COLLOID SYSTEMS Ih: XITROBENZENE BY FREDERICK STANLEY BROWPi APiD CHARLLS R. BURT
The chief cause of error in the freezing point method of determining niolecular weights of substances dissolved in organic solvents is the change of water content of the solvent in the course of the experiment. It has been shown1 t h a t this error can be eliminated by keeping the solution in contact wth 2 dehydrating agent such as phosphorus pentoxide. with water, or with a salt hydrate pair such a s T\;a2S04,0-10 H20: these keep the activity of the water constant a t a value dependent on their aqueous vapour pressure. K e are investigating the apparent molecular weights in nitrobenzene solution of "associated" substances-zlcohols, phenols, and organic acids. XO difficulty has been experienced in obtaining consistent results with wet or partially met solvent, and it has been found that the degree of association depends on the degree of moisture of the solvent. This makes it particularly desirable t o continue the work with the dry solvent, but here difficulties are encountered. The following is description of a typical experiment :The usual Becl ,nann freezing point method was used, except that about one gram of anhj ('rcus calcium chloride n a s placed in contact with the solvent in the inner tube: a current of dry air was used as an additional precaution against moisture. TTith a solution containing o 439 grams propionic acid in 2 0 05 grams of nitrobenzene, the observed depression changed rapidly with time, the following values being obtained successively:- I ozo", o 962", o 7 3 j 0 , o 68j0, o 630". o 596", o 560°, o jog", The solution became viscous, and on leaving for an hour, set t o an almost solid pasty mass. In other cases the depression does not change with time, but is obviously too small. h solution of o 365 grzms isoamyl alcohol in 19 16 grams of nitrobenzene, for experiments in the im'iist solvent, should give a depression of about one degree: in the presence of phosphorus pentoxide a value of o 040" was obtained. Again, the solution became viscous, and on standing an hour set PO firm that it showed no sign of flowing when the tube was inverted. Chemical reaction between the constituents can hardly explain this behaviour, since it occurs with many dehydrating agents (phosphorus pentoxide, calcium chloride, zinc chloride, alumina, and silica.) and with a wide variety of solutes. Also, j f a known weight of acid is shaken with calcium chloride and nitrobenzene, allowed t o set, then treated with ewess xvater, titration shows t h a t none of the acid has heen destroyed. The explanation seems t o be that powerful dehydrating agents necessarily have a great attraction for any molecule containing an hydroxyl group, as a result of which the solute is ads0rbed.I K e have shown that the amount of acetic or propionic acid removed from solution by calcium chloride varies continuously with the conRobertsand Bury: J. Chem. Soc. 123, 2037 ( 1 9 2 3 ) ;Brown and Bury: 125, 2 2 1 9 (1924). Calcium chloride forms compounds with many alcohols, b u t in dilute nitrobenzene solutions, in which it is insoluble, the renction seems to be limited to the surface, i. e. adsorption takes place. 2
COLLOID S1-STEJ16 IS S I T R O B E S Z E S E
1313
centration of the acid, in accordance with Freundlich's law of adsorption isotherm. Patrick and Jones' have obtainect similar results with silica as adsorbent, This adsorption must lower the surface tension of the dehydrating agentliquid surface: on vigorous stirring, such as occurs in the freezing: point experiments. the dehydrating agents are peptised and pass into solution as colloids. K i t h almost any organic acid, alcohol, or phenol as peptising agent, we have found it possible t o prepare colloidal solutions of phosphorus pentoside. calcium chloride, zinc chloride, or silica, from which t,hese substances
ADSORPTION \OFACETIC & PRCPIO~ICACIDS
FROM NITROBiNZENE BY CALCIUM CHLORIDE
1'0
LOG. CONCENTRATION OF SOLUTION
F1c.
I
cannot be removed by filtration. These filtered solutions show the Tyndall cone phenomenon. The csncentration of silica in these solutions is very m a l l , but with calcium chloride concentrations of several grains per roo grams of nitrobenzene are obtainable: these sols have been observed t o display brilliant Christiansen colours. Sufficiently concentrated soh of phosphorus pentoxide and calcium chloride set to gels on standing. These sols only exist in the absence of moisture, traces of Jvliich cause rapid coagulation :calcium chloride separates out as coarse crystalline needles.
