KI = 180
Kz
= 21,900
KS = 126,500
Kq
=
43,000
A wide scatter in points was noticed for the F3(X) and F4(X) plots and the data were reanalyzed in the form of a linear regression (8). K3 was found to be 129,268 and K4, 51,264. In comparison to the corresponding cadmium-succinic dihydrazide complexes (2), the zinc complexes tended to be more stable. Comparison of the K3 complexes disclosed a 5fold increase in stability for the zinc compared to the cadmium complex. From the data obtained, it appears that the dihydrazide complexes of zinc are more stable than the monohydrazide (8) G. W. Snedecor, “Statistical Methods,” 4th ed., Iowa State
College Press, Ames, Iowa (1964).
complexes. Also, the possibility of forming higher complexes is increased for the dihydrazide as compared to the monohydrazide complexes. The results found by Albert (3) and Fallab and Erlenmeyer ( 4 ) for the complexation between copper and isonicotinic acid hydrazide indicated that the reaction involved release of protons. Titration of the hydrazide with copper lowered the pH approximately 0.6 unit below the pH for copper alone. Titrations of the hydrazides studied with zinc were carried out beyond the 1 :1 mole ratio, and, the pH of the solutions did not drop below that for zinc alone. Complexation of zinc by these hydrazides appeared to involve the neutral hydrazide molecule. RECEIVED for review November 22, 1967. Accepted February 26, 1968.
Titration of Amides by Chlorination and Equilibrium Constant Evaluation with Constant Current Potentiometry C. 0. Huber and K. E. Smith’ Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee, Wis. 53201 VARIOUS TITRATION methods for determination of amides have been proposed. These include potentiometric and photometric acid-base titration in acetic acid or acetic anhydride ( 1 , 2), formation and titration of the corresponding amines (3), and ion exchange hydrolysis to acids followed by titration (4). In 1965 Post and Reynolds ( 5 ) proposed an amperometric chlorination titration of certain aliphatic and aromatic amides in a water-dioxane solvent using readings well beyond the equivalence point of the reaction. The work reported here presents a constant current potentiometric titration using the chlorination reaction in aqueous solvent. Several sources of error are examined and a method for measurement of the equilibrium constant is given. In constant current potentiometry the polarizing current is produced by a relatively high dc voltage in series with appropriate value resistors and the titration cell electrodes (6). In the case for the titration studied here a platinum cathode serves as the indicator electrode. The potential shift near the equivalence point is caused by reduction of excess chlorine at potentials considerably more positive than those occurring at the cathode before the equivalence point is reached. Platinum is also used for the anode. It undergoes virtually no change in potential throughout the titration. 1 Present address, Dept. of Chemistry, University of Iowa, Iowa City, Iowa. ~~
(1) T. Higuchi, C. H. Barnstein, H. Ghassemi, and W. E. Perez, ANAL.CHEM., 34, 400 (1962). (2) D. C. Wirner, Zbid., 30, 77 (1958). (3) S. Siggia and C. R. Stahl, Zbid., 27, 550 (1955). (4) T. M. Bednarski and D. N. Hurne, Anal. Chim. Acta, 30, 1 (1964). (5) W. R. Post and C. A. Reynolds, ANAL.CHEM., 37, 1171 (1965). (6) C. N. Reilley, W. D. Cooke, and N. H.Furman, Zbid., 23, 1223 (1951). 982
ANALYTICAL CHEMISTRY
The constant current potentiometry technique, in contrast to the amperometric titration proposed by Post and Reynolds (5), permits direct reading of the end point signal-Le., does not require graphical extrapolations based on a linear indicator electrode signal. As is often the case, the superior end point technique results in helpful observations concerning the titration reaction itself. These observations result in elimination of several sources of error and in improvement of titration accuracy and convenience. In addition, the titration data show that the equilibrium constant for the titration reaction is relatively low. The constant current potentiometric data at 200 FA allows the determination of end points reproducibly far enough beyond the equivalence point to yield stoichiometric accuracy. The potentials at lower currents permit evaluation of concentrations of reactants and products to allow estimation of the equilibrium constant. A value for the equilibrium constant for the propionamide chlorination reaction is presented. EXPERIMENTAL
Apparatus. The constant current potentiometric apparatus used was similar tot hat described by Reilley, Cooke, and Furman (6). The vessel used to prevent loss of chlorine through evaporation was a 125-ml Erlenmeyer flask fitted with a ground glass neck joint into which fits a complementary joint sealed onto the tip of a 2-ml self-filling micro buret. The titration vessel was covered to shield the titration solution from light. Two 20-ga. platinum wires were sealed through the vessel walls. The cathode was 8 mm in length and the anode was 15 mm. A calomel or other nonpolarized electrode could just as well have been used for the anode, but would have been less convenient. Solutions were stirred magnetically with a synchronous motor. Constant current was supplied by a 90-V battery with appropriate resistors in series. A commercial direct reading pH meter on the 1400
mV scale was used to follow the potential differences between the electrodes. Reagents. An approximately 0.5N solution of sodium hypochlorite was prepared by dissolving an appropriate amount of calcium hypochlorite in water followed by adding an equivalent amount of sodium carbonate and filtering to remove the insoluble calcium carbonate. The solution was standardized iodometrically. A change in titer of approximately 0.5 was evident over a one month period. All amides, with the exception of acetamide and benzamide, were Eastman white label reagents and were used as received. The acetamide was recrystallized twice in alcohol from technical grade. The benzamide was zone-refined (purchased from James Hinton, Valparaiso, Florida). All other chemicals were reagent grade. Procedure for Analysis by Titration. A 25-ml aliquot containing 0.5 mmole of amide in water was diluted with aqueous HCl to fill the titration vessel so that the final solution was 0.5M in HC1. The system was then sealed by placing the buret into position. A constant current of 200 pA was imposed across the electrodes. Pressure was applied to the top of the buret column by a rubber bulb and check valve assembly in order to cause the titrant to flow. Titrant increments were smaller near the end point, of the order of 0.002 ml or less. The end point was taken as the small increment which caused the potential to fall below 0.4 V for at least one minute.
