(20) ~ I E R R I LIt. L , C.? .ISD AICB.IIS, J . W.:J . P h y s . Cheni. 46, 10 (1942;. (21) POTSEY,J . , ASD SO.\D, R . IV.: 3. Textile Inst. 30, T157 (1939). (22) PRESTOS, W. C . : J. PhJ-s. Colloid Chem. 52, 84 (1948). C . I T . : J . Am. Chpm. SOC.68. 2160 (1916). (23) RALSTOX. W.,.\SD HOERR. (24) I T R I G H T , Ii. -I,,a k ~ ~.A. ~D..) SIVERTZ. ~ ~ . i-,. . i S D TARTAR, H. I-.: J. . h i . Cheni. 61, 540 (1939’.
SOC.
COA\GUL.kTIOS OF HYDROCS FERRIC OXIDE SOLS BY ELECTROLYTES ELIZ.1BETH F. TULLER’ Deportment
OJ‘
E.1 . E’ULlIER
.~XD
Chcmistr!/. IOUTS t o i c College, -4m e s . lorn Keceir ed October 2 . 1947
I.
I’HOTOJIEI‘RIC
J I E T H O D FOR THE DETERMIX-‘LTIOX O F ‘THE;
(?OAGCI,bTION
T*ALTTE O F ELECTROLYTES
ISTRODUCTIOS
The importance of coagulation phenomena in hydrophobic colloidal systems is attested by the large number of papers on the subject in the scientific literature. The “coagulation value” is generally expressed in terms of the millimoles of electrolyte, per liter of final volume of sol, required to produce coagulation. Since there are various definitions of the term “coagulation”, the actual number assigned to the coagulation value depends on the purposc and method of the particular investigation. The most basic method of determining complete coagulation is the direct observation of the phenomenon with the ultramicroscope. Since, hon-ever, this is a long tedious process usable only with dilute colloids and not easily adaptable to following the rate of coagulation, a number of other indirect methods have been employed. Of all the indirect methods, probably those involving some type of optical measurement are no^ the most frequently used. These measurements include turbidimetric, nephelometric, spectrophotometric, colorimetric, and photometric methods. The type of instrument used has gradually changed from the visual to the photoelectric type, so that the accuracy does not depend upon individual judgment. Mukherjee and Papaconstantinou (5) r e r e the first to use spectrophotometric and photometric methods. Since that time various types of these instruments have been employed. Sometimes the measurements \\-ere made only for the purpose of following the course of agglomeration as a means of studying such a phenomenon as the liinetics of coagulation. Khere the methods TI ere used to determine a coagulation value, the value was given in terms of the concentration Department of Cheniistrv. Wellesle\- Collegp, JVellesl~~, :+Inssarhu-
setts.
01 electrolytz neviwiry t o produce it cxtain efYect in n given time as determined by the methods mentioned above. That is, the limiting values of degrees of turbidity, angles of rotation, intensities of light, and other criteria are indicative of changes in the colloid. These methods are valid only for comparative piirposes but not for absolute data, that is, curves may be drawn indicating trends. I n an attempt to improve upon the above methods, Wanno\\- (10) described a procedure in which he first plotted the relative transmittancies against the time of coagulation for various concentrationa of electrolytes. He then plotted the relative transmittancies for a specified time (such a. 5 min.) against the activities of the coagulating (dominating) ion; the cuive is S-shaped with a sharp inflection point. The concentration of electrdyte at the inflection point is talcen to be the coagulating value. TVanno\\ and Hofl'mann (11) stated that this method gave absolute values, based on the fact that the curve obtained by plotting the number of particles in a given time again3t the activity coefficient of the coagulating ion shoxed about the same course, particularly the same inflection point, as did the corresponding c~irveof light transmission against activity coefficient. Thc I\ ork >honing thi- (.orrelation I\ as reported on hy Hoffmann arid Wannon ( 2 ) . Troelstra and K r ~ i y t( 7 ) took exception to the a h e method of \Vanno\\ for determining the eoagiilation point. they indicate. the inflection point for the curve based on transmission at 2 min. nil1 not he the san-e as that taken at 5 min., and so forth. They, therefore, do not lielieve that it is characteristic of the coagulation capacity of the electrolyte used. They also question the standard methoc! of investigating coagulation hy determining sediwentaticn in R given length of time, beca ye it indicate- not only coagilation 13 t a length of time for suhaequent 3edimentstion. Differences in rates of coagulaticn n ill be associated Trith the formation of different hinds of aggregates n-hich, in turn, settle a t different rates. Therefore, from the point of vie\\- of kinetics, Troelstra and Iiruyt suggest that it TI ould tie better to