Determination of Aromatic Compounds in Petroleum Products Ch rornatographic Method A \ U E L. CONR i D , Standard Oil CorrLpnny, Cleceland 6 , Ohio
i rapid chromatographic method for the deterniination of aromatic curnpouncib in hydrocarbon mixtures i n the gasoline and kerosene range utilizes a combination of silica gel separation and ultraviolet-stimulated fluorescence. The method is reproducihle to within "2% and requires a maximum of 45 minuteper determination. Olefins and other h?drocarhon* d o riot interfere. cator Connect bottle of iilica gel covered 511th gla- \roo1 to a varuum pump and pump for 10 minutes. Place plug of glass wool in bottom of tubing. Fill tube to within 15 em (6 inches) of top n i t h dry silica gel arid pack tube by tapping firmly for 10 minutes while rotating tube Particles of silica gel should shon no downnard movcmcnt at the end of the tapping period Insert cotton plug into top of tube To a 2- to 5-ml sample add 2 drops of Parasheen and place 111 top of the column. When the sample hai completely entei cd the column, nic asure the total length of its adsorption band. -4dd 5 mm. of silica gel and t a p gently Add ethyl or isopropyl alcohol equal to approximatel\ tuice the volume of the sample When the sample has moved d6wn the column a distance equal to tnice the length of its original adsorption band, direct ultraviolet rays on it Mark and measure accurately the limits of purple fluorescent laver and the limits of the entire sample. Repeat measurements a t 5-minute intervals as the bands move donn the column until thcir ratios are constant
H.1PID niethod )vas
rirtdt*(l for determiiiiiig the percentagc of aromatic compounds in iiunierous samples taken from various fractions of petroleum. Present methods for determining aromatic conttlnt of petroleum compounds are either tinic-consuming or inaccurate on account of interfering hydrocarbons. The use of silica gel percolation as recommended by Xair ( 2 , 5 ) wquires from 4 to 8 hours prr determination. The:preaent .I.S.T.M. acid absorption method (1 I and the Kurtz controlled acid absoiptiori method ( . b ) depend upon accurate determinations of olefinic content as well as mole?ular weight, both of which art> subject to numerous potential errors. Specific dispersion mpthods (5) have been found inaccwrate on samples boiling over H. wide tempcratiire rang^(' and on cracked stocks containing olefins. Tlie method described here is based upon the procedure dvveloped by hIair ( 2 , 5 )for separating aromatic hydrocarbons from hydrocarbon mixtures. IVhilc ttmploying the same limitations for the type of silica gt.1, ratio OF yilica gel t o sample, and rate of percolation, the author ha. developed a rapid procedure for estiniating the aromatic cont ent, xvliich employs a much smallel, sample and a smaller column. Instead of eluting the saniple from the column, the deterriiination was complrted through the use of fluorescing chromatograms while the saniple remained adsorbed on the silica gel. The fluorescence of thtk entirc adsorption band of the sample 011 the column was stiinulated by tlic addition of a trace of a tiydrocarbon-solublc fluorescing impurity. With ultraviolet radiation, chromatographic bands becamc visible demarkirig the area covered by the highly fluorescent aromatic compounds ttnd rhat covered by the other weekly fluorescing hydrocarbons. Sharp lines of demarcation becanit: visible n-hen the column was carefully packed and the separation was complete. Tlie ratio of the length of the aromatic hind to that of the whole sample \vas found to be equal to tho pt'rcentage of aromatic compounds i n the sample. These ratios i~niainedconstant as thr, saniplc pertvlxted through a spc~ificportion of the tube.
Calculation Length of purple aroniatic band T,ength of mtire sample--
100
=
5 aronlatlc*~
DISCUSSION OF >lETHOI)
Coiisitlerablt
