ADSORPTION BY DIATOMACEOUS FILTERSL EARL J. HOAGLAND AND JOHN E. RUTZLER, JR. Department of Chemistry, Cornell Universzty, Ithaca, New York
Received June $0, 1036
While attempting to use a Berkefeld filter candle to remove mother liquor from a crystalline solid, it was found extremely difficult to draw air through the candle by means of a vacuum pump after it was wetted by the aqueous mother liquor. Then it was observed that after washing the wetted filter with acetone it passed air readily; the acetone did not dissolve out a determinable amount of solid. It was thought that an investigation of this phenomenon might yield better insight into the behavior of these diatomaceous filters, and perhaps reveal why pore size determinations by the bubble method are erratic (1). Despite the extensive use of diatomaceous earth (3), few studies of its adsorptive power appear to have been made. All of the experiments were carried out with Berkefeld “N” diatomaceous candles, which are supposedly of medium pore size. The apparatus chain consisted of a vacuum pump connected to a 4-liter round-bottom flask which was in turn connected, by means of a T-tube, to a manometer and a 1-liter suction flask; between pump and suction flask was a by-pam which allowed evacuation of the suction flask without disturbing the 4-liter flask. The candle and its glass reservoir were fitted into the top of the suction flask by means of a one-hole rubber stopper. I n some experiments an air-tight connection was made between the open end of the candle reservoir and a Friederichs’ gas-washing bottle to control partially the water vapor content of the air entering the diatomaceous candle. Each unit of the apparatus was separable from the rest by stopcocks. The experiments were made by evacuating the system up to the Berkefeld candle, closing off the vacuum pump, opening the candle to the evacuated system, and reading the height of the mercury in the open arm of the manometer a t regular intervals of time. From the height of the mercury in the open arm of the previously calibrated manometer and the atmospheric pressure, the pressure within the system was determined for each interval of time. The 4-liter flask was used in order to provide a reliable and duplicable pressure difference between the bpen side of the 1 Presented a t the Twelfth Coiloid Symposium, held a t Ithaoa, 20-22, 1935.
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New York, June
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EARL J. HOAGLAND AND JOHN E. RUTZLER, JR.
candle and the evacuated side. In this manner runs were made using different candles and the same candle under different conditions. The first run was made using a new candle which had not been in contact with water, in order to determine the rate of flow of air through dry diatomaceous earth for comparison later with the rate through moist earth. This is candle No. 1. The next four runs with candle No. 1 were made using water, absolute ethanol, absolute methanol, and carbon tetrachloride. The data are given in table 1. The candle was completely dried each time before tests were made; and it was soaked thoroughly with the liquid to be tested, in addition to running 100 cc. of the liquid through it in each case. The gas-washing bottle did not contain any liquid during the runs of table 1. In this and the other tables representative data only are presented. In each run from twenty-five to fifty observations were made; all of these TABLE 1 Adsorption of liquids by candle N o . I CARBON TEIRACELOAIDE
AIR
ABSOLUTE ETHANOL
ABSOLUTE METHANOL
__ - __ -
___
1
Time
Pressure
Time
Pressure
Time
Pressure
Time
'ressure
Time
Preasure
seconds
mm.
seconds
mm.
seconds
mm.
eoonds
mm.
seconds
mm.