choose a certain degree of particle number obtained in a definite time as the criterion for the investigation of agglomeration. It i; more important t o knox the point at which deforn-ation of the double layer O C C L I ~ S ,even though it might not lead to complete coagulation. But, at least, a method should be chosen, sucli 21s the color change of a gold sol, nhich indicates a certain aggregation degree independent of the occurrence of sedimentation. I n addition to the difficulties described above, the choice of definiticn of the coagulation a130 cau.;es coiisiderable confusion. since upon occasion i t refers to an end point and a t other times t o a process. The authors, therefore, suggest the following terminology to clarify the iwie. -4gyZomerafion refers to the process of adhesion of the colloidal particles to form larger aggregates. Coagzdafion refers to the final stage of agglomeration x-hich leads to separation into tn-o obvious phases through sedimentation. CSP1:IZI\ILSTAL
The present paper deals with the development of a procedure to determine the coagulation value of hydrous ferric oxide sols, independent of the occurrence
of sedimentation. This procedure also permits the study of rate of agglomeration and the structure of the flocculate as evidenced by rate of subsequent sedimentation. The progress of the agglomeration was folloivetl by determining the change in t'ransmission of light by means of a KWSZ photometer purchased from the TTilkins-L4nderson Company. This photometer consists essentially of a double-cell, l~alanced-bridge system using photoelectric cells and glass hsorbent filters with 11 100-n-att lamp as a light source. The tubular cuvettes used xere held in the instrument by adaptors placed on the cai-riage. The filter chosen \vas the ,% 0 absorling a t 725 millimicrons; this filter gave the maximiin light transmittnnq- for the hydrous ferric oxide sol used in thc study. The standard employed was the colloid diluted to the appropriate concentraticn with distilled water. The per cent transmission of the agglomerating colloid was noted each minute for the first 3 min. nnd approximately every 5 min. thereafter unt'il coagiilation took place. Since it is well linonn that 1 difference in thc method of mixing thtt cwlloid m d tlie coagulating electrolyte \vi11 m u s e a difYei*ericein t h e agglomeration hehaviol., a ~ i n i f o i ~method n of mixing \\.as employed. T h e piweclui~cfollinvccl \vas t o add to the colloid a sufficient quantity of electrolyte t'o malie the I o t u l vo!iime 3 ml. The mixture x:is itnniccliately stirred nnd placed in the instrument. Tlic first reading of timxmission of light I V D ~taken :30 m a . nitel' the addition of the electrolyte. The rhoice of the amount of colloid usecl was on thc hasis of the final concentration tlcsired. The iisiial concentrat ions of volloiil cmployed \verci 20. 20, 30, 20. and ti0 per (wit. :~lthoiighin m n c c*asc> roncwitrations ai; \ O K as 5 per cent 1 ~ used. 1 ~ The hydrous ferric oxide sol rmployeci in tlir experimwth i 1 - a ~thc so-called Soiwii sol (6). The method (if pwparation \ v a s by the niodificat,ion described l q * 'ruller and Eblin (9). T h c procedure \vas further niotlifiecl by iising T-isking ce11oph:~nc sausuge casing, \vhicli allon-ed maximum purification of the sol in 24-48 hr. of dialysis. Since t,lw time of dialysis is only :I qualitative indication of purity. the purity (designated as 1') is expressed as the ratio of iron to [ahloride in terms of gram-equivalents; the higher the vahie of I'. the purer is thc sol. Full details as t o a11 of the ahoye points have hecn given hy Tuller (8). When agglomeration talirs place, absorption of light increases, causing a decrease in the intensity of light striking a photoelectric cell. This change of transmission vas recorded as the per cent transmission at a given time. Thus the complete vourse of agglomeration could he fol on-ed as a funct,ion of transmission and time iintil coagulation, and finally sedimentation, resulted. The transmission for 5 given sol decreases n i t h agglomeration until coagulation occ~irs,after which the t'ransmission increases. The second phase of this change in t,ransmission is related to the type of aggregate formed and the rate of sedimentation following coagulation. ,Ilthough Wannow (10) and others have mentioned that the curves of opacity or per cent transmission plotted against time shon- an increase in transmission of light after the original decrease, none of these investigators have concerned t'hemselves n-ith the possibilities of using the change in slope. In fact,,for the most part, they have failed even t o indicate the full course of the agglomeration.
