Determination of Some Chlorine-Substituted Methanes and Ethanes Infrared and Mass Spectrometric Determination R . B. BERNSTEIN, G . P. SEMELUK, A X D C . B. 4RENDS Department of Chemistry, Illinois Znstitibte of Terhnology, Chirapo, I l l .
1265 wave numbers, respectively. No suitable band for carbon tetrachloride was available over the accessible (sodium chloride) wave-length range. Reference mass spectral patterns and relative ionization efficiencies were measured and monoisotopic spectra were computed for each of the seven purified compounds. -4nalytical results with synthetic mixtures indicated that the mass spectrometric method was suitable over a wide range of concentrations; the infrared method was in some cases more sensitive. Accuracies better than k0.3 mole % were obtained w-ith both methods.
In the course of a kinetic study it w-as necessary to develop an analytical method for the determination of several chlorine-substituted compounds present at low- concentrations in an excess of chloroform. Double-beam infrared spectrophotometric techniques were compared with mass spectrometric methods for the analysis of a mixture consisting of small quantities of hexachloroethane, pentachloroethane, sym-tetrachloroethane, tetrachloroethylene, methylene chloride, and carbon tetrachloride in chloroform. Selected for the infrared determination were bands located at 680, 822, 1279, 912, and
I
X A kinetic study of the thermal decomposition of chloroform, it became necessary to determine the concentration of a variety of chlorine-substituted methanes and ethanes in the presence of excess chloroform. The possibilitj of analyzing complex mixtures of chlorinated hydrocarbons by the infrared spectrophotometric method has been pointed out by Urone and Druschel (4), who listed wave lengths characteristic of a large variety of these compounds and also demonstrated the validity of Beer's law for solutions containing as many as three selected components in iso-octane (2, 2 , 4-trimethj 1 pentane). I n actual practice, ho\T ever, hecause of the considerable overlapping of abqorption bands for these structurally similar molecules. it is not possible to carry out a successful determination for a mixture containing any large number of these compounds. Careful consideration must be given to the selection of the analytical bands, and good photometric precision is necessary to perform a fairly accurate analysis of complex mixtures even under the most favorable conditione. As an extreme case, for example, it is seen from the spectra of carbon tetrachloride and chloroform in the sodium chloride region that the infrared method is entirely unsuitable for thedeterminationof carbon tetrachloride in mixtures containing excess chloroform. Additional incompatible combinations are apparent from inqpection of the pure compound spectra. I n several cases the absolute sensitivity of the method for a given component is low, as evidenced by the small absorbancy indices for many of the potential]?- useful analytical txinds. On the other hand, it might appear that the mass spectrometric method would be ideally suited for such a determination because of the large range of molecular neights of these compounds. This investigation indicates the details involved in the application of both these instrumental techniques to the analysis of a mixture of seven chlorine-substituted compounds, and evaluates the relative merits of each from the viewpoint of accuracy and convenience. As might be anticipated, it was found that each of the methods possessed certain inherent advantages and that a combination of the two nould prove to be most desirable for precise results.
