ABSORMAL ADSORPTIOK BY FILTER PAPER BY RUBY RIVERS MURRAY
Evans' has reported a number of cases of apparently abnormal adsorption by filter paper. n'ith hydrochloric acid the amount adsorbed was apparently independent of to o 08 N . the concentration of the solution from o 007 It then increased to double the amount as the concentration of the solution rose to o 28 S and dropped off apparently to zero with o 36 S HC1 and with normal acid. In a second series the maximum adsorption was found a t about o 2 S HC1 and there was apparently no adsorption with A' 2 acid. A similar behavior was observed with copper sulphate solutions, the maximum adsorption occurring at about o 04 CuSOl and the adsorption dropping to zero for a o 13 11 solution. There seemed to be two possible explanations for these phenomena. The filter paper might undergo a change which cuts down the adsorbing power. In that case treating filter paper with a half-normal solution of hydrochloric acid and then diluting should not give the same result as treating originally with a dilute solution. The other possibility was that the solution might be adsorbed without change of concentration, in which case the method of analysis would show no apparent adsorption.? As a matter of fact neither of these hypotheses accounted for the data. The first thing to do was evidently to repeat Evans' results. Five grams of filter paper (No. j97, 9 cm, washed) were torn in shreds and added bit by bit to IOO cc hydrochloric acid in order to ensure complete wetting. The stoppered flask was shaken thoroughly but not too vigorously, because too violent shaking tends t o disintegrate the paper. The bottles were allowed to stand at ordinary temperatures for lengths of time varying from one hour to ten days before titrating. Since it was found that there were always some +\-
Jour. Phys. Chem., IO, 290 (1906). Leighton: Jour. Phys. Chem., 20, 3 2 (1916)
R u b y Ricers .Ilurray
62 2
particles of paper which did not settle out of the supernatant liquid, the solution was filtered through a single paper of the same sort as that used in the adsorption tests. Except when otherwise stated, I O cc of the filtered solution were drawn off in a pipette, the same pipette being used in all determinations. The solutions were titrated with caustic soda solutions slightly less in strength than the corresponding acid solutions. An ordinary burette was used which was cleaned carefully and was allowed t o drain one minute before reading. Evans used phenolphthalein as an indicator ; but methyl orange is safer for this sort of work, so runs were made both with methyl orange and with phenolphthalein. Evans states that “the amount of adsorption in a particular case was identical after various periods ranging from five minutes to a week.” This has been found not t o be quite accurate.
TABLE I 5 grams filter paper and IOO cc HC1. 2 5 cc portions titrated in 7-day runs and I O cc portions in other runs. Methyl orange as indicator -
G. equiv. HC1 per liter
0.986 0,489 0.216 0.986 0.489 0.216
0.986 0.489 0.216 0.986
0,489
0.216 0.986 0.489 0.216
Diff. in cc NaOH
M g HCl adsorbed
HC1per adsorbed gram paper
0.31
116.0 32.5 19.5 123.4 35.7 19.5 119.7 3.5.0 ‘3.9
23.2
0.21 0.20
0.33 0.22 0.20
0.32 0.21
0.19 0.32 0.20
0. I O
0.13 0.07 0.05
6 5 3 9 24.7 7 1 3.9 23.9 7 0 2.8
17.9
9 6
14.3 7.3 50.3
2 9 1 5 IO I
11.7
2 3
3.7
0 7
Time of standing
hour hour I hour 3 days 3 days 3 days 3 days 3 days 3 days 7 days 7 days 7 days I O days I O days I O days I I
In Table I are given the data with methyl orange as indicator.
In order t o call attention t o the experimental error
*-I buormal Au'sorptiou by Filter P a p u
623
in this sort of work, the difference in the cubic centimeters of S a O H required to neutralize before and after adsorption is gil-en in the second column. Since 2 5 cc portions were titrated in the seven day runs as against I O cc portions in the other runs, the differences for those runs are not directly comparable with the corresponding data for the other runs. In all cases the adsorption increases with increasing concentration. n'hile the values for one hour are practically the same as those for three days, there is a marked change when we come to the se\-en day runs. The adsorption has dropped to less than half its original value. This cannot be due to a dissolving of alkali from the glass because that would produce an apparent increase in the adsorption. Some change must therefore take place in the filter paper which decreases its adsorbing power for hydrochloric acid. The experimental error is so large in the ten day runs that it is impossible to decide whether the change was still in process, though this seems probable. It would have been interesting to have made a study of the change in adsorption, with increasing time and also to have determined whether a similar phenomenon is to be observed with purified cotton wool; but such experiments were outside the scope of this investigation. It was not deemed desirable to centrifuge the solutions; but it was noted that only about 70 cc solution could be poured off, so that about 30 cc solution were held in and about the fragments of paper. JVhile Evans does not state what kind of filter paper he used, it seemed probable that the trouble was due to his choice of indicator. Kothing is said in his paper about the boiling off of carbon dioxide and therefore two runs were made using phenolphthalein as indicator and not driving out carbon dioxide. The data are given in Table 11. When using phenolphthalein as indicator the apparent adsorptions for the approximately normal and half-normal solutions are much less than when methyl orange is used. In the first run no change in titration could be detected with the most concentrated solution. This confirms Evans. In
TABLE I1 5 grams filter paper and
IOO cc HCI. I O cc portions titrated. Phenolphthalein as indicator
-~ ____ ~
G. equiv. HC1 per liter
0.986 0.489 0.216 0.986 0.489 0.216
~~
-
~ ~
X I g HCl adsorbed
Diff. in cc iSaOH
~ ~~
~
-
Mg HC1 a d - ,
sorbed per gram paper
0.00
0.0
0.0
0.05
8.3 3.7
I .j
0.0j
0.03 0.04 0.05
’
__
~
Time of standing
7 days i
j
days
11.2
2.2
7 days 7 days
6.7 3.7
1.3
j
0.7
7 days
0.7
__
days
the second run the adsorption is apparently greater than for the more dilute solutions; but the total difference in titration before and after adsorption is only 0 . 0 3 cc IXaOH, which is pretty close to the limit of experimental error. It would be legitimate to call the adsorption “doubtful” just as Evans did. Since we have duplicated Evans’ results approximately by working under apparently similar conditions, it seems safe to conclude that the abnormal results obtained by Evans with hydrochloric acid were due to analytical errors. The general conclusions to be drawn from this work are: I . The adsorption of hydrochloric acid by filter paper increases with increasing concentration of the solution within the range studied. 2 . TT‘hen filter paper is allowed to stand with hydrochloric acid solutions for 7-10 days, some change occurs in the filter paper which decreases the adsorption to less than one-half its original value. 3. S o experiments have been made to determine what changes take place on longer standing than ten days. 4. The abnormal results obtained by Evans with hydrochloric acid are due to analytical errors. This work was suggested by Professor Bancroft and has been carried out under his supervision. Cornell Unioersity
