constant of about 7.5 and a high resistance. It should also be nontoxic and have a high or nonexistent fire point. It seems doubtful t,hat such a pure compound exists and various mixtures are now being examined, together with various surfactants, to determine a suitable compromise more acceptable than ether.
LITERATURE CITED
(1) gexa:,der, A. E., Johnson?P., “Colloid Science, pp. 300-3, Clarendon, Oxford, 1947. (2) Burden, J. P., Guy, V. H., Trans. Inst. Metal Finishing 35, 93 (1963). (3) Gutierrez, C. P., Mosley, J. R., Wallace, T. C., J . Electrochem.SOC.109, 923 (1962). (4) Koch, W., Sundermann, H., Arch. Ezsenhuetknw. 29,219 (1958).
(5) Pohl, H. A., J. Appl. Phys. 22, 869 (1951).
(6).Ralston, 0. C., “Electrostatic Separation of Mixed Granular Solids,” pp. 105-14, Elsevier, New York, 1961. (7) Rosenho!tz, L., Smith, D. T., A m . Mineralogzst 21, 115 (1936). (8) Strickland, R. D., ANAL.CHEM.34,31 (1962).
RECEIVED for review November 18, 1963. Accepted January 8, 1964.
Investigation of p-Nitrobenzhydrazide as an Acid -Base Indicato r G. J. PAPARIELLO and SEYMOUR COMMANDAY Research Deparfment, ClBA Pharmaceutical Company, Summit, N. 1.
b An investigation of p-nitrobenzhydrazide as an acid-base indicator has been made. The type of acids that can be titrated and the solvents that can be used with this indicator are reported on. This indicator is compared with existing acid-base indicators effective over the same pH range. The dissociation constants of the indicator have been spectrophotometrically determined as 2.77 and 11.17. A mechanism to explain the indicator character of this compound is offered.
M
of analysis for acid hydrazides were being studied in this laboratory when it was observed that one of these acid hydrazides, namely, p-nitrobenzhydrazide, turned from colorless to yellow upon addition of base and then back to colorless on addition of acid. The observation of this reversible reaction suggested the use of this compound as an acid-base indicator. Consequently, an investigation of this compound was initiated. I n this investigation a study of the characteristics of this compound as an indicator as well as a consideration of a mechanistic explanation for its action is made. ETHODS
EXPERIMENTAL
Reagents. All the acids used were Reagent Grade with the exception of benzoic acid and potassium acid phthalate which were Primary Standard Grade. p-Nitrobenehydrazide (Eastman, No. 3341) was recrvstallieed from ethanol. Melthg range” for the recrystallized material was 211’ to 212’ C. [literature value, 213’ to 214’ C. (S)]. A 1% solution of p-nitrobenzhydrazide in acetone was used throughout this investigation. -411 other indicators were used as 1% alcoholic solutions. 1028
ANALYTICAL CHEMISTRY
Sodium methoxide, O.lN, and 0.1N sodium hydroxide were prepared and standardized as indicated in the U.S.P. (6). Apparatus. A conventional glass indicating electrode and saturated calomel reference electrode were used along with a Beckman Model G p H meter for potential measurements in aqueous systems. I n nonaqueous systems the conventional calomel electrode was replaced by a platinum electrode or a modified calomel electrode having a salt bridge consisting of a saturated solution of lithium chloride in methanol. A Cary Model 11 recording spectrophotometer was used for the spectrophotometric determination of the dissociation constants. Titration of Various Acids. A weight of acid (0.5 to 2.0 mmoles) which gives a convenient volume titration is dissolved in the aqueous or nonaqueous solvent, and 1 to 10 drops, depending on the strength of the acid being titrated, of the 1% solution of p-nitrobenzhydrazide are added. The acid is titrated with standard sodium methoside or standard sodium hydroxide, and the color changes which occur are observed and recorded. Solvent Study. A weight of benzoic acid (1 to 2 mmoles), which gives a convenient volume titration, is dissolved in the solvent being studied, and 1 to 2 drops of the 1% solution of p-nitrobenzhydrazide are added. The titration of the acid with standard sodium methoxide or standard sodium hydroxide is followed potentiometrically. The color changes which occur during the titration are observed and recorded. Spectrophotometric Determination of Dissociation Constants. The general procedure that was followed for this determination was that of Albert’s and Serjeant’s (1). Since this substance is amphoteric, two analytical wavelengths are used-i.e., absorbance measurements a t both 353 mp and 263 mp
were needed to determine the two dissociation constants. The two wavelengths mentioned above were chosen because they reflected the species changes most readily. The equation used to determine the pKa is as follows:
Where axis the absorbancy index of the ion a t the analytical wavelength, U M is the absorbancy index of the molecule a t the same wavelength, and a is the absorbancy index of a mixture of the ion and the molecule a t this same wavelength. An ionic strength of 0.5 is maintained throughout this determination by the addition of the electrolyte, potassium chloride, Thus, any effect that the buffer salts might have on the activity of the determined ions is minimized. The buffer salts used are those recommended by Albert and Serjeant (1). Comparison to Existing Indicators. Both benzoic acid and succinic acid were titrated in an ethanol-water solvent using standard sodium hydroxide as titrant. Various indicators were used for this titration. Visual observation of the color changes were noted and recorded. Benzoic acid and methylphenidate hydrochloride were titrated in chloroform using standard sodium methoxide as titrant. Various indicators were used for this titration. Visual observation of the color changes were noted and recorded. RESULTS AND DISCUSSION
The preliminary work done with p-nitrobenzhydrazide indicated that 1 to 10 drops of the indicator were needed to give a good color change in a 50-ml. solution. The number of drops of indicator needed depends on the strength of the acid being titrated.
