V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3
1832
liberated iodine with thiosulfate has the additional advantage that for a given amount of iodide the required amount of reagent equivalents is three times as large as in previous methods. Although the titration of iodide in concentrations greater than 10-3M has not been discussed herein, the methods described can be used with necessary changes for larger concentrations.
Hume, D. N., and Harris, W. E., I N D .E N G .CHEM.,ANAL.ED.. 15, 465 (1943). (3) Kolthoff, I. M., and Jordan, J., J. Am. Chem. SOC.,74, 3 8 2 (2)
(1952). (4) Ihid., 75, 1571 (1953).
( 5 ) Kolthoff, I. M., and Lingane, J. J., “Polarography.” 2nd ed.,
Vol. 11, p. 946, New York, Interscience Publishers, 1952. (6) Laitinen, H. A., and Kolthoff, I. A f . , J. P h y s . Chem., 45, 1061 (1941).
ACKNOWLEDGMENT
Thanks are due Richard W. Ramette for measuring the current-voltage curves of permanganate and cerium. LITERATURE CITED
(1) Harris, E. D., and Lindsey, A . J., A n a l y s t , 76, 647 (1951).
(7) Lang, R., Z.anorg. u. allgem. Chem., 122, 332 (1922). ( 8 ) Ibid., 142, 229, 279 (1925). (9) Ihid., 144, 75 (1925). (10) Lewis, D., I N D .E N Q .CHEM.,A N A L .E D . , 8, 199 (1936). (11) Sadusk, J. F., Jr., and Ball, E. G., Ibid., 5, 386 (1933). (12) Vetter, K. J., Z . p h y s i k . Chem., 199, 22 (1952). RECEIIVED for review July 2, 1953.
.
Accepted September 17, 1953.
Titration of Weak Bases in Acetic Anhydride Solvent Mixtures JAMES S. FRITZ
AND MYRON
0. FULDA,
Zowa S t a t e CoZZege,
Ames, Zowa
The purpose of this investigation was to study the effect of titrating bases in organic solvents from which the last traces of water have been removed by the addition of excess acetic anhydride. For most tertiary amines and alkali metal salts, this increases considerably the sharpness of the break and rise in potential at the end point. A very large excess of acetic anhydride does not further increase the sharpness of the end point. The method proposed gives excellent results for tertiary amines including nitrogen heterocyclics of the purine, pyridine, pyridone, and thiazole type. Primary, secondary, and a few heterocyclic amines cannot be titrated. This method offers increased accuracy and broadens the scope of titrations in nonaqueous solvents to include weaker bases than formerly could be titrated.
I
T H.4P been recognized (3-5) that the presence of water de-
creases the sharpness with which bases can be titrated in acetic acid and other nonaqueous solvents. Because of this, the water in perchloric acid from which the titrant is prepared is often removed by addition of a calculated amount of acetic anhydride Some n ater still remains, however, since reagent grade glacial acetic acid commonly contains 0.3 to 0.5% water. The purpose of the present investigation was to study the titration of weak bases in nonaqueous solvents completely free from water. These conditions are practically obtained by titrating in solvents such as acetic acid or nitromethane containing 5 to 20% acetic anhydride. Titrations of tertiary amines in acetic acid containing acetic anhydride have been carried out (1, 7 ) , but no data are available concerning the effect of the acetic anhydride on the titration curve. REAGENTS AND SOLUTIOYS
Acetic acid, ACS grade glacial. Acetic anhydride, .4CS grade. Nitrobenzene, Matheson. Nitromethane, Eastman or illatheson. 1,s-Diphenyl-3-pentadieneone (dibenzalacetone). -4 0.1% solution in acetic acid. 1-Naphtholbenzein. -40.141, solution in acetic acid. Neutral red, 0.17, solution in acetic acid. Perchloric acid. A 0.1s solution is prepared by miung 8.5 ml. of i o to 72% perchloric acid n i t h enough acetic anhydride to remove all of the water. Let this mixture stand overnight, then dilute to 1 liter with acetic acid. Standardize against potassium acid phthalate. Triphenylmethanol (triphenylcarbinol). 0.1% solution in nitromethane. Samples were analyzed as received. Most were 98 to 100% pure. PROCEDURE
Dissolve a 0.3- to 0.8-meq. sample in 20 ml. of 4 to 1 nitromethane-acetic anhydride. Substances not readily soluble in
M O
,
I ML
1
I
ANALYTICAL CHEMISTRY
1838 dride. Each solution was then titrated with perchloric acid in glacial acetic acid. Figures 1 and 2 show that the rise in potential a t the end point is considerably greater for the titrations where acetic anhydride is present. Acetic anhydride mixed with the solvent also causes a sharper end point, as shown by the plot in Figure 3.
