~ for Elecvalues found earlier. From Table 11, K H =~0.546 trode 1, 0.241 for Electrode 5, and 0.034 for Electrode 3. Mattock ( 2 ) has shown that above 0.01 M Na+ ion concentration and for pH >5, the effects of pH changes would become unimportant for BH 68, BH 104, and NAS 11-18 glasses. His measurements were carried out using a conventional pH meter and the background medium of his solutions was enthanolamine hydrochloride. Since Electrode 1 (GEA 33) is known to be made from BH 104 glass (8),the findings here are in accord with Mattock's. Response to Light Variations. Electrodes 6 , 7, 8, and 10 were tested using a Mazda incandescent tungsten-filament lamp (60 watt, 240 V). The electrodes were placed in a reaction cell containing O.lm NaCl solution and magnetically stirred in the water-bath at 25 "C. We found that Ag, AgCl electrodes of the thermal and thermal-electrolytic type were not affected by light emitted from the Mazda incandescent lamp. Recently work by Moody et al. (9), and Milward (10) have confirmed our findings. The reference electrode was therefore left unshielded from the light. A glass window in the water-bath allowed the exposure of the electrodes to the lamp which was held about 20 cm away. (8) A. K. Covington and T. H. Lilley, Phys. Chem. Glasses, 8, 88 ( 1967). (9) G. J. Moody e f a/., Analyst, 94,803 (1969). (10) A. F. Milward, ibid., p 154.
The results are shown in Figure 1. A represents the time when the light was switched on, and B when the light was switched off. The emf at zero time was taken as the reference value. The emf was showing a steady value before the light was switched on. The subsequent increase in emf was more pronounced for the Beckman than for the GEA electrodes, In fact, for Electrode 7, the emf was still increasing after the light was switched off. Electrode 8 (GEA 33) was the least affected. Milward (10) has reported the effects of light on glass electrodes, He observed a decrease in the pH when the electrodes were illuminated with an Osram mercury vapor lamp, a Desaga Heidelberg T.L. illuminator, a Mazda infrared lamp, and sunlight. It should be noted that if one had plotted p N a us. time, a graph of similar shape as that shown by Milward would be obtained. In our case, an increase of about 6 mV would correspond to a decrease of 0.1 pNa unit. The effects of light on the glass electrodes as shown in Figure 1 are serious when accurate activity coefficients are to be measured. ACKNOWLEDGMENT
The authors thank R. A. Robinson for reading the manuscript and making several helpful suggestions. RECEIVED for review May 16, 1972. Accepted July 18, 1972. One of us, S. P., is grateful to the University of Adelaide for a University Research Grant.
Electrochemical Reduction of Mercury(I) and Mercury(II) on Platinum in Fused Sodium-Potassium-Nitrate Eutectic at 250 "C H. S. Swofford, Jr., and James Dietz School of Chemistry, Unicersity of Minnesota, Minneapolis, Minn. 55455
THEELECTROCHEMISTRY OF Hg(1) and Hg(I1) at stationary platinum microelectrodes in fused chloride media has been studied recently. We report below some of the results of our studies in non-complexing fused media (Na-KNO,, 250 "C). Laitinen and Liu ( I ) and Laitinen, Liu, and Ferguson (2) have observed a one-step, two-electron reduction of Hg(I1) to Hg(0) at a platinum microelectrode in Li-KCI eutectic at 450 OC. Hg(1) was reported to be unstable and to disproportionate to Hg(I1) and Hg(0). In the low-melting (70 "C) ternary eutectic A1C13-NaC1-KC1, Hames and Plambeck (3) have observed the reduction of Hg(I1) to occur in two, two-electron steps at a stationary tungsten microelectrode. Using half-wave potentials obtained in their work, they calculated the equilibrium constant for the disproportionation reaction (1) H. A. Laitinen and C. H. Liu, J. Amer. Chem. SOC.,80, 1015 (1958). (2) H. A. Laitinen, C. H. Liu, and W. S. Ferguson, ANAL.CHEM., 30, 1266 (1958). (3) D. A. Harnes and J. A. Plarnbeck, Can. J . Chem., 46, 1727 (1968).