Experimental Most of the experimental work has been sufficiently described above, hut it is desirable to record more fullv the experiments of ackorption, and on the J. Phys. Chem. 29,
I
(1925).
FRLI)L.RICE: S T 4 S L E T BROTVS . I S D CHARLES R. RCR'I'
I 3 14
variation of the concentration of t h e colloid with the conceiitration of the peptising agent. The nitrobenzene used in theqe e\perinients was fractionally distilled, fractionally frozen three times, and, imniediately before use, clistilled in vacuo from phosphorus pentouide. The anhydrous calciuin chloritic was prepared from r e c r y s t a l l i d heuahydra te hy dehydration in a current of hydrochloric acid gas, and was then alloired t o cool in a current of pure dry air: it reacted neutral to phenolphthalein arid to methyl orange. Acetic acid was twice frozen and then distilled : propionic acid and iso-amyl alcohol were fractionally distilled. T4BLE
1
-idsorption of Acetic Acid by ('alcium Chloride nretir acid PPI ~ o o grrrn. nitrobenzene
Giiinii
Illltlal 3. i 1 8
8 293 I1
91
14 i 6
r lnal
Giam. arctic acid ndsoi/xyl I g i a m titlc~iiinc*hlorid(~
PPI
I 'I
,(o
Calculatrd
066;
o 0667
0
I53
o 1-56
0
223
0 22j
3 462
0
7 733 10
99
I3 6;
o 292
0
284
In the adsorption experiments, closed tubes containing neighed quantities of nitrobenzme, calciuni chloride, and acid were kept in a thermostat at z .io('. for five hours, with occnsionzl gentle stirring: after settling, the clear liquid was decznted through a filter. the first portion of the filtrate was neglected and the remainder titrated with alkali. The results are shown in Tables I and 11, and are plotted on a logarithmic scale in Fig. I . The calculated values in the tables arc from the adsorption isotherni equation:a = l.(YI , the coefficients used being:11
for acetic acid propionic acid
0 0
1; 0180 ooojro
I1
0
949
0
384
('OLLOID SYSTEAIR I S S I T R O B E S Z E S E
131.;
The ~~nusuall\low values for I I are prohahl\- due to thc fact that t h e surface of a given n-eight of calcium chloride is riot constant: the Iiiore concentratctl qolutions have a disintegrating effect on the particles; this is specially notice:tl)le when the tubes are vigorously -haken. as in the following csperiments.
I
I
.
CONCENTRATION OF PEPTISING AGENT
Frc,. 2
TABLEI11 Yeriation of the Concentration of ('alciurn C'hl~ride K i t h the C'oncentration of Pepti3ing Agent Peptising agent
*Icetic acid
[so-amyl alcoliol
Coilcentration in gram. per I O O pram.: wliitioii
(;r:~insc d c i i i r n cvliloridc por roo grams solution
0.
0.
.;. 126
0.Ijj
9.260 13.80
0.30j
17.28
2
20.82
3.221
j .7 o j
181 ,461
I .
0.012
10.22
0.082
14.72
0 .j26
A number of tubes containing 2 0 grams nitrobenzene. 4 prams of calcium chloride. and varying quantities of acetic acid were shaken vigorously for about five niinutes, allowed t o settle over night. then filtered through a S o . 40 T h a t m a n filter paper. ('alciwii chloride was estiniated in the filtrate, the rewlts being shown in Table 111, and graphically, 111 Fig. 2 . It is clear that the peptising effect of acetic acid is sinal1 with low concentrations, but that it increases rapidly with the concentration of the acid. If the logarithm of the concentration of calcium chloride is plotted against the logarithm of the concentration of the peptising agent an approximately straight line results, which s h o w that these quantities are connected by an equation of the same form as Freundlich's adsorption isotherin. Similar results were obtained with iso-amyl alcohol as peptising agent. ~
Ed11nrd Dniaes Chemical Luhoroforics, rrizzersztzj College of TVnles, i l b e i yst11 y t h . d l n y 25, lY2i.
