copper, molybdenum, titanium, and vanadium gave high results. Some of this work was repeated, to determine if the interferences were stoichiometric, and some additional elements were studied with the results shown in Table V1. Aluminum, calcium, nickel, and zinc in amounts up to 2% cause no interference. Chromium causes slightly low results. Titrating in a n inert atmosphere and varying the time of standing prior t o titration did not eliminate the error. Copper, when present in 2y0concentration, causes a reproducible, but unexplained positive error. Titanium and tungsten are reduced to indefinite valence states and are subsequently titrated giving high results. At the suggestion of H. H. TT’illard, fluoride n-as added to complex the titanium(1V). The addition of 10 ml. of saturated sodium fluoride solution to samples containing 0.2 and 2.0% titanium reduced the errors by about half, but did not eliminate them. Vanadium gives high results corresponding to approximately a one-electron reduction. However, the end point is poor in the presence of vanadium, causing erratic results. Zirconium phosphate precipitates a t a 2% zirconium concentration and apparently some
uranium is coprecipitated to give lo^ results. VARIATIONSIN METHOD. Main’s (4) procedure was modified by the addition of sulfuric acid to prevent formation of a uranous phosphate gel. Another variable studied in some detail was the time of standing between reduction and titration. Samples containing excess stannous chloride for as long as 3 hours gave accurate titrations. Results on samples that had stood for up to 7 hours tended to be low, while samples standing overnight had no excess stannous chloride left. Erratic results were obtained when perchloric acid Tvas present during the reduction. This was expected in view of the knon-n tendency for stannous chloride to reduce the perchlorate ion in the presence of molybdenum (3) and other catalysts. Main attributes the low results n-hich were frequently obtained on samples requiring more th?n 30 ml. of titrant to the air oxidation of the ferrous iron during titration. He suggests adding dry ice to the titration flask to provide a n oxygen-free atmosphere during the titration. Early in this study, a comparison of titrations run 1%-ithand without the dry ice indicated no difference although the titrations required over
80 ml. of 0.05N dichromate. The titrant was added rapidly up to the end point. ACKNOWLEDGMENT
The use of the stannous chloride method was suggested by H. C. Anderson following a visit to the University of California Radiation Laboratory, Livermore Research Laboratory, where Main’s method &-asdiscussed. LITERATURE CITED
(1) Cooke, W. D., Hazel, F., LIcSabb, \$‘. bf., A N A L . CHEM. 22, 654 (1950). ( 2 ) Haight, G. P., Jr., Ibid., 23, 1505 (1951).
(3) Haight, G. P., Jr., Sager, W. F., J. Am. Chem. SOC.74, 6056 (1952). (4) Main, A. R., A N ~ LCHEJI. . 26, 1507 (1954). (5) Patterson, J. H., Ed. “Manual of Special Materials Analytical Laboratory Procedures,” Argonne Natl. Laboratory, ANL 5410 (March 19.5;). (6) Rodden, C. J., ed., Analytical Chemistry of the Manhattan Project,” Sational Nuclear‘ Energy Series, Division VIII, Vol. 1, Chap. 1, McGrawHill, T e w York, 1950. (7) Schre er, J. AI., Baes, C. F., Jr., AKAL. 25, 644 (1953). (8) Sill, C. W.,Peterson, H. E., Ibid., 24, 1175 (1952).
HEM.
RECEIVEDfor review lpril 24, 1958. -4ccepted December 8, 1958. Work done under AEC Contract S o . .iT(29-1)-1106.
Resolution of Isomeric Xylenes bY Gas-Liquid Chromatography ALBERT ZLATKIS, SU-YU LING, and H. R. KAUFMAN’ Department o f Chemistry, University of Houston, Houston, Tex.
b Several new polar stationary liquid substrates are suitable for the gas chromatographic resolution of m- and p-xylenes in aromatic mixtures. By using these liquids on aqua regiatreated C-22 firebrick or Chromosorb, separations can b e effected on short columns in reasonable emergence times. The aqua regia treatment allows the use of finer particle sizes, longer columns, and higher flow rates because of the lower pressure drop across the column.
A
mixtures containing benzene, toluene, ethylbenzene, and xylenes are currently analyzed by infrared (7) and ultraviolet ( I ) spectroscopy. Since 1952 (Q), gas-liquid chromatography has also been used to analyze aromatics. Although most C g Cg aromatics can be resolved by gas ROMATIC
Present address, U. S. Industrial Chemicals Co., Deer Park, Tex.
chromatography, Tn- and p-xylene hare resisted practically all attempts a t separation. The current approach involved a study of liquid substrates and the inert solid support. Suitable stationary liquid phases were selected to give a t least a noticeable resolution of m- and p-xylene; efficient columns using these liquids were then prepared by studying such factors as inert supports, particle size, flow rates, and percentage of stationary phase on the support. MATERIALS AND REAGENTS
Aqua Regia-Treated Firebrick. One hundred grams of C-22 firebrick (Johns-Manville) of 40 to 60 mesh were added t o 300 ml. of aqua regia. The mixture was heated so t h a t a mild reaction continued for about 1 hour. After cooling, the acid was removed by adding water and decanting several times. Then 300 ml. of dilute sodium hydroxide (5%) were added and de-
canted to neutralize any remaining acid, and finally the firebrick was thoroughly water-washed on a 60-mesh screen to remove remaining fines. The support was dried in an oven a t 110’ C. overnight and rescreened to the desired mesh size. A 20y0 loss in weight occurred during the over-all process. Stationary Liquid Substrates. Ethylene carbonate-1-nitronaphthalene mixture. This substrate is prepared by mixing equal quantities of ethylene carbonate and 1-nitronaphthalene and heating to 60’ C. Dimethylsulfolane. 1-Chloronaphthalene. All stationary liquids were put on the support in a 15 to 100 ratio. Methylene chloride was the solvent used to apply the substrate to the solid support. EXPERIMENTAL
The first stationary liquid phase to produce a separation of the xylene isomers was recently reported by Zlatkis, O’Brien, and Scholly (9): benzylVOL. 31, NO. 5, MAY 1959
* 945
Table I.
Relative Retention Volume Ratios and Separation Factors for Various Liquid Substrates (15% w./w.)
B.P., C.
d
Benzene 80.1 Toluene 110.6 Ethylbenzene 136.2 p X lene 138.4 m-&lene 139.1 o-Xylene 144.4 SU ( pand m-xylene) Q ( p - and m-xylene) R ( p - and m-xylene) 16-ft. benzyldiphenyl on 60 to 80 mesh U^
1.00 2.05 3.93 4.14 4.27 5.25 0.031 24.2 0.i6
1.00
1.00 2.35 5.00 5.45 5.i5
2.28 4.65 5.37 5.65 7.30 0,052 9.80
1.00 3.30
8.96
7.73 0.055 0.35
10.30 10.92 14.39 0.060 10.93 0.66
0.51 0.5z untreated Chromosorh, 100' C., 100 ml./min.
Pi(. l