15 30 45 60 75 90 105 120 135 150
184 241 464 545 607 649 680 696 709 718
10 20 30
54 88 118 148 181 21 1 245 277 312 344
10 20 30 40 50 60 70 80 90 100
46 90 129 161 190 216 241 263 292 318
10 20 30 40 50 60 70 80 90 100
73 128 168 202 235 265 291 320 348 371
30 210 510 810 1005 1200 1410 1606 1800 2010
-
40 50 60 70 80 90 100
- - __
-
---
i
94 38 57 84 103 124 152 169 5 208 240
points were used in plotting the curves of figures 1 and 2, and formed smooth curves. Using candle No. 2, which was new at the start, a study was made of the differences in rate of air flow through candles wetted by aqueous solutions. The data are given in table 2. Distilled water was used in all cases. All solutions were 1.23 M , and in each case the gas-washing bottle was filled with the solution being tested and connected with the candle reservoir to guard against clogging the filter by deposition of solid salt in the pores. Just before each run the candle was kept in the boiling test liquid until no more air bubbles escaped from it, cooled to room temperature, and placed in the system. From the time of immersion in the boiling test liquid until the start of the run the candle was kept constantly in contact with the liquid, 50 cc. of the test liquid being run through the candle into the suc-
ADSORPTION BY DIATOMACEOUS FILTERS
217
tion flask immediately prior to the start of the run. Using the by-pass around the 4-liter flask the suction flask was quickly evacuated again. The remaining liquid in the reservoir was then poured out and the run started. After each run hot distilled water was passed through the candle until the filtrate gave no test for the solute just used; then 1 liter more of hot distilled water was run through the candle, thus assuring the absence of the electrolyte. The data from table 2 are plotted in figure 1, the pressures being the pressures within the system at the given times. The differences in the pressures for the various liquids a t any given time are taken as measures
7
FIQ.1. Adsorption by filters of the relative amounts of adsorption by the diatomaceous filters. The differences shown in figure 1cannot be due to differences in vapor pressure, density, or viscosity, because they are not nearly in the correct order. The order of “clogging” of the candle was: sodium sulfate > sodium nitrate > sodium thiocyanate > water > sulfuric acid. If the effect were due entirely to changes in the water equilibrium, the order would either have to be the reverse of the above or the water curve would have to fall considerably further to the right. Experimental error is not a factor, because check runs made agreed to within 1.3 per cent. Probably owing to the small amounts of electrolytes adsorbed by silica,
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EARL J. HOAGLAND AND JOHN E. RUTZLER, JR.
few data are found in the literature. Gore (6) determined the amounts of adsorption of a number of salts and acids a t different concentrations by silica. Since diatomaceous earth is largely silica, it may be expected to behave roughly like silica. So far as one can determine from Gore's results, when recalculated for approximatelf molar solutions, the order of increasing adsorption for sodium salts is: sulfate < chloride < bromide < iodide. The molar sodium sulfate solution increased in concentration by 1.09 per cent. The other salts showed positive adsorption. In so far as it is permissible to reason from silica to diatomaceous earth, the experiments of Gore provide what appears to be the soundest explanation of the present 2esults. On this basis, sodium sulfate decreases the permeability of the candles to air mainly by an increase in the amount of water adsorbed by the diatomaceous earth. The effects produced by TABLE 2 Adsorption of liquids by candle N o . 2 SODIUX NITRATE
SODIC51 THIOCYANATE
SODISY SULFATE
___ ___ Time
Pressure
Time
Pressure
Time
Pressure
seconds
mm.
seconds
mm.
seconds
mm.
60 180 360 540 720 900 1080 1260 1440 1380
31.5 49.5 79.5 111.5 146.5 181.5 217.5 255.5 291.5 279.5
180 360 540 720 900 1080 1260 1440 1620 1800
-
-46.5 68.5 93.5 120.5 147.5 175.5 206.5 234.5 262.5 292.5 -
60 180 360 540 720 900 1080 1260 1380 1440 -
29.5 46.5 75.5 105.5 136.5 170.5 207.5 240.5 262.5 274.5 -
' ~
SULFURIC ACID
Time
-I Pressure
Time
econds
m7n.
seconds
60 240 420 600 900 1200 1500 1800 1920 2040
28.5 48.5 70.5 94.5 134.5 179.5 224.5 268.5 288.5 306.5
60 180 360 540 720 900 1080 1260 1380 1500
--mm
.
