The reference electrode was Hg/HgzS04(s)/NazS04(lF)and was connected to the solution compartment through a fineporosity fritted-glass disk (2 mm X 2 mm), a compartment containing reactant solution (which served as a salt-bridge) and a second fritted-glass disk. The reference and saltbridge compartments were filled using a syringe and sealed with tight-fitting Teflon plugs. RESULTS
Figure 2 shows a typical cyclic current potential curve obtained with a solution of Fe(II1)EDTA. The curve conforms in all respects with the relevant theory (5). Table I presents representative coulometric data obtained with standard solutions of Cu(II), Cu(II)EDTA, and Fe(I1I)EDTA by integration of chronoamperometric currents. The applicability of this electrode for study of reactions occurring at potentials near that of the background reaction has been confirmed by experiments with the Cr(II1)-Cr(I1) couple (Figure 3). The cyclic current-potential curve displays clear separation between the Cr(II1) reduction wave and the background reaction. The apparent reversibility of the Cr(II1)-Cr(I1) couple is considerably greater in thin-layer
(Ep,oathodio = -0.8, Ep,anodio = -0.6 volt SCE) than in semiinfinite voltammetry (Ep.osthadio = -0.95, Eg.anodie = -0.45 volt SCE) owing to the smaller current densities inherent in the thin-layer approach. The electrode described can be used to measure reactant adsorption by adopting the approach previously employed with the platinum thin-layer electrodes (6): The mercury pool is washed several times with the reactant solution in the micrometer-buret by repeatedly extruding small volumes onto the electrode surface and then removing the solution, but not the mercury. After the last extrusion of reactant solution the coulometric experiment is performed and the charge required for the consumption of all of the reactant in the thin layer is augmented by whatever reactant has been adsorbed on the electrode during the pre-rinsing. When the pre-rinsing procedure is not employed, the charge required is unaffected by specific adsorption, provided that adsorbed and unadsorbed reactant are electroactive over the same range of potentials.
RECEIVED for review October 12, 1967. Accepted December 21, 1967. This work was supported in part by a Grant from the National Science Foundation.
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(5) A. T. Hubbard and F. C. Anson, ANAL.CHEM., 38,58 (1966).
(6) A. T. Hubbard and F. C. Anson, J. Electroanal. Chem., 9, 163 (1965).
Thermometric Atnalysis of Mixtures of Water and Miscible Organic Compounds M. Y. Spink and C. H. Spinkl Juniata College, Huntingdon, Pa.
16652
THEANALYSIS OF BINARY solutions of water and miscible organic compounds-such as alcohols, ketones, esters, and amides-has been carried out by several methods, including gas chromatography ( I - j ) , refractive index measurement (4, 5), and polarography (6, 7). Because each method is restricted either with regard to range of composition or type of compound, complementary procedures are desirable. The basicity of a number of organic functional groups differs rather widely from that of water, as evidenced by protonation equilibria in strong acid solutions (8). If these differences in basicity also appear as differences in the enthalpies of protonation, the heat effect accompanying the introduction of an aqueous solution of the organic base into a strong acid solution would be sensitive to the composition of the binary mixture. 1
Present address, State University of New York, Cortland, N. Y.
13045 (1) C. Bluestein and H. N. Posmanter, ANAL.CHEM.,38, 1865 (1966). (2) J. E. Zarembe and I. Lysyj, Zbid.,31, 1833 (1959). (3) 0. F. Bennett, Ibid.,36,684 (1964). (4) A. E. Karr, W. M. Bowes, and E. G. Scheibel, Ibid.,23, 459 (1951). (5) W. M. Spicer and L.H. Meyer, Ibid.,23,663 (1951). (6) R. E. Van Atta and D. R. Jamieson, Ibid.,31, 1217 (1959). (7) M. E. Hall, Zbid.,31, 2007 (1959). (8) E. M. Arnett, Prog. Phys. Org. Chem., 1, 223 (1963).
