RATES OF COAGULATION. I AUTOCATALYSIS AND SOL PURITY F. C. HILDEBRAND AND C. H. SORUM Laboratory of General Chemistry, University 0.i Wisconsifi, Madison, W i s c o n s i n Received November lY, 1933
In a series of studies involving the use of the photoelectric cell as a means of measuring rates of coagulation, Desai (1)and Desai and Pate1 (2) observed definite evidence of an induction period in the coagulation process. Since the extent of this induction period was found to decrease with increased purification of the sol employed, it seemed pertinent to repeat Desai’s studies, using the presumably highly purified sols prepared in this laboratory (4,5 ) . The light source used was a 300-watt “Champion” tungsten-filament bulb which was operated on 60 volts a t 1.78 amperes. The light from this lamp, which was cooled by placing it in a water bath through which passed a copper coil containing running water, was passed through a lens (f = 6 cm.) placed a t a distance equal to its focal length from the filament. In series with the lamp were a key, a variable 10-ohm resistance, and an ammeter which could be read to 0.01 ampere. The light, which could be shut off by a shutter placed directly in front of the lens, then passed through a metal tube set in a thermostat to an inner container in the thermostat. Here were placed, in order, a filter of a 2 per cent solution of copper nitrate contained in an optical glass absorption cell (10 X 100 X 100 mm.), the reaction cell (10 X 100 X 70 mm.), also of optical glass, and a photoelectric cell. This last was a product of the General Electric Company (P. J. 23) containing cesium as the sensitive material, which when operated on an anode potential of 90 volts delivered a maximum peak anode current of 5 microamperes. This current was passed through a Leeds and Northrup wall galvanometer whose sensitivity was 1655 megohms. Readings of the galvanometer deflection were taken on a scale 2.5 meters along its arc, placed at a distance of 7 feet from the galvanometer mirror. The source of current for the lamp was a bank of storage batteries whose output current was maintained a t a constant value by means of the variable resistance mentioned above. 809
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EXPERIMENTAL
Preliminary experiments showed that the ferric oxide sol which was used was not affected by light after an exposure of from three to four hours, and that pure electrolyte solutions were without appreciable effect on the
I
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FIG. 1. COAGULATIOX RATESWITH SODIUMCHLORIDE
intensity of the transmitted light. Before each determination, a b1,ank was run using 10 cc. of sol diluted with 20 cc. of distilled water in the reaction cell. The necessary time was allowed for all external conditions to become constant and the galvanometer reading was then taken. The aero point was obtained by cutting off the light with the shutter, and the
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RATES OF COAGULATION. I
difference between these two values taken as the standard deflection for the sol under investigation as a measure of the opacity of the pure sol with no added electrolyte.
Sol 0 No,SO,
Time in Minutes 0
IO
20
30
40
50
Bo
70
90
FIG.2. COAQULATION RATESWITH SODIUMSULFATE
Twenty cc. of electrolyte was then placed in a 125-cc. Erlenmeyer flask, the time taken as being the zero time for the run, and 10 cc. of sol pipetted into the flask. The reaction mixture was then quickly transferred to the reaction cell and the cell placed in its proper position in the inner container of the thermostat. As nearly as possible, the same mixing technique was employed in every determination so as to eliminate any effect due to stirring.
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F. C. HILDEBRAND AND C . H. SORUM
At various times, readings were taken on the galvanometer scale and the differences from the standard deflection plotted against the time, giving the curves shown in figures 1 to 5 .
Sol 0 Na, PO,,
Tim@in Minutes k
I
10 FIG.
3.
20
30
CO.4GULATION
40
50
RATESWITH
60
70
00
0
SODIUM P H O S P H A T E
DISCUSSION
Since the decrease in intensity of the transmitted light is a measure of the increase in flocculated sol, the slopes of the various curves give a direct measure of the rate of coagulation a t any given time. The plotted results show no induction period; neither do they show any autocatalytic tendency (3).
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In all curves there is an evident tendency for the rate to drop off toward the end. Since, in keeping with Desai’s technique, the sol-electrolyte mixture was not stirred, this is no doubt caused by a counter-clarification due to sedimentation of the flocculated material. The actual appearance
Sol
c
NP c1
Time 10
20
30
40
in Minutes u)
60
70
$0
FIa. 4. COAQULATION RATESWITH SODIUM CHLORIDE
of a floc at the bottom of the cell, and the fact that this dropping off in the rate of decrease of intensity of the transmitted light was more pronounced when the beam of light was shot through the top of the cell and less pronounced when shot through the bottom substantiates this contention. In view of the fact that agglomeration is presumably the result of colli-
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F. C. HILDEBRAND AND C. H. SORUM
I
2
3
d
5
6
FIG.5. COAGULATION RATES WITH SODIUM CHLORIDE
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RATES O F COAGULATION. I
Sol B No, SO,
FIG.6 .
ORDER OF THE COAGULATION PROCESS
THE JOURNAL OF PHYSICAL CHEMISTRY, V O L . X X X V I I I , SO.
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F. C. HILDEBRAND AND C . H. SORUM
sions between two or more particles, it is to be expected that the reaction might be of second order. In the event that it is second order a straight line should result if the reciprocal of the direct galvanometer reading, which reading is a direct measure of the concentration of unflocculated sol, is plotted against time. Such graphs are shown in figure 6. The results seem to warrant the conclusion that the coagulation process is essentially second order. The results of studies with stirred sol-electrolyte mixtures will be presented in a later paper. SUMMARY
1. The rate of coagulation of highly purified ferric oxide sols by various electrolytes has been studied by means of the photoelectric cell using the technique developed by Desai. 2. There is no evidence of an induction period in the flocculation process; the process does not appear to be autocatalytic. This is in agreement with a prediction contained in Desai’s experimental results. 3. The flocculation process appears to be essentially of second order. REFERENCES (1) DESAI:Trans. Faraday SOC.24, 181 (1928). (2) DESAIAND PATEL: Trans. Faraday SOC.26, 126 (1930). (3) MEHTAAND JOSEPH:3J. Indian Chem. SOC.10, 177 (1933). (4) SORUM: J. Am. Chem. SOC.60, 1263 (1928). (5) SORUM: Kolloicl-Z. 68, (3) 314 (1932).