Table I. Constant Current Potentiometric Titration of Primary Amides Mean apparent No of trials Amide Std dev Z purity, Z Acetamide 0.37 100.0 6 0.59 99.3 5 Benzamide 6 n-Butyramide 0.18 100. 1 0.30 95.8 11 Propionamide 0.34 98.9 6 o-Toluamide 0.30 99.9 7 Benzamidea Zone refined sample. Q
Table 11. Comparison of Titration Data for Small Samples ml HOC]/ MHCIO mmole amide Amide, mmoles 0.420 2.366 0.500 0.420 2.375 0.200 0.420 2.42 0.100 0.042 23.03 0.100 0.042 23.66 0.050 0.042 23.70 0.020 0.042 24.7 0.010
RESULTS AND DISCUSSION
Analysis by Titration. Although the end point was reached somewhat slowly for some samples, the end point signal itself was rapidly achieved and was unmistakable-i.e., slowness in the titration procedure was due to reaction rate rather than delay in electrode response. The results for the titration of several primary amides are given in Table I. These results were obtained with 0.5 mmole samples. Table I1 shows that samples as small as 0.01 millimole (0.73 mg propionamide) can be titrated with 0.05N sodium hypochlorite with only a slight decrease in accuracy. The time required to complete a titration was about 10 minutes. The concentration of hydrochloric acid is not critical, but at concentrations below about 0.01 M the rate of reaction is so slow that premature end points are almost unavoidable. Because the formation of chlorine in solution is fast, the primary reaction of interest in the titration is the equilibrium reaction between molecular chlorine and the amide yielding the N-chloroamide and hydrochloric acid. Because the equilibrium is not sufficiently complete, measurable amounts of chlorine are present in solution before the equivalence point is reached. It was found, for example, that when a constant current of 50 or 100 pA was used the end points were premature. When the current was 200 pA, however, the excess chlorine being measured was sufficient to produce stoichiometric titration accuracy. The larger blank titration which resulted did not impair the convenience of the titration because for most samples a blank titration would be necessary in any case. Equilibrium Constant Measurement. In addition to measurement of titration end points and blanks at 200 pA to perform analysis, end point and blank readings were also taken at a current of 100 pA. This was conveniently accomplished by a selector switch connecting the appropriate magnitude resistor in the current circuit. The equilibrium constant for the chlorination reaction can be evaluated with titration and blank titration end point data at 100 as well as 200 pA. The equilibrium can be expressed as:
K =
[RCONHClI[H+I[Cl-I [RCONHJC~ZI
The equilibrium concentrations were ascertained as follows : MHClO(VlO0 -
[RCONHCI] =
VIOOo)
u
(4)
[Clz]
=
M H C 10 VI00
u
where terms are: MHClo,molarity of titrant M H +and Mcl-, stoichiometric initial concentrations Vloo, 100 pA end point Vlooo,100 pA end point blank Vzoo,200 pA end point Vzooo, 200 p A end point blank u, titration volume In the procedure used MHC~O (VIOO- V L O O O ) is negligible compared to M H + Uand Mcl-u, so Equations 3 and 4 can be written [H+] = MH+ and [CI-]
= Mcl-
and can write:
K =
[(Vzoo -
--( VVlooo)ME+Mc~I 0 0 - V1oo0)1 MHcloVlooo
V(VIO0 VZOO0)
VOL. 40, NO. 6, MAY 1968
(7) 983
The equilibrium constant was determined for propionamide chlorination using Equation 7 with data from 10 different titrations. The titration volume was 120 ml and did not change appreciably during titration. The amounts of amide titrated varied from 0.01 to 0.5 mmole and HC1 concentrations from 0.5M to 3.OM. Both 0.420M and 0.042M hypochlorite titrants were included in the series of 10 titrations. The log,,K value for the chlorination of propionamide computed from these data is 5.0. Standard deviation is 0.2 logl& units. Activity effects have been ignored in these computations. No previous evaluation of this constant was found in the literature. Previous workers (5) have suggested the use of 20 dioxane as solvent for this titration. Our results showed that the dioxane reacts slowly with the chlorine causing unstable end
points and excessively large blanks. It was therefore eliminated in favor of an aqueous solvent system. The need for a closed system was suggested when a chlorine odor was detected above an open titration vessel. Therefore, the sealed system described above was employed. Comparison titrations confirmed that end points in an open vessel yield two to three per cent positive errors. The authors are of the opinion that the causes for most of the one per cent per minute loss of chlorine reported by Post and Reynolds (5) are accounted for in the results presented here. RECEIVED January 2, 1968. Accepted February 15, 1968. This work supported in part by National Science Foundation Undergraduate Research Participation Program (GY-960).