i
850-
80*
I
4502
3
5
4
6
7 I 400
ML OF O I N HCIO,
Figure 2.
Titration of Caffeine
A . I n nitromethane-acetic anhydride B . In nitromethane
I
0
2
I
3
I
ML OF WATER'
Figure 4. Addition of Water to Perchloric Acid Solutions
ary amines. (The latter, however, do not interfere in the titration of other bases if they are completely acetylated by heating with acetic anhydride beforehand.) It is a valuable method for tertiary amines and for the alkali metal or tertiary amine salts of most acids. Included with tertiary amines are numerous important nitrogen heterocyclics of the purine, pyridine, pyridone, thiazole, and other types. Figures 5 and 6 show titration curves for several typical bases, Table I gives quantitative data. An interesting compound which can be quantitatively titrated is triphenylmethanol (Figure 5 ) . This is perhaps the first example reported of a quantitative titration of an alcohol as a base. Although many nitrogen heterocyclic compounds can be titrated, acetic anhydride appears to enter into disturbing side
ML OF O I N
Figure 3.
HC104
Differential Plots
A . Lithium nitrate i n acetic acid B . Lithium nitrate i n acetic acid-acetic anhydride C. Caffeine i n nitromethane D . Caffeine i n nitromethane-acetic anhydride
It is believed that the complete removal of water by acetic anhydride causes the considerable improvement in end points noted above. When water is present, a t least some of the H A C + Clod- is probably converted to H30fC10~-,which is a considerably weaker acid. The data plotted in Figure 4 give further experimental support to this argument. Here it will be noted that the addition of a small amount of water to a water-free solution of perchloric acid causes a considerable drop in potential. Also, in the titration of caffeine almost identical curves were obtained in nitromethane containing 5, 20, and 50% (volume) acetic anhydride. This infers that 5% acetic anhydride removes all of the water, and more acetic anhydride therefore causes no further improvement in the end point. SCOPE
As excess acetic anhydride is present during the titration, it is evident that this method cannot be used for primary and second-
Table I. Potentiometric Titrations in 4 to 1 Nitromethane-Acetic Anhydride Purit Founx, Compound
%
Benzothiaaole
97.8 97.7
Caffeine
99.8 99.6
5 ,7-Dichloro-8-quinolinol
95.8 96.2
1-llethyl-2-pyridone
95.1 96.1
Sicotinamide
99.8 99.6
8-Xitroquinoline
95.1 94.5 94.5
Quinoxaline
97.8 97.5
Theobromine
99.3 100.0 100.0
Theophylline
99.2 99.2
Triphenylmethanol
100.5
100.2 100.0
1839
V O L U M E 2 5 , NO. 12, D E C E M B E R 1 9 5 3
Table 11. Titrations in 4 to 1 Sitromethane-Acetic .4nhydride Using Visual Indicators Compound Benzothiazole Caffeine I-Methyl-2-pyridone Nicotinamide Quinoxaline
Indicator Triphenylmethanol Seutral red Neutral red Triphenylmethanol Triphenylmethanol
I
/
METHYL VIOLET
WOLET
a -NAPHTHMBENZEIN
YELLOW
YELWN~CJ?EEN
TRIWENVLCARBINOL
,
I
I
500
IML
II
4
M L . OF O.IN HC104
Figure 5. A.
