VISCOSITY OF THE SILICIC ACID GEL-FORMING MIXTURES BY MATA PRASAD, 6. M. MEHTA AND J. B . DESAI
Prasad and Hattiangadil have shown that when solutions of sodium silicate and of acids (or acidic ammonium acetate) are mixed together, crystalloidal silicic acid first formed goes over to the colloidal state and then follows the coagulation of the colloid solution by the electrolytes present in the mixture. Prakash and Dhar2 have shown that the viscosity measurements of the jelly-forming mixtures reveal ( I ) the passage of the crystalloidal substance into colloidal state ( 2 ) the gradual neutralisation of the charge on the colloidal micelles (3) the formation of the specific structure of jellies. The present investigation was undertaken with a view to distinguish these three stages in the process of formation of the silicic acid gels. The viscosity of silicic acid sols with and without the addition of electrolytes has been measured by Dhar and Chakravartys who find that with increasing quantities of the electrolytes the viscosity of the sol at first falls, then rises to a maximum and again falls. D h a i concludes that the degree of hydration and the viscosity of the sol increase as the charge on the colloidal particles is decreased. Thus it appears that the viscosity measurements of a colloidal system undergoing coagulation can also be utilised to measure the degree of hydration of the colloidal particles.
Experimental Scarpa’s apparatus5 modified by Farrows has been adopted with the following changes: (i) the connection of the guard tube opening the viscometer cylinder to the atmosphere has been kept a t the top of the ground-glass stopper and not inside the cylinder thus ensuring constancy of the concentration of the solution under investigation (ii) ground-glass stoppers have been substituted for the wooden and rubber corks used by Scarpa and Farrow. The viscometer with the guard tube is enclosed in an electrically heated air thermostat maintained at 40°C within o.oz°C. The dimensions of the viscometer used are:
*
(i) Volume of the bulb between the two fixed marks. . . . , . . . 3 . 1 7 C.C. (ii) Diameter of the capillary. . . . , . . , . . , . , . . . . . . . . . . . . . . . .8.9 mm. (iii) Length of the capillary.. . . . . . J. Indian Chem. SOC.,6, 893 (1929). * J. Indian Chem. So?., 6, 391 (1929). 3 Kolloid-Z., 44, 225 (1928). J. Phys. Chem., 29, 1556 (1925). 6 Gacz., 40,271 (19x0). 6 J. Chem. SOC., 101, 341 (1912).
. 7 . 5 cms.
I
VISCOSITY O F SILICIC ACID GELFORMING MIXTURES
1385
The viscometer bulb of a small volume was selected because the rate of increase of viscosity of the gel-forming mixtures with time was found to be very great at a later stage of gel-formation and hence a viscosity reading could be taken in as short a time as possible. Solutions of sodium silicate and acetic acid were prepared as described in the previous communication. The gel-forming mixtures were prepared by mixing equal volumes (20 c.c.) of sodium silicate and acetic acid solutions and the viscosity of the mixtures with (i) different silica content and (ii) different amounts of the acid was measured a t different intervals of time till the gel set. The time of rise “tl” and the time of fall ‘%” were measured by an accurate stop-watch. Another stop-watch started at the time of mixing the gelforming constituents indicated the time since mixing a t which the time of rise and of fall were measured. The mean of the time a t which the mixture was made to rise and that a t which it reached the lower mark while falling, was taken to indicate the time ‘IT”at which the viscosity reading of the mixture was taken.
TABLEI A. Alkaline gel-forming mixtures Silica content-4 per cent C = o.35N pH = 9.86 T 9 x IO‘
3’ 26“
91 1 2 ” 12’
7’’
43l’ 31’ 9” 25’
C = 0.36 N (pH = 9.74) ?x I
T
7929 8187 8271
5’ 9” 11’35’’ 16’38”
9326 9965
20) 2
I”
jo’ 25“
12350
26’53” 30‘ 21’‘
52’ 0’’
12670
40’
56’ 7” 59’54“
13490 14460 16970 18270 19x30 27790 30460 34810 39680 53520
7 0 ‘ 41“ 7 5 ‘ 41“
78’ 6” 97‘ 18”
9’’ 103’55“ IOOl
108’
0”
114‘45‘’
119’40”
a
C
8991 9274 9788 10360
0.37 N (pH = 9.03)
T
I06
2’ 42”
6’ 16’’
7 ‘ 44If 14’40“
IZIjO
18’5 2 ” 21’33’’
II500
34If 43’ 0” 56’24” 58‘ 50’’
