Quantitative Analysis of Low-Boiling Phenols by Capillary Column Separation of Trimethylsilyl Ethers SIR: Aisimple and rapid procedure has been de\ oiopeti for gas liquid chromatographic separation of the components which exist in the low-boiling phenols (180'-240' C). Quantitative formation of trimethylsilyl ethers of the phenols is obtained by reflux with hexamethyldisilizane in the presence of a silica catalyst. The latter ensures complete conversion of hindered phenols such as 2,6-xylenol. Complete (baseline) separation of all peaks of a known mixture is obtained through the use of a capillary column coated with pure 2,4xylenol phosphate. Routine separation of low-boiling tar acid fractions is generally accomplished using columns packed with high-boiling esters. Most of these give no separation of m- frorn p-cresol. Some of the higher boiling phenols also are not separated. Sassenberg and Wrabetz (11) show a fair separation of phenol through 3,4-xylenol using diA(3,3,5trimet hylcyclohexyl) p ht halate. Separation of m-and p-cresol was accomplished by Paterson (10) by the use of a tricresyl phosphate-phosphoric acid substrate. Brooks ( 3 ) used 2,4-xylenol phosphate for separation of close-boiling isomers I n most cases, attempts to use capillary columns for separation of free phenols have been unsuccessful as the result of wall effects which cause excessive tailing. The addition of nonionic detergents to coating substrates (1) tends to eliminate this effect. In our laboratories, fairly good separations were obtained using capillary columns coated in this manner. However, in all cases the column life was very short. Most of the difficulty encountered in the analysis of phenols results from the highly polar hydroxyl groups. Better
Table I.
I
I
10
15
t
TIME
Figure 1.
ANALYTICAL CHEMISTRY
Retention time, min.
8.4 11.6 12.2 13.4 15.0 16.1 17.6 18.2 18.8
results are obtained by chromatographing derivatives such as alkoxy (2) and trimethylsilyl ethers (9). The latter can be prepared by refluxing the sample with hexamethyldisilazane. One difficulty is the failure of hindered phenols such as 2,6-xylenol to react completely. Grant and Vaughan (8) were partially successful in circumventing this difficulty by using acid catalysis. A trace of hydrochloric acid vapor, introduced into the reaction flask, substantially increased the yield of 2,6-xylenoltrimethylsilyl ether. Other workers (6,?') have added a trace of trimethylsilyl chloride to the hexamethvldisilazane as an acid catalyst in pyridine solution. This results in complete silylation of some hindered phenols. By using dimethylformamide in place of pyridine ( 5 ) ,even more hindered phenols such as 2,4,6-tri-t-butylphenol are silylated. In a recent paper ( L ) , we described a simple
20.3 20.8
21.5 23.7 34.8 39.0
Area 2800 2700 4140 5330 324 2720 1270 680 3950 1233 567 273 483 384 246
yo Found 10.3 10.0
15.3 19.7 1 2
10.0 4.7 2.5 14.6 4.6 2.1 1 .o
1.8 1.4 0.9
20 (MIN.)