+zoo-
+loo-
I
i
-looi
I
INDICATOR CHANGED C O L O R
u'
0-
,
-200 0
I ,
4
8 0.1 N NoOMe, ml.
16
I2
Figure 1. Potentiometric titration of benzoic acid in chloroform, illustrating the point at which p-nitrobenzhydrazide changes ccdor
In most cases an indicator blank is not needed. To determine the capabilities of this indicator, it was nezessary to titrate various acids in various solvents. Consequently, benzoic acid was titrated in water, water-alcoholic mixtures, and nonaqueous solvents. A concentration range of 1 to 2 mmoles of the acid was used. The concentration of acid seemed to have little effect on the indicator end point. All titrations were run in triplicate. The observations and results obtained in this study are summarized in Table I. Obviously, chloroform or a water-ethanol mixture is the best solvent to use with this indicator. These titrations were simultaneously followed by potentiometric methods to ascertain a t what point on the titration curve a visible color change occurred. The point a t which a distinct color change occurs can be seen in li'igures 1 and 2 for chloroform and wster-ethanol, respectively. It is evident that the potentiometric end-point and the indicator's color change are coincidental. The results of titrations of various Br$nsted acids in the two solvents of choice-i.e., chloroform and ethanolwater-are shown in Table 11. It can be seen that the carboxylic acids and some hydrochloride salts of amine bases can readily be titrated. On the other hand, phenols cannot be titrated since the indicator is a strong enough acid to compete with the phtnol for the basic titrant. In the course of thidi work it has been established that the pH range of color change (transition range) for this indicator is 8.2 to 9.5. The color transition range is quite dependent upon the concentration of the indicator. The normal rule of thumb, which states that the indicator transition range usually lies one unit of p H on each side of its pKl,, is only followed if one uses 1 or 2 drops of the indicator solution. It was felt that this indicator should be compared to other indicators which operate in this approximate pH range. Table I11 is a summary of the results obtained
0.1 N NoOH, ml.
Figure 2. Potentiometric titration of benzoic acid in 1 :1 ethanol-water mixture, illustrating the point at which p-nitrobenzhydrazide changes color
Table
I.
Results of Titrations Using p-Nitrobenzhydrazide as Indicator in Various Solvents
Solvent N ,N-Dimethylformamide Methanol Chloroform 50% ISOropano1-50% propyPene glycol Acetone 50% Ethanol-50% water Water
Table II.
%.Benzoic acid found
Observationa No definite color change Fair color change Excellent color change Blank titration reqwed
2
100.3 0.3 100.1 f 0 . 1 100.2 f 0.6
** 00.1. 2 99.8 * 0 . 2
Faint end point Excellent color change Good color change
100.1 100.0
Titration of Various Acids Using p-Nitrobenzhydrazide as Indicator
Acid found, yo Acid Ethanol-water Chloroform Acetylsalicylic acid 99.7 i 0 . 2 100.0 i 0 . 2 Salicylic acid 100.2 i 0 . 2 99.6 f 0.4 Methylphenidate hydrochloride 96.5 f 0 . 5 100.0 f 0 . 4 Naphaaoline hydrochloride Resorcinol Thymol Succinic acid
Indefinite 0.0 0.0
99.9
100.4 i 0.2
* 0.1
from this study. It is apparent that in most cases p-nitrobenzhydrazide is superior to thymol blue, a-naphtholbenzein, and thymolphthalein. A stock solution of p-nitrobenzhydrazide was found to be completely stable over a three-month storage period. No signs of degradation or of loss in indicator character were noted during this period. The spectrophotometric examination of p-nitrobenzyhydradde in various buffer solutions revealed the existence of two isosbestic points, one a t 304 mp and the other at 273 mp (Figures 3 and 4). Prior to this examination it had been assumed that this compound was involved in only one equilibrium, that between the colored species and the colorless species. The discovery of a
0.0 0.0
...
