Application of Constant Current Potentiometry to Nonaqueous Titrations of Organic Bases GLENN R. SVOBODA' Chemistry Department, University of Wisconsin, Madison, Wis.
b The general techniques of constant current potentiometry have been extended to the nonaqueous titration of organic amines. A vacuum tube voltmeter was used to measure the potential between two platinum indicator electrodes, polarized by a constant 1 -pa. current. Peak-shaped titration curves were obtained which permitted direct indication of the end point from the meter readings. The electrode system gave reproducible results without pretreatment. A series of bases was titrated with perchloric acid in m-cresol, using m-cresol and acetonitrile (1 to l ) as the sample solvent.
Z (4,
ERO-CURRENT potentiometric non-
aqueous titrations of amine salts 6 ) , aliphatic amines (IO), and aromatic amines (9)have been described. A review of solvents and titrants used for nonaqueous titration of organic amines is given by Fritz and Hamniond ( 8 ) . All of these cited potmtiometric investigations have involved the measurement of the potential between glass and calomel electrodes with a p H meter using the general techniques of conventional p H titrations. Results of an investigation of constant current nonaqueous potentiometric titrations of organic acids (8) indicated that it might be possible to extend the method to similar titrations of organic bases. The results of such a study are presented in this paper. APPARATUS AND REAGENTS
used were practical grade materials obtained from Eastman. PROCEDURE
The titrations were performed using the general techniques of constant current potentiometric titrations (6). Half-cell titration curves were obtained by measuring the potentials of each of two platinum electrodes with respect to a reference electrode. The reference
:" G
/+
D
VOLU ME Figure 1 . Titration curves of benzylamine (left) and aniline (right) in a variety of sample solvent compositions A, E. m-Cresol 8, F. m-Cresol and acetonitrile ( I :1 1 C, G. Acetonitrile D. m-Cresol and methanol (2:l 1
The preparation of the apparatus and electrodes has been described (8). 'The base samples were generally the best grade commercially available. No further purification was attempted except with diphenylguanidine, which was used as the primary standard for thp standardization of perchloric acid. Diphenylguanidine was purified by recrystallization twice from 95% ethyl alcohol, twice from toluene, and drird a t 100' C. Both the acetonitrile and the m-cresol Present address, Freeman Chemical
Corp., Port Washington, !Vis.
1638
ANALYTICAL CHEMISTRY
tllectrocle was a side-arm calomel electrode containing acetonitrile saturated with lithium chloride. The side arni was fitted with an ultrafine fritted disk. Such a salt bridge arrangement did not rliminate the possibility of chloride contamination. However, the presence of negligible quantities of chloride ion had no appreciable effect on the accuracy or precision of the method. Using this electrode system, the potential was measured when the polarizing current was passed between the two platinum electrodes] the platinum anode and the reference electrode, and the platinum cathode and the reference electrode.
Although these measurements were made on different solutions, the reproducibility of titrations of replicate samples did not allow serious displacement of the titration curves along the volume axis of a composite plot of the results of the type shown previously ( 8 )
RESULTS AND DISCUSSION
Preliminary Considerations. As this ivork was oIiginally conceived it was expected t h a t the shift il potential of the polarized platinur 1 anode nould be in the sanie directio 1 as the shift of the polarized platinum cathode when t h e acidity of the solution was changed (3, 8). Honever, it was expected that the direction of potential shift would be opposite to that noted for the titration of organic acids (8). A peak-shaped titration curie was expected when the potential v a s measured between the two platinum electrodes. However, nhen constant current potentiometric titrations were performed in acetic acid, peak-shaped titration curves were not obtained, and the results were not reproducible. Consideration of the possible anodic electrode reactions of acetic acid indicated that the Kolbe hydrocarbon synthesis is highly efficient in nonaqueous solutions of fatty acids a t smooth or platinizedplatinum or gold electrodes ( I ) . S o r was i t possible to obtain peak-shaped titration curves with the ube of neutral solvents such as acetone, acetonitrile, or chloroform. It was evident, then, that a noncarboxylic, acidic solvent must be used. Such a choice was m-cresol. Solvent a n d Titrant Systems. The effect of different ratios of nicresol in acetonitrile or methanol as the solvent was next investigated. The inclusion of the latter solvents was desirable t o reduce t h e viscosity and to ensure better mixing during a titration. The results of this study are shown in Figure 1, where curves B and F , obtained with a solvent composition of 1 to 1 m-cresol and acetonitrile, show the largest potential change at the end point. For all othei titrations cited in this work, the solvent composition was 1 to 1 m-cresol and acetonitrile.