Graphs ale piewited in figurcl 1 shoning the per cent light trarlamissiorl as a function of time on addition of electrolyte to the sol. The letter X indicates the Sorum sol and the suhbcript the original batch chosen. However, different solsfor exzlrnple, -4: and -&-\yere found to give results in close agreement under otherwise identical conditions. The concentration of the sol is given in terms of volume of original so1 per total volume; for example, 80 per cent refers to 80 ml. of the original sol diluted to 100 ml. vith n-ater The symbol P refers to the purity of the sol and has been previously defined. It will be noted that the transmittancy decreases as agglomeration prod. 0 1 esses and then increases at the point of coagulation. It \\-as found that the rate of increase of transmittancy is indicative of the bubsequent rate of sedimentation. When cuvettes were removed from the instrument at the instant of change of slope, no sedimentation vas observablc. Sedimtlntatiori sufficient to shoir- 2-3 mm. of clpar supelmatant liquid occ.uriet1 from 15 see. to several hours following the break in the (wrve. Flocs (vikible particles) were observed in every case just as the time of the vhange of slope, but not before. For convenience, the time a t which the break occurs has been denoted as the critical time or t,. The critical time is therefore the coagulation time, i.e., the time when agglomeration is complete but sedimentation has not yet occurred. The wries of graphs in figure 1 illustrates the types of changes of light transmission that take place \\-hen an electrolyte is added to a colloid. Figures la through IC show the changes in the transmission as the critical time or coagulation time is reached more rapidly. I t shoulcl be noticed that as the critical time becomes smaller, the rate of change of transmission becomes greater. This indicates not only that agglomeration is more rapid hut that sedimentation takes place more rapidly. This very general statement is most accurately applied to colloids differing only in concentration of the added electrolyte. The concentration and purity of the sol affect the critical time and late of sedimentation. The increase in transmittancy for the 80 per cent so1 (figure Id) does not increase as rapidly, folloTving the critical time, as n i t h the 50 per cent sols (figues l a to IC). Mo:'eover, the mo:'e pare concentrated sol (figure le) shows a sharper change than does the less pure sol. These differences are associated with the types of coagulum formed and are indicated not only by changes in light transmittancy but also by the rate of sedimentation. In the case shown in figure Id, the rate of change of light transmission during agglomeration is very rapid and the critical time has a small value; yet the increase in transmission is very sloiv. When examined, this colloid showed a granular, dense coagulum which settled very slon 1y. This is in contrast to the flocculent, massive, rapidly settling coagulum of the 50 per cent sols (figures l a to I C ) . The coagulum of the highly concentrated sol becomes more massive and flocculent as purification proceeds and hence settles more rapidly, as indicated in figure le. If the colloid is of such nature that coagulation causes a tendency toward gelation, the critical time is frequently indicated by a sharp decrease in transmission before a rapid increase. This is illustrated by figure If, in which the sol was
COAGUL.\TIOS
O F FERRIC OXIDE SOLS
0
lu,
Y
do: !
c
D
l
D
*
C
u
NOlSSIWSNVtll lN33t13d
792
KLIZABETH F. TCLLER AiVD E. I. FULMER
coagulated n-ith potassium ferricyanide. The sharp decrease is due to a sudden packing effect occurring a t the coagulation point and before sedimentation takes place. With some preparations of sols and high concentrations of polyvalent ions coagulation ultimately produced gelation such that there was just a steady decrease in transmission until Sedimentation was nearly complete. The decrease in transmission \vas due to agglomeration and the packing following coagulation but gave no indication of the coagulation point. The critical time, therefore, can be determined only on colloids not shon-ing gelation tendencies upon coagulation and is indicative only of the time needed for coagulation, not of the type of coagulum or the rate of sedimentation. It was found that the transmission curves were exactly reproducible for 48 to 72 hr. following dialysis before the aging effects on the cdloid were sufficient to produce a noticeable change in the properties of the system. For an additional 48 to 72 hr. the critical time \vas the same, but the total change in slope on either side of this point had started to vary. By this time the aging effects were producing a noticeable difference in the properties of the colloid. After this period of 5-6 days, the critical time had changed. Thus the transmission curves and the critical time may he used as an indicatioii of the effects of aging on the colloid. The critical time v a s also found to he related to the kinetics of wagulation. Coagulation phenomena are generally divided into tvio types, rapid and slow. Rapid coagulation is usually characterized by a rapid attainment of complete agglomeration follon ed by quick sedimentation and is produced by relatively high concentrations of a giTen electrolyte. Slow coagulation, on the other hand, is attained by relati~elylon- conccntrations of electrolyte?, and s 1 0 ~sedimentation follows complete agglomeration. Theie, holyever, are merely yualitative indications of the rate of coagulation and, as has becn emphasized in the earlier discussion, are npt always adequate. That is, under certain circumstances, high concentrations of clectrolytes may prodwe rapid and complete agglomeration but sedimentation mag- be relative13 c,1 O T \ . The use of the critical time in differentiating hetveen rapid and sloiv coagulation is shown by the data in table 1 and figure 2, relating the critical times to concentrations of three electrolytes:-sodium chloride, sodium sulfate, and potassium ferricyanide. Figures 2b, 2d, and 2f show that, within the limits marked by asterisks, the critical time is a simple exponential function of the concentration of the electrolyte, that is, the logarithm of t , is a linear function of the concentration of the electrolyte. The deviation from this linear function which occurs with very rapid coagulation, i.e., when the value of t , is small, may lie due to limitations of the method rather than to any difference in behavior of the colloid. The high-value times not falling within the exponential function, however, are due to slow coagulation. In other words, the extent of the rapid coagulation is indicated by the range of the exponential function, while the slon- coagulation is indicated by critical times too long to be included in the linear-relationship. The range of critical times over which the exponential function was applicable varied with the electrolyte used. For example, the length of the straight-line function is shorter for the univalent electrolytes, as exemplified by sodium
793
COAGUL.kTIOS O F FERRIC OXIDE; SOLS
TA4BLE1
Critical time,t e , as a f u n c t i o n of the concentration of electrolyte for sols of $0 per cent -~
__
concentration a n d p u r i t y ( P ) us indicated .._ . _ ~___
SOL
A, ( P = 1 4 . 7 ) . .. . . . ,
0.27 S a C l *O. 32 0.40 *O .48
49 19 14 10
1.60 1.28 1.15 1.00
A, ( P = 475) . . . . . . . .
