divalent cation electrode or calcium selective electrode for improving the sharpness of the end point because the organic liquid ion exchanger is soluble in most organic solvents. When a solid state electrode (for instance, the lead ion selective electrode) is used, their addition may be desirable. p H effect. The effect of p H o n complex formation can be studied by plotting the change in the potential us. the p H of a solution that contains a metal ion and T T H A a t a given ratio. It is possible to predict the optimum p H range for complex formation when the stability constants of TTHA complexes are known. Some results with the divalent electrode are shown in Figure 5. Accuracy and Precision. I n the TTHA titrations using divalent cation electrode, a n accuracy of 1.0% o r better was obtained as shown in Table 11.
Precision of the method was tested by making eight titrations for lead, calcium, zinc, and magnesium, respectively. Standard deviations of 0.0027 for lead, 0.0028 for calcium, 0.0022 for zinc, and 0.0028 for magnesium were obtained. I n the titration of calcium and manganese using calcium ion selective electrode a n accuracy of 2 . 0 x or better was obtained.
RECEIVED for review April 1, 1970. Accepted July 15, 1970. Presented before the 195th National ACS Meeting, Division of Analytical Chemistry, February 23, 1970, Houston, Texas. One of us (EAM) thanks the Ford Foundation for the FordOriente University fellowship.
CORRESPONDENCE Alkylation and Gas Chromatography of Aqueous Inorganic Halides
EXPERIMENTAL
ammonium were products of Eastman Organic Chemicals, Rochester, N. Y . , except for a few of the salts which were prepared by neutralization of the hydroxides with aqueous acids. The cation exchange resin was Dowex 50W-X4, 200 to 400 mesh (Dow Chemical Company, Midland, Mich.). The resin was converted t o the tetraalkylammonium salt with excess tetraalkylammonium chloride and then washed exhaustively with water. Gas Chromatography. A Barber-Colman Model 5000 gas chromatograph with l/An. (0.d.) coiled stainless steel columns and a thermal conductivity detector was used. Three porous polymer packings were employed as stationary phases : Chromosorb 101, 80-100 mesh (Johns-Manville Company, New York, N. Y . ) in a 10-ft column, Porapak Q, 80-100 mesh, and Porapak T , 100-120 mesh (Waters Associates, Framingham, Mass.) in 6-ft columns. In all cases the vaporizer was maintained a t 360 “C and the detector was held above the column temperature. The helium flow rate was 75 ml per minute with Chromosorb 101 and as listed in Table I for Porapaks Q and T. The thermal conductivity detector was operated at 215 mA with an attenuation factor of 1. A 0-5 mV recorder was used. The Chromosorb 101 column was held at 125 “C and the Porapak columns were held at the temperatures listed in Table I. Cation Exchange. In a typical example, the cations of a freshly-prepared solution, which was 0.25M each in fluoride, chloride, bromide, and iodide as the sodium or potassium salts, were exchanged for tetramethylammonium ion o n a charged Dowex-50 column at room temperature. In order to prevent crystallization of some of the salts, it sometimes was necessary t o hold the microsyringe, the solutions, the ion exchange effluent fractions at 68 “C in a TempBlok heater (Lab-Line Instruments, Chicago, Ill.).
Reagents. The hydroxides, fluorides, chlorides, bromides, and iodides of tetramethyl-, tetraethyl-, and tetra-n-propyl-
RESULTS AND DISCUSSION
(1) R. W. Moshier and R. E. Severs, “Gas Chromatography of Metal Chelates” Pergamon Press, Oxford, 1965. (2) R. M. Bethea and M. C. Meador, J. Chromatogr. Sci., 7 , 655 (1969). (3) T. Hashizume and Y . Sasaki, A d . Biochem., 15, 199 (1966). (4) A. W. Hofmann, A m . , 78, 253 (1851). ( 5 ) Ibid., 79, 11 (1851). (6) J. Haslam, J. B. Hamilton, and A. R. Jeffs, Analyst, 83, 66 ( 1958). (7) J. MacGee and K . G. Allen, Steroids, 16,79 (1970).
The retention data for the products of degradation of the tetraalkylammonium halides and hydroxides on Porapak Q and for the methyl series on Porapak T are listed in Table I. Separate samples of methanol, water, methyl fluoride, methyl bromide, methyl iodide, ethyl bromide, and triethylamine as dilute solutions in suitable solvents were chromatographed for comparison with the retention times of our products. The methyl fluoride was prepared by the metathesis of methyl
SIR: Gas chromatography of inorganic cations has been demonstrated by taking advantage of the volatility of certain metal chelates ( I ) . Aside from the gas chromatography of anhydrous acid gases ( 2 ) and a trimethylsilyl derivative of phosphate (3), we know of no gas chromatography of inorganic anions or of derivatives easily prepared from them. Hofmann first demonstrated the alkylation of inorganic iodide by the thermal degradation of its tetraalkylammonium salts in 1851 ( 4 , 5). Gas chromatography of alkyl iodides was demonstrated in 1958 (6). Hofmann’s degradation has been exploited in the preparative chemistry and gas chromatography of the products arising from the thermal degradation of tetramethylammonium salts of biochemical interest (7). The successful application of the degradation procedure to the gas chromatography of inorganic halides is reported here. The cations of an aqueous solution of the halides are exchanged for tetraalkylammonium ions by cation exchange. On injection of an aliquot of the exchanged solution into the vaporizer of a gas chromatograph, the salts are decomposed by the thermal elimination of trialkylamine. When the alkyl group is methyl, the products are trimethylamine and the methyl halides. When the alkyl group is ethyl o r n-propyl, the products are trialkylamine and the alkyl halides except for the fluoride salts. These salts decompose to yield the olefins of the alkyls, hydrogen fluoride, and the trialkylamines. The degradation products are separated from each other on a porous polymer column.