D I P H E N Y L GUANIDINE
Figure 4. Titration curve of quinidine using two
QUINIDINE
polarized platinum electrodes Theoretical end points indicated
9 0.5
w VOLUME TITRANT Figure 2. Titration curves of diphenylguanidine in m-cresol and acetonitrile ( 1 :1) vs. 0.1N perchloric acid in ( A )acetonitrile, ( E ) rn-cresol
For obvious reasons it would also be desirable to decrease the viscosity of the titrant. Thus, a titrant consisting of perchloric acid in acetonitrile and a similar titrant in m-cresol were investigated. The results shown in Figure 2 indicate that a larger potential change a t the end point is obtained when the titrant composition is 0 . W perchloric acid in m-cresol. This effect rnust be considered in the titration of very weak bases, but if the titration technique is to be applied to stronger bases, a titrant composition of 0.1N perchloric acid in acetonitrile could readily be used. Stability of the titrant, 0.lN perchloric acid in m-cresol, used
0.5
ML,
-1
VOLUME TITRANT Figure 3. Titration curves for a number of organic amines Diphenylguanidine n-Butylamine C. Benzylamine D. Diethylaniline E. Ethylaniline F. Aniline G. 1-Naphthylamine H. m-Nitroaniline I . p-Nitroaniline A. 8.
~
for all other titrations cited in this work, was checked over a 4 5 d a y period, during which time a 1% decrease in normality was noted. Titration Characteristics. The results of a study of the half-cell reactions of the constant current nonaqueous titration of organic bases were similar to those of the previous treatment of organic acids ( 8 ) , except that with the bases, the anode shifts to more anodic potentials before the cathode shifts. I n both types of titrations, measurement of the potential between the two polarized platinum electrodes resulted in peak-shaped titration curves. That the peak actually corresponds to the end point of the titration of the bases was checked by the use of crystal violet (where possible), an indicator recommended for the type of titrations described. The titrations chosen t o demonstrate the method are shown in Figure 3. Table I demonstrates the agreement of theoretical and experimental end points. I t is interesting to compare the K B values listed in Table I with the general shape of the titration curves shown in Figure 3. The shape of the titration curves before the peak depends almost entirely on the potential change of the anode. With bases the strength of benzylamine, the anode potential shifts 300 to 400 mv. before the end point. Thus, when two platinum electrodes are used, a potential rise is observed before the end point. As weaker bases are titrated, the shift in anode potential decreases and a smaller rise in potential before the end point iq observed, as is the case with aniline.
Table I.
Since the cathode potential is not affected appreciably until after the solution becomes basic, the cathode shift is practically identical for all bases the strength of, or stronger than, diethylaniline. However, as progressively weaker bases were titratede.g., 1-naphthylamine-the cathodic potential shifts were significantly smaller until this effect ultimately limited the extent to which this method could be applied to very weak bases. m- and p-nitroaniline could be titrated (with difficulty), but o-nitroaniline is such a weak base that no potential shifts could be detected. Titration of Polyfunctional Amines. It was not possible to determine both end points of polyfunctional amines such as p-phenylenediamine or quinidine. A typical titration curve is shown in Figure 4 where quiiiidine was titrated. The first theoretical end point is in the area where the potential rise begins to level off. The point before the potential decrease corresponds exactly to the sum of the two theoretical end points. Such an effect might be expected in the titration of a mixture of weak bases (in the proper range of basicities) where the total basicity can be titrated as accurately as any of the individual bases. In this respect the present work also would be expected to parallel the results of the titration of mixtures of weak acids (8). Electrode Reactions. It would be somewhat premature at this time t o postulate electrode reactions responsible for the potential shifts of the anode and cathode. One might expect t h a t the cathode would follow
Analytical Feasibility of Method as Demonstrated by Organic Bases Titrated and Arranged in Order of Decreasing Ke Values
Compound Diphenylguanidine n-Butylamine Benzylamine Diethylaniline Ethylaniline Aniline 1-Naphthylamine m-Nitroaniline p-Nitroaniline
KB
... 4.1 x 2.0 x 3.6 x 1.3 x 3.83 x 8.36 x 4 x 1 x
10-4 10-5
10-8 10-9 10-lO lo-" 10-11 10-12
Calcd. End Point, M I . Primary standard 4.17 4.71 5.18 3.45 5.01 5.21 4.60 4.59
Exptl. End Point, M1. ... 4.03 4.75 5.20 3.45 5.05 5.23 4.59 4.55
Potential axes shifted for clarity VOL. 33, N O . 12, NOVEMBER 1961
1639
t h e addition of hydrogen ion, whereas one or a combination of several possible reactions might be responsible for t h e anode potential shift. Further work is indicated. CONCLUSION
Simplicity of end point detection and equipment needed makes this titration technique a valuable extension of the constant nonaqueous potentiometric titration of organic acids (8). The accuracy and precision of the method are comparable to, or better than, those of any of the zero-current potentiometric or indicator titrations of bases in nonaqueous solvents previously reported (2). This technique is adapt-
able to automatic titration with the type of instrument described earlier ( 7 ) . Thus, this technique is more convenient than zero-current potentiometric methods. Another valuable aspect of the technique is the ability to determine accurately total basicity of polyfunctional amines. ACKNOWLEDGMENT
The author is grateful to Irving Shain for helpful comments and assistance in the preparation of this manuscript. LITERATURE CITED
(1) Allen, M. J., “Organic
Electrodc Processes,” Reinhold, New York, 1958.
