replicate runs) waa attained with many of the compounds investigated, and, although with poorer accuracy, very high sensitivities were demonstrated. The best solvent system for the reduction of mononitro compounds was 4-to-1 methanol-to4.5N aqueous lithium chloride solution, but other methanolwater solutions also proved acceptable. Although the main problem encountered in this study-the background current of the solvent systemcould not be eliminated, o u t g ~ i n g , rapid stirring, and pre-electrolysis of the
supporting electrolyte solution reduced the interference to tolerable limits. REFERENCES
R. s., Fuman, N* H.,ANAL. CHEY.27, 1182 (1955). (2) Bran& w. w., DeVnes, J.E.7 Gmtz, E.,Ibid., 27, 392 (1955). (3) DeVries, T., Ivett, R. W.; IND. ENG. CHEM.,ANAL.ED.13,339 (1941). (4) Ehlers, V. B., Seage, J. W., ANAL. CHEM.31, 16 (1959). (5) Findeis, A., Dissertation Abslr. 17, (1)
1909 (1957).
(6) Koniecki, W., Linch, A., ANAL.C ~ M 30, 1134 (1958). (7) Lingahe, J. J., IND.ENG. CHEY., ANAL.ED.16, 150 (1944). (8) Lingane, J. J., Small, L. A., ANAL. CHEM.21, 1119 (1949). (9) Tirouflet, J., Bull. SOC. chim. Frame 1956, p. 274. (10) Vanderzm, C. E., mgell, W. F., ANAL.CHEM.22,573 (1950). (11) waller, J., Chm. (LO&) 72, 787 (1955). (12) wolthuis, E., s., L., ANAL. 263 1238 (1954).
RECEIVED for review March 27, 1959. Accepted August 13, 1959.
Application of Constant Current Potentiometry to Nonaqueous Titrations of Weak Acids IRVING SHAIN and GLENN R. SVOBODA Chemistry Department, University o f Wisconsin, Madison, Wis.
b Nonaqueous titrations of weak acids were investigated using the general techniques of constant current potentiometry. Two platinum indicator electrodes were polarized by a constant 1 -pa. current, and the potential between the two electrodes was measured with a vacuum tube voltmeter or with a pH meter. In most cases typical peakshaped titration curves were obtained which permitted direct location of the end point from the meter readings. Although the potential-determining electrode reactions are complex, the electrode system gave reproducible results without pretreatment. Several different weak acids were titrated with tetrabutylammonium hydroxide, using acetone as the solvent.
I.o
R
investigations of the nonaqueous titrations of weak organic acids have led to many new applications and refinements of the technique. Reviews have been included in several papers (2, 3, 11). Almost all of the work reported involved the measurement of the potential between glass and calomel electrodes with a p H meter using the general techniques of oonventional p H titrations (2,3, 6). However, other electrode systems have also been investigated using zero current potentiometry such as platinum-calomel and platinum-platinum (6),glass-silver (f4), and platinum (10% rhodium)graphite (8). The results of Harlow, Noble, and Wyld (6) indicated that both preanodized and precathodized platinum electrodes respond to changes in effective acidity in nonaqueous solvents.
C
VOLUME TITRANT
Figure 1. Titration curves salicylic and benzoic acids A.
ECENT
2F
6. C.
of
Potential of platinum anode with respect to S.C.E. Potential of platinum cathode with respect to S.C.E. Potential between the two platinum electrodes
VOLUME TITRAM
Figure 2. Titration curves of several weak acids using two polarized platinum electrodes A.