On the basis of previous work using thermometric analysis (9-12), it was felt that this general type of methodology would
be suitable for the analysis of simple water-organic mixtures. This study describes a procedure for the analysis of aqueous solutions of methanol, acetone, 1-propanol, and methyl acetate based on measurement of the heat evolved when the binary mixtures are added to 80% sulfuric acid. EXPERIMENTAL Apparatus. The calorimeter was a 250-ml Dewar flask fitted with a 600-rpm synchronous stirrer, a 100-pl Hamilton syringe with Chaney adaptor, and a Sargent 2000-ohm thermistor probe (S-81620). The syringe was modified by attaching a glass tip to the barrel. The thermistor probe formed one arm of a Sargent dc Wheatstone bridge (S-81601), the output of which was fed to a Varian 1-mV recorder (Model G40A). To ensure control of the environmental temperature, the Dewar flask was placed in a 25.0" C constant temperature bath. Reagents. All chemicals were reagent grade. The organic liquids were further purified by careful distillation through a 70-cm fractionating column. The sulfuric acid solutions (9) H. D. Richmond and J. A. Eggleston, Analyst, 51, 281 (1926). (10) L. H. Greathouse, H. J. Janssen. and C. H. Havdel. ANAL. CHEM., 28, 357 (1956). (11) J. C. Wasilewski, P. T-S. Pei, and J. Jordan, Ibid., 36, 2131 (1964). (12) H. J. Keily and D. N. Hume, Ibid.,36, 543 (1964). -
VOL. 40,
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22
n
22.0
18.0
1:
0 IW V
n a w a n
14.0
METHYL ACETATE
a
0
,q7 W
8 10.0
n
0.02
E
I- PROPANOL
-0
6.0
0.2 0.4 0.6 0.8 VOLUME- FRACTION
0
1.0
Figure 1. Recorder deflection os. volume-fraction for injection of MeOH-H20 mixtures into 40,60, and 80 wt % H2SO4
0.60
0.80
1.000
Figure 2. Thermometric calibration curves for aqueous solutions of methanol, acetone, 1-propanol, and methyl acetate in 80 wt % H2S04
RESULTS AND DISCUSSION
Figure 1 shows plots of recorder deflection us. volumefraction for methanol-water mixtures at several different sulfuric acid concentrations. Because 80 % sulfuric acid offers the most useful sensitivity and linearity, all subsequent measurements are made at this acid composition. Higher acid concentrations cause less efficient stirring because of the increased viscosity of the solution. The shapes of the calibration curves at the different sulfuric acid compositions probably reflect differences in protonation enthalpies and changes in the structure of the binary solutions with overall changes in volume-fraction. Calibration curves for aqueous solutions of methanol, acetone, 1-propanol, and methyl acetate are shown in Figure 2. Curves for acetamide and glycine solutions are not included because of the relatively small change in the thermal response with change in composition. Because methyl acetate is not completely miscible with water, there is a gap in the ANALYTICAL CHEMISTRY
0.40
VOLUME-FRACTION
were made up by weight with deionized, distilled, COrfree water. Procedure. In order to analyze unknown samples, a calibration curve was constructed using solutions of known composition. The composition variable was volume-fraction, defined as the number of milliliters of organic compound divided by the sum of the volumes of water and organic component. The syringe was filled with 50 pl of the known sample, and the liquid pulled back from the glass tip so that no contact of the sample with the acid occurred until injection. Time was allowed (usually about 4 minutes) for the syringe and its contents to equilibrate thermally with the 342.0 grams of 80% sulfuric acid in the Dewar flask. When the sample was injected, a deflection, which is proportional to the heat evolved, was noted on the recorder. The measured heights of the deflections were then plotted us. volume-fraction in order to construct the calibration curve. The thermal response due to the injection of an unknown sample was compared with the calibration curve in order to determine unknown concentrations.