2232
Hgz+
+ Hgo -+
Hgz*+
(1)
to be 3.6 X lo3. In a similar system Torsi and coworkers ( 4 , 5 ) cite both electrochemical and spectrochemical evidence for the existence of the Hga2+ion. The voltammetric behavior of Hg(1) and Hg(II), at platinum electrodes, in fused nitrate solvents has not been reported. Mazzocchin et al. (6) potentiometrically determined the standard electrode potentials for the Hg/Hg,*+, Hg2*+/Hg2+couples in Li, NaKNO, eutectic at 150 "C, and reported values of $0.060 V and $0.270 V, respectively, cs. a 1.0-molal Ag/Ag+ reference electrode. Mercury(1) was indicated as stable in the melt for at least a few hours, and a value of 2.24 X l o 2 was reported for the equilibrium constant for Reaction 1 above. The electrochemical oxidation of mercury electrodes in fused Na-KNO, eutectic (7) and in fused Na-KNOJ eutectic con(4) G. Torsi and G. Marnantov, Inorg. Nucl. Chem. Lett., 6, 843 ( 1970). (5) . , G. Torsi, K. W. Fung, G. M. Begun, and G . Marnantov, Inorg. Chem. 10,'2285 (1971); ( 6 ) G. A. Mazzocchin. G. G. Bornbi. and M. Fiorani, J . Electroanal. Chem., 17, 95 (1968). (7) H. S. Swofford, Jr., and C. L. Holifield, ANAL.CHEM.,37, 1513 (1965). .
I
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
Table I. Estimation of Standard Potential (Eo) from HalfWave Potential (Ell2) for the Reduction of Hg(I1) to Hg(1) at the RPDE. (Data from typical experiment) Eii2, [volts L‘S. Ea, [volts 0s. Conc. Hg(II), m M Ag/Ag(I)I Ag/Ag(I)I 0.58 0.237 0.421 0.93 0.244 0.417 1.87 0.271 0.430 3.74 0.283 0.425 5.76 0,297 0.429 8.15 0.301 0.425 11 .o 0.305 0.423 14.5 0.315 0.426
I
I
I
15
a
B9
‘0
m
taining excess chloride ion (7-9) has been previously studied. We have studied the behavior of mercury in fused NaKNO, a t 250 “ C . Some of the results of these studies at rotating (RPDE), and stationary (SPE) platinum electrodes are reported below. EXPERIMENTAL
Procedure. General procedures for experimental work in this solvent have been reported (IO). Experiments were conducted in 30- x 140-mm borosilicate glass cells immersed in a bath of molten Na-KNO, maintained a t 250 “C. An inert atmosphere of dried, oxygen-free gas (Nq or Ar) was maintained in the cell at all times to prevent outside contamination. The criterion of melt purity is a residual current of less than 0.5 pA recorded at the rotating disk electrode ( A = 8.8 x lo-, cm2, w = 160 rad sec-I) over the potential range of 1.O V to - 1.O V cs. the Ag/Ag(I) reference. This criterion ensures that NOz-, H20, and other impurities electroactive in this region are not present in significant quantities. Voltammograms were obtained using a 3-electrode multi-purpose electrochemical instrument constructed from operational amplifiers as previously reported (1 I ) . Current-voltage curves were recorded with either a Moseley 2D-2M X-Y recorder or a Tektronix type 564 storage oscilloscope with a Polaroid camera. Electrodes. All potentials were measured with respect t o a 0.07M Ag/Ag(I) reference electrode isolated from the bulk solution by an asbestos wick. A platinum flag or a large-diameter platinum wire contained in a separate fritted borosilicate glass compartment served as the auxiliary electrode. For linear scan voltammetry, a platinum wire (d = 1.3 mm) was sealed into soft glass tubing and cut to give a n exposed surface area of about 0.23 cm2. The rotating platinum disk electrode was prepared by sealing 1.O-mm platinum wire into 6-mm soft glass tubing and then grinding flat. This electrode was calibrated in a standard solution of Fe(II1) in aqueous 1NHCI (12). Reagents. All reagents used were A R grade or the equivalent. Mallinckrodt N a N 0 , and K N 0 3 were used as the solvent. Fisher HgS04, Hg(NO&. H20, and Hg2(N03)2.2 H 2 0 were used as solutes [after drying under vacuum over Mg(C104)2for several days],