Although these authors did not postulate any mechanism through which these pretreated electrodes would become sensitive to acidity changes, it seemed reasonable to assume that the preanodized electrode was a platinumplatinum oxide electrode (?', I 2 ) , and that the precathodized electrode was a type of hydrogen electrode. (The actual electrode processes are considerably more complex.) The sensitivity to changes in acidity was not permanent, however, and Harlow, Noble, and Wyld indicated that the electrodes had to be reconditioned before each titration. It was expected that this disadvantage could be overcoine if a small constant
Benzoic acid 6. Acetic acid C. Dichloraacetic acid D. Salicylic acid E. p-Nitraphenol F. m-Nitrophenol G. Methyl salicylate
current were passed through the solution during the titration. The results of such a study are presented in this paper. APPARATUS
The titrations were carried out using the general techniques of constant current potentiometric titrations (10). The current source consisted of two 45volt batteries in series with the proper VOL. 31,
NO. 1 1 ,
NOVEMBER 1959
a
1857
resistors to furnish approximately 0.5, 1, 2, 3, 5, or 10 pa. of polarizing current through the cell. Except for those mses in which the effect of variation of thc polarizing current was investigated, all the titrations were carried out using a I-pa. current. The potential between the two platinum electrodes was measured with either an inexpensive vacuum tube voltmeter (approximately 10-megohm input impedance) or a Beckman Model H-2 p H meter. The electrodes were made by sealing platinum wire (0.016 inch in diameter) into soft glass tubing so that about 1 cm. of wire was exposed. All other details of the titration procedure were conventional and were similar to those used by other workers (.2,5). I n several titrations the potentials of the platinum electrodes were measured individually with respect to a reference electrode. Because a stable reference electrode which was also capable of handling the necessary currents could not be readily found, a three-electrode system was used. The third (reference) electrode was a Beckman fiber-type calomel electrode containing, in the outer compartment, acetone saturated with lithium chloride. This electrode was connected to the titration cell by means of a salt bridgeluggin capillary arrangement containing a solution of the tetrabutylammonium salt of the acid being titrated. (Actually, a solution of the wid which had previously been titrated just to the end point was used.) The solution in the salt bridge was changed often enough to eliminate all possibility of contamination of the solutions titrated. Using this electrode system, the polarizing current passed between the two platinum electrodes as before, and the potential was messured in three ways after each addition of titrant: between the two platinum electrodes, and between each platinum electrode and the reference electrode. By making all three measurements on the same solution a t the same point in the titration, erroneous displacement of the titration curves along the volume axis (Figure 1) was eliminated. The potentials were measured by a very high input impedance “follower” cirruit----based on DeFord’s (4)wo& with analog computer amplifiers-which, in turn, fed a vacuum tube voltmeter. Currentcvoltage curves were obtained \iith a general-purpose voltammetric rclsearch instrument similar tu that described by 1)eFord (4) and also based on analog computer amplifiers. ,. I hc, working electrode was a conventional rotating platinurn electrode :isscmbly (1080 r.p.ni., 1-cm. platinum wire, 0.016 inch in diameter, at right angles to the axis of rotation). The counterelectrode was a large platinum electrode immersed in the solution. The reference electrode was similar to that described above. The circuit was arranged so that the current flowing between the working electrode and the counterelectrode was measured as a function of the potential between the working electrode and the reference electrode. The rate of voltage scan was 5 mv. per Decond. 1858
ANALYTICAL CHEMISTRY
t - T
Figure 3. Titration curve of a mixture of dichloroacetic and acetic acids using two polarized platinum electrodes
Z
W
t 0
I
1.0
I
2.o
I
3.0
VOLUME TITRANTML
I 4 .O
REAGENTS
The acid samples were generally the best commercially available. No further purification was attempted. The tetrabutylammonium hydroxide (methoxide) (1) was prepared according to the procedure of Cundiff and Markunas (8). RESULTS AND DISCUSSION
It was expected, when this work was originally conceived, that the shift in potential of the polarized platinum anode would be in the same direction as the shift in potential of the polarized platinum cathode when the acidity of the solution was changed (6). Furthermore, the only previous investigation in which platinum electrodes were purposely polarized, in nonaqueous acid-base titrations, was reported by Phillip (9) who obtained conventional S-shaped titration curves. Therefore, the titrations were performed by using individual platinum electrodes-either anode or cathode-us. one of several forms of modified calomel electrodes. The titration curves obtained under these conditions showed definite potential shifts in the region of the end point. However, the potentials were not reproducible, and attempts to keep chloride out of the titration vessel by means of suitable salt bridges, introduced so much resistance that several volts ZR drop developed between the electrodes. Nevertheless, the major shift in the potential of a platinum cathode occurrtd significantly earlier in a titration than when the same titration was repeated using the platinum electrode as an anode. I n order to be sure that this effect was not caused by some error in pipetiing (these observations were made on different titrations) a three-elwtrode system was devised so that there could be nc error in correlating the individual titration curves with each other along the volume axis. Such titration curves are shown in Figure 1. I n addition, the titration curves obtained when the potential was measured between the two platinum electrodes during the same titration are included. For the acids indicated, the cathode shifts to more cathodic potentials significantly before the anode
2.21 2.0Ln
5 9
1.8-
Ai
4
5
6 Q
1.61.4-
1.2-
1,
bq
-p_I
g , ,h, VOLUME TITRANT
Figure 4. Effect of variation of polarizing current on titration curves of acetic acid Polarizing current in p a .
