reagents. Freundlich observed that “the maximum viscosity of an &Il(OH) 3 sol which is completely precipitated by the addition of an electrolyte, increases proportionately to the amount of colloid particles in the sol,” a result
* Contribution S o . 29 from the Cohb Chemical Laboratory of the University of Virginin, Trans Faraday Soc., 9, 34-107 (1913). J Coll. Sei. Imp. Univ. Tokyo, 25, S o . 8 (1908). A n n , 89, I j 6 (1854).
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J O H S H. T O E AND EGBERT R . FREYER
in agreement with previous experiments on suspension and emulsions; and, furtheimore, that “the maximum viscosity is to a certain degree dependent upon the nature of the precipitating electrolyte,” the results pointing t o a lyotropic series “in so far as the viscositj, is smaller with the anions XOa, CXS than with sulphate, succinate, etc.” In these measurements no attention was paid t o the hydrogen ion concentration of the sols. Recently much work has been done on the effect of the hydrogen ion concentration on various properties of colloids. Loeb’ studied the effect of the hydrogen ion concentration on colloidal protein substances. Tartar and Gaily2 have made a study of the effect of hydrogen ion concentration on the precipitation of mastic and gamboge sols hy acids and by salt solutions and found that “while different amounts of acids are required, each sol is precipitated at a quite definite hydrogen ion concentration regardless of the concentration of the colloid.” “The neqative ion of the acid is without effect.” ‘(Various potassium and aminonium salts precipitate the sols at the same concentration provided the hydrogen ion concentration is kept approximately constant, The precipitating values of the salts vary directly as the concentration of the colloid at the same hydrogen ion concentration. The stabilizing or peptizing effect of the ion bearing a charge similar to that of the colloidal particle has been shown t o be very limited if it exists at all.” On the other hand, Michaelis and Hirabayashi3 studying the precipitating power of salts when added to mastic sols at various hydrogen ion concentration find that the anion has sowe e f e c t , the activity depending upon the hydrogen ion concentration of the sol. I t had previously been observed by Linder and P i ~ t o n and , ~ TYhitney and Oberj that in the precipitation of a negative suspensoid, such as arsenious sulfide, by neutral salts, some of the positive ions are carried down by the precipitate, and that there is a simultaneous increase in the acidity of the supernatant liquid. Blum6 found from observations made with a hydrogen electrode and with suitable indicators that “the precipitation of aluminum hydroxide by ammonium hydroxide is complete when [H+]= 10-6.5 to 10-7.5, points approximately defined by the color change of methyl red and of rosolic acid,” and that “considerable aluminum hydroxide remains unprecipitated when the solution is just alkaline to p-nitrophenol [color change at [H+] = IO-^ approx.], while a smaller amount, but still appreciable, is redissolved when the solution is just alkaline to phenolphthalein” [color change a t [H+] = IO-^ approx.]. Quite recently Bradfield7 has studied the relation of hydrogen ion concentration to the flocculation of a colloidal clay. He found that hydrochloric, J Gen. Physiol., 1, 2, 3 (1918-21); “Proteins and the Theory of Colloidal Behavior,” 2nd ed (1925). J Am. Chem SOC.,44, 2212 (1922). Kolloid -Z., 30,209 (1922). J Chem. S o c , 67,63 (1895) j J. Am. Chem SOC 23, 842 (1901). 6 J. Am Chem SOC.. 38, 1282 (1916), also in a little more detailed form as Sci Paper KO. 286, Bureau of Standards (1916’ jr J. Am. Chem. SOC.,45, 1243 (1923)
VISCOSITY O F OXIDE HYDROSOLS
I391
sulfuric, phosphoric, and acetic acids produced flocculation at the same hydrogen ion concentration but that greater acidity was required with citric acid.’ In the case of potassium chloride and potassium hydroxide mixtures the amount of electrolyte required to produce flocculation was ten times greater at a p H of 8 than at a p H of 6.2. Further increases in alkalinity had no effect. “Dipotassium phosphate mixtures showed a similar variation in flocculating power with changes in hydrogen ion concentration.” The phosphate curves differ, however, from the chloride curves in three respects: ( I ) the break occurs at a much higher hydrogen ion concentration, ( 2 ) the electrolyte requirement is about one-third higher, and (3) the curve is not linear after it reaches the maximum but shows a secondary minimum at pH 9. j. Bradfield concludes these “results indicate that the flocculating power of potassium salts is influenced by the nature of the anion even when compared at the same Sorensen value. Secondary reactions seem to be responsible.” Although much work hzs been done on the effect of h) drogen ion concentration on the viscosj ty of proteins sols and sols of other organic substances, little or no work seems to have been done with inorganic sols. It is true much work has been done on the effect of electrolytes on inorganic sols, but with few exceptions the effect of the hydrogen ion was not taken into consideration. The work recorded in this paper was designed to study the effect of the hydrogen ion concentration on the viscosity of hydrosols of inorganic substances. Hydrosols of aluminum. chromic, 2nd ferric oxides were employed.