+
VOLTAMMETRY OF Hg(I1) AT RPDE
An R P D E is well suited t o these studies. First, the theory of convective diffusion t o a rotating disk has been described
5
0 0.50
0
0.25
E volts vs A g / A g ( I ) Figure 1. Current-voltage curve (RPDE, A = 8.8 X lo-, cm2, w = 159 rad sec-’, v = 0.002 V sec-I) of 2.44mM Hg(I1) in Na-KN03 eutectic at 250 “C by Levich (13) and permits precise determination of diffusion coefficients. Second, the forced convection a t the rotating disk minimizes thermal convection which is often a problem when working at elevated temperatures. Finally, the construction of the electrode permits rapid mechanical cleaning and polishing of the electrode surface during experimental work. A current-voltage curve for a solution 2.44 X 10-3M in Hg(II), recorded a t the RPDE, is shown in Figure 1. The two steps in the reduction are expected to be :
+ 2eHg2++ 2.2-
2Hgz+
--
HgP
(2)
2Hg0
(3)
Classical wave analysis (14) confirms that Reaction 2 is a reversible, two-electron reduction of Hg(I1) yielding dimeric Hg(1) product. First, a plot of -log i/(id - i)’ cs. E is linear with a slope of 2.3 RT/2F (0.052 V at 250 “C) as predicted for a second-order reaction (corresponding first-order plot is nonlinear). Second, as expected for a second-order reaction, the half-wave potential is concentration-dependent. An anodic shift of RT/2F (0.052 V at 250 “ C )for every tenfold concentration increase is observed, as predicted, in the range studied (0.5-10.0 mM) (Table I). According to Shuman (I 5), Equation 4 relates the standard potential, Eo, to the half-wave potential, E I / ~ .
(8) M. Francini, S. Martini, and C. Monfrini, Electrochim Metal., 2, 325 (1967). (9) W. O’Deen and R. A. Osteryoung, ANAL. CHEM.,43, 1879 (1971). (10) H. S. Swofford, Jr., and H. A. Laitinen, J. Electrochem. Soc., 110, 814 (1963). (1 1) R. B. Fulton, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1968. (12) P. J. Lingane, ANAL.CHEM.,36, 1723 (1964).
(4) (13) V. G. Levich, “Physicochemical Hydrodynamics,” Prentice Hall, Englewood Cliffs, N.J., 1962. (14) L. Meites, “Polarographic Techniques,” 2nd ed., Interscience, New York, N.Y., 1965, Chap. 4. (15) M. S. Shuman, ANAL.CHEM., 41, 142(1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
0
2233
200
100
a
I
I
0
-100
0.45
0.35
0.25 0.50
E volts vs A g / A g ( I )
Figure 2. Forward (a) and reverse (b) voltammogram (SPE, A = 0.244 cmZ,u = 0.075 V sec-I) of 13.7mM Hg(1I) in Na-KN03 eutectic at 250 "C
.25 0 E volts vs A g / A g ( I )
Figure 3. Current-voltage curve (RPDE, A = 8.8 X cm2, w = 145 rad sec-l, u = 0.002 V sec-l) of Hg(1) in presence of Hg(0) in Na-KN03 eutectic at 250 "C. Concentration of [Hg(I) Hg(II)] is 5.4mM
+
From our values of Elizwe estimate Eo for the Hgz2+/Hg2+ couple to be $0.424 V. As predicted by the Levich equation (13),
id
=
0.62 nFAD2/3y-11%1i2C
where i = amperes, C = moles/cm3, and y = 0.0189 cm2/sec ( I I ) , the limiting current is directly proportional to concentration (over the range studied) and to the square root of rotation rate (available range 25-325 rad sec-l) (see Table 11). Hence, the first step of the reduction of Hg(I1) is thought to be a reversible, diffusion-controlled, electrochemical reaction at the RPDE. The second step in the reduction of Hg(I1) is evidently the two-electron reduction of dimeric Hg(1) to Hg(0). For a n electrode reaction of this type, a plot of log (id - i) us. E would be expected to be linear with RT a slope of 2.3 - (0.052 V a t 250 "C). Such a plot is linear 2F and has a slope of 0.049 V. A large deviation