37.5 55.5 86.0 118.0 152.0 189.0 227.0 265.5 291.5 320.0
sodium nitrate and sodium thiocyanate appear to be due to adsorption of electrolyte superimposed on the adsorption of water; potassium nitrate was adsorbed considerablymore (data not obtained for sodium nitrate) than the corresponding halogen salts in Gore's experiments. The position of the sulfuric acid curve in figure 1is about what one would expect from the fact that Gore observed no effect with a molar solution of this acid; it falls closer to the water curve when a correction is introduced for a slight difference in temperature which existed. I n all other cases the maximum difference in temperature was one degree from 27OC. The data from table 1 are plotted in figure 2, curves 1, 2, and 3. Curve 1 was obtained by plotting the data for methanol, while curve 2 represents the data for ethanol and carbon tetrachloride, the two being so close together that they are represented by one curve. Curve 3 was obtained
ADSORPTION BY DIATOMACEOUS FILTERS
219
by plotting the data for water in candle 1. Check runs agreed within 1 per cent. The main point of interest is the large difference between the curves for the organic liquids and that for water, the amounts of adsorption being of entirely different orders. ALSO there appears to be s c p e specific adsorption as between the organic liquids, since vapor pressure differences do not entirely account for the results. In a differently arranged experiment, toluene and acetone behaved similarly to the above three organic liquids, did not differ much between themselves and, by comparison with water under those conditions, were only slightly adsorbed. Since the vapor pressure of toluene is about that of water, the differences between
N6. t 4 T I M E IN
0 0
500
I
0
ism
swo
FIG.2. Adsorption by filters the behavior of the organic liquids and water cannot be accounted for on that basis. The air curve falls to the left of curve 1, of course. Ewe (5) points out that filters for sera prepared from the same sample of kieselguhr treated in the same way show considerable variability in performance. This is probably due to differences in pore size and consequent differences in amounts of adsorption. Table 3 contains interesting data on the behavior of candle No. 3, also an “N” candle, toward water. The data are plotted in figure 2, curve 4. There is a tremendous.differencein the adsorption of water by this candle and candle No. 2. With candle No. 3 the pressure in the system was below the vapor pressure of water a t
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E A R L J. HOAGLAND AND J O H N E. RUTZLER, JR.
that temperature for over thirty minutes. The candle was virtually plugged by adsorbed water. The water curve in figure 1 is different again, there being less adsorption than in either of the other two cases. These results were duplicated. From this it appears that identity of behavior is not to be expected as between any two given Berkefeld candles. It might be worthwhile for the bacteriologist to standardize his candles by the amount of water they will adsorb, using the above method. Microscopic examination of candles No. 1, 2, and 3 revealed the reason for the differences in the amount of water adsorbed by them. It was found that with increasing adsorption of water there had been an increasing amount of destruction of the diatoms, probably during the process of manufacture. In other words, the fragments of the diatoms were smaller with increasing adsorption, which probably means smaller pore size and which certainly means an increased specific adsorbing surface. TABLE 3 Adsorption of water by candle N o . 3 TIYE
~
PRESSURE
seconds
mni.
15 90 240 600 900
20.5 21 5 21 5 21.5 22.5
-1'
'1
11
TIME
1I
PRESSURE
seconds
I
mm.
1200 1800 3000 4500 7200
1
23 23 26 28 29
I
I
1
5
5 5 0
0
We can now see why pore size determinations in ultrafilters are erratic. Bechhold (2) gives two methods for the determination of pore sizes of ultrafilters, which he calls the air transpiration method and the rate of transfusion of water method, respectively. The air transpiration method falls down in such filters as the Berkefeld, where the water is strongly adsorbed; two different liquids would give two different values depending on their relative adsorptions, and two different filters would give different values which would express both a difference in pore size and a difference in adsorption. Measurements such as those of Einstein and Muhsam (4), in which ether was used instead of water, are probably free from this objection, because it is not to be expected that the ether is strongly adsorbed by the Berkefeld filter. In the rate of transfusion method no account is taken of the effect of adsorption. This method is defective to that extent. It seems clear that more concordant results on pore size should be obtained by the use of a non-adsorbed or slightly adsorbed liquid, and the pore size should be greater the less the adsorption. The general results of this paper are as follows : 1. Diatomaceous earth adsorbs water strongly.
ADSORPTION BY DIATOMACEOUS FILTERS
221
2. Berkefeld filters of the same rating show great variation in behavior when wetted by water, probably because the diatoms are crushed to different degrees. 0 3. Salts are adsorbed by diatomaceous earth, sodium thiocyanate being less strongly adsorbed than sodium nitrate. 4. There is quite strong negative adsorption of a sodium sulfate solution by the Berkefeld filter. 5. Organic liquids tested were adsorbed less than water. 6. A method is suggested for the evaluation of Berkefeld filters for bacteriological work. 7. Certain pore size determinations on ultrafilters are shown to be defective. REFERENCES (1) BANCROFT: Applied Colloid Chemistry, p. 227. The McGraw-Hill Book Co., New York (1932). (2) BECHHOLD: Colloids in Biology and Medicine, p. 101 (1919). (3) CALWRT:Diatomaceous Earth (1930). (4) EINSTEINAND MUSHAM:Deut. med. Wochschr. 49, 1012 (1923). (5) EWE:J. Lab. Clin. Med. 6, 538 (1920). (6)GORE:Chem. News 69, 23 (1894).
