I n the absence of interfering organic materials in the water, sample preparation time is at a minimum. When interfering materials are present, they first must be destroyed by an oxidation treatment. Regardless of the nature of the samples two or more samples can be analyzed hourly with good repro-
ducibility and at sensitivities 100 times greater than that obtainable with atomic absorption procedures. RECEIVED for review August 11, 1969. Accepted December 29,1969.
Coulometric Titration of Acid Hydrazides with Pressuremetric End Point Detection D. J . Curran and James E. Curley Department of Chemistry, Unicersity of Massachusetts, Amherst, Mass. 01002 The application of pressure measurements to end point detection for coulometric titrations is demonstrated by the oxidation of acid hydrazides with electrogenerated bromine. Approximately 6- to 10-mg samples of isonicotinic-, p-toluic-, or phenyl acetic acid hydrazides and adipic dihydrazide, all favorable cases, have been titrated with an accuracy of a few parts per thousand and a precision of about five parts per thousand. Linear-segmented type titration curves result when the transducer output voltage is plotted against microequivalents of titrant generated. The slopes of the lines of the titration curves yield useful information concerning stoichiometric or kinetic complications in the titration reaction. An extreme example is the case of a compound believed to be succinic dihydrazide which apparently reacts with bromine but does not produce any nitrogen. Other cases where complications were found were: carbohydrazide, 1,2diacetyl hydrazine, isobutyric acid hydrazide, and malonic dihydrazide.
methyl red end point detection techniques for the coulometric determination of isonicotinic acid hydrazide in pharmaceutical preparations. This paper deals with the titration of various acid hydrazides with electrogenerated bromine using the pressuremetric end point method of Curran and Driscoll (9). Acid hydrazides are oxidized by electrogenerated bromine in the anode compartment of a modified H-cell to yield nitrogen according t o (IO):
2Br,
0
0
II
I1
+ R C N H N H L + HsO -,RCOH + Ns(g) + 4HBr (1 )
Simultaneously, hydrogen gas is generated in the cathode compartment : 2H+
THE COULOMETRIC DETERMINATION of acid hydrazides with electrogenerated chlorine or bromine has been investigated by a number of workers. Several end point detection methods were employed. Kalinowski (1-3) titrated isonicotinic acid hydrazide with chlorine liberated by the electrolysis of hqdi ochloric acid. Decolorization of methyl red placed in the anode compartment of the titration cell signaled the end point. Krivis et al. (4, Kawamura and coworkers (9,and Olson (6) titrated various hydrazides using amperometric end point detection techniques. Wijnne and coworkers (7) used a dead stop technique for the coulometric titration of isonicotinic acid hydrazide which had been separated from other components by thin-layer chromatography. Stoicescu and coworkers (8) employed both the amperometric and the
+ 2e-
+
Hp(g)
(2)
Thus, before the equivalence point of the titration, nitrogen and hydrogen are both evolved, but after the equivalence point only hydrogen is evolved. EXPERIMENTAL
(1) K. Kalinowski, Acra. Polon. Pliarm., 11, 113 (1954); Chem. Abstr., 48, 14123 b (1954). (2) K. Kalinowski, Przem. Cliem., 10, 73 (1954); Anal. Abstr., 3, 528 (1956). (3) K. Kalinowski, and Z . Zwierzchowski, Acta. Polorz. Pharm., 20, 309 (1963); Anal. Abstr., 11, 1470 (1964). (4) A. F. Krivis, E. S. Gazda, G . R. Supp, and P. Kippur, ANAL. CHEM.,35, 1955 (1963). (5) F. Kawamura, K. Momoki, and S. Suzuki, Bull. Fac. Eng., Yokohama Nut. Unio., 4, 123 (1955). (6) E. C. Olson, ANAL.CHEM., 32, 1545 (1960). (7) H. J. A. Wijnne, E. Bletz, and J. M. Frijns, Plzarm. Weekbl., 102 (38), 959 (1967); Chem. Abstr., 67, 102844d(1967). (8) V. Stoicescu, C . Ivand, and H. Beral, Reo. Chirn. (Bucharest), 1968, 19 (8), 484; Chem. Abstr., 70, 6568h (1969).
