the sulfite impurities of reagent grade hydrochloric acid may be calculated from only one point of the calibration curve by w e of the following equation:
% suEte
=
(C
- 8.528)(0.00240)
PV
(7)
where c
=
analytical concentration of chlorate, moles per liter X 106
A = absorbance of solution W = weight of 38% (w./.cv.) hydrochloric acid used, grams
The sulfite impurity of reagent grade hydrochloric acid, Batch I, for each of the calibration curve points shown in Figure 3 has been calculated and is listed in Table 111. The average of these results is 0.000070%.
Similarly, hydrochloric acid may also contain small amounts of free chlorine. Because this oxidizing impurity and the reducing impurity would not be compatible, only one impurity would exist in the solution. If the term c - 8.528 in Equation 7 is negative, chlorine is in excess and its content may be calculated by :
yo chlorine
=
(8.52A - ~ ) ( 0 . 0 0 2 1 3 )
w
(8)
There were no oxidizing impurities in three batches of hydrochloric acid examined during the course of this investigat’ion. ACKNOWLEDGMENT
The author thanks Frances D. Chang and Lois L. Taylor, who performed
some of the determinations required for this paper. LITERATURE CITED
(1) Boltz, D. F., Hollpd, W. J., “Colorimetric Determnation of Nonmetale,” D. F. Bolta, ed., pp. 172-3, Interscience, Kew York, 1958. (2) Joint .Army-Navy Specification for
Ammonium Perchlorate, JAN-A-192 (March 1, 1945). (3) “Scott’s Standard Methods of Chemical Analysis,” N. H. Furman, ed., 5th ed., Vol. I, pp 274-6, Van Nostrand, New York, 1939. (4) Youden,. W.,, J., “Statistical Methods for Chemsts, pp, 40-2, Wiley, New York, 1951.
RECEIVED for review November 20, 1959.
Accepted August 5, 1960. One phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract NASw-6, sponsored by the National Aeronautics and Space Admhktration.
A N e w Spectrophotometric Procedure for the Microdetermination of Methyl Chloride MARGARET REDFORD-ELLIS and JAMES E. KENCHI Departmenf o f Pathology, Manchester University, Manchesfer, England
b A procedure, based on the Fujiwara reaction for chloroform, is described whereby methyl chloride is converted into an unknown material absorbing at 365 mp by treatment with pyridine and alkali. Quantities of methyl chloride in the range 2 to 80 pg. either in gaseous form or in aqueous solution can be measured accurately. The procedure is nonspecific and all alkyl the molar halides tested reactedj absorptivity of the chromogens produced by different alkyl halides differed greatly. The nature of the reaction has been investigated by using pyridine homologs. Although it was not possible io isolate the reaction product, the evidence suggests that N-methylpyridinium chloride combines with one or more molecules of pyridine at the a-position of the latter.
Methods employed for the determination of halogenated hydrocarbons in air and in biological tissues have been various and relatively nonspecific. For high concentrations of methyl chloride in air (40% v./v. or more), Allison and Meighan (1) absorbed the methyl chloride in acetic acid and measured the change in volume. Smaller quantities in the range 0.5 to 10 mg. have been
measured by conversion of the organic halide to inorganic halide by combustion, hydrolysis, or reduction with hydrogen, followed by determination of inorganic halide by titration, nephelometry, or colorimetry. Greater sensitivity appeared possible using the Fujiwara (3) reaction, which takes place when one of many halogenated hydrocarbons or derivatives is
t
to manometer
to pump c
T
PRESEST INVESTIGATION was undertaken because a sensitive met,hod was needed for the determination of methyl chloride in order to study its metabolism. The determination of methyl chloride is complicated by the fact that it is a gas at ordinary laboratory temperatures and is toxic to man (9). HE
1 Present address Department of Chemical Pathology, $he Medical School, University of Cape Town, South Africa.
Figure 1. Diagram of vacuum-line assembly used for handling methyl chloride (not to scale) VOL. 32, N O . 13, DECEMBER 8960
*
APPARATUS
Nitrogen cyimder
3.way tap & A i r
I
Manifold
Figure 2. Apparatus used for carrying out reaction under strictly anaerobic
conditions; several units may b e accommodated on manifold Flexible connections made with Silastomer 156. First bubbler contalns 987e (v./v.) pyridine-
The vacuum-line assembly for handling methyl chloride is shown in Figure 1. The flasks forming the vacuumreservoir are each of 5-liter capacity. The calibrated bulb, L , has a capacity of about 80 ml. A11 taps are of highvacuum quality, and both these and the ground glass joints are lubricated with high-vacuum silicone grease. The tubes M and N are of approximately 100-ml. capacity. The apparatus for carrying out the final stages of the reaction in an oxygenfree atmosphere is shown in Figure 2. Connections are made with 8 silicone tubing, Silastomer 156 (Greengate & Irwell Rubber Co., Ltd., Ordsall Lane, Salford, England), which is more resilient than most silicone tubing and is unaffected by pyridine. The apparatus used in the experiments on reaction of methyl chloride with water is shown in Figure 3; the round-bottomed flask has a capacity of 25 ml. The spring-loaded t a p has a bore of 4 mm. All absqrption spectra were determined using the Unicam spectrophotometer. REAGENTS
ethyl alcohol
heated with pyridine and alkali, forming colored compounds. A number of modifications have been proposed in attempts to increase the specificity of the reaction, but none of these modifications is specific for methyl chloride and the most sensitive claims to measure 5 to 100 pg. of alkyl halide. Further modifications were made by which methyl chloride is converted into a different product absorbing strongly a t 365 rnp and the reaction mechanism was studied.