Experimental A p p a r a t u s . A11 glass ware used in this work was Pyrex, except the hydrogen electrode vessel and tqhe viscometer. Calibrated volumetric apparatus was employed whenever the accuracy of the rneasurement justified it. The viscometer was an Ostwald capillary type. This was kept in a water-bath maintained a t 2 j oi o . o j o . Effiux times were measured with a stop-clock which gave readings easily estimated t o 0.1second. The viscometer was thoroughly cleaned and dried before each new portion of so1 was added. The hydrogen ion concentrations were determined potentiometrically with a Leeds and Korthrup Type K potentiometer. Preparation of the Hydrosols. The aluminum, chromic, and ferric oxide hydrosols were prepared by adding approximately normal ammonium hydroside solution, slowly and with vigorous stirring, to dilute solutions of the corresponding chlorides. The amount of ammonium hydroxide added in each case corresponded to about half the stoichiometric equivalent, the excess of metal chloride acting as a peptizing agent and insuring a water-clear sol. The sols were then purified by hot dialysis according to the method of Keidlee2 Each sol \vas poured into a large beaker and diluted t o about 400 cc. A large parchment bag, through which distilled water circulated (one liter per hour), 1 -4. B. K e i r has just publishcd a papcr iJ. Chcm. Foc.. 127, 2245 (1925)) on the coagulation of a Prwsian Flue 801 k-y hydroFrn ions. Hydrochloric, sulfuric, acftic, and citric acids wpre wed. Ccntrary to E r a d f t l d , he f c d s t h a t citric acid a r r r a r s to behave normally. J. Am. Chem. Soc., 38, 1270 (1916); Neidle and Barab: 39, 71 (1917).
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JOHK H. YOE 9 N D EGBERT B. FREYER
was then suspended in the beaker until the water level inside was I or 2 cm. higher than the level of the outside liquid, the inside level being kept constant by means of an automatic siphon. The top of the dialyzer was covered to prevent contamination by dust. The temperature of the sol was maintained a t 6oo-8o0. The sols of aluminum and chromic oxides were dialyzed hot from the start, but that of the ferric oxide was first dialyzed at ro3m temperature about 24 hours, after which the temperature was raised and kept a t 60’-80’. This insured a perfectly clear brownish red sol. In each case dialysis was continued until only a minute amount of chlorzde i o n remained, as indicated by the silver nitrate test. About one week of continous dialyzing was required. -4lthough dialysis was continued in each case until only a minute amount of chloride 7‘011 remained, it should be remembered that each of the particles in the hydrosols consists of a complex of the oxide and a soluble metal chloride. Disregarding the hydrate water, the formulae for them may be roughly m i t t e n as follows: xX1203.yXlC13;xCr203.yCrC13:xFe203.yFeC13. The chromic oxide hydrosol was perfectly clear and a deep green. The aluminum oxide hydrosol was faintly opalescent. The oxide content of each sol was determined gravimetrically according to Blum’s methodl for the estimation of aluminum as oxide. The sols vere then accurately diluted with distilled water to the desired percentage concentration and were kept in stoppered Pyrex flasks as “stock solutions.” By suitable further dilution additional concentrations were obtained. The following concentrations of the three hydrosols were employed: 0.06. 0.03, and 0.01 j percent oxide content. Two ferric oxide hydrosols of 0.85 and 0.12 percent, respectively, were also used. ;Ifethocl of PTocedzcre. Two portions of the dialyzed sol of the desired concentration were withdrawn from the standard solution. One portion was put in the viscometer, allowed to come to the temperature of the bath, and the viscosity measured by observing the rate of effius through the capillary. Timings were made with a stop-clock accurate to 0 . I second (graduated t o 0 . 