Reagents. Solutions were prepared using laboratory distilled water that had been redistilled from alkaline permanganate. All chemicals, except acid hydrazides, were reagent grade. Stock anolyte solution was 0.75M in HCl and 1 M in KBr. A 3M HCI solution was used as catholyte. Isonicotinic acid-, p-toluic acid-, and phenyl acetic hydrazides were obtained from Aldrich Chemical Co., Inc., Milwaukee, Wis. Carbohydrazide, 1,2-diacetyl hydrazine, adipic- and malonic dihydrazides were obtained from Olin Mathieson Chemical Corp., New Haven, Conn. Samples of isobutyric acid hydrazide, carbohydrazide, malonic dihydrazide hydrochloride, and succinic dihydrazide were supplied by L. A. Carpino, University of Massachusetts at Amherst. All acid hydrazides were purified by successive recrystallization from ethanol except malonic dihydrazide which was precipitated from ethanol with concentrated HCl and isobutyric acid hydrazide which was recrystallized from a 1 :1 ethyl ether-water mixture. Solutions were made by dissolving accurately weighed samples in 3M HC1. Aliquots (9) D. J. Curran and J. L. Driscoll, ANAL.CHEM.,38, 1746 (1966). (10) W. A. Waters, “Mechanisms of Oxidation of Organic Compounds,” John Wiley and Sons, New York, N. Y., 1964, p 84.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
373
Table I. Analytical Data for Acid Hydrazides Elemental analyses
I I1 I11
IV V VI VI1 VI11 IX
Coulometric titration with amperometric end point Taken, Found, No. of detn. Compound mg mg 6.78 6.50 3 Isonicotinic acid hydrazide 9.46 9.18 6 pToluic acid hydrazide 7.18 7.16 3 Phenyl acetic hydrazide 5.88 5.80 4 Adipic dihydrazide 4 0.909 0.820 Carbohydrazide 3 6.04 5.71 1,2-Diacetyl hydrazine Malonic dihydrazide 1.345 1.280 3 hydrochloride 4 1.331 1.023 Succinic dihydrazide 4 2.501 2.482 Isobutyric hydrazide
U
U
Figure 1. Schematic of pressuremetric titration equipment A . Sargent Model IV constant current source B. Titration cell C . Tri-R Model MS-7 submersible magnetic stirring motor D. 3-Way stopcock E. Datametrics Series 511-3, 0-1 psi, Barocel Pressure Sensor F. Waterproof box encasing pressure sensor G. Reference cell H. Cenco Model 97200-1 constant temperature bath I. Datametrics Type 700 power supply J . Datametrics Type 1015 signal conditioner K . Double pole, double throw switch L. Electronic Scientific Industries Model PVB 300 Potentiometer M . Photovolt Linear Log Varicord Model 43 Servorecorder
of these solutions were analyzed using the amperometric method of Kawamura et at. (5). Analyses for nitrogen were performed by the Office of Research Services, Chemical Analysis Laboratory, University of Massachusetts. Results of both sets of analyses are summarized in Table I. The identity of each hydrazide, except succinic dihydrazide, was confirmed by comparing its infrared spectrum recorded on a Model 137 Sodium Chloride Spectrophotometer (PerkinElmer Corporation, Norwalk, Conn.) with that published in the literature (11). The infrared spectra of succinic dihydra-
0.5 0.5 0.5
0.9 0.6
’
Purity,
Theoretical,
Found,
7 2
ZN
ZN
95.7 96.9 99.7 98.5 90.2 94.4
30.6 18.6 18.6 32.2 62.2 24.1
30.4 18.6 18.3 32.3 61.8 23.8
94.8 76.9 99.3
27.3 38.4 27.4
25.1 23.7 27.4
Purity based on N found, % 99 100
98 100
99 99 94 61 100
zide obtained from two different sources indicated that these compounds were identical. It was assumed, therefore, that the compound was succinic dihydrazide. Differential scanning calorimetry data for carbohydrazide were taken with a Model DSC-1 (Perkin-Elmer). Thermograms of carbohydrazide were taken on a thermobalance consisting of a n RG Electrobalance (Cahn Instrument Co., Paramount, Calif.), a Power Proportioning Temperature Programmer Model 240-M 25(F& M Scientific Corp., Avondale, Pa.), a Marshall furnace (Marshall Products Co., Columbus, Ohio), and a Model HR-100 XY recorder (Houston Instrument Co., Houston, Texas). Apparatus. AMPEROMETRIC ENDPOINT TECHNIQUE. These titrations were performed using a conventional H-cell with fritted glass disks and a saturated KC1 salt bridge to separate anolyte from catholyte. The anolyte was stirred with a Teflon (Du Pont) coated stirrer bar. Two l-cmz platinum flag electrodes were used to generate bromine and hydrogen, and platinum wire was used for amperometry with two polarizable electrodes. A Sargent Model IV Constant Current Source (E. H . Sargent and Co., Chicago) provided 19.65 mA of current. A 200-mV polarizing signal and the measurement circuitry for electrolysis current was provided by a Sargent Model P Ampot. PRESSUREMETRIC END POINTTECHNIQUE. The equipment for pressuremetric end point detection is shown schematically in Figure 1. It consists of the constant current source, pressure transducer system, potentiometric readout device, servorecorder, and constant temperature bath. The titration cell and its mirror image, the reference cell, are modified H-cells. One-cmz platinum flag electrodes were sealed to glass tubing and ring sealed to T 29/42 outer member glass joints. These are secured with a gas tight seal