a t the foot of the wave is undoubtedly due to mercury being deposited on platinum at an activity of non-unity. This phenomenon is not peculiar t o this system, and has been previously reported (16). At concentrations greater than about lmM, this second reduction wave becomes increasingly distorted. In all probability, this is due to alloying as evidenced by a black film on the electrode surface (removable only by mechanical polishing). Hg-Pt alloys are known at these temperatures (17), and the problem of alloy formation at platinum electrodes in fused media has been previously reported (16). As with the first reduction wave, the total limiting current
depends directly upon concentration over the range studied (0.5-10mM) and varies with the square root of electrode rotation rate. The data then indicate the second step in the reduction [(Hgzz+to Hg(O)] to be a two-electron, diffusioncontrolled, process which is nearly reversible at low concentrations. An equation similar to Equation 4 can again be used to calculate the standard potential, En,from available half-wave potentials (14). The En of the Hg/Hgiz' couple was estimated to be 0.198 V 6s. the Ag/Ag+ reference. Assuming that the diffusion coefficients for both species are equal, their calculated value is 0.14 f 0.01 X 10-5 cm2 sec-1. Comparable values are 0.23 X 10-5 cm2 sec-', and 0.51 X 10-5 cm2 sec-' for Pb2" and Cd*+,respectively, in Na-KNO, eutectic a t 263 "C (18). LINEAR SCAN VOLTAMMETRY
LSV has been applied to analysis in fused media (19, 20). The short time experiment minimizes problems of thermal convection and fouling of the electrode surface, both common problems in molten salt electroanalysis. A useful relationship applicable to LSV analysis is (21):
In this equation, c is the scan rate in volts sec-I, X ( a l )is a dimensionless parameter, and the other symbols have their (18) D. Inman and J. O'.M. Bockris, J. Electroanal. Chem., 3, 126
(1962). (16) H. A. Laitinen and R. A. Osteryoung in "Fused Salts," B. Sundheim, Ed., McGraw-Hill, New York, N.Y., 1964, Chap. 4. (1 7) M. Hansen, "Constitution of Binary Alloys," McGraw-Hill, New York, N.Y., 1958, p 832. 2234
(19) G. Mamantov, J. M. Strong, and F. R. Clayton, Jr., ANAL. CHEM., 40, 488 (1968). (20) W. K. Behl, J . Electrochem. Soc., 118, 889 (1971). (21) R. S. Nicholson and I. Shain, ANAL.CHEM.,36, 706 (1964).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
4;
usual electrochemical significance. Values of X ( a t ) for various potentials have been published by Nicholson and Shain for the first-order case (21), and by Shuman for the second-order case (15). Cyclic voltammograms were recorded for various concentrations of Hg(II), an example being shown in Figure 2. Assuming that the reaction is secondorder, the diffusion coefficient of Hg(I1) was calculated to be 0.13 ==I 0.01 X lo+ cm2 sec-l, which is in good agreement with results obtained at the RPDE. The shape of the linear scan voltammogram (curve a) is also characteristic of the second-order process. Values for - E p ) of 0.079 V agree well with the value of 6.080 V predicted by Shuman (after correction for temperature). Finally, a plot of log C US. Ep yielded a slope of 0.048 V in agreement with the value of 0.052 V predicted for the second-order reaction. The peak potential was independent of sweep rate over the range studied (0.025-0.30 V sec-I), and the peak height was directly proportional to concentration. The shape and position of the curve obtained on reversal (curve b) supports the above contention. These observations confirm the reduction of Hg(I1) to dimeric Hg(1) to be reversible under these experimental conditions on a platinum surface.