2 sec.). The recorded reading was the mean of several timings which checked t o within 10.2 sec., except in those cases where precipitation had occurxed. In the latter cases the precision mas not quite so good, and varied with the extent of precipitation. In each case the hydrogen ion concentration and the viscosity were measured concurrent15 in order to avoid any possible effect due to “aging” of the sol. Having obtained the p H and viscosity of the dialyzed sol, j o cc. of the latter were withdrawn from the standard solution and to it added, drop by drop and ryith stirring, a few drops of ammonium hydroxide solution. The sol was then divided into two portions and its viscosity and hydrogen ion concentration measured at once as above described. Similarly, other portions of the standard solution were withdrawn, the pH increased by adding a little ammonium hydroxide, and the viscmity and hydrogen ion determined con1 J. -Am, Chem. SOC.,38, 1282 (1916) and also RS Sei. PaFer 286, Bureau of Standards (1916).
currently, until a pEI range of ahout z,.;(’?) t o 9 ha8tll m n covered. The pH value of ahout 2 . 5 found for the freshly tlialyzetl sols indicates a surprisingly high acidity. The ferric oxide hydrosols termed “piire” ljy Thomas ant1 Frieden,’ i. e., calculated on t’he basis 3f nioles or’ Fe2C): t o nioles of Fe(’13 giving a ratio of ahout twenty one t o one, all g a w R pH value of 1.0. The velocity of ejflux of water \vas measured a t frcquenr intervals during. the investipation to insure aga.inst an accidental error in the readings. The very slight dilutions of the sols hy the ntltlition of small amounts (usually less than I per cent, hy vol,) of amnioniiiiii hytlroxitle solution produced no measurable change in their viscosities a s intlicatetl by yiscosity determinations on n-ater t o which had been added similar amounts of amnioniuni hydroxide solution. Results and Discussion The results of the experiments in this investigation are shown graphicn811y in Figs. I-.;. I n the case of sols containing 0.06, 0.0,;~ and 0.01.; per cent oxide, respectively, t,he rela,tive viscosi’ies. 7 , a t 2 5 ’ are plcttetl ggainst pH values (Figs. I . z 2nd 3 ) . On account of the tliffictiltj- in obtaining reliable p H nieasiireinmts on sols more concentrated t h m 0.06 per cent, in the case of ferric oxide sols containing 0.8.; ant! 0.12 per cent Fe20s, respectively, the change in viscositJ- is plotted ,.igainst cc. of 0.006 S ammonium hydroxide :rtlded (Figs. 4 :>nd ;I. =Ilthough 120th the hydrogen ion concentration :inti the viscosity were alwayc measured concurrently in order t o avoid [iny posihle effect clue t o “eging” cf the sol. experiments showed that, the ykcosity remained constant for a t least cex-era1 n-eeks in the c a w cf sols having a pR ahout 6.;or less. &4bovea pH of 6.5, however, the sols n-ere unstahle and gave a,n increasing viscosity upon “aging.” even whcn allowetl to c t m d only a few minutes. In such cases the viscosity wa,s determined as rapidly as po.ssible after changing the sol t o the desired hydrogen ion concentration. A4ta pH of 7 . 0 and ehove, precipitation occurred. hut a marked difference in the rate of settling n-as okxervetl among the three sols. Upon adding nnimoniuni hydroxide to the aluminum cxitie so1 no i-isihle change could lie detected TYith each addition. until suddenly the whole sol liecame very turhid. The precipitr,te settled in about cn hour. I n the case of ferric oxide sol, the precipitation was very gradual. Eech sinal1 adc!ition of aiiiiiioniiini h;\-droxitle .solution produced a temporary nebulous precipitate which soon tlisappeared, leaving the sol slightly more turbid. until finally t1r.o phases could easily he clistinguishecl. The precipitate settled rapid! , only a fen- minutes being required. The chromic oxide sol behaved like the fcrric oxide sol in so far :is it showed a very gradual increaFr in turbidity upon increasing pW. but it w a s iinlikc the !a,tter in that the precipitate settled very slowly, requiring several hours in some cases. An inspection of the curves shows that the change in viscosity with increase in pH j s very miall until a pH of about 7 is reached. -It this point n
* J. Am. Chcm. Poc., 45, 2 5 2 8
(1~23).