25
-'
I
I
35
40
Separation of trimethylsilyl ethers
Analytical Results of Gas Chromatographic Separation of Trimethylsilylated Phenols
Component (1) Phenol ( 2 ) +Cresol (3) m-Cresol ( 4 ) p-Cresol ( 5 ) o-Ethylphenol ( 6 ) 2,5-Xylenol ( 7 ) m-Ethylphenol (8) 3,5-Xylenol ( 9 ) 2,4-Xylenol 10) p-Ethylphenol 11 ) 2,6-Xylenol 12) 2,3-Xylenol 13) 3,4-Xylenol 14) 2,4,6-Tri methylphenol 15) 2,3,6-Tri methylphenol
1880
I
% Added 9.9 9.9 15.2 19.5 1.2 9.9 4.9 2.5 15.0 4.7 2.0 1.0
1.8 1.5 0.9
silylating procedure for rapid quantitative analysis using silica or other acidic oxides as catalysts in the absence of solvents. In our procedure, 2,6-substituted 1-butylphenols, however, apparently do not form ethers. I n the work described in this paper, we have arrived a t conditions for complete (baseline) separation of all the phenols commonly encountered in the low boiling tar acid fraction. The use of silica catalysis ensures the complete conversion of the more hindered phenols. EXPERIMENTAL
Reagents. Hexamethyldisilazane was purchased from Peninsular ChemResearch, Inc., Gainesville, Fla. Phenol, hfallinckrodt; reagent grade was used. The alkylphenols were obtained from Consolidation Coal Co. and were 99-10070 pure by analysis. Silica (Sea Sand) was purchased from Fisher Scientific Co. Pure 2,4-xylenol phosphate (b.p. 350' C. a t 100 mm.) was prepared by the reaction of 2,4xylenol with phosphorus oxychloride. Apparatus. A flame ionization chromatograph of conventional design is used employing a Research Specialties Corp. Electrometer, Model 605-3. A 0.020-inch i.d. X 0.0625-inch 0.d. x 300-foot stainless steel capillary column coated with 10% 2,4-xylenolphosphate in ethylene chloride is used a t 125' C . At a column inlet helium pressure of 25 p.s.i.g., a split flow of 250 ml. per minute and a column flow of 10.5 ml. per minute occurs a t 125°C. Sample injections of 0.1 rnl. are made. The apparatus for preparation of trimethylsilyl ethers consists of a 3-inch test tube connected by 24/40 standard joints to an open end air condenser 18 inches long. This apparatus is kept in
the oven a t 110” C. until immediately before ube. 5-mv. recorder coupled to an Instron Integrator is used for digital readout of peaks. Procedure. Ai mixture of 1 ml. of sample, 1.5 ml. of hexamethyldisilazane, 0.5 gram of sand, and 0.5 gram of Drierite is refluxed for 45 minutes on a 200” C . sandbath, cooled, and a 0.1-ml. sample injected into the chromatograph. Peak areas are integrated and percentages of individual peaks evaluated as fractions of the total integrand, ignoring area factors. A \
RESULTS AND DISCUSSION
measured percentages. The peaks exhibit very little tailing and are all well separated. The hindered phenols (having o,o’-methyl substituents) are present in too small a proportion to demonstrate completeness of reaction. This is dealt with along with theoretical considerations in our previous paper (4). ACKNOWLEDGMENT
The alkylphenols were furnished by Donald C. Jones of the Research Division of Consolidation Coal Co., who also prepared the 2,4-xylenol phosphate. LITERATURE CITED
(1) Averill, W., 2nd Int. Symposium of
Figure 1 is a chromatogram of an accurately proportioned mixture of pure phenols listed in Table I. Areas listed are the digital areas multiplied by attenuation factors. Good agreement is obtained between known and
Gas Chromatog., East Lansing, Mich.,
1-7 fl961). ( 2 ) Bergmann, G., Jentasch, D., Angew.
Chem. 70, 192 (1958). (3) Brooks. V. T., Chem. & Znd. (London) 1959, i3i7. ( 4 ) Freedman, R. W., Croitoru, P. P., ANAL.CHEM.36, 1389 (1964).
(5) Friedman, S., Kaufman, M . L., Wender, I., J . Org. Chem. 27, 764-5 (1962). (6) Friedman, S., Steiner, W. A., Wender, I., Fuel 40, 33-45 (1961). ( 7 ) Friedman, S, Zahn, L., Kaufman, M. L., Wender, I., Bur. Mi.zes Bulletin 609, (1963). (8) Grant, D. W., T ’ a u g e , G A , “Gas Chromatography, 1961, M. van Swaay, ed.. pp. 305-14, Butterworths, London, 1962. (9) Langer, S. H., Pentages, P., Wender, I., Chem. & Ind. (London) 50, 1664-6 (1958). (10) Paterson, A. R., “Gas Chromatography,” (2nd Int. Symposium, Analysis Instrumentation Ilivision of the Instrument SOC. of America, June 1959), H. J. Noebels, ed., pp. 223-6, Academic Press, Sew York, 1961. (11) Sassenberg, W., Wrabeta, K , 2. A n d . Chem. 184, 423-7 (1961). ROBERTW. FREEDMAN GEORGE 0. CHARLIER Consolidation Coal Co. Library, Pa.