Table 11. Verification of i - u ~Dependence / ~ for Reduction of Hg(I1) to Hg(0) at RPDE. [ A = 8.8 X 10-3 cm*, C = 1.0 m M Hg(I1)I il, PA, (measured at -0.05 V DS. Ag/Ag+) a, (rad sec-1) h/a1/2, (A rad-”%secl’t) 3.35 3.77 4.77 5.83 6.80 7.77 8.28 9.63 10.70
26.4 33.4 53.5 81.7 111 146 165 224 275
to Hg(I1). Under these conditions, Hg(1) is found to be stable in this melt as Hg22+. Using the standard potentials calculated above, we can calculate the equilibrium constant for the disproportionation
VOLTAMMETRY OF Hg(1)
Samples of Hg2(N03)2.2H20 added to the melt were rapidly and quantitatively converted to Hg(I1) under the normal experimental conditions. The presence of metallic mercury, condensed on the cooler surfaces of the cell, indicated that, as expected, the other product was Hg(0). When the experiment was performed in a sealed cell, in the presence of large amounts of metallic mercury, the composite currentvoltage curve (Figure 3) was recorded at the RPDE. The portion of the composite wave above the zero current axis corresponds to the reduction of Hg(I1) to Hg(1) and the portion below this axis corresponds to the oxidation of Hg(1)
6 . 5 2 x 10-7 6.51 6.51 6.46 6.48 6.47 6.47 6.42 6.44 Av 6.48 i.0 . 0 3 X lo-’
K=
[Hgz ‘+I [Hg2+l[Hgol
The value calculated is 1.5 X lo2. This compares to K = 3.6 X lo3 obtained in the A1Cl3-KC1-NaCl eutectic (3), and the value of 1 . 1 X l o 2 calculated using aqueous standard potentials (22). RECEIVED for review April 26, 1972. Accepted July 14, 1972. (22) W. M. Latirner, “Oxidation Potentials,” 2nd ed., PrenticeHall, Englewood Cliffs, N.J., 1952.
Negative Ion Mass SpectrometryA New Analytical Method for Detection of Trinitrotoluene J. Yinonl and H. G . Boettger Jet Propulsion Laboratory, California Institute of Technology, Pa,radena, Calif. 91 I03
W. P. Weber Department of Chemistry, University of Southern California, Los Angeles, Calif. 90006 THEDETECTION of poly-nitro aromatic compounds concealed in airline baggage, such as trinitrotoluene (TNT), is an extremely relevant analytical problem, which requires a highly sensitive as well as specific method for its solution. One of the most sensitive analytical devices available for the detection of trace quantities of material is the mass spectrometer. The use of positive ion mass spectrometry is difficult because it is approximately equally sensitive to all types of volatile organic compounds, so that TNT, which is in the presence of all sorts of other organic molecules-such as those derived from perfume-must first be separated by gas chromatography and subsequently identified by its complete mass spectrum.
Reagent grade T N T was purchased from Eastman Kodak Co., Rochester, N.Y. The negative ion mass spectra were determined with an AEIiGEC MS9 high resolution mass spectrometer, modified for negative ionization ( I ) . The sample was admitted into the source using the direct insertion probe. Negative ion mass spectra were recorded at various
NRC Resident Research Associate, on leave of absence from the Weizrnann Institute of Science, Rehovot, Israel.
(1) J. Yinon and H. G. Boetiger, Znt. J. Muss Spectrum. Zun Phys., in press.
Negative ion mass spectrometry, on the other hand, is extremely selective in its sensitivity. With this thought in mind, we have examined the negative ion mass spectrum of TNT. EXPERIMENTAL
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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