Deviations of the slopes from ideal values probably are caused by adsorption phenomena such as that found with 2-mercaptoimidazole (4). The log plots, therefore, support a one-electron reaction similar to a thiol, such as ergothionine ( I I ) , rather than the expected two-electron thione reaction. Further support for the one-electron mechanism was obtained from the controlled-potential electrolysis experiments. At pH 2.9, the reaction was found to involve 0.92 electrons/ molecule and, at pH 8.1, the value was 0.93. These data corroborate a one-electron reaction. The electrolysis product was isolated and analyzed, with the results establishing its formula as the mercuric dimercaptide, Hg(SR)z. This corresponds to similar findings by Stricks, Kolthoff, and Heyn-
RECEIVED for review May 20, 1968. Accepted September 20, 1968. Manuscript abstracted in part from a dissertation submitted by E. S. G . to St. Joseph College, Hartford, Conn., in partial fulfillment of the requirements for a Master of Arts degree.
(11) V. Preininger, M. Ctirnoch, and A. Santav);, Ado. Polarography, 3, 1087 (1960).
(12) W. Stricks, I. M. Kolthoff, and A. Heyndrickx, J . Amer. Chem. SOC.,76, 1515 (1954).
drickx (12) that compounds of the type RSHg are unstable in aqueous solution and decompose to (RS)2Hg and Hg. No cathodic wave attributable to an N-oxide reduction was found at any pH level. By contrast, pyridine N-oxide and 2-chloropyridine-N-oxideboth give cathodic waves. The presence of sulfur in 1-hydroxypyridine-2-thionemust stabilize the N-oxide linkage to reduction.
Determination of Hydrogen in Partially Deuterated Water Maurice M. Kreevoy and Thomas S . Straub
School of Chemistry, University of Minnesota, Minneapolis, Minn. THEDETERMINATION of the isotopic composition of partially deuterated water has been the subject of a number of investigations. Earlier methods, generally based on density, are described and discussed by Kirshenbaum ( I ) . Later methods have been based on a variety of physical and chemical properties, and numerous references are given by Leyden and Reilley (2). A previous method based on the spectrum in the 1900 nm region (3) lacks the precision, and, for many purposes, the convenience of the present method, although it has the advantage of using much smaller samples. The present note describes a method based on the spectrum of water in the 800-1300 nm region (4). Liquid HzO has peaks at 970 and 1192 nm and a valley at 1065 nm (4). A semiempirical relation is developed among AIl92,A1065, and xH. (Absorbance at wavelength, A, is A x . ) In this relation A1065 is, effectively, used in the place of a suitable reference material, which is unavailable. The method offers an accuracy of -BO2 in XH, the atom fraction of ordinary hydrogen. Since the analysis is carried out in 1-5 cm quartz spectrophotometer cells, it is particularly convenient for applications where other spectrophotometric measurements have been made on the same samples, as in many measurements of rates. EXPERIMENTAL
All spectra were made on a Beckman DK-2 spectrophotometer. Its wavelength scale was calibrated by means of the major water vapor lines at 1362, 1375, 1382, and 1400 nm (5). These were observed by scanning the transmitted energy in one beam with the cell compartment empty and the chopper turned off. Only air was used in the reference beam. A suitable reference substance would have to have no absorbance in the region of interest and a refractive index identical with water.
Densities used in the determination of isotopic composition were determined by a standard pycnometric technique (6) using a 50-ml pycnometer. Deuterium oxide was obtained from a number of commercial suppliers and was redistilled before density determinations. RESULTS
Figure 1 suggests that, for DzO, A1065 and AlI92are equal. Since the two wavelengths are so close together, it is reasonable to assume that scattering at the windows and by dust is the same at both of them. With this observation and assumption the quantity AA, defined as A1192 minus A1065 should be due entirely to absorption by HOD and HzO. Assuming that AeHoD( e 1 ~ 9 2-~ elOejHoD) ~ ~ is exactly half of AeHzO,then A A at constant pathlength, b, should be proportional to xR. Molar absorbtivities are designated enM where M i s the absorbing substance and n is the wavelength. Figure 2 shows that this relation is not obeyed with useful accuracy. If AeHoais given by rAeHzo,r being a constant independent of isotopic composition, A A is given by AA =
xH30
214
ANALYTICAL CHEMISTRY
+
UAeH’Ob.