Adsorption of Cobalt( 111) Trisethylenediamine at Mercury Electrodes SIR: In a recent publication, Anson ( 1 ) reported that he was unable to
detect adsorption of C ~ e n at ~ +8, hang~ ing drop mercury electrode using the potentiostatic current integration method (2, S), whereas we (4) had reported detection of a small surface excess by the chronopotentiometric method. He concluded that adsorption of this ion is absent, and attributed our finite intercepts in i r us. l/i plots to double layer charging effects. The following statements are made in an effort to clarify, in so far as possible, this apparent disagreement. I n the measurement of chronopotentiometric transition times, an effort is made to eliminate double layer charging from each transition time measurement. Admittedly, this is not necessarily an exact procedure because of possible errors in the graphical methods used in evahating transition times. However, we satisfied ourselves of the adequacy of the method we used by determining a set of transition times for C d + 2in the same range of i and 7 as for C ~ e n and ~ + ~finding that the i7 us. l l i plots indeed had negligible intercepts. It should be noted that negligible intercepts were a l ~ o observed for C0en3+~under certaiin conditions, notably in the absence of C1- which should enhance adsorption of the cobalt species by ion pair formation with adsorbed chloride. The potentiostatic current integration method, on the other hand, makes no provision for removing the double layer charging current from each individual measurement. Instead, the intercept of the Q us. t 1 I 2 plots includes
the sum of current required for double layer charging plus reduction of the adsorbed solute. Therefore, any change in double layer charging caused by the adsorption of reactant cannot be taken into account in running a blank. As a result, the sensitivity of the method is considerably decreased and the erroneous conclusion that adsorption is absent may be reached in cases in which adsorption occurs to a limited extent only, particularly if adsorption leads to a decrease in double layer capacity. However, increases as well as decreases can occur, particularly with charged adsorbates. Independent evidence exists for adsorption of Coen3f3 ion. The electrocapillary curve for an equimolar mixture of Coen3C12 and Coen3Cla in 1.lf NaC104 and O.1M en shows a shape typical of cation adsorption when compared with the supporting electrolyte which contains a concentration of chloride ion (added as NaCl) equivalent to that added with the cobalt cations. The deviation between the two curves begins a t -0.10 to -0.15 volt (US. S.C.E.) and increases with increasing negative potential. Because of the reversible behavior of the couple, the open circuit potential is a function of the ratio of concentrations of the two forms of cobalt complex in solution. With C ~ e n alone, ~ + ~ the open circuit potential was about -0.18 volt (4) at which point the electrocapillary curve shows only a small amount of adsorption. This may be the reason that Anson failed to detect adsorption. The unexpected result in our work was that in an equimolar mixture of Coen3+3and
C ~ e n ~a +smaller ~, amount of adsorption of C ~ e n was ~ + observed ~ than in a solution containing only the osidized form, Two opposing effects can be visualized. Because of the more negative open circuit potential, increased adsorption of cations would be expected. On the other hand, primary adsorption of chloride, together with ion pair formation between chloride and the cobalt (111) trisethylenediamine cation, appears to play an important role. The primary adsorption of chloride would be decreased by the more negative open circuit potential. In any case, the electrocapillary curves indicate a greater amount of total adsorption a t more negative potentials. I t would appear, both from the chronopotentiometric observations and the impedance behavior discussed below that the adsorption of cobalt(I1) is greatly favored over that of cobalt(II1) from an equimolar mixture. Unfortunately, the adsorption of cobalt(I1) could not be studied because of kinetic complications in the oxidative electrode process ( 4 ) . I n brief, the electrocapillary behavior appears to give unequivocal evidence for adsorption of the cobalt trisethylenediamine comple\- but it does not distinguish between the relative amounts of cobalt(II1) and cobalt(I1) adsorbed a t any potential. The anomalous faradaic impedance behavior of the C0rn3+~-Coen,+z couple can be explained by assuming additional electron exchange between adiorbed reactants and electrode ( 6 ) . The anomaly in faradaic impedance could be represented by a large capacitance in series with a small resistance, the com VOL. 36, NO. 9 , AUGUST 1964
1881