JOHK H. TOE AND EGBERT B. F R E P E R
I394
very rapid increase in viscosity occurs, reaching a maximum a t a p H of i . 3 , 7.0, and 7.9 in the case of the 0.06 per cent sols of aluminum, chromic, and ferric oxides, respectively. The corresponding 0.03 per cent sols have a maximum viscosity at pH 7.9, 7 . 0 , and 8.4, respectively. The 0.01; per cent sols of aluminum and chromic oxide had a maximum viscosity at pH 8.0 and
FIG.I Aluminum Oxide Hydrosol Circles are o 067, A h 0 3
Crosses are o
Triangles are o 03% .41203
015s hh08
7 . I , respectively. The corresponding concentration of ferric oxide was not determined. During the pH measurements of the dilute sols (0.03 and 0.01: per cent oxide) the potassium chloride of the calomel half cell diffused into the sols and caused partial precipitation. To avoid this, the saturated potassium chloride bridge was eliminated, liquid contact being maintained just long enough t o take an E. 11.F. reading after the cell had reached equilihrium. This procedure did not give the highest degree of accuracy. Jloreover, since all the sols mere precipitated at a pH of about 7 , the viscosity measurements at pH and above show some irregularity as seen in the viscosity-pH curves. I n the following table the maximum viscosit) of each of the sols is given. together v i t h the corresponding pH: 1Iauimum 1 is-cosity Per r r n t R20a o 06 0 03 O O I
H! droeol
pH at Max. 1-ivosity Per cent R,O, j
0.06
0.03
xXl?Oa. yAlC13 I 39 1.16 I 07 7.9 7.2 7.0 7.0 I 06 1.325 I 12j xCr203. yCrCls 1.06; -I 13 x F e L 0 3 yFeC13 . 7.9 8.4 I n 1906 Einstein' developed the following formula for the viscosity of liquid having suspended in it small, rigid, spherical particles : 7j
where
'7'
(I
+ Kf)
=viscosity coefficient of 7' =viscosity coefficient of Total volume of f =ratio Total volume of 7j
'.inn. Ph>-sik. 19, 289 (19061.