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Table I.
Products Arising from the Thermal Degradation of Tetraalkylammonium Solutions
Relative retention times (water Alkyl group Ethyl
=
1.00)
Methyl Stationary phase Q" Tb Q Temperature, "C 150 150 150 200 Helium flow, ml per min 65 56 65 60 Water (minutes) (1.18) (2.28) (1.18) (0.75) Methanol 1.77 1.40 ... ... Ethylene ... ... 0.53 0.63 Propylene ... ... ... ... Alkyl Fluoride 0.71 0.27 ... ... Alkyl Chloride 1.87 0.75 3.90 2.84 3.27 1.36 7.28 4.53 Alkyl Bromide 7.23 2.87 16.67 8.53 Alkyl Iodide 6.70 1.81 NDc 18.05 Trialkyl Amine a Porapak Q. Porapak T. Not determined, because the retention time was too long for a meaningful value. c ND. iodide by mercuric fluoride a t 0 "C (8). The product was collected in hexane held in a solid carbon dioxide-ethanol bath. Figure 1 shows that the gas chromatographic elution pattern obtained from an aliquot of the ion exchange effluent (Figure 1B) is identical with the pattern evolved when a solution prepared from the tetramethylammonium salts is subjected to analysis (Figure 1A). Comparisons of the gas chromatographic peak areas of ethyl bromide and methyl iodide from the alkyl halides in hexane and t'le aqueous tetraalkylammonium salts shows that the temperature of the vaporizer is important. At a vaporizer the yield of ethyl bromide is essentially temperature of 360 "C, quantitative w'lile that of methyl iodide is 46%. Increasing the reaction temperature to 450 and 500 "C (our upper limit) increases the yields of methyl iodide to 59 and 86%, respectively. Broadening and tailing of the methyl iodide peak generated from the salt suggests some resynthesis and redegradation of tetramethylammonium iodide takes place on the column. Only limited success has been achieved in this laboratory in demonstrating the alkylation of other inorganic anions. Encouragement in the form of the production of trialkylamines has been obtained in several cases, so further investigations into the use of other stationary phases is indicated. Success in such a study could be of much value in the application of gas chromatography to inorganic analysis. Three stationary phases were examined in this study. I n our hands a 6-ft Porapak Q yielded a higher number of theoretical plates than did a 10-ft Chromosorb 101 column, but the retention times of the products on the Chromosorb 101 column were such that a better illustration could be obtained with this latter column. For these reasons, most of the retention data given in Table I are from the Porapak Q studies, and Figure 1 was obtained from our Chromosorb 101 studies. Extrapolation of the data presented in the manufacturer's literature (9)offered a hope that Forapak T would retard water sufficiently to allow the use of a n electron capture detector, but such was not the case (Table I). A thermal conductivity detector was used in these studies because we wished t o show the solvent, water. Lack of sensitivity required relatively high concentration of the salts. (8) A. L. Henne and T. Midgley, Jr., J . Amer. CAem. Suc., 58, 884 (1936). (9) "Porapak," Waters Associates, Inc., Framingham, Mass., 1965
Propyl
Q 200 60 (0.75) ...
240 54 (0.67)
...
... ...
1.42
1.35
5.32 8.74 16.74 ND
3.47 5.18 8.88 34.12
...
A -
...
CH31
(CH3CI
1
1
0
I
I
4
I
I
I
I
I
I
I
8 12 16 20 MINUTES
0
4
8 12 16 20 MINUTES
Figure 1. Methylation and gas chromatography of inorganic halides A . Six 1 1 of an aqueous solution, which was 0.25M each in authentic tetramethylammonium fluoride, chloride, bromide, and iodide, was injected into the chromatographic unit B. Six p l of the cation exchange effluent described in the Experimental section was injected into the chromatographic unit. The stationary phase was Chromosorb 101
Much greater sensitivity can be expected with an electron capture detector if the water can be kept out of the detector. A stationary phase that would retard water longer than the alkyl halides could allow bypassing the detector after elution of the halides. We know of no stationary phase that will accomplish this separation, but a porous polymer more polar than Porapak T may allow for gas chromatographic analysis of aqueous halides in the parts-per-billion range. JOSEPHMACGEE G. ALLEN KENNETH Medical Research Laboratories Veterans Administration Hospital 3200 Vine Street, Cincinnati, Ohio 45220 and the Departments of Biological Chemistry and Internal Medicine College of Medicine, University of Cincinnati Cincinnati, Ohio
RECEIVED for review May 7,1970. Accepted August 28,1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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