A.
B.
c.
10 5 2
D. 1 E . 0.5
shifts, and the potential between the two platinum electrodes exhibits a marked peak. That the peak actually corresponds to the end point of the titration was also checked by the use of thymol blue and azo violet-indicators normaliy reconimended for the titrations described-and in addition, by thc current voltage curves obtained. The titrations chosen to demonstrate the method are shown in Figure 2. The shape of the titration curve before the peak depends almost entirely on thc potential change of the cathode. With acids of thc strength of salicylic acid the cathode potential shifts 200 to 30C mv. before the end point. Thus, when two platinum electrodes arv used. : F potential rise is observc(1 IJcfore thc end point. As weaker acids are titrated. however, the shift in cathode potentia: decreases and a smaller rise i n potentia! is observed before the eud point. W h i acetic acid is titrated, the shift in cathode potential is essentially con,pleted before the titration is started. Because the anode potential is no: affected appreciably until after the: solution becomes basic, ths anode shi;:
i.8 1
Figure 5. Effect of input impedance of potential measuring device A.
pH meter Conventionol vocuum tube voltmeter
1
VOLTS,VS. S.C.E. 12 0.6 I
I
0 I
Figure 6. Anodic current-voltage curves of salicylic acid solutions in acetone
From Figure 2, it would appear that differentiating titrations might be performed on mixturel of two acids, one of which is of the strength of acetic acid is practically ideiitical for all acids of or stronger, and the second is of the strength similar t,o, or stronger than, acetic acid. However, as progressively strength of p-nitrophenol or weaker. Such a titration would depend on a weaker acids art: titrated-e.g., nitropartial shift of the anode potential when phenols-the . d u t ion becomes alkaline the stronger component is titrated, and enough so that part of the anodic on the completion of the anode potential potential shift takes place before the shift when the second component is titration starts. This last effect ultimately limits the extent to which this titrated. These titrations have been method can be applied to very weak only partially successful because of the normally unimportant shifts of the acids. A nietl~ylsalicylatc: solution can entire titration curve along the potential lw titrated (with difficulty) but phenol axis. is such a n-rak :Icid that all potential shifts take place M o r e addition of base. Effect of Variation of Polarizing Current. The effect of varying the The shape of tJIiepeak and the magnipolarizing cui,rent on the titration tude of the pckential change a t the peak curves of acetic acid using two of these titrzition curves are reproduciplatinum electrodes is shown in Figure ble. H o w v w , t.he actual potential 4. Although the total shift in the ineasured may vary up to 100 mv. from anode potential increased after the one replicate i,itration to another. end point, as the polarizing current This effect, apparently caused by \vas increased, the curves were genervariations in surface characteristics of ally more roundrd a t the higher the platinum electrcdes, merely results currents. For the particular size elecin shifting the titration curves along thc trodes used in these titrations, a I-pa. vertical (potential) axis. It has no current seemed most satisfactory. This i>ffcct on the accuracy of the titration. corresponded to a current density of ihiririg any one titration the potential about 8 pa. per sq. cm., based on the rc,:itlings are assiimctl very rapidly and gross surface area of the electrode. art’ st:tble. The shift of the titration curves along the Titration of Mixtures. As prepotential axis was due to both a change dicted from Figure 2, it is not possible in electrode reaction potentials and to to differentiate hetween various acids the I R drop in the solution. of the strength of acetic acid or Effect of Input Impedance of the stronger ivhen t.hey are present in a Potential Measuring Device. Bemixture. The only difference because the shape of the titration curve tween the tit>ration curves of the obtained depends to some extent on individual acaids in t,his group appears maintaining a constant polarizing curin thr cathock potential shift before the rent through the cell, some distortion cnd point. These potential changes are results, if significant current is drawn in general rather small and not very by the potential measuring device. sh:irp. Thi- i,s clrrnonstrated (Figure 3) Conventional vacuum tube voltmeters \\.hen B riiisture of tlichloroacetic and have an input impedance of the order :icetic acids \\ere titrated. The rise in of 10 megohms and the current drawn potential near the center of the curve may be as much as 10% of the 1-pa. rnrrc~spnii~li.(approximately) to thtx polarizing current. This effect is sh0n.n ,lichloroact~tic:icid concentration. Th(h in Figure 5 , where titration curves f n r peak corrcymntfs twxtly to the sum of salicylic acid arc’ compared. The lower the two acitl concentrations. Thus thta input impedance device caused a slight total acidity of a complex mixture of rounding of the mrve a r d a decrease in iveali acids (in the proper range of :tcidities) can be titrated as accuratrl~. the peak height. ‘This was not scrioiis for L: titration of 3 moderately weak acitl :is any of the individual acids. 6.