system, liquid (continuous phase), particles (disperse phase), system
001j
8.0
7.1 -
any
VISCOSITY O F OXIDE HYDROSOLS
I395
and K = a constant, which Einstein a t first made equal to I but later’ changed to 2 . 5 , after Bancelin had checked the equation experimentally, using spheres of gamboge, and obtained a constant of 2.9. Hatschek? deduced a similar formula for the viscosity of liquids in which are suspended small, rigid spheres whose volume is less than 40 per cent of the total volume of the system. His formula differs from that of Einstein only in the numerical constant, which is 4.5 instead of 2 . j . Harrison3 tested Hatschek’s formula
FIG.2 Chromic Oxide Hydrosol Circles are 0 . 0 6 5 CrKk
Triangles are 0.03% CrzOa
Crosses m e o.o~jC’, Cr.07
with qtarch suspensions in water and found it t o hcld good up to 3 0 per cent of disperse phase. On the other hand Odbn,.‘ working with sulfur sols, found the viscosity with particles about I O p p approsiniately 50 per cent greater than that of sols with particles I O O p p . Hatschekj says that the increase in viscosity with increase in dispersion is probably due to the adsorption of a film of liquid around the particles so that the “effective volume” is the slim of two factors: “volume of actual disperse phase (probably, but not necessarily proportional to the weight) plus volume of adsorption envelope.” This explanation. in the case of Odbn’s sulfur scls at leasi, seems to he a very probable one and was not considered when Einstein’s and Hatschek’sformulas were deduced. Since the radius of the particles and the distance between them do not appear in the above formula, it is obvious that the viscosity is independent of the size of the particles and is a linear function of the volume of the disperse phase only. From this it would seem that an increase in viscosity of aluminum chromic, and ferric oside-hydrosols would represent. in pari at least, an increase in degree of hydration, using the term “hydration” in its most general sense. The exact mechanism of this increase in hydration is yet to be satisfactorily esplained. It seems probable that part of the water may be in (I) true chemical combination, ( 2 ) part adsorbed on the surface 2
4
Kolloid-Z., 9, 154 (19111 . I- Poc., 9. So (1913j. J. Soc. Dyers niid Colonrists, 27, April (1911). Z. physik. Chein., 80,709 (1912). Iiolloid-Z., 11, 280 I 1912); .we also Trans. Faraday Soc., 9 . 80 (1913)
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J O H N H . TOE B S D EGBERT B. FRETER
and in the capillaries (perhaps even causing a swelling), and ( 3 ) part occliitictl in the voids when the particles agglomerate into loose aggregates. I n this connection we quote Rancroft :I “If the suspended particles aggregate into chains, the viscosity will he increased very much. If the particles form larger ppherical pz,rticles lyhich are homogeneous, there will be a decrezse in the
-
i’
3 Ferric Oxide Hydrosol. FIG.
Circles are o 06‘
Trinngle3 ‘ire o 0
Fe.03
3
FelO? ~ ~
viscosity, because of the decrease in the surface anti consequently in the amount of bound ivater. If, however, the particles simply agglomerate loosely into spherical masses. the viscosity will increase because the water in the voids inside the spherical agglcmerates no longer counts as free v-nter. \Ye shall therefore expect t o get an increase of viscosity as a result of agplomeration when the effect of aggloineration is not t o increase the size of homogeneous tlro1;s.” Referring t o viscosity changes in some gelatine sols Bancroft’
FIG,
1
Feiiir O\ide H > d r o d . c
1
2
Fr,O, ~ ~
further remarks: ”The iinportant point is that the increase in viscosity does go hand in hand with an increase in agglomeration which must mean that increasing agglomeration involves decrease in the amount of available free water.” The general character of the viccosity -pH ciiryes for the hydrosols of aluminum, chromic, and ferric oaides indicate that very little increase in hydration occurs with increase in pH until finelly a critical point (about pHi) is reached; then rapid aggloiiieraticn takes place (n-ith consequent rapid increase in viscosity) with further increase in rH. finally reaching a ma\;imum (pH about 8), and beyond this point paitial conlefcence takes place thereby forcing out part of the “imprisoned” n a t e r , the latter once more becaming a part of the external phase and the viscosity eccordinply lowered. S o t 211 of 1 2
“lpglied Colloid Chrmistry,” 192, ‘1921). Ibitl. p. 19j.
the water taken u p n t the critical pH range ivoultl lj? re1e:isetl hy a partial coalescence antl. hence. the Tit!- curve n-ould not drop to its original value. If mcst of the cccluc!etl iveter is (>si;elletl fairly rapitlly just lieyontl the critical pH range. the visccsity curve ivoultl tlrop steeply ant1 then fla,tten out t o a n a p p r o s i m a t e l ~constant ~ valuc. As noted in Figs. I-.;. all of the curves, with the esccption of the (!illite (o.06 ant1 0.03 I;cr c m t ) sols of ferric ositle. .show a repitl tlrop after 11 inp the niasimi:ni visco?ity antl then fl~3.tten out t o :mi approxinir.i-elJ- cmstnnt vnluc.
0
0.5
I, 0
I t
C C 0 . U 0 6 N NHAON ADDED FIG.
5
Fci,ri