to the T 29/42 inner member joints blown onto the tops of each cell. The titration and reference cells are connected to the transducer cia lS/9 ball and socket joints. The Barocel Modular Pressure Transducing System (Datametrics Division of C.G.S. Scientific, Waltham, Mass.) is described by Curran and Driscoll (9) and by Curran (12). The system detects pressure differences between the titration and reference cell and presents this difference as a voltage. The titration cell, reference cell, stopcock, stirring motor, and pressure transducer which was enclosed in a waterproof case fashioned from Lucite and sheet aluminum, were mounted o n a ring stand and kept at uniform temperature by immersing them in a constant temperature bath which was regulated at 27 i 0.005 “C. PROCEDURE FOR PRESSUREMETRIC END POINTDETECTION. Ten milliliters of anolyte and a 5-ml aliquot of the acid hydrazide to be determined were pipetted into the anode compartment of the titration cell. To prevent the solution from flowing into the anode compartment, sufficient catholyte
(1 1) “Sadtler Standard Spectra,” Sadtler Research Laboratories, Philadelphia, Pa., 1969. 374
Re1 std dev, % 0.02 0.01 0.2 0.1
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
(12) D. J. Curran, J. Chem. Edur., 46, A465 (1969).
was pipetted into the cathode compartment so that the liquid level of the catholyte was higher than that of the anolyte. The reference cell was prepared in the same manner except that distilled water was substituted for reagents. Each cell was connected t o the stopcock and the ball and socket joints were secured with clamps. The cells, stopcock, stirring motor, and pressure transducer were then immersed in the constant temperature bath. The stopcock was turned so that there was zero differential pressure between titration and reference cells, and the transducer system's output voltage was adjusted to zero. The stopcock was next set so that the titration and reference cells were no longer ported t o each other and stirring of the anolyte was begun. After a few minutes a steady pressure base line was obtained. Generally, four increments of bromine were generated both before and after the end point. The output voltage was continuously monitored using the stripchart recorder. When a steady recorder trace indicated that equilibrium had been reached for a n increment of titrant added, the output voltage was accurately measured with the potentiometer. Figure 2 depicts a tracing of the strip chart recording for a titration of p-toluic acid hydrazide. End points were obtained by plotting transducer output in volts us. microequivalents of bromine generated.
T
metric titrations which yield one gaseous product. Before the equivalence AP is given by : AP = (aitRTK/NF)/[(V,)IK
F o r volumetric titrations in which the solution and gas phases are assumed ideal and the equilibrium constant for the titration reaction is large, Curran and Driscoll(9) have shown that plots of volume of titrant us. change in pressure yield linear-segmented type titration curves. The change in pressure, A P , in the titration cell before the equivalence point was given as :
AP = k
+ K(V,),)IVt + [ ( P r ) ~ ( V o ) ~ / V o 2+1 V t
(5)
I n this work, two gaseous products are generated before the equivalence point. Nitrogen is produced in the anode compartment and hydrogen at the cathode. The number of electrochemical equivalents is it/NF = m. Substituting into Equation 4:
+ nroRT) (6)
[(P,)I/TIAT (3)
where a is the ratio of moles of gaseous product to moles of titrant, C t is the molar concentration of titrant; and V cis the volume of titrant in liters, R is the gas law constant, K is Henry's law constant, nrois the number of moles of water in the reactor vessel, and T is the absolute temperature. The subscript I denotes the initial condition, that is before any titrant is delivered. V , is the gas phase volume, and P, is the pressure in the titration cell. Kronfeld (13) derived the analogous equation for coulo-
+ n,RT]
(4) where i is the titrant generation current in amperes, t the titrant generation time in seconds, N the number of electrons involved in the electrode process, and F the value of the Faraday. After the equivalence point, no more gas is generated, and the change in pressure is constant, or:
THEORY
AP = [aCIKRT/(RTn,
"
Figure 2. Tracing of stripchart recording of pressure transducer output during titration ofp-toluic acid hydrazide
(cux,mRTKx,)/[(V,),KH,
F o r a given experiment, all the terms o n the right hand side of Equation 6 except m are constants, thus: AP = (K2
+ Ka)m
(7)
The transducer system measures the difference in pressure, AP', between the titration and reference cells. If the pressure in the reference cell is equal to the pressure in the titration cell before any titrant is generated, AP' = 0 and Equation 7 is valid u p to the equivalence point. Otherwise, a n arbitrary constant, Kq, is introduced into Equation 7 :
(13) J. Kronfeld, M. S. Thesis, University of Massachusetts, Amherst, Mass., 1966.