hexachloroethane were obtained from the Matheson Co. The chloroform was U.S.P. grade, and the carbon tetrachloride was technical grade. sum-Tetrachloroethane, methylene chloride (dichloromethane), chloroform, and carbon tetrachloride were purified according to the procedures given by Vogel ( 5 ) . Pentachloroethane and tetrachloroethylene were dried over calcium chloride and fractionated in a small packed column under reduced pressure. The hexachloroethane was used without further purification, as it was found that the infrared spectrum of a sample purified by sublimation Tvas indistinguishable from that of an unsublimed sample. Refractive indices of the purified liquids agreed to within 2~0.0003of the literature values. Mass spectrograms of the purified compounds indicated no anomalous peaks whose magnitude exceeded 0.02% of the maximum peak, with the exception of small peaks (2% are indicated in the figure. Certain qualitative features are immediately apparent on in-
spection of the monoisotopic patterns. First’, as anticipated, no detectable parent peak was found for carbon tetrachloride and hexachloroethane. In a study of the mass spectra of carbon tetrafluoride, silicon tetrafluoride, and sulfur hexafluoride, Dibeler and Mohler (2) found that, dissociation by electron impact rather than formation of t,he molecular ion was by far the most probable occurrence. It is of interest, to note from Table VI that the molecular ion is the most abundant. species in the spectrum of t,etrachloroethylene, however. For carbon tetrachloride, chloroform, and methylene chloride, it is seen t’hat the most probable process is the rupture of C-CI bond by electron impact. However, for hexachloroethane and sym-tetrachloroethane, the cleavlink is most probable. I n the case of pentaage of the C-C chloroethane, both processes occur iTith equal probability. Certain regularities are also noted in the ionization of the pairs of moleculespentachloroethane-chloroformand tetrachloroethanemethylene chloride. For pentachloroethane and chloroform the relative probabilities of removal of a hydrogen atom, a chlorine atom, and a hydrogen chloride fragment, compared to molecule ion formation, are seen to be 0.28, 78, 1.1, and 0.56, 43, and 1.9, respectively, while for the pair tetrachloroethane and methylene chloride these values are 0.08, 0.97, 0.23 and 0.035, 1.3, 0.13. The relative stability of t’he molecular ion compared to the dissociated ion of largest abundance is seen to decrease in the series tetrachloroethane, pentachloroet’hane, and hexachloroethane and likewise in the series methylene chloride, chloroform, and carbon tetrachloride, as the number of electronegative chlorine at,oms is increased. This is reasonable in the light of the high electron affinity of the chlorine atom and the lessened tendency toward removal of an electron to form the molecular ion as the number of chlorine atoms is increased. ACKYOWLEDGMEYT
The authors appreciate the valuable cooperation of D. V. Kniebes with regard to the mass spectrometer. Partial support of this work by the A4tomicEnergy Commission is gratefullv acknowledged. LlTERATURE CITED (1)
Dibeler, V. H., and Bernstein. R. B , J . Chem. P h y s , 19, 4 0 4 (1951).
(2) Dibeler, 1‘.H.. and Mohler, F. L., J . Research,~at2.BzLr.Standards, 40, 25 (1948). (3) Inghrarn, M., Phys. R e v , 71, 560 (1946). (4) ?ne, P. F., and Druschel, M . L., A N ~ LCHEM., . 24, 626 (1962). (5) T ogel, A . I., “Textbook of Practical Organic Chemistry,” p. 174, London, Longmans, Green and Co., 1948.
RECEIVED for reviea July 7,
1952.
Accepted October 13, 1952
Thermal Stability of Potassium Acid Phthalate EARLE R. CALEY
AND
ROBERT H. BRUNDJNl
Department of Chemistry, T h e Ohio S t a t e Liniversity, Columbus, Ohio
P
OTASSIUM acid phthalate has been widely accepted as the most satisfactory substance for standardizing base solutions, but a lack of agreement exists as to its thermal stability. The experiments of Hendrixson (1) indicate that it is stable a t temperatures up to 150’ C. but Kolthoff and Stenger (3) caution against drying above 126 O C. because loss of phthalic anhydride may occur. Hillebrand and Lundell ( 2 ) recommend 120” C., and others-Willard and Furman ( 4 ) , for example-specify 110” C. as the most suitable drying temperature. For USP in acidimetry, the Kational Bureau of Standards, in it6 certificate of analysis for potassium acid phthalate, recommends that the salt, after being 1
Present address, U. S. Military Academy, West Point, N. Y .
crushed to about 100-mesh size, be dried a t 120’ C. for 1 to 2 hours. The bureau found no significant change in weight after 2 hours a t this temperature. The purpose of the present investigation was to study the thermal stability of potassium acid phthalate in the temperature range 110” to 200” C. Because of the way it is usually dried for use as a primary standard, only its stability in an air oven a t normal atmospheric pressure was investigated. DECOI\IPOSITION PRODUCTS
I n the temperature range here considered the products of the thermal decomposition of potassium acid phthalate are water,