*1.20 x 1.25 x 1.28 X 1.31 X *1.36 X 1.40 X
lo-'
35 25 18 14 30 9
1.54 1.40 1.25 1.15 1.00 0.95
10-5 K , F e ( C S ) s 10-5 10-6 10-5 10-5
16 25 8 2 2
1.66 1.40 0.90 0.30 0.30
A8 ( P = 9 9 ) . .
*5.32 X 5.43 x 5.60 x *5.80 x 6.00 x
10-1 li,SOi 10-4
lo-$ lo-*
.~
Sol A 5 P = 14.7
Sol A 5 P = 14.7
0 2 03
0.4 0.5
M NaCl Pa
-
100
Sol A 7
0.6
0.2 0.1
0.2 03 0.4 0.5 M NaCl
2b
S o l A7 P . 475
;I\ 2c
8 1.0 J
I .6 I.
5
2d
2e
5.5
M ( X IO5 1 K3Fe ( a ) S 2t
FIG 2 . Ciitical tiiiie (fJ as u iutlctioilot t h e iiiolxrit\- oi electrolyte for sols of 20 per cent roilcentintion a n d puiity ( P ) R S inchrated
794
ELIZABETH F. TULLER .iND E. I. FULMER
chloride, than it is for potassium sulfate and potassium ferricyanide. I n general, it was observed that the critical times for the univalent electrolytes extend at the most from 5 to 35 min., while those for potassium sulfate and potassium ferricyanide have ranged from 3 to 50 min. That is, the zone of rapid coagulation is greater for a polyvalent effective ion than for a monovalent ion. Owing to the complexity of the colloidal system and the resultant hysteresis phenomena associated with it, a t this time it is probably impossible to designate an absolute coagulation value. Since many concentrations of electrolytes rray produce coagulation in a given colloidal system, it seems more logical to use for comparison purposes critical times and the concentrations of electrolytes corresponding to those times. Within the limitations imposed by hysteresis, probably the concentration of electrolyte needed to produce a given critical time for a colloid system defined in terms of method of preparation, purity, and concentration should be reasonably constant. It certainly is adequate as far as using critical times and the corresponding concentrations of electrolytes for comparison purposes. Caution must be observed to insure that any times taken for comparison are all from the same general type of coagulation, rapid or slow. SUMMARY
I n order to clarify confusion in the use of the term “coagulation” the following definitions have been proposed: Agglomeration refers to the process of adhesion of the colloidal particles to form larger aggregates; coagulation refers t o the final stage of agglomeration which leads to separation into two obvious phases through sedimentation. Agglomeration of hydrous feriic oxide sols I\ as folloned by a photometric procedure. The transmittancy of light decreases as agglomeration progresses and then increases a t the point of coagulation. ‘The rate of increase of transmittancjis indicative of the subsequent rate of sedimentation. The tirne a t which the direction of the transmittancy curve reverses direction is the time for coagulation and is designated t,, the critical time. The value of t, is, within limits, an exponential function of the concentration of electrolyte, that is, log t, is a linear function of the concentration of electrolyte. This linear relation holds only through the range of rapid coagulation. For values of t , greater than the linear range, slo\r- coagulation is indicated. Critical times within the range of rapid or slow coagulation may thus be chosen as a basis upon v-hich to compare the concentrations of electrolytes required to produce coagulation of a given sol. 11. RELATIOX OF
THE: P U R I T Y O F THh S O L S TO T H E B U R T O h - B I S H O P 1 l l ~ L E IK‘TRODUCTIOiX
Burton and Bishop ( I ) in 1920 formulated a rule relating the concentration of hydrophobic colloids to the coagulation value of an added electrolyte. The so-called Burton-Bishop rule is as follon-s: ( 1 ) The coagulation values of univalent ions increase n-ith decreasing sol concentration. ( 2 ) The coagulation values of bivalent ions remain almost constant regardless of sol concentration.