AP' = (K?
+ K3)m + K 4
(8)
Table 11. Titration Results with Pressuremetric End Point Detection
Taken," mg 6.50 9.18 7.16 5.80 0.820 5.71
Compound I Isonicotinic acid hydrazide I1 p-Toluic acid hydrazide I11 Phenyl acetic hydrazide IV Adipic dihydrazide V Carbohydrazide VI 1,Z-Diacetyl hydrazide VI1 Malonic dihydrazide hydrochloride 1.280 VI11 Succinic dihydrazide 1.023 2.482 IX Isobutyric hydrazide Based on amperometric analysis. * Based on mg taken listed in column 3 of Table I. No end point observed.
Found, mg
6.47 9.15 7.18 5.84 0.818 5.15 1.192 2:397
No. of Re1 std
Detn. 4 6 4 3 5 7
dev, Z 0.3 0.4 0.2 0.5 0.4 4.5
2 3 5
4.3 ... 6.0
Relative accuracy,a
Purity,b
Z I
z
-0.3 -0.3 +0.4 +0.6 -0.2 +0.6
95.7 96.7 100.1 99.1 90.0 95.0
-7.0
88.1
-3.5
95.8
...
...
Slopes of titration curves, mV/peq Before After end end point point 12.4 8.7 11.7 8.1 12.2 8.3 12.9 9.4 14.2 9.4 11.1 8.5 9.0
... 8.4
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
7.3 9.6c 6.3
375
4-,
I
/
O 2-i
C-i
I
0
I
50
,
I
IC0
150
200
1
250
300
Figure 4. Pressure response to electrogeneration of hydrogen gas
p q u i valent s Bromlne Gene rated
Figure 3. Pressuremetric titration curve of p toluic acid hydrazide with electrogenerated bromine Transducer range setting: output)
Curve A . Cell not presaturated with hydrogen Curve B. Cell presaturated with hydrogen Transducer range setting: X 1
X 1 (1 psi = 5 volts
The transducer output, E, in volts, is directly proportional to AP‘ so that Equation 8 becomes:
E = (K5
+ K d m + K7
(9)
After the equivalence point, the only gas produced is hydrogen, and the transducer output is:
E=Kg+Ks
cathode to anode compartment. Other metal ions also form complexes with acid hydrazides (15, 16) so their use as cathodic depolarizers was not further attempted. To determine if the theoretically expected amount of gas was being evolved during a titration, the theoretical values of the titration curve slopes before and after the equivalence point were calculated. Before the equivalence point the slope is obtained from Equation 6 as:
(10)
(APjm) before equivalence point
A plot of transducer output in volts us. m yields a linearsegmented type titration curve. The end point for a given titration is taken at the intersection of the lines described by Equations 9 and 10.
