lower overvoltage of that state). I n each case, a lessening of surface crowding can increase the adsorption-preferred-. reaction-preferred conversion rate and, also, in the latter case the rate becomes accelerated as more product is generated. At short equilibration times, or low bulk concentrations, the smaller reactant surface excess provides less surface crowding, allowing a more rapid shift to a high conversion rate and producing a less prominent potential spike. Higher surface escewes, produced by increased equilibration times or bulk concentrations, prohibit attainment of a more rapid conversion rate and the spike is enhanced. Ultimately, in the smoothed wave, the reactant surface excess is completely consumed at the adsorptionpreferred potential. This physical picture, consistent with all the experimental results (including the current density effect), is supported by the fact that the adsorption-desorption kinetics in this system are apparently slow, making slow processes occurring within the adsorbed film more plausible. I n view of the above results, several cautions concerning polarographic adsorption waves seem warranted. Because the phenylmercuric ion prewave is shown to be governed by adsorption of the reactant rather than the product,
considerable care is indicated in interpretation of polarographic pre- and postwaves for irreversible systems. I n addition, while electrocapillary or polarographic prewave-height measurements might superficially appear to be well behaved and to give reliable r, data, i t is evident that in any adsorption system displaying the slow adsorption rates typical of phenylmercury ion, such approaches to surface excess data would be invalid. Last, the surface coveragedependent wave-shapes observed for phenylmercury ion indicate that the ultimate appearance of an adsorption wave in an irreversible polarographic (or chronopotentiometric) process can be largely a matter of adsorption or surface-orientation rate processes. LITERATURE CITED
(1) Benesch, R., Benesch, R. E., J . Am. Chem. Soc. 73, 3391 (1951). (2) Benesch, R. E., Benesch, R., J . Phys. Chem. 56, 648 (1952). (3) Bondi, A.,Zbid., 68, 441 (1964). (4) Brdicka, R., 2. Elektrochem. 48, 278 (1942). (5) DeFord, D. D., 133rd Meeting, ACS, San Francisco, Calif., April 1958. (6) Delahay, P., Trathtenberg, I., J . Am. Chem. SOC.79, 2355 (1957). (7) DeMars, R. D., Shain, I., ANAL. CHEM.29, 1825 (1957). (8) Herman, H. B., Tatwawadi, S. V., Bard, A. J., Zbzd., 35, 2210 (1963).
(9) Hush, N. S., Oldham, K. B., J . Electroanal. Chem. 6, 34 (1963). (10) Laitinen, H. A., Chambers, L. M., ANAL.CHEW36, 5 (1964). (11) Lorenz, W.,2. Elektrochem. 59, 730 (1955). (12) Lorenz, W., RIuhlberg, H., Ibid., p. 736. (13) Lorenz, W., RIuhlberg, H., 2. Physik. Chem. (Frankfurt) 17, 129 (1958). (14) Rlunson, R. A., J . Electroanal. Chem. 5, 292 (1963). (15) RIunson, R. A,, J . Phys. Chem. 66, 727 i1962). (16) Rlurray, R. W., J . Electroanal. Chem. 7, 242 (1964). (17) RIurray, 1%.W., Gross, D. J., 149th Meeting, ACS, Detroit, Rlich., April 1965. (18) Osteryoung, R. A,, ANAL. CHEM. 35, 1100 (1963). (19) Pauling, L., "The Sature of the Chemical Bond," 3rd ed., Cornell Univ. Press, Ithaca, Y. Y., 1960. (20) Iteilley, C. &'., Schmid, R. W., Lamson, D. W., ANAL. CHEM. 30, 953 (1958). (21) Iieinmuth, W. H., Zbid., 33, 322 (1961). (22) Takemori, Y., Rev. Polarog. (Kyoto) 12, 63 (1964). (23) Tatwawadi, S. V., Bard, A. J., A N IL. CHEY.36, 2 (1964). (24) Vojir, V., Collection Czech. Chem. Commun. 16, 488 (1951). RECEIVEDfor review May 11, 1965. Accepted August 2, 1965. Work supported by Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant T o . AF-.4FOSR-584-64.
Use of Differential Scanning Calorimetry for the Analysis of Chloride-Bromide Mixtures SIR: The heat of fusion of an ideal solid solution of the type A,X,-B,X, or A,X,--Amy, is directly proportional to the concentration of solute ion. This property can be utilized for the determination of chloride-bromide mixtures in the complete concentration range of O-lOO%. I n the procedure described below, solutions containing both chloride and bromide are precipitated with silver nitrate, forming solid solutions of silver chloride-bromide. The heat of fusion of the mixed crystal is then determined, and the per cent chloride or bromide is obtained from a previously prepared standard curve.
