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 potenthat during a titration the &e tial 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
accurate determination of one organometallic compound in the presence of another ordinarily entails
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):
Grams of C&Lir in aliquot = vol. Ce" soln. X NCe+' soln. X 90 55 x 110
+ 202
PROCEDURE
L i a- L i
+
L i o o~ - o o L i
HE
1860
0
ANALYTICAL CHEMISTRY
L
i
o
o
e
L
i
+
L
i
e i 2L
0--
~ O
~
-
i
O
oo L
i
+ ZH+-.
L i o a- o L i
H
-
0
+ 42 ~ i *~
+
H 2ce+4-.+
O = a = o
+ 2 H + 2Ce +a +
Apparatus. Leeds & Northrup potentiometer, platinum reference electrode, standard calomel electrode. Reagents. Dry oxygen, 0.5M sulfuric acid, 0.1000N primary standard
1
1
10 YL
I
I
w wok
20 30 CEMTL SOLL~IOW ADDED
Figure 1. Typical titration of oxidized and hydrolyzed Sample of p-phenylenedilithium using cerate solution
grade cerium(1V) ammonium nitrate (ammonium hexanitratocerate) solution. Preparation of Standard. The cerate solution was prepared after Mellon (4) by dissolving 54.83 grams of ammonium he?ranitratocerate in a little 0.5M sulfuric acid and diluting to 1 liter with water. Preparation of hphenylenedilithium. React 3.73 grams of lithium wire over a &hour period in 130 ml. of pentane (36’ to 37’ C.)solvent with 36 grams of butyl bromide t o form a 1.06N solution of butyllithium as determined by acid titration of a hydrolyzed aliquot. Stir 100 ml. of this filtered solution and reflux with 11.8 grams of dibromobenzene over 24 hours to effect the exchange (9). Method. Remove from a reaction mixture to be analyzed 2 ml. or any other convenient volume of the p phenylenedilithium solution or suspension, and dilute with anhydrous diethyl ether to 50 ml. Saturate the ethereal solution with dry oxygen by passing oxygen into the solution through a filter stick or fritted disk for about 3 minutes, stirring the solution constantly. Hydrolyze the solution by adding 50 ml. of 0.5N sulfuric acid and extract the prepared hydroquinone into the water layer by shaking the two solutions together. (For very small amounts of the dilithium compound, it may be necessary to evaporate the ether to ensure the extraction to the water layer.) Place the calomel and platinum electrodes in contact with the water solution and stir it with a magnetic stirrer. Connect the electrodes to the potmti-
ometer and titrate in the ceric solution from a buret. Note the potential and volume of ceric solution added. At the end point the potential will increase about 400 mv. with the addition of 0.5 ml. of titrant. A self-balancing recording potentiometer could be used to save plotting time, provided the range of the instrument exceeds 400 mv. Comparison of Methods. The gravimetric method used in the past (9) for the analysis of p-phenylenedilithium was compared with this method after only a 16-hour exchange.
Sample 1
2 3 1 2 3
CO, Reaction 0.121N 0,452N 0.191N This method 0.146N 0,563N 0.229N
Calcd. % Yield 48 48 47 58 60 56
quiqone, any good method for the dctermhation of hydroquinone could be substituted ( 1 , 6 ) for the potentiometric titration with the loss of some accuracy. Scope of the Method. As the method described here depends on the formation of a quinone, any dimetallated aromatic which can be oxidized to a diphenol capable of being oxidized to a quinone should be assayable by this method. In this category fall either ortho or para dilithium and di-Grignard reagents as well as similar compounds of other active metals. In addition, compounds with the two metal atoms on di!Terent rings but in positions to form quinones should be analyzable by this method. Cornpowids in this category include 2,2’dilithiodiphenyl, 2,4’dilithiodiphenyl, etc. With certain compounds some coupling has been observed (6)to take place during the reaction with oxygen, and the results of this method may be low by an amount equivalent to the extent of the coupling reaction. Interferences. As the method does not involve an acid-base titration, the presence of lithium hydroxide or other hydrolysis products does not interfere. Alkyllithiums and m-phenylenedilithiums give oxidation and hydrolysis products which cannot be oxidized by ceric ion; therefore, the method is selective for 0- or p-phenylenediithiums in the presence of other types of monometallo-organics. A typical titration is illustrated in Figure 1. ACKNOWLEDGMENT
One of the authors (R.R.O.) thanks the Sherwin-Williams Co. for financial support. LITERATURE CITED
The lower values for the gravimetric analysis might be expected bemuse of the difficulties in extraction and purification of the terephthalic acid. In this case, the acid was separated and purdied by vacuum sublimation. DISCUSSION
Because this method depends on the oxidation of the dilithium compound and subsequent hydrolysis to form hydro-
(1) Furman, N. H., Wallace, J. H., Jr., J. Am. Chem. SOC.52, 1443 (1930). ( 5 ) Gilman, H.,Haubein, A. H., [bid.,
66,1515(1944). (3) Cilman, H.,Langham, W., Rloorc, F. W.,Ibid., 62,2327-35 (1940). ( 4 ) Mellon, M. G., “Quantitative .4nalysis,” p. 442,Thomas Y.Crowell Co., New York, 1955. (5) Muller, E.,Topel, T., Ber. 72, 2i3 (1939). (6)Sastri, T. P.,Gapala Rao, G., AX.AI.. CHEW163, 263 (1958). RECEIVED for review March 27, IO>!). Accepted August 14, 1959.
VOL. 31, NO. 1 1, NOVEMBER 1959
e
1861