( ~ N ~ R T K N ~ ) / [ ( V ’ )nI K JN r l~
RESULTS AND DISCUSSION
Table I1 summarizes the results of the coulometric titrations utilizing the pressuremetric end point technique. Coulometric titration with amperometric end point detection was chosen as the comparison method because the chemistry of the two methods is identical. The precision and accuracy of the pressuremetric technique is a few parts per thousand in the most favorable cases. A titration curve forp-toluic acid hydrazide is shown in Figure 3. The initial pressure difference between titration and reference vessels is not exactly zero because of their slightly different gas phase volumes. The curvature of the line during the initial stages of titrant generation is caused by the solubility of hydrogen gas. Figure 4 shows the results of experiments involving only the generation of hydrogen gas in the titration cell. Curve A results when hydrogen gas is generated in the cell without presaturation with hydrogen, and Line B, when the cell is presaturated. It would be convenient to use a cathodic depolarizer that did not liberate a gas when reduced. Under these circumstances, no curvature would be caused by hydrogen generation, and the segment of the titration curve after the end point would be a horizontal line. Copper sulfate was employed as a cathodic depolarizer during the investigation of isonicotinic acid hydrazide, but low results were obtained. The error was probably due to complex formation between the hydrazide and cupric ion (14) diffusing from the (14) S. Fallab and H. Erlenmeyer, Helu. Chim. Acta., 36, 6 (1963). 376
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
+
=
+
+
(~H,RTKHz)/[(V~IK nURT1 H, (11) After the equivalence point, only the term involvjng hydrogen evolution contributes to the slope. The following values were used to calculate these slopes: a H 2= 0.50 mole/ equivalent, c y N 2 = 0.25 mole/equivalent, R = 0.0821 liter atm/degree mole, T = 300 OK, (V,), = 0.012 liter, nzo = 1.39 moles. Henry’s law constants for nitrogen and hydrogen were estimated to be 1.04 X l o 4atm and 7.64 X lo3atm, respectively, by extrapolation of values for Bunsen coefficients, f i j , tabulated by Linke (17), and conversion to Henry’s law constants according to (18):
Kj
(22414p)/Mpj
(12)
where p is the density of water and M its molecular weight. The calculated slopes before and after the equivalence point atm/peq and 1.14 X atm/peq, respecare 1.73 X tively, which corresponds to transducer outputs of 12.7 mV/peq and 8.3 8 mV/peq on the X1 multiplier. The experimental values for the slopes of the titration curves or the three aromatic acid hydrazides, isonicotinic-, p-toluic-, and phenyl acetic- (Table 11), compare favorably with the theoretically predicted values. The excellent precision and accuracy with which these compounds were titrated is a reflection of the ease of oxidation caused by the donation of ~~~~
(15) A. F. Krivis, G. R. Supp, and R. L. Doerr, ANAL.CHEM.,37, 52 (1965). (16) A. Albert, Experientia, 9, 370 (1953). (17) W. F. Linke, “Solubilities, Inorganic and Metal Organic Compounds,” 4th ed., D. Van Nostrand Company, Inc., Princeton, N. J., 1958, Vol. I, p 1078, and Vol. 11, p 577. (18) G. W. Castellan, “Physical Chemistry,” Addison-Wesley Publishing Company, Inc., Reading, Mass., 1964, p 286.
electrons by the aromatic ring to the hydrazide reaction site (19). 1,2-Diacetyl hydrazine and isobutyric acid hydrazide were titrated pressuremetrically with only fair' accuracy and precision. McKennis (20) reported that 1,2-diacetyl hydrazide did not react with iodate to yield nitrogen, and Krivis and coworkers ( 4 ) reported that it did not react with electrogenerated bromine. The average slope of the titration curve before the end point was 12 % less steep than expected. For isobutyric hydrazide the slope before the end point was 30 % lower than expected. These facts suggest either that with these compounds the reaction was slow and pressure measurements did not correspond to equilibrium values for the chemical reaction, or that a competing side reaction occurs which consumes bromine but does not liberate nitrogen. Of the three dihydrazides titrated, malonic-, succinic-, and adipic-, only the latter reacted as expected, yielding good precision and accuracy. The average values of the slopes of its titration curves were slightly higher than expected. Although the amperometric method indicated that succinic dihydrazide reacted with bromine, no break in the pressuremetric curve was discernible. The average slope of the lines was 14.2% greater than the expected value corresponding t o only hydrogen evolution, indicating that perhaps some nitrogen is being evolved, or that the solubility of bromine has been exceeded. When a thousand-fold excess of bromine was added to a n acidic solution of succinic dihydrazide, vigorous gas evolution was noted. Products of the titration of succinic dihydrazide with electrogenerated bromine were extracted into ethyl ether. IR spectra and thin-layer chromatograms developed with CCI1 on silica gel indicated that the expected product, succinic acid, was not present. Clearly the oxidation does not proceed according to Equation 1. Malonic dihydrazide