Separation and Identification of Aromatic Carbonyl Compounds as Their 4-Nitrophenylhydrazones by Paper and Thin-Layer Chromatography Ethel D. Barber and Eugene Sawicki U.S. Department of Health, Education, and Welfare, National Center for Air Pollution Control, Cincinnati, Ohio 45226 As A PORTION of a program for the study of aromatic carbonyl compounds, 4-nitrophenylhydrazones were used as suitable derivatives for the identification of such compounds in automobile exhaust by paper and thin-layer chromatography. Fracchia, Schuette, and Mueller (1) have shown the possibility of the presence of benzaldehyde in automobile exhaust by gas liquid chromatography. Although the 2,4dinitrophenylhydrazones have been widely used to identify carbonyl compounds, the 4-nitrophenylhydrazones show many more fluorescent and phosphorescent characteristics (2). In the work described here, the 4-nitrophenylhydrazones were prepared (3), and chromatographed on paper and silica gel in various systems. EXPERIMENTAL
Apparatus. Spectrophotometric measurements were made on a Cary Model 14 and Cary Model 15, manufactured by Applied Physics Corp. Standard chromatographic equipment including plates, applicator of fixed thickness, and developments tanks were obtained from Brinkman Instruments, Inc. Low-temperature fluorescence and phosphorescence of the compounds of glass-fiber paper were examin$d in a Chromato-vue cabinet, which is equipped with 3660A, 2537A, and white-light lamps (Kensington, Scientific Corp.). Reagents. Chemicals were reagent grade except where otherwise noted. Methanol was refluxed and distilled over silver nitrate made alkaline with sodium hydroxide (4). Chloroform was refluxed and distilled over 5 grams of 4(1) M. F. Fracchia, F. J. Schuette, and P. K. Mueller, Environ. Sci. Technol., 1, 915-22 (1967). ( 2 ) E. Sawicki and H. Johnson, Microchem. J., 8, 85-101 (1964).
(3) S. McElvain, “The Characterization of Organic Compounds,”
MacMillan, New York, 1949, p 199. (4) 0. Buyske, L. H. Wilder, P. Wilder, Jr., and M. Hobb, ANAL. CHEM., 28,911 (1956). 984
ANALYTICAL CHEMISTRY
nitrophenylhydrazine and 2.5 ml of glacial acetic acid per liter. The 4-nitrophenylhydrazine derivatives were prepared by the method described by McElvain (3). Schleichter and Schuell paper, 2043 B gl was used for the chromatograms. Silica gel, according to Stahl, manufactured by E. Merck, Darmstadt, Germany, was used for the thin-layer chromatography. Procedure, Paper Chromatography. The 4-nitrophenylhydrazones of the various carbonyl compounds were separated in the following systems: (A) papers impregnated with 35 formamide-65 ethanol by Crump’s system (5) of cyclohexane-benzene-dipropylene glycol in the ratio of 70 :30 :3 VjV; (B) papers impregnated with 50% N,N-dimethylformamide by Schnitt’s system (6) of dibutyl ether-N,Ndimethylformamide-tetrahydrofuran in the ratio of 85 :15 : 4 VjV; (C) papers impregnated in 25 N,N-dimethylformamide-75 % ethanol by Schnitt’s system (6); (D) papers impregnated with 2 0 x formamide-80Z ethanol with cyclohexane-cyclohexane-formamide in the ratio of 15 :12:7. Schulte and Storp’s system (7) of methanol-acetic acid in the ratio of 9.25-0.75 (VjV) was tried, but the results were unsatisfactory. Determination of RF Values. Solutions of pure 4-nitrophenylhydrazones of approximately 1 mg per ml of 9 5 z ethanol were chromatographed at 20” C by the procedure of ascending chromatography (8, 9). The spots were visible and could be detected very easily with white light. The results are recorded in Table I. Low-temperature fluorescence and phosphorescence studies were performed according to Sawicki and Johnson (2).
x
z
( 5 ) B. G. Crump, J. Chromatog., 10,21-8 (1963). (6) W. Schnitt, ANAL.CHEM., 28, 249 (1956).
(7) K. E. Schulte and C. B. Storp, Fette Seifen Anstrichmittel, 57, No. 1, 36-42 (1955). (8) A. P. DeJonge, Rec. Truu. Chim., 74, 260 (1955). (9) A. P. DeJonge and A. Verhage, Ibid., 76,221 (1957).