tassium bromide. Prepare 0.1M solutions of each. Rocedure. Prepare standard binary solutions of KC1 and K13r in Precipitate t h e t h e range O-lOO%. halides from a slightly acidic medium with silver nitrate solution (10% excess), heat until t h e precipitate settles, filter and wash with very dilute nitric acid. D r y the solid a t 110" C. for several hours. (The precipitate
should be shielded from sunlight during the procedure.) Punch out disks from a thin sheet of mica with an appropriate size cork borer and use these disks in place of the aluminum sample pans supplied with the instrument. (This will prevent the reaction of molten silver halide with either the aluminum pan or the metal sample holder.) Place a mica disk in the sample and in the reference holder,
EXPERIMENTAL
Apparatus. A Perkin-Elmer DSC-1 differential scanning calorimeter, Perkin-Elmer Corp., Norwalk, Conn. Reagents. Reagent grade silver nitrate, potassium chloride, and po-
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ANALYTICAL CHEMISTRY
Figure 1. Differential scanning calorimeter traces fob A. AgBr, B. 24.6% AgCl in AgBr, C. 64.0% AgCl in AgBr, D. ASCI. Sample weights: A. 3.17 mg., B. 3.61 mg., C. 3.99 mg.,D. 3.23 mg.; heating rate 20" per minute
Figure 2. Heat of fusion as a function of weight % chloride
The accuracy of the method can be no better than the accuracy of the determination of AH fusion, which is generally =& 1% with the Perkin-Elmer DSC-1 (2). Interferences should be handled in the same manner as in the standard gravimetric procedure for chlxide or bromide (1). ACKNOWLEDGMENT
The author thanks Norman Poginy for the preparation of the precipitates. t
0
10 20 30 40 50 60 70 80 90 WEIGHT PER CENT SILVER CHLORIDE IN SILVER BROMIDE
add a weighed amount of silver halide.
LITERATURE CITED
100
(1) “Scott’s Standard Methods of Chemical Analysis,” N. H. Furman, ed.,
Sixth Edition, T’ol. 1, p. 327, Van Nostrand, Princeton, V. J., 1962. (2) Watson, E. S., O’Neill, M. J., Justin, J., Brenner, N., ANAL.CHEY.36, 1233 (1964).
RESULTS AND DISCUSSION
JACOB BLOCK^ chloride to bromide ratio, Figure 1.) Plot AH fusion Per P m US. weight % silver chloride, and use this plot as the standard curve. Determine the heat of fusion of unknown mixtures of chloride and bromide in a manner similar to above and find the weight per cent chloride from the standard plot.
ions forming ideal solid so1uiions“in all proportions will yield straight line plots, and therefore can be analyzed by this method. This very probably would include lead and barium as the sulfate, and chromate and sulfate as the barium Salt, as well as Certain Organic mix& crystals and metal alloys.
Olin Research Center Olin Mathieson Chemical Corp. New Haven, Conn. 06504 Present address, W. R. Grace & Co., Research Division, Washington Research Center, Clarksville, Md. 21029. RECEIVEDfor review June 29, 1965. Accepted August 12, 1965.
Chronopotentiometer with Compensation for Extraneous Currents SIR: Chronopotentiometric theory, though explicit, is based upon the fundamental assumption that all of the electrolysis current advances the reaction of the electroactive species of interest. This assumption is never entirely valid in conventional chronopotentiometry because other processes occur concurrently with the electrolysis of the electroactive species. Prominent among these processes are the charging of the electrical double layer a t the electrodesolution interface, the electrolysis of minor or major components of the medium (exclusive of the electroactive species of interest), and the electrolytic reduction or oxidation of the electrode itself. Moreover, these extraneous processes proceed a t variable rates and to variable extents and, hence, the current efficiency for the desired electrode reaction is a variable quantity, seldom subject to measurement or calculation, Lingane (2), and later Bard ( I ) , proposed methods for determining the effects of these undesired reactions so that appropriate corrections for them could be applied. Their corrections offer considerable improvement, but they are empirical in nature and are not always satis-
Time Base
I+?
Ret Cent.
CELL:
Counter
II
Figure 1. Block diagram of chronopotentiometer with compensation for extraneous currents
factory. The net result is that there have been few truly analytical applications of the chronopotentiometric technique and these have generally dealt with relatively large concentrations of the electroactive species-concentrations which are readily accessible by other methods.
The instrument described in this communication is intended to cause the electrolytic reaction of interest to proceed as if the current efficiency were essentially loo%, based upon the selected current. Two chronopotentiometric cells are utilized, the first of which contains the electroactive species plus a suitable supporting electrolyte and the second of which contains only the supporting electrolyte. These are labeled Cell I and Cell 11, respectively, in Figure 1. When Cell I1 is disconnected, Cell I is operated in the conventional chronopotentiometric fachion. X selected electrolysis current (denoted iconst in Figure 1) is caused to flow between the working and counter electrodes, and the potential difference between the working electrode and a reference electrode is monitored as a function of time. This potential difference is applied to the Y input of an X-Y plotter through the voltage follower (amplifier 4) connected to “Ref.” An accurate time base, generated by amplifier 5 and its integrator circuit, is applied t o the X input. Hence with Cell I1 disconnected, the instrument functions conventionally, and an uncompensated VOL. 37, NO. 1 1 , OCTOBER 1965
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