Potentiometric Titration in Nonaqueous Solutions Solvents 4 . E. RUEHLE, Bell Telephone Laboratories, Yew 1-ork, N. Y.
T
HE successful use of the alcohols as titration media has permitted the direct determination of acidity of various materials not soluble in water. Ethyl alcohol (3'). n-butyl alcohol ( 6 ) , and amyl alcohol (6) have all been used, the latter two in the virtual absence of water. The present work Tas undertaken in an attempt to utilize the solvent properties of the ethylene glycol monoalkyl ethers (CellosolJ-es) and other
the most widely used solvents for numerous organic compounds and mixtures. [Dioxane, as well as numerous other solvents, was rejected recent'ly (4) for potentiometric titrstion because of it's low conductivity and low solvent power for oxidized oils. The conditions in this case, however, were different than the present one.] Apparatus
The appai,atub wed consisted of a Leeds B- Sorthrup Type I< potentiometer and 2500-e galvanometer, a saturated calomel half-cell, a quinhydrone electrode, a titration cell, and a storage system for alkali ( 1 , 7). Titrations have also been carried out n.ith the thermionic titrimeter ( 1 ) .
0 40
0 32
Reagents 0.24
SOLVEKTS. The ethylene glycol monomethyl ether \vas purified by treating the commercial product as fol1on.s: Add 30 grams of animal charcoal to 4 liters of the solvent,, stir, and filter. Add an additional 30 grams of the charcoal and 500 grams of anhydrous sodium sulfate. Allow to stand overnight, add 20 grams of sodium wire, and distill in t~7cuoat a temperature of 75" C. The other solvents n-ere obtainable commercially in a relatively pure state. . ~ L K A L ISOLUTIOS.-40.05 11- solution of potassium hydroxide in the purified ether, prepared and stored as with butyl alcohol solutions, was used as a standard reagent for the titrations in this solvent. The stability of such a solution is comparable to that of similar butyl alcohol solutions (7). The sodium butoxide in butyl alcohol used in part of the viork was prepared and stored as previously described ( 7 ) . L I T m m r CHLORIDE SOLUTIOSS.Lithium chloride (c. P.) was dissolved in the purified solvent (or in n-butyl alcohol) by reflusing 200 grams of the salt with 1 liter of solvent. ACIDS. The acids used were of c. P. quality with the exception of the benzoic acid, which \?-as Bureau of Standards material and was used for standardization of the reagent. H L A K K ON ~ ~ E A G E S The T Sblank . titration on 100 cc. of Solvent containing 50 mg. of quinhydrone usually !!-as found to be 0.G re. or less of 0.05 S reagent.
V O L U M E O F A L K A L I IN CUBIC C E N T I M E T E R S
FIGFRE1.
TITR.4TIOSs IN
ETHYLENE GLYCOLfifOSOMETHYL ETHER
solvents in acidity determinations for subktances not appreciably soluble in the alcohols already studied. X a n y titrations were performed with solutions in the monomethyl ether (methyl Cellosolve) ; a fen- were tried in the monoethyl ether with equal success; t h e higher ethers have not been investigated but ought to offer no difficulties, should their solvent properties prove especially desirable. In addition to the pure soh-ent system employing methyl Cellosolve, several mixed systems have been used with success. I n these cases acetone, anisol (CsH5.0CH3),and 1,kdioxane (C4HgOP) have each been used to enhance the solvent action of n-butyl alcohol. The n-butyl alcohol is apparently necessary to ensure normal behavior of the quinhydrone electrode. Each of these diluents has solvent properties not exhibited by butyl alcohol. Thus, anisol is an excellent solvent for asphalts and pitches (6).dioxane readily dissolves the heavier oils, while acetone is one of
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FIGVRE 2. 130
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T I T R A T I O S S I S ACETOSE-BCTYL
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ALCOHOL
01
ANALYTICAL EDITION
MARCH 15, 1938
13 1
Experimental ETHYLENEGLYCOLNONOMETHYL ETHER. Solutions of benzoic, trichloroacetic, and acetic acids were prepared approximately 0.05 1V in strength. Ten cubic centimeter aliquots were titrated in 100-cc. volumes of solvent to each of which 10-cc. portions of the lithium chloride solution had been added. Sharp end points were obtained (Figure l), and the titrations were characterized by steady potentials and rapid assumption of equilibrium after each addition of reagent. eo0
800
FIGURE 4. TITRATIOSS IN ~,~-DIOXASE-BIJTYL ALCOHOL
DIOSAXE-BUTYL ALCOHOL. The same instability of the quinhydrone electrode as was encountered in anisol solutions Tas also found with dioxane, but in this case too i t disappeared when the solvent n-as mixed with butyl alcohol. Titration curves in this mixture are shown in Figure 4.
Summary FIGURE 3. TITRATIOSS IS ANSOL-BUTYLALCOHOL
Samples of cellulose nitrate and of cellulose acetate were also titrated. These materials are readily soluble in the glycol ethers and do not interfere with the proper functioning of t h e quinhydrone electrode. Aliquots of acetic acid added to solutions containing cellulose acetate were completely titrated, showing the feasibility of acidity determinations of weak acids in this solvent. ACETOSE-BUTYLALCOHOL. For practical purposes it is useful to employ sodium butoxide in butyl alcohol as the reagent ( 7 ) ,especially if i t is in constant use for other purposes i n the same laboratory. Also i t is advantageous to add 10 cc. of a saturated solution of lithium chloride in butyl alcohol to each 100 cc. of acetone before titration, especially if a potentiometer is to be used. Titrations of benzoic acid, acetic acid, and salicylic acid, using the quinhydrone electrode as the indicator are shown in Figure 2. The titers were the same as those obtained in butyl alcohol solution, using the same reagent. ASISOL-BUTTLALCOHOL.Pure anisol is unsatisfactory as a medium for the quinhydrone electrode, because of apparent instability of the quinhydrone as the end point is approached. The inhtability is characterized by the appearance of a bluegreen color and by drifting potentials. K h e n sufficient butyl alcohol is present the quinhydrone electrode behaves normally and the solvent powers of anisol for asphalts and pitches are not seriously impaired. Titrations of several acids and of a typical asphalt ale shown in Figure 3.
Use has been macle of the monomethyl ether of ethylene glycol as a medium for the potentiometric determination of acidity of materials soluble in this solvent. Acetone, anisol, or 1,4-dioxane can be used to enhance the solvent poTvers of butyl alcohol without interfering with the functioning of the quinhydrone electrode in this solvent. This extends the possibilities of acidity determination to materials soluble in such solvent mixtures.
Acknowledgment The author wishes to thank L. A. Wooten for his helpful criticism and advice in this work.
Literature Cited Clarke, \Tooten, a n d Compton, ISD. ESG. C H E J f . , Anal. Ed., 3, 321 (1931). Demarest and Rieman, Ibid., 3, 15 (1931). Iiremann and Schijpfer. Se$e, 6 (Xos. 35-8) (1922). Rescorla, Carnahan, and Fenske, ISD. ESG. CHEM.,Anal. Ed., 9,505 (1937). Seltz and McKinIiey, ISD. E X . CIiEx.. 20, 542 (1928). Seltz and Silverman, Ibid., Anal Ed., 2, 1 (1930). F o o t e n and Ruehle, Ibid., 6, 149 (1934). R E C E I V E DDecember 28 193i.