Observations Good end point Good end point Slow color change in ethanol-water Faint end point in chloroform Indefinite end point in ethanol-water No titration No titration Good end point
second isosbestic point indicates th presence of a second equilibrium. Obviously this compound is amphoteric, The apparent dissociation constants, pKa, for these acid-base equilibria have been determined to be 2.77 f 0.03 and 11.17 f 0.02. The equilibrium a t the higher p H is, of course, the one which involves a color transition. The equilibrium reaction in acid solution is postulated as follows: H H+
O\c,N-"2 I
I
+ 04N' 0 VOL 36,
H 0,
y
/N--"Z
I
0 ' /N+' 0 - .
NO. 6, MAY 1964
1029
5
w 0
4 z
a m u) 0
m
275
250
300
325 350 WAVELENGTH (mp)
375
400
Figure 3. Absorption spectra of p-nitrobenzhydrazide at various basic, pH values 1. pH 13. 8.48
2.
pH 11.50.
3.
pH 11.06.
The primary amino group of an acid hydrazide has been reported as basic and capable of protonation (4), lending strong support to the equilibrium shown above. In basic solution, abstraction of a proton leads to a resonance hybrid. This resonance hybrid requires less energy for excitation and therefore
Table 111.
Indicator Cresol Red
Thymol Blue
pH 10.49.
5.
01 225
pH
2 50
1.
pH 4.47.
absorbs a t a longer wavelength than the neutral molecule, thus making this acid hydrazide a useful acid-base indicator. The equilibrium in basic solution is postulated as follows:
o+NLoL
I
V
Yellow
Comparison of p-Nitrobenzhydrazide to Established Indicators
Transition range4 Acid titrated 7.2- 8.8 Benzoic acid Benzoic acid Succinic acid Methylphenidate HCl 8.0- 9.6 Benzoic acid
Solvent Ethanol-water Chloroform Ethanol-water Chloroform Ethanol-water
End point Excellent Excellent Excellent Good Rather indefinite
Benzoic acid Chloroform Poor Methylphenidate HCl Chloroform Indefinite Phenolphthalein 8.2-10.0 Benzoic acid Ethanol-water Good Benzoic acid Chloroform Very good Succinic acid Ethanol-water Good Methylphenidate HCl Chloroform Good a-Naphtholbenzein 8.5- 9.8 Benzoic acid Chloroform Very good Methylphenidate HCl Chloroform Fades Thymolphthalein 9.3-10.5 Succinic acid Ethanol-water Good Methylphenidate HCl Chloroform Fades p-Nitrobenzhydrazide 8.2- 9 . 5 Benzoic acid Ethanol-water Good Benzoic acid Chloroform Very good Succinic acid Ethanol-water Good Methylphenidate HCl Chloroform Good 4 “The Merck Index of Chemicals and Drugs,” 7th ed., p. 1568, Merck and Co., Rahway, N. J., 1960. a 50y0 Ethan01-50% water.
1030
ANALYTICAL CHEMISTRY
275 300 WAVELENGTH ( m p )
325
1
D
Figure 4. Absorption spectra of p-nitrobenzhydiazide at various acidic pH values
’+
o// N+ LoColorless
4.
2.
pH 2.96.
3.
pH 2.03.
4.
pH 1
The resonance hybrid is described by the structures. The anion as drawn should contain the following structure: \
C=N-N, I
/
as in p-nitrobenzaldehyde hydrazone. The ultraviolet absorption properties of the anion and the hydrazone should consequently be very similar. This was found to be the case since the absorption maximum for the hydrazone is 347 mp (log a = 4.1) ( 2 ) ,and that for the anion is 351 mp (log a = 3.7). As expected, the ortho- and metanitrobenzhydrazides do not exhibit a definite bathochromic shift with a change in pH. The reason for this is that the orthoand meta-forms cannot exist in the mesomeric forms as drawn above for the para-compound. ACKNOWLEDGMENT
The authors are grateful to N. Finch for his advice and interest in this work. LITERATURE CITED
(1) Albert, A., Serjeant, E. P., “Ioniza-
tion Constants of Acids and Bases,” 69, Wiley, Kew York, 1962. (2y.G rammaticakis, P., Bull. SOC.Chim. France 17, 690 (1950). (3) Ibid., 22, 659 (1955). (4) Sah, P. T., Rec. TTUV.Chim. 59, 1036 (1940). (5) “United States Pharmacopeia,” 16th Revmion, p. 1082, Mack Publishmg Co., Easton, Pa., 1960. RECEIVEDfor review October 28, 1963. Accepted January 24, 1964.