'Table 1.
AI1 reagents are analytical grade unless stated otherwise. Methyl Chloride. T h e gas used in these studies was manufactured by P.C.I., Ltd., England. It was homogeneous as tested b y gas chromatogra-
ne. I n preliminary experiyrid.ine was found to be unory 0cvii;g to contamination with a-picoline. T h e difficulty vas overcome by taking advantage of t h e relative ease of oxidation of t h e alkyl side chains in pyridine homologs. Pure pyridine (hlersey Chemicals Ltd., Liverpool, England), shown by gas chro-
Incubation of N-Methylpyridinium Chloride with and Alkali under Various Conditions (Analytical grade of pyridine used; not all from the same batch) CoefMean @ Concns. ficient of No. of Tested, Variation, Atmosphere Pcg./Ml. % Tests ~~~~~~~~
Air Commercial Nz
4,710 (250) 19,600
15
18
18.4
22
9
1.B.6
Prepared oxygen-free NI
23 ,100
10
9
21.0
26
3
13.8
19
0.5-7
8.1
6
5-9
3.8
43
0.5-18
9.1
(1,500)
27,300 (740) 19,800 (350) 26 ,600 (410) 31, 400b (440)
Calculated from concentrations of It"-m&hylpyridinium chloride used. Pure pyridine used.
1804
e
ANALYTICAL CHEMISTRY
Figure 3. Apparatus used for studying reaction of methyl chloride with water matography to contain less pyridine homologs than analytical grade pyridine, is distiIled slowly through a beadpacked fractionating column; the fraction boiling below and a t 116' C. is collected. This fraction is mixed with one tenth volume of 40y0 (v./v.) aqueous sodium hydroxide solution, and potassium permanganate is added using the apparatus shown in Figure 4, thereby avoiding coating of the permanganate with manganese dioxide. The presence of alkali prevents volatile picolinic acids blocking the apparatus. Following oxidation, the pyridine is dried over solid potassium hydroxide and redistilled over phosphorus pentoxide through a bead-packed fractionating column. The product is homogeneous as shown by gas chromatography. Alcoholic Potassium Hydroxide. A 0.25N solution of potassium hydroxide in ethyl alcohol (British Drug Houses, Ltd.). T h e solution is discarded if a t all yellow; i t keeps satisfactorily a t 0' C. for several weeks. Oxygen-Free Nitrogen. Oxygenfree nitrogen (less than 30 pap.m. oxygen, British Oxygen Gases) is passed through a heated column of copper-kieselguhr (4). Before routine use t h e column is stabilized by heating i t in a stream of nitrogen for a few days. DEVELOPMENTOF ~ E T H I M
(700) Commercial Na treated with alk. pyrogallol Commercial oxygen-free Na
n
The reaction of N-methylpyridiniurd chloride with pyridine and alkali was first studied. The best solvent for maintaining homogeneity and adequate sensitivity was 98% pyridine-ethyl alcohol. The reaction sensitivity increased with increasing concentration of alkali and optimum results were obtained using 0.25N alcoholic potassium hydroxide. The reaction product showed a major peak at 365 mp and s smaller, broader peak at 450 mp. The reproducibility and intensity of the major peak were extremely sensitive to tho presence of oxygen even in trace
amounts. As shown in Table I, improved results were obtained with progressive removal of oxygen from the system, and under the final conditions the peak a t 450 mp disappeared. The reaction of methyl chloride with excess pyridine was first order with respect to methyl chloride and was virtually complete after 18 hours at 60' C. Initially methyl chloride was handled in a gas buret, but it was more convenient to use the calibrated bulb method reported in the next section.
b Figure 5. Absorption spectrum of product d e rived from methyl chloride under standard conditions
DETERMINATION OF METHYL CHLORIDE
The foregoing studies on the reaction of methyl chloride with pyridine to form N-methylpyridinium chloride, and the reaction of this compound with potassium hydroxide in pyridine-ethyl alcohol form the basis of the method now described. The apparatus for delivering a measured amount of methyl chloride into, a euitable reaction vessel is shown in Figure 1. A tube with a stopcock, M, is fitted in position a t K to act as a methyl chloride reservoir. The manifold, methyl chloride reservoir, calibrated bulb, L , and vacuum reservoir are evacuated. Methyl chloride is admitted r i a stopcock A and is condensed by cooling M in liquid nitrogen. The system is evacuated again (