COAGULATION OF FERRIC OXIDE SOLS
795
( 3 ) The coagulation values of trivalent ions vary directly with sol concentration. Since the time of the formulation of this rule there has been considerable discussion as to the extent of its applicability, particularly in regard to the colloids known as the hydrous osides. Gp until the time of the n-ork of Judd and Sorum (3) it was generally believed that the hydrous osides \\-ere an esception to the rule. Judd and Sorum, hen-ever, used a specially purified hydrous ferric oside sol and shon-ed that the Burton-Bishop rule held. They ascribed this correlation of concentration of the colloid and coagulation value of the electrolyte to the high purity of the colloid. -4lthough Sorum’s 11-orkwas challenged several times, it was confirmed by Kauffmann (4) and Tuller and Eblin (9). Weiser and Alilligan (12) pointed out that the Sorum effect is to be expected; the purer sol is more unstable to electrolytes and upon dilution becomes relatively more stable. Hence the sol formed by dilution of an unstable preparation n-ould be relatively more stable to univalent coagulating ions than the sol formed by dilution of an impure highly stable preparation. EXPERIRZEKTAL
( a ) Sodium chloride as the electrolyte The course of the agglomeration of hydrous ferric oxide sols was follou-ed photometrically, using the techniques described in Section I of this paper. The critical time, f,, \\-as used as a criterion for coagulation of a hydrous ferric oside sol, the purity of the sol \vith respect to the chloride-ion concentration, and the coagulation value of an added electrolyte. Nost of the data \\-ere obtained in the region of rapid coagulation, i.e., the region in which the critical time is an esponential function of the concentration of electrolyte. The similarities and differences in slow and rapid coagulation uere shon-n and will be discussed later. Several critical times n-ere determined for each concentration of the sol, and data were obtained for three to six different concentrations for each sol purity. Data are given in table 2 for the critical time, t,, as a function of the concentration of sodium chloride and of the purity, P , and concentration of sol -A5. Values are given for ?n and b, through the range of application of the linear relation : c
=
m log t,
+b
in which c = concentration of electrolyte in millimoles per liter and t, = critical time. The asterisks in the table indicate the demarcation betn-een rapid and slow coagulation; in all cases a t least one slon coagulation value and a limiting value is given. The limiting value is the concentration of electrolyte which will just produce agglomeration of the colloid, as indicated by a steadily decreasing transmittance. The value of the intercept, b, is the extrapolated coagulation value for a critical time of 1 min. TVhile siich yaliies cannot he obtained esperimentallg a t 1 min., they do serve a usefnl purpose in correlating the various results for rapid coagulation.
796
ELIZABETH F. TULLXIt AND E. I. FULhlER
Data for P = 47, 119, and 510 are plotted in figures 3, 4, and 5 . For values of P less than 119 the straight lines tend t o intersect a t a common point, as illusTABLE 2 Critical time, t c , as a j u n c t i o n of concentration of sodium chloride and of the puritv, P , and concentration of sol As SOL
P
Concentration
ger cenl
20
14.7
40
50
60
80
47.0
I
20
OSCESTRATION OF NaC1
millimoleslliter
m
b
min.
240 272 *320 400 480
Limiting value 45 1.65 19 1.28 14 1.15 11 1.04
-731
1240
240 276 *300 360 420 480
Limiting value 39 1.59 20 1.30 15 1.18 12 1.08 9 0.95
-500
955
250 280 *300 350 400 450
Limiting value 32 1.51 19 1.28 13 1.11 10 1.oo 7 0.84
- 360
760
250 270 *280 320 360 400
Limiting value 32 1.51 20 1.30 15 1.18 11 1.04 8 0.90
- 348
730
250 260 *280 300 320 360
Limiting value 55 1.74 20 1.30 14 1.15 10 1.00 5 0.70
- 130
450
150 240 *272 304 320 400
Limiting value 30 1.48 22 1.32 15 1.18 1.11 13 6 0.77
-24.5
590
COAGULATION OF FERRIC OXIDE SOLS TABLE 2-Continued SOL
P
JNCENTRATION
Concentration
per cenl
40
50
60
80
119
20
40
OF Sac1
IC
zillirnoles/liler
min.
LOG IC
m
b
170 240 *285 300 336 360
Limiting value 35 1.54 17 1.23 15 1.18 10 1.00 8 0.90
-21.2
550
190 200 *260 280 300 320
Limiting value 40 1.850 20 1.30 15 1.18 12 1.08 9 0.95
-16.2
475
210 240 *254 280 300 320
Limiting value 34 1.53 24 1.38 15 1.18 10 1.00 7 0.84
- 125
425
230 240 *260 280 300
Limiting value 50 1.70 23 1.36 19 1.15 8 0.90
-88.0
380
65.0 70.0 *i6.8 78.4 85.0
Limiting value 65 1.81 33 1.52 26 1.41 11 1 .04
-19.0
101
60
Limiting value 60 1.78 21 1.32 14 1.15 10 1.00 8 0.90
-48.5
165
Limiting value 80 1.90 25 1.40 14 1.15 9 0.95 5 0.70
-21.5
115
io
*92 102 108 116 50
I
70 r*
(0
*85 90 95 100
T U L E 2-Continued I
1 CONCESTRATIOL
SOL
P
OF
Concentration
p e r cent
60
80
20
40
60
80
20
SaCI
1
m:limoles,iiter
~
LOG IC
m
6
mis.
