ALCOGEL OF SILICIC ACID BY BASRUR SANJIVA RAO AND Y.. S. GL’RVRAJA DOSS
Attempts to replace water by alcohol in the hydrogel of silicic acid have given contradictory results. Graham’ considered the complete replacement of water by alcohol possible, while Neuhausen and Patrick2 could not secure complete replacement and concluded that the residual small percentage of water in silica gel is held with a force which exceeds that exhibited between atoms of many stable compounds. Firth and Purse3 found the replacement more complete by a dynamic method than by the static method adopted by Neuhausen and Patrick, the final water content of a sample of gel being 0.577~. They concluded that under the most suitable conditions, ultimate replacement was not impossible. Results of a comparative study of three methods of replacement of the water in silica by alcohol are described in the present communication.
Experimental The silica gel employed in the work was prepared by a method which according to Firth and Purse yields a gel showing the best replacement. To a solution of pure sodium silicate, having a density 1.118,was added an equal volume of 3 S hydrochloric acid and the gel allowed to set at the room temperature. The gel was then washed free from sodium chloride and dried at IOO’C. The moisture in the gel was 6.87,. The alcohol used was refluxed with barium oxide, distilled and then treated similarly with metallic calcium till the product was found t o be “absolute.” The three methods of replacement employed were, (a) T h e “statzc” method: One gram of the gel was immersed in joo cc. of absolute alcohol. The stoppered “ether” bottle containing the alcohol and gel was placed in a desiccator charged with phosphorus pentoxide. The bottle was occasionally shaken. The alcohol was replaced three times by fresh supplies at intervals of two days. (b) T h e “dynamzc” method: The method described by Firth and Purse was followed. (c) T h e Soxhlet extraction mcthod: The gel was placed in the alundum thimble of a Soxhlet apparatus and extracted with absolute alcohol. The alcohol was maintained in the anhydrous state by the addition of pure metallic calcium t o the flask. A guard tube of barium oxide protected the alcohol from atmospheric moisture. ?
J. Chem. SOC.,17, 318 (1864); Pogg. Ann., 123, j29 (18643. J. Am. Chem. SOC., 43, 1844 (1921). J. Phys. Chem., 30, 617 (1926).
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ALCOGEL OF SILICIC ACID
For the determination of the water in the alcogel, Firth and Purse used samples having 2 5 to 30% alcohol. We found that the relative proportions of water and alcohol could be more accurately determined if the alcoholic content of the sample used for analysis, was reduced to about 8% by heating the gel at 180°C.in a current of pure and thoroughly dried nitrogen. The total volatile matter in the gel (alcohol water) was determined by ignition of the gel to constant weight at blast lamp heat. The alcohol content was estimated by the usual combustion methods of organic chemistry and the percentage of water in the gel got by difference. We thus obviated the difficulty in securing the complete removal of water from the gel in the combustion boat -a process which requires very strong heating.
+
Results % of EtOH
production of alcogel
wt. of gel
combusted
wt. of C02
“dynamic”
0.5881 0.4279
0.0988
8 8
(76 hours)
0 0714
__ 8.7 8.75
“static” ( 8 days)
0.3906 0.4856
Method of
0,0944 0.1227
13.3
SC loss on
5 of water
9.4
0.65
ignition
13.55
in gel.
0
30
13.2
13.25
“Soxhlet” Gel A (69 hours)
0.2128 0.3188
0.0283
6.95 6.93 6.94
7.15
0.21
0.0421
-
Gel B (53 hour4
0.3220 0.2330
0.0419 0.0304
6.92 6.82
7.21
0.39
Discussion The gel employed by us appears to have been of the same quality as that used by Firth and Purse since, on replacement of water by the dynamic method, residual water in the gel was 0.65% while in an identical experiment Firth and Purse got a gel with 0.57% water. Our experiments confirm the view of the authors that the “dynamic” method gives a more rapid elimination of water than the “static” process. The superiority of the dynamic method can be easily accounted for. I n this process of replacement of water, the distribution of water between the liquid in the gel and the alcohol-saturated air passed over it, causes an effective loss of water as the volume of air passed is very considerable (450 litres in 3 daysthe rate of air flow in our experiments being I O O cc. per minute). I n the “static,’ experiments, however, the water in the gel gets distributed over a comparatively small volume-the volume of alcohol used by Firth and Purse being only 1 5 0 cc. per gram of gel, the replacement being repeated three times.
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BASRUR SANJIVA RAO AND K. S. GURURAJA DOSS
Furthermore, the flow of air over the gel surface would reduce the thickness of the stationary film and thus lead to greater efficiency. I n our experiments we used a much larger volume of alcohol (500 cc. per gram of gel) and we were able to secure a greater replacement-the final product having only 0 . 3 water. ~ ~ Of the three methods employed for replacing the water in gel, the extraction method adopted by us seems to be the best in point of efficiency and convenience. In our experiments the Soxhlet used to siphon out once in zominutes, thus the quantity of absolute alcohol that came in contact with the gel was about 40 litres. That the water content could not be reduced to below 0.2% in spite of the contact of the gel with 40 litres of absolute alcohol shows how difficult is the ultimate replacement of water. summary
(I) Three methods of replacing the water in silica gel by alcohol have been examined. The most convenient, method has been found to be the extraction of (2) the gel with absolute alcohol in a Soxhlet apparatus. This gave a final product having 7Yc alcohol and 0.21%~ water. (3) The ultimate replacement of t,he water in silica gel by alcohol is extremely difficult. Central College,
Bangalore, S. India, AugZist 19, 1951.