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0.20 ,
middle region of composition. Because small volumes of samples are added to the sulfuric acid solutions, a number of injections can be made into the same Dewar flask of acid without affectink the magnitude of the deflections. For example, the injection of 50 p1 of 0.60volume-fraction methanol into 80 % sulfuric acid produces a deflection of 15.88 divisions. After 23 injections into the same acid solution, the value of the deflection is 15.93 divisions. Similar results at other volume-fractions of methanol show that after 20 to 25 injections into the same acid solution, the values of the recorder deflections are not statistically different from the first-injection value. It is necessary, however, that the temperature within the Dewar flask remain at 25.0' f 0.2" C. Insertion of a cold glass rod after six to eight injections (corresponding to a temperature change of about 0.2" C) is generally sufficient to cool the acid solution. If some care is taken in preparing the 80% sulfuric acid stock solution and in weighing the acid into the Dewar flask, the calibration curve need not be reconstructed when a new Dewar flask of acid is used. However, for quick analyses it is possible to construct a well-defined calibration curve over the appropriate range of concentration and to determine six to eight unknown samples within the same Dewar flask of acid. Thus, depending upon the analytical needs, several approaches are available. The relative standard deviation of the results of injecting five different samples of 0.800-volume-fraction 1-propanol into 8 0 z sulfuric acid is +0.3%. Two additional trials involving the same five samples give relative standard deviations of ~ t 0 . 3and 10.5%. At the 0.2-volume-fraction level of the organic component, the relative standard deviation is about &0.5%. With this level of precision, the minimum distinguishable difference in composition is 0.005-volume-fraction unit. In the range 0.15 to 0.90 volume-fraction, the average relative errors for the analyses of known samples of methanol, 1-propanol, acetone, and methyl acetate are 0.13, 0.55, 0.40, and 0.56z, respectively. Both the precision and accuracy
are in part dependent upon the slope of the calibration curves. Based on a 0.5% relative error in making up the standard solutions by volume, and a 0.3 relative error in the readability of the recorder deflections, the error in the slope of the calibration curves could be as high as 0.8 %. This error in the slope would result in a maximum relative error of about 1 in the determination of an unknown. The results of this study generally fall within that range of error. A larger number of calibration points in the vicinity of the unknown concentration improves results. For instance, the methanol determinations are based upon calibrations at every 0.05-volume-fraction unit. The other mixtures are analyzed using calibration points at every 0.1-volumefraction unit. The relative error in the methanol analyses averages 0.1 %, compared with 0.5% for the other mixtures. The analytical accuracy of the method can be affected by the presence of one or more contaminants. Each component or impurity contributes to the measured heat according to the type of compound and its concentration. Thus, the effect of a
particular contaminant must be investigated. However, based on the overall sensitivity of the method, it is improbable that interfering substances at about 0.01-volume-fraction unit or less cause noticeable changes in accuracy. In conclusion, the described thermometric method for the analysis of miscible organic-aqueous mixtures provides good accuracy and precision, requires very little sample, and is relatively simple for routine problems. A large range of composition is suitable for analysis, although dilute solutions of either component provide limited accuracy. The procedure can be useful for the analysis of water in organic solvents, and can readily apply to on-stream analytical problems involving binary aqueous mixtures. RECEIVED for review August 16, 1967. Accepted December 14, 1967. Paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 6, 1967. Work supported in part by Petroleum Research Fund of the American Chemical Society.