60 77 *92 96 112
Limiting value 28 1.45 13 1.11 11 1.04 7 0.84
-81.0
184
70 78 *84 96 100 120
Limiting value 51 1.71 30 1.48 16 1.20 14 1.15 5 0.70
-43.5
150
24.0 38.0 "40.0 43.2 44.8
Limiting value 50 1.70 23 1.36 13 1.11 10 1.00
-12.2
56.8
30.0 35.0 *38.4 40.8 42.0 45.0
Limiting value 80 1.90 30 1.48 18 1.25 13 1.11 7 0.84
-10.5
53.8
26.0 3-1.0 *36.0 38.4 40.0 42.0
Limiting value 63 1.80 36 1.56 27 1.42 21 1.32 15 1.18
-15.5
60.5
28 35 440 42 44
Limiting value 63 1.so 16 1.20 10 1.00 8 0.90
-11.5
54.2
22.4 24.0
Limiting value 40 1.60 21 1.32 18 1.25 15 1.18
-28.8
58.0
12.0 16.0 *18.0 21.6 24.0
Limiting value 45 1.65 22 1.34 11 1.15 11 1.04
-19.7
14.0 18.0 *20.0
40
tc
~
__
44.5 ~
798
799
COBGULATIOK O F FERRIC OXIDE SOLS
TrlBLE 2-Continued I
I
SOL
I b
P
Concentration
p e r cent
60
80
510
20
40
50
60
80
mdlimoles/liter
min.
12.0 14.0 "16.0 18.0 20.0 21.6
Limiting value 50 1.70 26 1*41 17 1.23 12 1.08 9 0.95
-12.7
33.7
10.0 14.0 *16.0 16.8 18.0 20.0
Limiting value 55 1.74 34 1.53 21 1.32 14 1.15 8 0.90
-7.2
26.5
8.0 12.0 * l 4 .4 16.0 17.6
Limiting value 30 1.48 17 1.23 12 1.08 9 0.95
-12.0
29.0
4.5 6.0 *7.2 9.0 12.0
Limiting value 1.64 44 26 1.42 19 1.28 11 1.04
-12.9
25.5
4.5 6.0 *9.0 10.0 11.0 12.0
Limiting value 40 1.60 16 1.20 13 1.11 11 1.04 9 0.95
-11.6
23.0
4.0 6.0 '7.2 8.0 8.8 10.0
Limiting value 32 1.51 22 1.34 18 1.25 15 1.18 12 1.08
-11.2
22.0
3.0 5.0 T.2 8.0 9.0 10.0
Limiting value 33 1.52 15 1.18
-10.4
19.4
13
1.11
10 8
1.00 0.90
800
ELIZABETH F. TELLER AXD E. I. FULMER
13oN
SIlORIlllYI
CO.IGULATIOS O F FERRIC OXIDE SOLS
80 1
802
EL1ZIDI;TH F. TULLER A S D E. I. FULJIER
COAGULATION OF FERRIC OXIDE SOLS
803
trated in figure 3b. The point of intersection may be within or just at the zone of rapid coagulation. If i t lies vithin the zone of coagulation, the concentration of electrolyte for s l o coagulation ~ nil1 be in the reverse order to that indicated by the intercept b. If the point of intersection is beyond the zone of rapid coagulation, the order of coagulation values may be, but are not necessarily, in the reverse order of that for rapid coagulation. Figure 3b illustrates a case in Ivhich slo~vcoagulation values are the inverse of rapid coagulation. For highly purified sols, that is, P is greater than 150, there is a tendency for the straight lines to become parallel as illustrated in figure 5b, and the curves of t , against c also do not intersect as seen in figure 5a. This means that the order of coagulation values is the same for rapid as for slon- coagulation. For sols of intermediate purity, from 50 to 150, there is a scrambling effect as shown in figure 4. This case s h o m the relationship of coagulation values to concentration of the colloid when the coagulation values are not varying directly with or inversely to the concentration of the colloid. The decreasing order of the coagulation values in the most rapid coagulation is with concentrations of sol of 60, 40, 80, 50, and 20 per cent. The order of the coagulation values in the slow coagulation as indicated by the limiting values is not the exact inverse but decreases 11-ith the concentration in the order of 80, 50, 20, 40, and GO per cent. In any case in which the scrambling takes place, the intersection of lines never occurs so nearly at a common point as in other cases in which the lines do intersect. According to the Burton-Bishop rule, the coagulation value of the univalent ions increases with a decrease in the concentration of the colloid for any given sol. An examination of the intercept values in table 2 shows that the BurtonBishop rule holds for all but two of the samples of the sol. In these two cases v-here the values of P are 119 and 151, there is a scrambling of the order of the coagulation values compared to the concentration of the colloid, but the tendency is tonard a reversal of the Burton-Bishop rule. Examination of the limiting values as an indication of the order of the slov coagulation values shons that the Burton-Bishop rule is not obeyed until P for the sol is about 500. In summarizing the results for sodium chloride it can be stated that, in rapid coagulation, the Burton-Bishop rule is followed in all cases except in the range of purity of approximately 50 to 150 where there is the scrambling effect; in slow coagulation the Burton-Bishop rule is apparent only if the sol has a purity of 250 or higher. ( b ) Potassium sulfate as the electrolyte Potassium sulfate n-as employed as the source of the divalent ion. 117th this electrolyte, sol concentrations of 40 per cent or less must be used. \Tit11 higher concentrations, either gelation occurs or the break bet\\-een rapid and slow coagulation is so sharp as to preclude obtaining sufficient points. That ib, coagulation was so rapid as to give a critical time of 1-3 min., or \vas so slon as to give no coagulation in 1-2 hr. With concentration of sol less than 40 per cent, no difficulty was met in determining sufficient critical times to determine a straight line.
