the laboratory. This type of nieasurement appears to be very satisfactory as it has a good reproducibility, is insensitive to variation of flow rate, and possesses an excellent long-term stability.
of the Special Instrumentation Division who made the conductivity bridge used in this work, and particularly TV. R. Thompson for improving it. Appreciation is also expressed to other personnel of this division whose cooperation made this work possible,
ACKNOWLEDGMENT
The author thanks the members of the Electronic and Nucleonic Group
LITERATURE CITED
(I) Harley, J. H., Wiberley, S. E.,
“Instrumental Analysis,” p. 265, Wiley, New York, Chapman & Hall, London, 1954. (2) Xoller, C. R., “Chemistry of Organic Compounds,” p. 633, W. B. Saunders Co., Philadelphia, Pa., 1955. (3) JTherry, T. C., DeFord, D. D., J . Control Eng. 5 , 119 (March 1958). RECEIVED for review September 15, 1959. Accepted March 28, 1960. Division of Industrial and Engineering Chemistry, 136th Meeting, ACS, Atlantic City, K.J., September 1959.
Conductometric Titration of Very Weak Bases in Aqueous Medium FRANC0 GASLlNl and LUClO ZION NAHUM Research Division, Cartiera Vita Mayer &
b Twenty-five weak bases with ionization constants between 1 O-s and 10-l’ have been conductometrically titrated. The determinations are carried out b y dissolving the samples in an excess of aqueous acetic acid and titrating with trichloroacetic acid. Sharp angles are obtained a t the equivalence point and results are accurate. The method has been applied to the differential titration of diacid bases and mixtures of monoacid bases which have been resolved in the presence of ethyl alcohol.
V
TEAK bases are not satisfactorily titrated in aqueous niediuni by the usual conductometric methods (I), because hydrolysis causes very obtuse angles a t the equivalence point and large roundings in its vicinity. Therefore, many authors have reported nonaqueous titrations of very iveak bases using high-frequency conductometric apparatus. These methods have been recently summarized by AIcCurdy and Galt (4), who investigated solvents or solvent mixtures nhich increased the sharpness of the equivalence point angles, thus improving the accuracy. Very \T-eak acids can be conductonietrically titrated with lithium hydroxide, using an aqueous solution of ammonia as a solvent ( 2 ) . By this procedure, which considerably increases the ionization of the acids, the intersection angles were as satisfactory as those given by strong acids using the usual conductometric method, and very weak acidic groups, which ordinarily were not revealed, could be determined. In this work, this same principle was applied to the conductometric titration of very weak bases, titrating these compounds a i t h a strong acid and using an aqueous solution of a weak acid as a solvent.
ERP
Co.,Milan, Italy The presence of an excess of the weak acid, AH, strongly increases the dissociation of the base, B, shifting the following equilibrium
B
+ AH e B H + + A -
toward the right. The portion of the titration curve before the equivalence point represents the change in conductance due to the replacement of the weak acid anions by the strong acid anions, the former combining with the hydrogen ions of the titrant. After the equivalence point, the conductance increases sharply as a consequence of the rapid increment of the free H + ions of the excess titrant. Therefore, the difference beh e e n the mobility of the disappearing anion and that of the anion of the
W V
i llrntl
VOLUME OF T I T R A N T Figure 1. Conductometric titration of weak bases in aqueous acetic acid with 2N trichloroacetic acid I. Pyridine II. Quinoline 111. Aniline
titrant, as n-ell as the ionization constant of the latter, is important in determining the sharpness of the intersection angle. Hence it appears advisable to use as titrant an acid having a 1017 anionic mobility and a large ionization constant and, as solvent, a weak acid with a high anionic mobility. The ionization constant of the weak acid must be much smaller than that of the displacing acid, but large enough to allow a satisfactorily high dissociation of the higher base being titrated. The aqueous titration of several water-insoluble bases is made possible by this procedure. Aniong the various couples of strong and weak acids tried in thePe experiments, results were best with trichloroacetic acid as titrant and aqueous acetic acid as solvent. Some bases were also titrated with benzenesulfonic acid; the intersection angles were of the same order of magnitude as those obtained with trichloroacetic acid, the higher anionic mobility of the benzenesulfonic acid being counterbalanced by the higher ionization constant. Figure 1 shows the curves for the conductometric titration of three bases of different strengths. The decrease of the ionization constant causes larger angles a t the equivalence point, but the curves are still sharp enough to allow the location of the equivalence point with great accuracy. For a given base, the variables which determine the value of the equivalence point angle are the base and acetic acid concentrations. Varying the sample concentration from 50 to 150 meq. per liter and the acetic acid concentration from 200 to 900 meq. per liter the value of the equivalence point angle is not greatly influenced. It is possible to produce sharper angles, increasing the acid concentration a t constant base VOL. 32, NO. 8, JULY 1960
1027
Table 1.