14500
241 2 5 ”
15090 20480
2
7I ’ 43’l 75‘ 33” 78‘ 0’’
30490 38420
22000
a
7’ 44” 29’53”
7
x
10‘
8736 9426 9924 13020 15630 18870
23200 32290 a
1386
MATA PRASAD, S. M. MEHTA A N D J. B. DESAI
TABLEI
i
B. Acidic gel-forming mixtures Silica content-4 per cent C
C = 0.55 N (pH = 5.28) T q X 106
= 0.50 N
(PH = 5.4) T q X
106
3’ 7’‘ 5 ’ 40” 18’48” 24’ 5 ” 30’56” 32’ 29” 34‘ 8”
4’ 49” 6’ 13”
8330 8529 IO’ 8” 10970 1 1 ’ 55” 13390 14’ 4” 19570 16’ of’ 0:
37’ 45‘‘ 40‘ 3’’ 41‘ 50’‘ 43‘ 11’’
7702
7617 8007 8636 10390 11190 12200
15360 19280 23390
C = 0.60 N (PH
=
T q 5’ 14” 14/15’’ 19’12’’ 41‘ 2’’ 50’21” 53’45” 55’41’’ 60‘ 5” 62‘ 5” 64’ on
5.19) X IO^
7667 7602 7617 9466 11950 13840 15250
21190 28270
a
0:
C = 0.65 N (pH = 5 . 1 1 ) T q X 106
6’ 46” 20‘21’‘
25’”’’ 32’53’ 41’ 7” 44) 36l‘ 49’36” 54’41’’ 59’ 8”
61’
25‘‘
7767 7789 7929 8307 8945 9371 10150 11160 12730 13690
70’ 20‘’
20810
73’ 2” 74‘38“ 76‘ 0“
31810
26200
0:
TABLE I1 A: Alkaline gel-forming mixtures Silica content- j per cent C = 0.40 N (PH
T 8’ 17” I O ’ 37l‘ 24’ 21’’ 31’ 54” 39’ 30’’ 45’ 22’’
53’ 24” 60’ 44” 63‘ 5 “ 70’ 42” 76’ 44’’ 80‘ I 5’‘ 87’ 40’‘ 93’ 13” 97’ 28” 101’
os
=
9.97) q x
106
9303 9561 11050
C = 0.41 N (pH = 9.86) T 11 x
9588 10290 11160
59’’
12180
12070
12’
12970 14100
2 0 ’ 50’’
15960 17970 18750
25’
59’‘ 16“
35’
22”
22500
26980 29950 39330 51390 63450 a:
106
2’29” 6‘ I ” 9’ 6“
22’
39’ 50’’ 42’ 40”
C = 0.43 N (PH = 9.74) T q x
14960
3’ 6’’ 4‘ 45” 6’46” 9’ 7” IO’ 30”
I 5820
12’ IO”
17410 30860 44510 a
II
106
190
12620
15330 21210
26610 a
VISCOSITY O F SILICIC ACID G E L F O R M I N G MIXTURES
I387
From tl and tz (expressed in tenth of a second) the viscosity is calculated from
. . . where K = q,,.
(t’tl~ztz)w
. .
. .
.
. .
.
(i)
the calibration data of the apparatus, obtained
from pure distilled water.’ Taking the value of q w to be 0.006535 (c.g.s. units) a t 4ooC from Thorpe and Rodger’s data2 the value of K was found t o be 7118.5 X IO-*. The coefficient of viscosity of the various gel-forming mixtures was then calculated from (i) and the results are given in the following tables, in which c represents the concentration of acetic acid added. Curves in which viscosity is plotted against time have been drawn and one of the set is shown in Fig. I .
TABLE I1 B. Acidic gel-forming mixtures Silica content-5 per cent C = 0.65 N (PH = 5.3) T q X 106 9665 3’ I ” 6‘ I O ” 10750 8‘ 2 ” 11980 9’ 55” 14160 1 2 ’ 13’’ 19950 13‘30” 0:
C =
0.70
N
(PH = 5 . 2 ) T q X 106 3’ 6“ 8999 9620 8‘ 47” 10600 13’”‘’ 16’ 53” 12260 18‘ 47’‘ 13680 2 0 ’ 16’’ 15980 23’ 5’’ 20240 24‘ 32” 23 7 5 0 26’ 0’’ a
C = 0.75 N (PH = 5.15) T q x 106 2 ‘ 58” 8806 6‘ 4” 9126 19’ 38” 10970 23’ 3“ 12480 26’ 54” 15430 ‘29‘ 2 ” 18340 31’ 34” 24550 34’ 16’’ 28720 37’ 5“ a
c
= 0.80
T
q X
N
(PH = 5.1) IOO
3’ 18” 8869 8’ 48” 8855 13‘ 6‘‘ 9133 20’42’’ 9965 26’ 38” 11040 29’ 52’’ 12060 33’ 40” 13690 35’ 34” 15130 37’ 43’‘ 17680 40‘ 15” 22190 43’ 35l’ 34420 4 j ‘ 50” a
Discussion of Results It appears from the curves shown in Fig. I that the viscosity increases slowly for some time after mixing the gel-forming constituents and afterwards the rate of increase becomes very rapid. The slow increase in viscosity may be due t o the formation of the colloidal particles in the gel-forming mixtures and the rapid increase may correspond to the neutralisation of charge and consequent increased hydration of the particles and to the formation of definite structures in gels. Curves plotted with logarithm of viscosity against time are not straight lines as found by Prakash and Dhar3 but are continuous curves having nearly the same shape as those shown in Fig. I . Cf. Scarpa: LOC.cit. and Farrow: LOC.cit. *Phil. Trans., 185, A 11, 397 (1894). LOC.cit.
J
1388
MATA PRASAD, 8. M. MEHTA AND J. B. DESAI
The first portion of the viscosity curves for the acidic mixtures is less steep than that for the alkaline ones and this may indicate that the rate of formation of the colloidal particles in the former mixtures is slower than in the latter ones. Further, the steepness of the curves for the two types of mixtures beyond a certain point shows that the increase in the viscosity of the acidic
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