Polarography in N,N-Dimethylacetamide Alkali Metal Ions and Ammonium Ion Viktor Gutmann, Manfred Michlmayr,’ and Gerald Peychal-Heiling Institut f ur Anorganische Chernie, Technische Hochschule Wien, Getreidemarkt 9, A-1060 Wien,Austria N,N-DIMETHYLACETAMIDE (DMA) has not been used extensively as a solvent for polarographic purposes. Although thallium, lead, cadmium, and zinc ions have been investigated in this solvent (I), and a preliminary report on DMA as possible polarographic solvent has been published (2), no systematic studies have been made on alkali metal ions. In order to establish the relationship between the polarographic half-wave potential Eliz and the donor number of the solvent (3), studies in DMA have been carried out. Though the dc polarographic behavior of alkali metal ions is well known in aqueous solutions (4-8), little information is available on the oscillopolarography of these simple ions (8-10). The polarographic behavior of alkali metal ions in acetic acid anhydride (II), ace1 Present address, Department of Chemistry, University of California, Riverside, Calif. 92502
(1) S. Musha, T. Wasa, and K. Tani, Reuiew of Polarography (Japan), 11, 169 (1963). (2) L. 0. Wheeler and D. W. Emerich, J . Miss. Acad. Sci, 9, 63 (1963). (3) V. Gutmann, G. Peychal-Heiling, and M. Michlmayr, Znorg. Nucl. Chem. Letters, 3, 501 (1967). (4) J. Heyrovskg, Chem. Listy, 16, 256 (1922). ( 5 ) E. S. Peracchio and V. W. Meloche, J. Am. Chem. SOC.,60, 1770 (1938). (6) I. Zlotowsky and I. M. Kolthoff, IND. ENG. CHEM., ANAL. ED., 14, 473 (1942). (7) 1. Zlotowsky and I. M. Kolthoff, J. Am. Chem. SOC.,64, 1297 (1942). (8) A. A. VlEek, Collection Czech. Chem. Commun. 20, 413 (1955). (9) J. E. B. Randles and K. W. Sornerton, Trans. Faraday SOC., 48, 951 (1952). (10) L. Treindl, Chem. Listy, 50, 154 (1956). (11) V. Gutmann and E. Nedbalek, Mh. Chem., 89, 203 (1958).
tone (12), dimethylsulfoxide (13, 14), ethylenediamine (13, morpholine (16), liquid ammonia (17, 18), N,N-dimethylformamide (19), N-methylacetamide (20), propanediol-l,2carbonate (21), acetonitrile (22), propionitrile ( 2 3 , isobutyronitrile (24), benzonitrile (23), and phenylacetonitrile (23) has been reported. EXPERIMENTAL
A Polariter P04b (Radiometer, Copenhagen) was used for dc polarography at the dropping mercury electrode @ME) and at the stationary mercury electrode (SME); ac experiments were carried out by means of the Polaroscope P 576 (Kiiiik, Prague). A Kalousek commutator, as adapted by RBlek (25), was used to check the reversibility of the electrode processes. The capillary used was supplied (12) J. F. Coetzee and Wei-San Siao, J. Inorg. Chem., 2, 14 (1963). (13) V. Gutmann and G. Schober, Z . Anal. Chem., 171, 339 (1959). (14) G. Schober and V. Gutmann, “Advances in Polarography,” I. S. Longmuir, Ed., Pergamon Press,London 1960, p 940. (15) G. Schober and V. Gutmann, Mh. Chem., 89,401 (1958). (16) V. Gutmann and E. Nedbalek, Ibid., 88, 320 (1957). (17) H. A. Laitinen and C. J. Nyman, J. Am. Chem. SOC.,70,2241 (1948). (18) H. A. Laitinen and C. E. Shoemaker, Zbid., 72,4975 (1950). (19) G. H. Brown and R. Al-Urfali, Zbid., 80, 2113 (1958). (20) L. A. Knecht and I. M. Kolthoff, J . Znorg. Chem., 1,195 (1962). (21) V. Gutmann, M. Kogelnig, and M. Michlmayr, Mh. Chem. in press. (22) I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. SOC.,79, 870 (1957). (23) R. C. Larson and R. T. Iwamoto, Ibid., 82, 3239, 3526 (1960). (24) J. F. Coetzee and J. L. Hedrick, J. Physik. Chem., 67, 221 (1963). (25) M. Kalousek and M. Rklek, Collection Czech. Chem. Commun. 19, 1099 (1954). VOL 40, NO. 3, MARCH 1968
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