Base Morpholine yPicoline a-Picoline /%Picoline Pyridine p- Anisidine p-Phenetidine Phenylhydrazine p-Toluidine Quinoline p- Aminophenol l,l0-Phenanthroline Aniline o-Toluidine o-Phenylenediamine o-iinisidine o-Aminophenol m-Anieidine m-Sminophenol Creatinine Creatine a-Alanine Glycine
Titration
A\cet'ic Acid Concn., Equiv./ ~ K B Liter 2.64 0.726 1.79 0.874 8.05 0.874 8.15 0.874 8.64 0.726 8.71 0.726 8.74 0.436 8.80 0.726 8.82 0.726 9.00 0.726 ... 0.726 9.14 0.726 9.34 0.726 9 48 0.363 9.48 1.020 9.51 0.436 9.66 0.436 9.80 0.726 9.80 0.436 10.43 0.874 10.72 0.726 11.29 1.460 11.65 1.460
of W e a k Bases Type of
Curve I I I I I I I I I I1 I I1 I11 I11 I1 I11 I1 I11 I11 I1 111
...
...
Meq. Found 6 100 6 220 6 153 6 140 6 143 6.330 6 300 6.137 6 025 4 470 4.503 6.153 6 083 6.263 6 233 6.370 6 317 6.073 6 133 6.093 6 167 6.450 6 423 3 110 3 197 6 267 6.320 4.317 4 330 4.343 4 233 6 307 6 327 4 230 4,307 6 018 6 015 3 837 3 907 9 850 10,090 9.690 9 317 Taken 6.110 6.200 6.096
Recovery, c-/ G
99 100 100 100 99 98 99
8 3 9 0 5 2 3 98 9 98 5 99 2 101 0 101 2 99 6 102 8 99 2 100 3 97 5 100 3 98 2 100 0 101 8 97 6" 96 3"
Titration in 507, water-ethyl alcohol. Table II. Titration of Mixtures of W e a k Bases
Meq. Mixture Diethylamine Pyridine
A~KB 5.74
Pyridine G1y ci n e
3.01
a-Picoline .hiline
1.29
p-;inieidine o-Anisidine
0.80
p-Toluidine o-Toluidine
0.66
Pyridine p-Toluidine
0.19
Pyridine p-Anisidine
0.07
Diet'hylamine a-Picoline Aniline
1028
ANALYTICAL CHEMISTRY
Taken 2.930 3.100 2 260 3 747 3 433 2 777 2 897 3 173 2 480 3 860 2.987 3.333 3.017 3.650 3,840 2.493 2.993 3 043 3.917 2 . 777 2.310 3.777 3.460 2.777 3.163 2.873 3.053 3.047 3.433 2.680 3.010 2.977 2.357 4.360 3.430 2.747 2.083 2.773 2.697
Found 2,853 3.080 2 217 3 640 3 390 2 753 2 865 3 100 2 440 3 713 2.950 3.310 2.920 3.683 3.793 2.497 2 963 2.980 3.900 2 . 770 2.263 3.700 3.347 2.780 3.123 2.917 3.037 2.993 3.420 2.637 3.017 2.883 2.330 4,320 3.383 2.757 2.123
2.760 2.610
Recovery,
53 97.4 99.4 98.1 97.1 98.7 99.1 98.9 97.7 98.4 '36.2 98.8 89.3 96.8 100.9 98.8 100.2 99.0 07.9 99. 6 99.7 08.0 98.0 96.7
100.1 118.7 101.5 99.5 98.2 99.6 98.4 100.2 96.8 98.9 99.1 98.6 100.4 101.9 99.5 97.2
concentration as 11ell as the base concentration at constant acid-base ratio. I n the fornier case, the increase of the acetic acid concentration shortens the straight-line poi tiori of the neutralization curve. For this reason both concentrations TI ere selected t o produce sharp enough equivalence point angles, without too large io~i~idingain its vicinity. EXPERIMENTAL
Apparatus. Resistance nieasurements were made \yith an alternating current Wheatst'one bridge supplied through a n insulation transformer. The characteristics of the bridge w r e : frequency, 50 c.P.s.. sensitivity, 0.057,. Cell capacity was balanced by additional capacitors in parallel t o the variable resi st ance. The cell u-as a modified Jones and Bollinger cell ( 3 ) . The elect'rodes consisted of trvo disks of platinized plat'inum (diameter 2.3 em.) facing each other at a distance of 16.2 cni. The cell was immersed in a water bath therniostatically controlled ivit'hia i 0 . 0 0 2 " C. of the working temperature. The automatically filled microburet had a capacity of 5 nil. anti 0.01-ml. graduations. Procedure and Reagents. I n all titrations, 40 nil. of aqueous or 50% aqueous ethyl alcohol solution containing acetic acid and the sample, in concentrations as indicated. \$-ereused. Determinations were carried out a t about, room t,eniperature and in a nitrogen atmosphere, titrating with 2.Y trichloroacetic acid. Reagent' grade samples of the basrs were titrated as received. after drying. The measured resistance values were between 1000 and 20.000 ohms. Their reciprocals were corrected for volume variations. TITRATION OF M O N O A C I D BASES