(acetic acid or stronger), but it affcctetl the ease of end point detection for very weak acids. For many cases, h o \ v e v t ~ , an inexpensive vacuum tube voltnicter would be suitable for the perfornutncc of these titrations. Electrode Reactions. Polarogi.:rms of various solutions of weak acids and their tetrabutylammoniuni salts in acetone were obtained by using :I rotating platinuni electrode in an attempt to define the elcctrode processes. These processes are rather coniplicakd, and further stuc1ic:s are now in progress. Preliminary results indicated, however, that the important cathode reaction is the reduction of tlissolved oxygen, and that if oxygen is renioved from the solution, the cathode potcntial shifts to very negative (cathodic) values. Cathodic polarograms were obtained on air-saturated solutions comparable to those encountered during a titration of salicylic acid with tetrabutylaniinonium hydroxide. Each solution contained 0.1M tetrabutylammoniunl perchlorate as the indifferent electrolyte,. Because the oxyren limiting current was very high (more than 25 pa.) and the polarizing current used for the titrations was only 1 pa., the potential a t which the current reached 1 pa. in each polarogram was measured for compnrison purposes (Table I). Although the potentials were reproducble to only about 30 niv. in thew preliminary expt,riments, the behavior confirms the obsrrvations made in tht, titrations with the three-electrode systems--i.c., the major change of thc cathode potenti:rl occurs before the entl point. The polarogranis observed on anodic* potential scans are very complex, and i l l fact appear to be different for each acitl studied so far. Only some preliniinary polarographic observations on solutioris containing salicylate are presented hcrc,. Figure 6 indicates the type of polarographic behavior encountered. Curvcs A and B were obtained on a solution coiltaining 10-3M salicylic acid with 0.lM tetrahutylammonium perchlorate as the indifferent electrolyte. Curve B vxs
Table I. Potential at Which Current Reaches 1 pa. on Cathodic Polarograms of Air-Saturated Salicylate Solutions
Potential Modified
m.
Solution0 10-1.W salicylic acid 10-3M salicylic acid 10-4M salicylic acid
Indifferent electrolyte onll. 10-3df tetrabutylammonium hydroxide
S.C.E. -0 “1 -0.36 -0.60
-0 66 -0 7 ,
Each solution contained 0.1.11 tetrabutylammonium perchlorate as t h e indifferent electrolyte. 0
VOL. 31, NO. 1 1 , NOVEMBER 1959
1859
obtained on the first anodic sweep using a clean platinum electrode-i.e., an electrode which had previously been held a t reducing potentials. Upon repeating the potential scan without cleaning the electrode, curve A was obtain&. W e B could be repeated by setting the electrode a t cathodic potentials for a few minutes before starting the scan. This behavior indicates that some sort of a surface film is produced by the electrode reaction taking place during the anodic peak. This film may be either a product of the electro-oxidation of salicylate or a platinum oxide film, or a combination of both. In any event, the surface film inhibits further electrode reaction and the potential corresponding to the 1-pa. polarizing current used during the titration can be correlated with curve A . Essentially, the same behavior, with no significant changes in the anodic potentials or in the shapes of the polarograms were observed with 10-2 and lO-*M solutions of snlicylic acid. Upon the addition of tetrabutylammonium hydroxide to make the solution approximately 10-3M in base, curve C was obtained, indicating that the film is either soluble or permeable in basic solution.
