1so0
ANALYTICAL CHEMISTRY
these crystals were due to thermal decomposition of the benzene derivative or were actually an addition compound of the original material. These two compounds are not reported in the tables, and, a t least for the present, they must be listed as unknown. In the case of 1,4-diphenylbenzene, there was no such doubt; no addition compound formed with this substance. Addition compounds were formed in several cases of 1,4-disubstitution, such as 1,4-dimethylbenzene, 1,4-diaminobenzene, 4-methylaminobenzene, and 4-aminobenzoic acid. From these facts, it must be concluded the effect of 1,4-disubstitution is unpredictable, Kot all of the cases under Rule ( d ) have been tested, but sufficient data have been obtained to warrant prediction as to the cases not tested. Since the amino group is the most powerful addition compound promoting group, and since 2,4-dinitroaminobenzene did not form a 2,4,7-trinitrofluorenoneaddition compound, Rule ( e ) can be considered to hold. Rule ( f ) follows from the fact that 1,3,5-tribromobenzene and 1,2,4,5-tetrachlorobenzenedo not form addition compounds. Rule ( 9 ) is deduced from the fact that diphenylmethane and triphenylmethane as well as dibenzylamine and tribenzylamine do not form addition compounds although the groups are electron donating groups. I n these compounds because of the tetrahedral nature of the central atoms, coplanarity is impossible. I n the case of 3-methyl-4-tert-butyI phenol, an addition compound should form, since all of the groups are electron donating. It is assumed that the addition compound does not form with this substance because of the bulky tert-butyl group, therefore, Rule ( h ) has been formulated. Since only one compound has been tested, Rule ( h )can be only tentative.
It is not possible a t this time to make statements regarding the theoretical interpretation of the above results. Equilibrium constants, composition diagrams, and thermodynamic data must be determined before quantitative statments can be made regarding the effects of substituents on addition compound formation. These electronic properties of the substituents may be taken as a clue to the necessary requirements for addition compound formation. Work is in progress to determine the thermodynamic data and phase diagram data for this class of compounds. ACKNOWLEDGMENT
The authors gratefully acknowedge the generous supply of 2,4,7-trinitrofluorenone donated by the Dajac Laboratories, Division of Monomer Polymer, Inc., Leominster, Mass. LITERATURE CITED
(1) Hildebrand, J. H., “The Solubility of Sonelectrolytes,” 3rd ed.,
New York, Reinhold Publishing Carp., 1950.
(2) Kofler, L., and Kofler, A, “Mikromethoden eur Kennzeichnung
Organischer Stoffe und Stoffgemische,” Innsbruck, Universitatsverlag Wagner, 1948. (3) Laskowski, D. E., Grabar, D. G., and McCrone, W. C., ANAL. CHEM.,25, 1400 (1953). (4) Remick, A. E., “Electronic Interpretations of Organic Chemistry,’’ New York, John Wiley and Sons, 1949. RECEIVED for review April 15, 1954. Accepted June 17, 1954. Based upon a part of t h e thesis t o be submitted b y Donald E. Laskowski t o the Graduate School of Illinois Institute of Technology in partial fulfillment of t h e requirements for the degree of doctor of philosophy.
Determination of Traces of Copper in Germanium By Activation Analysis GUSTAV SZEKELY Physics Laboratories, Sylvania Electric Products, lnc., Bayride,
The electrical characteristics of germanium are affected by the presence of copper in the germanium lattice. By an activation analysis as little as y of copper in germanium can be determined. The neutron irradiation of germanium samples, containing copper, results principally in the formation of radioactive isotopes of germanium, arsenic, and copper. Copper-64 is separated chemically, and the activity due to it serves as a measure of the copper content of the samples analyzed. The method is specific, and the result of the analysis is not affected by contamination introduced by sampling or during the chemical procedure.
T
HE electrical characteristics of semiconductor devices, such as transistors, are highly sensitive to the presence of traces of impurity atoms in the semiconducting material. The presence of copper in pure germanium renders the latter a p-type conductorthat is, conducting by means of holes. Furthermore, the operation of a transistor depends upon a certain minimum lifetime which the current carriers must possess; the presence of copper in germanium was found to increase the rate of recombination of holes and electrons ( 2 ) . Because copper can readily enter the germanium lattice [large diffusion coefficient (3,b ) , abundance of sources of copper contamination], this element is recognized as an important contaminant in germanium.
N. Y.
In attempting t o find an analytical method applicable to the submicrogram range, one is confronted by the limit of detection of the more usual methods, such as absorption or emission spectroscopy. The activation analysis, described here, is specific and can be used t o determine as little as lo-‘ y of copper in germanium. PRINCIPLES
The principles of activation analysis are now well known (1). Briefly, a sample is bombarded by nuclear particles-e.g., neutrons-at a flux for a period of time sufficient t o produce an optimum amount of a radioisotope from the trace constituent to be determined. The decay rate of the isotope, registered by a counter, serves as a measure of the quantity of trace constituent originally present in the sample. Frequently, the major and other minor constituents also become activated during bombardment, which may necessitate a separation of extremely small amounts of the radioisotope t o be counted from large amounts of other radioactive isotopes formed. One way to achieve this is to effect chemical separations after adding a known amount of the stable element (“carrier”) to entrain its radioactive isotope. The stable isotopes thus carry the radioactive trace through the chemical procedure. From the yield with which the carrier is recovered and its counting rate, the quantity of radioactive trace present may be
V O L U M E 2 6 , NO. 9, S E P T E M B E R 1 9 5 4
1501
Table I. Naturally Occurring Isotopes Qe'3
Abundance,
R
(stable) 2 0 . 4
Ge'2 (stable)
27.4
Ge'a (stable) 7 . 8