Table I lists the analytiial results of the monoacid bases successfully titrated. Each figure represents the mean of a t least t n o determination.; from the values obtained, a mean deviation of 0.62% and a coefficient of variation of 0.19% Ivere calculated. The conductometric titration curve obtained for each compound is related to one of the curves show1 in Figure 1 by the Roman figure in the 4th column of the table. Concentrations of about 6 meq. in 40 nil. of solvent were ubually used; for some poorly soluble baqes it \vas necessary to employ lower concentrations. Bases having ionization constants smaller than could not be titrated in aqueous acetic acid because of the extensive hydrolysiq which resulted in a practically complete curvature of the neutralization line. The-e compounds
I
I
.II
I
I
I
I
I
/
I
2
=t
/’ I
3
i
6
i
VOLUME OF TITRANT Figure 2. Conductometric titration of diacid bases with 2 N trichloroacetic acid Solvent, 50% aqueous ethyl alcohol; sample acetic acid approximately 74 rnmoles/liter; approximately 0.44 equivalent/liter 1. Cinconine 2. p-Phenylenediamine 3. Nicotine
could be successfully titrated using acetic acid in 50yo ethyl alcohol-water mixture as a solrent and a higher base roncentration (about 10 nieq. in 40 nil. of solvent). Ethyl alcohol decreases the, hydrolysis of the n-eak bases and sufficirntly long straight portions of neutralization line were, therefore, obtaiiicd ; the concoinitant decrease of the 11aw tli+sociation causes larger interwction angles. L7nsatisfactorj- results due to lack of solubility or precipitation during the titration n ere obtained n i t h the follon-ing vcry neak bases: m-toluidine, 0- and m-phenetitlinc, and m- and pvliloi onniline. TITRATION
OF
concentration, concentration,
the base, neutralizing the leveling effect of the acetic acid, thus increasing the sharpness of the first end point. Data from the titrations of nicotine, cinconine, and p-phenylenediamine are illustrated in Figure 2. The values of titrant corresponding to the second equivalence point are, within experimental errors, twice as big as those corresponding to the first equivalence point.
DIACID BASES TITRATION OF MIXTURES
The titi :ition curves of the inolioacid I)n\es (Figwe 1) slion a rounding in the ic’gion of the end point due to the salt hydrolj 61s. I n this case this hydrolysis docs not :iffeet the intersection point location, but does exert a n unfavorable influence nhen a diacid base is titrated. I n fact, the straight-line portion of the second neutralization line becoiiics too Jiort and the end point cannot be accurately intcrpolated. Also in this case, the difficulty can be owrconie by using aqueous ethyl nlcohol as a solvent. E t h j l alcohol decreases the hydrolj sis and makes it po-ible to obtain longer straightline portionq. Moreover, ethyl alcohol d e c r e a w both ionization constants of
These titrations were performed in aqueous ethyl alcohol, to overcome the inconvenience due to the hydrolysis phenomena mentioned above for diacid bases. The titration curves (Figure 3) show that in titrating bases with pKB 10, the angle a t the first equivalence point beconics sharper as can be set’n comparing curve 7 (pyridine-glycine) with curve 6 (diethylamine-pyridine) . Curve 8 is related t o the ternary mixture: diethylamine-picoline-aniline, which nas successfully resolved. Three equivalence points w r e obtained although the first intersection angle was very obtuse. The data in Table I1 show the results obtained by titration of binary mixtures having decreasing differences in pKB values. From these data a mean deviation of 0.69% and a coefficient of variation of 0.28% were calculated. LITERATURE CITED
(1) Britton, H. T. S., “Conductometric Analysis,” IT. G. Berl, ed., LIPhysical Methods in Chemical Analysis,” Vol. 11, pp. 51-104, Academic Press, S e w York. - > 1SFil. - -
(2)