X H ~ D
(1)
The mole fractions of HzO and HOD, XH,O and X H O D , can be evaluated in terms of X H and K, the equilibrium constant governing the redistribution reaction, HzO
K
+ Dz0 e 2 HOD
(2)
If K is 4.00 as suggested by the rule of the geometric mean (7), they are XHiO
and xHOD
(1) I. Kirshenbaum, “Physical Properties of Heavy Water,” McGraw-Hill, New York, 1951. (2) D. E. Leyden and C. N. Reilley, ANAL.CHEM., 37, 1333 (1965). (3) W. E. Keder and D. R. Kalwarp, ibid., 38, 1288 (1966). (4) M. R. Thomas, H. A. Scheraga, and E. E. Schrier, J. Phys. Chem., 69, 3722 (1965). (5) E. K. Plyler, Phys. Reo. (2), 39 77 (1932).
AeHlob
=
=
(3)
XH2
2 XA(I-
XH).
(4)
Combination of Equations 1, 3, and 4 leads to (6) D. P. Shoemaker and C. W. Garland, “Experiments in Physical Chemistry,” McGraw-Hill, New York, 1962, p 126. (7) J. Bigeleisen, J. Chem. Phys., 23, 2264 (1955).
.7
t
I 1
.6
.5
.4
.3
.2
0.0
.1
x,
.1
.2
.3
.4
.5
(Density)
Figure 2. The success of Equation 5 (0)and the failure of simple proportionality ( 0 ) in obtaining x H from AA 0.0 900 1000
1200
WAVELENGTH,
1300
The solid line represents a perfect fit
1400
nm
Figure 1. Water in a 1-cm cell (A) and DzOcontaining 1 atom per cent H in a 2-cm cell (B), both with air as a reference
Table I. Effect of Sohtes on AAH10/bH20 Solution A A H ~ o / ~ H cm-’ ~o, 0.497 Water 0.454 1M H$Oa 3M HC1 0.444 2M NaOH 0.444
Curve C is air us. air
XH
=
+
(Q - 2rQ r2)1/2 r 1 - 2r
(5)
where Q is bHZoAA/bAAH,o and b is a pathlength. Figure 2 shows the general acceptability of Equation 5 , taking 0.4205 for r. Altogether XH was determined for 16 mixtures, the isotopic composition of which was also determined from density. The average difference between the two values of X H was 0.0025 and the largest difference was 0.008. Since the xH values determined by density have an imprecision of -.001, the average inaccuracy in the XH determined spectroscopically would seem to be about ,002. It should be noted that the form of Equation 5 requires that at least one “insignificant” figure be carried through intermediate stages of the calculation in order to avoid loss of a significant figure in the result. DISCUSSION
above .003. Also, the numbers of positive and negative deviations are equalized by taking 0.4205 for r . With 0.421 taken for r there are 3 times as many negative as positive deviations, and with 0.420 there are no negative deviations at all. The value of AAH20/bH?0is about 0.5 cm-I, but it is somewhat temperature sensitive, and probably also somewhat sensitive to instrumental variables, SO it should be redetermined as a part of each series of X H determinations. Wavelength calibration of the spectrophotometer is generally unnecessary, as A A can be taken as the difference between the peak and the valley. The exact wavelengths are slightly temperature sensitive in any event. While no systematic survey of interferences has been carried out, Table I shows that A A H ~iso not too much changed even by rather substantial concentrations of inorganic salts. The same conclusion can be drawn from the results of Bonner and Woolsey (9). This suggests that the method will be applicable should be redeterto such solutions, although AAHzC)/bHzO mined for each solute and concentration. Organic solutes containing C-H bonds probably disturb the equality of A1192 and AIoe6 even in the absence of hydroxylic hydrogen. However it seems likely that an appropriate correction to Q could be made.
It has recently been shown that K, in fact, is 3.76 A 0.02 rather than 4.00 (8). This has the effect of making X H ~ O always higher and XHOD always slightly lower than the values given by Equations 3 and 4. This is one of the effects which reduces the numerical value of r and introduces some systematic error into values of XH. However, with a value of r adjusted to minimize these discrepancies, this error is less than .002, even in the worst cases (which occur when XH is about 0.1, 0.5, and 0.9). Since these errors seem to be less than the experimental scatter, and since the introduction of the best value of K makes Equation 5 much more cumbersome this has not been done. The average discrepancy in XH is a rather sharp function of r. With r taken as either 0.420 or 0.421, it rises from .0025 to
RECEIVED for review August 16, 1968. Accepted October 9, 1968. We are pleased to acknowledge the support of the National Science Foundation through grants GP 5088 and GP 7915.
(8) J. W. Pyper, R. S. Newbury, and G. W. Barton Jr., J . Chem. Phys., 46,2253 (1967).
(9) 0. D. Bonner and G. B. Woolsey, J . Phys. Chem., 72, 899 (1968). VOL. 4 1 , NO. 1, JANUARY 1969
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