This again confirm earlier ob-rvations that during a titration the &e potential remains easentially constant until the solution becomes basic, at which time the change in anode potential would correspond to a shift from curve A to curve C. A further investigation of the electrode reactions involved is now in progress. CONCLUSION
The ease of detection of the end point and the simple equipment required make this titration a valuable analytical method. The accuracy is a t least as good as for other nonaqueous titratim methods for individual weak acids, and the possibility of accurate determination of the total acidity of certain weak acid mixtures complements the differentiating titrations of Fritz and Yamamura (6). I n addition, these titrations are readily adaptable for use with the automatic titrator described earlier for constant current potentiometric titrations (IS). LITERATURE CITED
(1) Cluett, M. L., ANAL. Cmm. 31, 610 ( 1959).
(2) CunditT, R. H., Markunas, P. C., Zbid., 28,792 (1956). (3) Deal, V. Z., Wyld, G . E. A., Ibid., 27,47 (1955). (4) DeFord, D. D.,Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Cali., April 1958. (5) Fritz, J. S.,Yamamura, S. S., ANAL. C ~ M29,1079 . (1957). (6)Harlow, G. A., Noble C. M., Wyld, G. E. A., Ibid., 28, 784 (1956). (7) Kolthoff, I. M.,Tanaka, N., Zbid., 26,632 (1954). (8) Malmstadt, H. V., Vassallo, D. A., Zbzd.,31,206 (1959). (9) Phillip, B., Chem. Tech. (Berlin) 9, 581 (1957). (10)Reilley, C. N., Cooke, W. D., Furman, N. H., ANAL.C ~ M23, . 1223 (1951). ' (11) Riddick, J. A,, Zbid. 30, 793 (1958). (12) Ross, J. W., Shah, i.,Zbid., 28, 548 (1956). (13) Shah, I., Huber, C. O., Zbid., 30, 1286 1958). (14) Ya ubik, M. G., Safranflki, L. W., Mitchell, J., Jr., Ibzd., 30,1741 (1958).
k
RECEIVED for ieview July 2, 1959. Accepted August 20, 1959. Preaented in part a t the Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., hlarch 1959. ported in art by funds received Atomic Aergy Commission under Contract No. AT (1 1-I)&.
EAkL?E
Determination of p-Pheny lenedilit hium by Potentiometric Titration with Cerium(lV) Nitrate Solution A. F. CLIFFORD and R. R.
OLSEN
Deportmenf of Chemistry, Purdue University, lafayeffe, I d .
b A quick convenient method to determine accurately even small amounts of p-phenylenedilithivm in the presence of monometallo-organics has been developed which involves the oxidation and hydrolysis of the dilithium compound to form hydroquinone. Potentiometric titration of the hydroquinone formed is carried out with standard cerium(lV) nitrate solution using a standard calomel electrode and a platinum reference electrode. Intermediates and side products cannot interfere with the determination, as the method involves the titration of only the oxidized dilithium compound. The method should be amlicoble to the analysis of any dilithium compound capable of yielding a quinone.
T
either reactions to form widely differing compounds, then separation and purification of such a mixture (8,S), or a selective reagent or reaction for one of the organometallics. A convenient and quick method was required for the determination of pphenylenedilithium obtained in the exchange reaction between n-butyllithium and p-dibromobenzene (8) where excess n-butyllithium and pbromophenyllithium are present along with the dilithium compound. The method reported here determines only the organometallic present as pphenylenedilithium and is best summed up in the following equations (6): L i a- L i
1860
0
ANALYTICAL CHEMISTRY
+
L i o o~ - o o L i
HE accurate determination of one
organometallic compound in the presence of another ordinarily entails
+ 202
L
i
o
o
e
L
i
+
L
i
e 2L
~
-
i
0--
~ O
-
O
oo L
i
+ ZH+-.
L i o a- o L i
H
i
0
+ 42 ~ i *~
+
H 2ce+4-.+
O = a = o
+ 2 H + 2Ce +a +
Grams of C&Lir in aliquot = vol. Ce" soln. X NCe+' soln. X 90 55 x 110 PROCEDURE
Apparatus. Leeds & Northrup potentiometer, platinum reference electrode, standard calomel electrode. Reagents. Dry oxygen, 0.5M sulfuric acid, 0.1000N primary standard
