1994 fact that consistent agreement was obtained tends to support the reliability of the thermal conductivity method to within the error mentioned. One serious disadvantage in the determination of deuterium by the thermal conductance of a compound is that the compound must be completely free of impurities. The reasons for this can be seen by examining the theoretical expression for R as given by Trenner (6)
nherr C, = specific heat a t constant volume u = molecular diameter C, = mole fraction of the ith kind of molecule 11 = molecular weight
If, for example, one had present in diborane 1y0of pentaborane as an impurity, the apparent deuterium content could be in error by as much as 10% if the conductance of the sample were used for analysis. On the other hand, decomposition of such a sample of diborane and subsequent analysis of the resulting hydrogen would give a result which would be in error by only 1.5%. Consequently, the direct analysis method has rather severe purity requirements and these a t times cannot be too easily attained with the boron hydrides. The situation in pentaborane is somewhat more favorable since the curve for fractional deuterium content in the pentaborane versus R is less steep than in the corresponding diborane case. The bulk of this work has involved deuterium and the conductance method has been applied routinely in such problems. Recently, however, it has become necessary to use boron-10 in some of this work, so the authors have made some exploratory studies of the feasibility of using the thermal conductance technique for measuring the boron-10 content of some of these gaseous
ANALYTICAL CHEMISTRY boron hydrides. In the case of diborane it was found that in going from 18.8 to 96% boron-10, the ratio R changed by 2%. This gives a curve which in comparison to the one in Figure 1 is much steeper but the effect is large enough to determine the boron-10 content probably to an accuracy of about 2%. -4s it now appears, the main disadvantage in using this method for boron-10 analysis lies in the stringent purity requirements. In pentaborane, although no measurements have as yet been made, it is expected that the slope of the curve will be smaller, permitting a more accurate determination of the boron-10 content. ACKNOWLEDGRIEhT
The authors wish to acknowledge their indebtedness to Joseph
E. Todd, of this laboratory, for the preparation and purification of tetraborane and dihydropentaborane. The trimethylborazolc, triethylborine, and sec-tributylborine was obtained through the courtesy of George W. Shaeffer, St. Louis University. The mass spectrometric measurements referred to in this paper were very kindly made by Len-is Friedman, Rrookhaven Sational Laboratory. LITERATURE CITED
(1) Bigeleisen,J., Perlman, A I . L., and Prosser, H. C., A ~ A I ,CHEY., . 24,1356 (1952). J . A m . Chem. SOC.,75, 2 2 s (2) Burg, A. B., and Stone, F. G. (1953). (3) Maybury, P. C., and Koski, 11'. S., ,J. Cheni. P h y s . , 21, 742 (1953). (4) Sewton, A. S., AIanhattan District, Declassified Document C724MDD (January 1947). (5) Shapiro, I., Weiss, H. G., Skolnik, S., and Smith, G . 13. L., ,J. Am. Chem. SOC., 7 4 , 9 0 1 (1952). ( 0 ) Trenner, N. R., J.Chem. P h y s . . 5 , 3 8 2 (1937).
RECEIVED for review March 31, 1064. -4ccepted .4ugust 17,
Polarography of Thiourea-Correction Bn error in the paper on "Polarography of Thiourea" [.4sa~. CHEM.,26, 724 (1954)] has been independently pointed out by H. L. Kies, R. F. hlakens, and E. W. Hobart. The slope of the line in Figure 3 should be 0.026 instead of 0.052 as originally indicated. This would make the ratio a / b = 1 instead of 2 as reported. The suggested reaction and product are therefore in error. -4corrected interpretation is given below. The reaction of thiourea with mercury at the dropping mercury electrode can be expressed as
E = Bo
i + 0.06 log ___ u - i)" -
(id
At the half-wave potential ( E l / > i) ,= id/2
El I'
=
E'
'id12 ~
(id'2)C
a =
306
-A
log ( i d i s , and a straight-line
plot of E1 2 us. log (id)2would not be expected. The same is true for higher values of a. Figure 3 in the paper "a: a straight-line plot with a slope of 0.026, which is in fair agreement I\ ith the 0 03 above. If a = 2, then the reaction can be given as 2
SH, \C-S-
? + hH2
At any point on the polarogram described by this equation, the concentration of the reaction product would be i and thiourea concentration i d - i. The polarogram could then be described by the following equation:
-+ 0.06 - log a
1, Eli2 = E', and no change of E, 3 u ith thiourea concen0 06 tration would be expected. If a = 2, El/* = E" 2 log zdr and a plot of log t d us. E, 2 as given in Figure 3 of the original paper would be expected to result in a straight line having a slope of
If
0.03. If a = 3, Eliz = E"'
The potential can t,hen be expressed as
1954
+ Ilg
- [(k-S)2 + +-
SHz
Hg]
2e
A more detailed account of the work of Liberti and Sartori mentioned in the paper has been received. The results given here are in agreement with their findings [Liberti, A,, and Cervone, E., Ann. chim., 42, 481-90 (1982).3 R. L. EDSBERG
Burroughs Corp Paoli, Pa.