804
ELIZ.4BETH F. TULLER AKD E. I. FULJIER
TABLE 3 Coagiilation raliies of potassium sztlfccte a s a function of t h e p u r i t y and concentration of sol A 7 SOL
P
OSCESIRATIOS O F K2SOI
LOO to
Concentration ~
28.9
ger cent
ndlimuleslliler
miii.
5
0.171 0.190 0.208
31 16 8
1.49 1.20 0.90
0.180 0.189 0.197
20 14 11
1.30 1.15 1.01
10
-0.058
1
-0.057
1
20
30
40
60.3
5
10
20
30
10
0.176 0.184 0.192 0.208
19 14 10 5
1.28 1.15 1.00 0.70
0.168 0.181 0.183 0.189
32 19 18 14
0.312 0.318 0.324 0.360
0.260
~
0.255
1
~
-0.053
0.245
1.50 1.28 1.25 1.15
-0,058
0.255
14 12 10 3
1.15 1.08 1.00 0.40
-0.07
0.395
0.190 0.199 0.204 0.219
32 20 14 7
1.50 1.30 1.15 0.84
-0.040
0.250
0.180 0.186 0.189 0.197
23 14 12 7
1.36 1.15 1.08 0.84
-0.033
0.225
0.206 0.211 0.216 0.240
20 16 12 3
1.30 1.20 1.08 0.48
-0.041
0.260
0.245 0.252 0.260 0.266
30 18 11 8
1.48 1.25 1.04 0.90
-0.038
0 I300
0.300 0.306 0.310 0.324
29 20 12 3
1.45 1.30 1.08 0.48
-0,023
~
,
0.335
805
COAGUL-4TIOX OF FERRIC OXIDE SOLS
TABLE 3-Contin1tcd SOL
P
1
ONCENTRATIOS
OF
Concentration
per
114
cetit
5
10
20
30
201
5
10
20
KzSOd
miilimoles/iiter
''
LOG t c
m
b ~-
~
min.
38 16 12 10
1.58 1.20 1.08 1.00
-0.045
0.262
0.173 0.180 0.189 0.198
17 13 8 5
1.23 1.11 0.90 0.70
-0.048
0.232
0.168 0.175 0.185 0.192
17 12 7 5
1.23 1.08 0.84
0.iO
-0.047
0.225
0.210 0,224 0.238
25 12 5
1.40 1.08 0.70
-0.040
0.267
0.190 0.192 0.199 0.219
23 21 12 7
1.36 1.32 1.08 0.84
-0.035
0.237
0.180 0.185 0.193
19 15 9
1.28 1.18 0.95
-0.034
0.225
0.168 0.175 0.180
25 15 10 7
1.40 1.18 1.00 0.84
-0.028
0.208
30 21 18 12
1.48 1.32 1.25 1.08
-0.035
0.230
28 21 17
1.45 1.32 1.23
-0.041
0.205
31 22 18 16
1.49 1 .31 1.25 1.20
-0.041
0.202
5
10
'
.
0.190 0.208 0.214 0.219
40
300
1
0.144 0.148 0.151 0.153
TABLE 3-Continued 6
m
LOG t c
P
:oncentration
per
ceizt
20
30
40
nillimoles, l i i e i
?nil%
0.120 0.128 0.131 0.136
25 16 13 11
1.40 1.20 1.11 1.04
-0.043
0.180
0.140 0.147 0.153
38 25
1.58 1.40 1.20
-0.041
0.205
0.144 0.149 0.153 0.157
30 20
1.38 1.30 1.23 1.08
-0.041
0.205
16
li 12
T%BI,L 4 CoagulatLon Lalues of p o t c i s s z u m j e r zcganzde cis a f u n c t z o n of the p z t r z t y and concent! atzon of sol A , SOL ~
P
CONCEYTRATIOX O F E;rFe:CS!s
'oncentration
,
5
10
15
20
52.2
5
10
15
-1
I
25 19 15
1.40 1.28 1.18
-0.0034
0.038i 0,0396 0,0405
30 15 10
1 .48 1.18 1.oo
- 0.0043
0.0448
0.0555 0.0560 0.0565
16 12 9
1.20 1.08 0.95
-0.0040
0.0603
0.0662 0.0669 0.0677
10 6 4.5
1.00 0.78 0.65
- 0.0042
0.0704
0.0214 0.0220 0.0228
20 14 8
1.30 1.15 0.90
- 0.0033
0,0258
0.0335 0.0332 0.0349
28 15 10
1.45 1.18 1.00
- 0.0031
0,0380
0.0154 0.0460
36 14 10
1.56 1.15 1.00
- 0,0035
0.0500
35 16 11
1.54 1.20 1.04
- 0.0038
0.0653
0.0219 0.0225 0.0228
0.0464 20
LOG l c
min.