(2).Fritz, J. S., Hammond, G. S., “€Juan-
titative Organic Analyses,” Wiley, New York, 1957. (3) Harlow, G. A., Noble, C. M., Wyld, G. E. A., ANAL.CHEM.28,784 (1956). (4) Pifer, C. W., Wollish, E. G., Zbid.,
24.300 (1952). ( 5 ) Pifer, ‘c. ‘C. LV., LV., W Wollish, O~ E. G., J . Am. Pharm. Assoc. 40,609 (1951). (6) Reilley, C. N., Cooke, W. D., Furman, N . H., ANAL.CHEM.23, 1223 (1951). ( 7 ) Shain, I., Huber, C. O., Zbid., 30, 1286 (1958). (19581. ((88 ) Shain, I., Svoboda, G. R., Zbid., 31, 1857 (1959). 185 ( 9 ) Siggia, si-- , S.,, Hanna, J. G., Kervinski, I. R., R., Ibid., 22, 22,1295 1295 (1950). (10) Wagner. Wagner, C. D.. B Brown, R. H., Peters, E. D., J . Am. Chem. SOC.69, 2611 (1947).
RECEIVED for review February 24, 1961. Accppted Augrwt 17, 1961.
Differential Thermometric Titrations and Determination of Heats of Reaction BRUCE C. TYSON, Jr.,l W. H. McCURDY, Jr.,2 and C. E. BRICKER3 Department o f Chemistry, Princeton University, Princefon, N. 1.
b An apparatus utilizing thermistors for conducting thermometric titrations, which measures the difference in temperature between the reaction vessel and a blank solution, is described. To eliminate a high impedance recorder and simultaneously to realize the sensitivity of high resistance thermistors, four 100,000-ohm thermistors are connected in parallel for each of the detectors in the differential circuit. The response of the electrical circuit is analyzed mathematically in order to achieve a linear response of the circuit to differences in temperature between the two sensing elements, Because of its linear response, electrical power can be used as a standard for calibrating the apparatus, so that heats of reactions can be measured rapidly and simply.
D
the past eight years there has been considerable interest in thermometric titrations (9, 15-16, 18, 20, 21, 28-50)-that is, titrations in which the course of the reaction is followed by observation of the heat absorbed or liberated. The literature on this type of titration has been well URIKQ
1 Present address, Strategy and Tactics Analysis Group, U. S. Army, Bethesda, Md. 2 Present address, Department of Chemistry, University of Delaware, Newark, Del. 3 Present address, The College of Wooster, Wooster, Ohio.
1640
ANALYTICAL CHEMISTRY
covered in two excellent review articles (12, 27). Many thermometric methods for following titrations have been limited because the temperature-sensing device has been too insensitive or because heats other than just the heat of reaction have been detected. It seemed reasonable t h a t thermometric methods could be applied more widely if a more sensitive circuit could be designed that would measure only the heat of reaction. With such an apparatus it should be comparatively easy to determine calorimetric data for calculating heats of reaction. A differential thermometric apparatus seemed to be the best solution to this problem. Muller and Stolten (28) and Higuchi et al. (11) have used differential thermistor circuits for thermometric determinations of molecular weights, and Pakulak and Leonard (24) have used a shunted differential thermistor circuit in differential thermal analysis. I n the differential apparatus designed for this work, temperature-sensing devices were placed in both the sample and blank solutions. Sensitivity greater than that previously reported for thermometric titrations has been obtained and extraneous heat effects, such as stirring and heats of dilution, have been greatly minimized. This apparatus has been used to follow 14 d 8 e r e n t reactions and the heat of the reaction has been calculated for 11 of these reactions.
EXPERIMENTAL
Chemicals. Common chemicals were of reagent or analytical grade. The hydroxyamines were supplied by the Commercial Solvents Co. and were purified by distillation except for tris(hydroxymethy1) - aminomethane which needed no purification and for 2 - methyl - 2 - amino - 1,3- propanediol which was recrystallized from acetone (26). Apparatus.
Motor-driven burets similar to ones designed by Lingane (19) and by Jordan and Alleman (15) were used. -4 60-cycleJ 110-volt, 1r.p.m. synchronous motor from the Holtzer Cabot Co. was geared so that a 16-thread-per-inch screw was turned at 4 r.p.m. This screw, in turn, drove a threaded brass block which pushed the plungers of two horizontally mounted 5-ml. syringes. Only one size of syringes and one set of gears were used, but others could have been substituted to vary the delivery rate of the titrant. The delivery rate, which was 0.01110 ml. per second for each buret, was determined by measurement of the weight of titrant of known density delivered during a determined number of revolutions of the gear driven by the synchronous motor. During each calibration, the delivery rates of the two burets were found to be the same within 1 part per thousand, although the syringes were not a specially matched pair. I n fact, a third syringe was substituted and no appreciable change in delivery rate was found. These delivery rates were checked periodically over a period of 7 months and the mean deviation of all of the values was 1 part per thousand. Each syringe was connected to a