Titrations in 80% Xitromethane-2070 Acetic Anhydride
Triphenylmethanol.
B.
Purity % IndiPotenticator ometric 97.1 97 7 99.4 99 7 95.2 9.5.(i 99.6 99.7 97.8 97.7
Quinoxaline. C.
CCLORLESS m Y E L L O W
RNK m m C W L E S S
RED
I
DIBENZALACETONE
CCWRLESS
I 500
Benzothiazole
I
I 550
600
I
I
650 700 MILLIVOLTS
I
150
YELLOW I
800
8,b
Figure 7. Approximate Transition Ranges of Indicators in 80% Nitromethane-20% Acetic Anhydride
S50
ML
I OF
I rnl ~~
0 I N HC104
I
J
Figure 6. Titrations in Various Solvents .4. B.
-
I-\lethyl-Z-pyridonc in nitromethane-acetic anh) dride Sodium acetate i n acetic acid-acetic anhydride
C. 8-Nitroquinoline in nitromethancacetic anhydride
reactions with some. Quantitative results with benzotriazole, 3-methoxypyrazole, phenazine, pyrimidine, or pyrrole could not be obtained. IVDIC4TORS
Although potentiometric titrations are usually better for colored compounds and for very xeak bases, use of visual indicators is often very convenient. Xethyl violet, I-naphtholbenzein, neutral red, triphenylmethanol and dibenzalacetone are all useful indicators for titration of bases in nitromethane-acetic anhydride. -4potentiometric curve should first be carried out to establish the equivalence potential and thus choose the proper indicator. Figure 7 shows approximate transition potentials for these indicators. These data were obtained by titrating magnesium acetate with 0.1X perchloric acid; also by titrating various weak organic bases with 0.111’perchloric acid. For titrations carried out using different ionic strengths than those reported here, the transition point of the indicator may be appreciably different (3, 6). Data for visual titrations of several amines are given in Table 11. DISCUSSION
Sitromethane was widely used in the present work for several reasons. I t is readily available and is practically water-free.
I ML.
-I
I
I
Y L ff O I N K I O (
Figure 8. A. B. C. D.
Titration Curves for l-.Methyl-2-Pyridon e and Caffeine
1-Methyl-2-pyridone i n acetic acid-acetic anhydride I-Methyl-2-pyridone nitromethane-acetic anhydride Caffeine in acetic acid-acetic anhydride Caffeine i n nitromethaneacetic anhydride
Its high dielectric constant makes it possible to obtain steady potentials using a fiber-type calomel electrode. Use of nitromethane-acetic anhydride often gives sharper end points than are obtainable with acetic acid-acetic anhydride. Figure 8 illustrates this effect for caffeine and l-methyl-2pyridone. Salts such as lithium nitrate and sodium acetate gave just as good results in acetic acid-acetic anhydride. LITERATURE CITED (1) Blumrich, K. G., and Bandel, G., Angew. Chem., 5 4 , 374 (1941). (2) Conant, J. B., and Terner, T. H., J . A m . Chem. SOC.,52, 4436 (1930). (3) Hall, N. F., and Conant, J. B., Ibid., 49, 3047 (1927). (4) Markunas, P. C., and Riddick, J. *4.,ANAL. CHEM.,23, 337 (1951). (5) Pifer, C. W., and Wollish, E. G., Ibid., 24, 300 (1952). (6) Seaman, W., and Allen, E., Ibid., 23, 592 (1951). (7) Wagner, C. D., Brown, R. H., and Peters, E. D., J . Am. Chem. SOC.,69, 2609 (1947). RECEIVED for review May 27, 1953. Accepted August 31, 1953. Presented before the Division of Analytical Chemistry a t the 124th Meeting of the SOCIETY,Chicago, Ill. Contribution KO.269 from AMERICAN CHEMICAL the Institute of Atomic Research and Department of Chemistry, Iowa State College, Ames, 107%-a. Work performed in the Ames Laboratory of the -4tomic Energy Commission.