per cenl
26.9
tc
0.0595 0.0608 0.0614
I
0.0268
1
COAGULATION O F FERRIC OXIDE SOLS
507
Data for the coagulation of sol Xi a t several purities by potassium sulfate are given in table 3. The slopes of exponential functions determined for various concentrations of a given purity of sol shon- that the lines tend to be parallel. This indicates that the order of coagulation values for both slo~vand rapid coagulation will be the same. The coagulation values, represented by b, were found to decrease slightly and then increase as the concentration of the sol increased. This slight minimum occurred a t 10 or 20 per cent concentration of the sol. That is, the order of the intercept values indicated that potassium sulfate had a tendency to act like an electrolyte ivith a univalent dominating ion when coagulating lo~vconcentrations of the sol; at higher concentrations the electrolyte had a tendency to behave as though the dominating ion lvere trivalent. These tendencies were shown throughout the complete range of sol purities used and u-ere a t a minimum with highly purified sols. In conformity xvith the BurtonBishop rule, the coagulation values of potassium sulfate show an intermediate position between the monovalent and trivalent ions.
( c ) Potassium ferricyanide a s the electrolyfe Investigation of potassium ferricyanide as the added electrolyte was hampered by tivo factors: inability to use high concentrations of the colloid due to the quick change from rapid to slov- coagulation, and the gelation caused by agglomeration of the sol. Both of these changes have been discussed in previous sections of this paper. However, sufficient data were obtained, and are listed in table 4, to show that the exponential function of critical time against the concentration of the electrolyte gives a series of parallel lines from different concentrations of the same sample of sol. As with potassium sulfate, this indicates that the coagulation values of potassium ferricyanide n-ill be in the same order for different sol concentrations in both rapid and s l o coagulation. ~ The data also show that the coagulation values of the electrolyte vary directly with the concentration of the colloid, a result Ivhich is in agreement with the BurtonBishop rule. SUMMARY
It has been found that critical times may be used in determining the relationship of the sol concentration to the coagulation values of an electrolyte. The order of the intercepts of the straight lines obtained by plotting the concentration of electrolyte against the logarithm of critical time indicates the order of the rapid coagulation values. If the straight lines are approximately parallel, the order of coagulation values in both rapid and s l o coagulation ~ are the same. Hou-ever, if the lines intersect, the coagulation values in slov coagulation may he, but are not necessarily, in the inverse order to those of rapid coagulation. If the dominating ion is univalent, the order of the rapid coagulation values is in accord with the Burton-Bishop rule with all purities of the hydrous ferric oxide sol except hetween approyimately 50 and 150. However, the order of sloir- coagulation values for univalent electrolytes agrees n-ith the Burton-Bishop relation only if
808
JOHN F. DHEYER
the purity is 250 or higher. If the dominating ion is divalent, the behavior of the electrolyte is intermediate between the monovalent and trivalent ions, in accord with the Burton-Bishop rule. Although, for reasons previously noted, complete investigation could not be made of coagulation with a dominating trivalent ion, the data show that rapid coagulation values vary directly with the sol concentration, as has been expressed in the Burton-Bishop relation. Further investigation is being made of the trend toward a minimum coagulation value found when potassium sulfate was the added electrolyte. The cause of the scrambling of the coagulation values in some sols when the dominating ion was univalent is being investigated. REFERENCES (1) BcRrroN, E. F.,A V D BISHOP,E.: J . Phys. Chem. 24, 701-15 (1920). (2) HOFFM~N, , A N D WANNOW., H. A . : Kolloid-Z. 83, 258-62 (1938). C. H . : J. Am. Chem. SOC. 62, 2598-2602 (1930). (3) JUDD,R. C . , ASD SORUM, (4) K a u ~ ~ ~ aV. s sH.: , Kolloid-Z. 93, 86-103 (1940). (5) MUKHERJEE, J. s., A N D PAP%CO~STANTINOU, B. C.: J Chem. soc. 117, 1563-8 (1920). (6) SORUM, C . H.: J. Am. Chem. SOC.60, 1263-8 (1928). (7) TROELSTRA, S. .4.,ASD KRUYT, H. R . : Kolloid-Beihefte 64, 225-6 (1943). (8) TULLER, E. F.: Doctoral thesis, Iowa State College, 1946. E. F., A X D EBLIN,L. P.: J . Phys. Chem. 49, 9 (1943). (9) TULLER, (10) WAXNOV~, H . -4.: Kolloid-Z. 77, 46-53 (1936). (11) WANNOR,H. 8 . ,AND HoFFarANs, I