exhibited erratic behavior. Three of five titration attempts yielded plots consisting of intersecting straight lines, but the average slope of the straight lines corresponding to the initial increments of bromine generated was 17.5 % less steep than the average slope of the lines corresponding to the latter increments of bromine generation. The intersection of these lines showed no sensible relationship t o the expected equivalence point. The precision and accuracy of the other two titrations are poor, and the slope of the titration curve before the end point is only 7 2 z of the expected value. The erratic behavior of this compound, the inability to titrate succinic dihydrazide with the pressuremetric end point technique, and the excellent accuracy and precision and predictable behavior of adipic dihydrazide lead us to speculate that the chain length of these dihydrazides is a factor in their chemical behavior toward bromine. Adipic dihydrazide, which has four methylene groups separating carbonyl carbons, behaves like a monofunctional hydrazide. Malonic dihydrazide, with only one methylene group, and succinic dihydrazide with two, may undergo a n intramolecular interaction such as cyclization between hydrazide functionalities which prevents evolution of nitrogen and therefore titration of them pressuremetrically. The agreement between amperometric and pressuremetric end points for analyses of carbohydrazide is excellent but the calculated percent purity for the compound is only about 90 in either case. Our amperometric results are in agreement with those of Krivis et a / . ( 4 ) who proposed that carbohydrazide crystallizes from solution as the hemihydrate. With ~~
- -~ _ ~
_
_
~
(19) B. Singh and S . S. Sahota, J . Itidion Chern. SOC.,38,569 (1961). (20) H. McKennis, Jr., J. H. Weatherby, and E. P. Dellis, ANAL. CHEM.. 30, 499 (1958).
this change in molecular weight, a purity of nearly 100% can be calculated. However, neither differential scanning calorimetry nor thermogravimetric analysis gave any indication that water of hydration was lost on heating the compound from room temperature to its melting point of 153.8 "C. Elemental analysis for our samples of carbohydrazide gave results of 13.4% C, 6.2% H, 61.8% N, and 17.9% 0 which are in good agreement with the theoretical values for the anhydrous compound of 13.3% C , 6.7% H, 62.2% N, and 17.8 % 0. The hemihydrate would contain 12.0% C , 7.1 % H, 56.1 % N, and 24.0% 0. This evidence suggests that the compound is not the hemihydrate, but rather that the anhydrous material takes u p bromine reproducibly with a ratio of 3.60 moles of bromine per mole of carbohydrazide rather than the expected ratio of 4.00. CONCLUSIONS
This work demonstrates the feasibility of using pressuremetric end point detection for coulometric titrations. T h e method is capable of good accuracy and precision for the coulometric titration of a number of acid hydrazides with bromine. However, it has also been shown that this oxidation reaction cannot provide a general analytical method for the analysis of this class of compounds. The use of both amperometric and pressuremetric end point techniques is valuable because the former monitors the concentration of titrant and the latter product formation. Differences between the analyses employing the two methods may indicate undesirable side reactions or kinetic complications. The possibility of direct electrochemical oxidation of acid hydrazides cannot be ruled out. Very little information is available but the work of Iversen and Lund (21) indicates that some compounds will oxidize directly a t the electrode. Information concerning the mechanism and stoichiometry of these electrochemical reactions is needed. It is interesting to note the possibilities of using the pressure transducer system as a coulometer. The slope of the line for Curve B of Figure 4 is 11.6 mV/Feq. This is two thirds of the slope that should be observed for a hydrogenoxygen or hydrogen-nitrogen coulometer. Thus, the expected slope for either coulometer for this particular cell is 17.4 mV/peq. The range setting used to obtain the data of Figure 4 was X 1. If the transducer system were operated on the highest sensitivity setting ( X 0.001), then either the hydrogen-oxygen or the hydrogen-nitrogen case would yield a slope of 17.4 Vlpeq. A tenth of a volt can be measured t o 1 % or better accuracy and precision. This signal would be provided by 5.75 x 10-3 peq, which corresponds to about 5.75 X coulomb. Optimum design of a cell for this purpose might enable measurement of as little as 6 X 10-j coulomb. ACKNOWLEDGMENT Thanks are due to L. A. Carpino for supplying some of the samples investigated. We also acknowledge initial studies on isonicotinic acid hydrazide by J. Kronfeld. RECEIVED for review October 9, 1969. Accepted December 29, 1969. This investigation was suppoited by Research Grant GM-12284 from the Public Health Service, U. S. Department of Health, Education, and Welfare. Thanks are due to the Analytical Chemistry Division of the American Chemical Society for a Summer Fellowship awarded to J.E.C. (21) P. E. Iversen and H. Lund, ANAL.CHEM., 41, 1322 (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
377
