1182
Anal. Chem. 1984, 56,1182-1183
(6) Barron, R. E.: Fritz, J. S. Reactive Polymers 1983, 1 , 215. (7) Cantwell, F. F.; Puon, S. Anal. Chern. 1979, 5 1 , 623. (8) Pohl, C. A.; Johnson, E. L. J . Chrornatogr. Sci. 1980, 18, 442. (9) Cassidy, R. M.; Elchuk, S. J . Chrornatogr. Sci. 1983, 27, 454.
RECEIVED for review August 29,1983. Accepted February 16,
1984. Operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-83. This research was supported by the Director of Energy Research, Office of Basic Energy Sciences. This paper was presented at the Minnesota Chromatography Forum in May 1983.
Determination of Milligram Amounts of Sulfur in Hydrocarbons by Constant Current Coulometry S a b r i M. Farroha, Albertine E. Habboush,* and Makarim N. Micheal Department of Chemistry, College of Science, University of Baghdad, Baghdad, Iraq
A new method for transforming trace amounts of elemental sulfur dissolved in hydrocarbons quantltatively to thiosulfate is developed. The sensltlvlty and accuracy of the method were checked by titratlng 1 m L samples of sodlum thlosulfate to 6.85 X lo-' containlng sulfur in the range of 3.22 X g in a 100-mL solution of phosphate buffer and potassium iodide coulometrlcally wlth an amperometrlc end polnt detectlon technique. Synthetlc standards wlth free sulfur ranglng from 5.135 X lO-'to 2.16 X g dissolved In hydrocarbon were transformed quantltatively Into sodium thiosulfate (according to optlmum conditions found during this investlgatlon). Sodium thiosulfate solutions thus formed, contalnlng from 2.06 X to 8.64 X lo-' g of elemental sulfur per 1 mL, were titrated following the same technique. The results obtalned showed a percentage error ranging from 0.27 % to 2.77 % by weight whlle the relatlve standard deviatlon dld not exceed 5%.
Few instrumental techniques have been introduced for the determination of elemental sulfur in hydrocarbon (1-4)such as polarography, square wave polarography, and spectrophotometry, but these methods faced various difficulties, lack of sensitivity and a limited range of detection. The present investigation concentrated on the quantitative transformation of trace amounts of elemental sulfur dissolved in hydrocarbon into a measurable form, and a coulometric method was devised for the measurements. The sensitivity, precision, and accuracy of the method were investigated through the determination of series of samples of standard sodium thiosulfate and synthetic standard solution. EXPERIMENTAL SECTION Apparatus. A Metriplex Universal Coulometer Analyzer, Type OH-404, was used which consists of three main units: a po-
tentiostatic galvanometric unit, a coulometer-integrator unit, and an end point sensor and controller unit. The electrolysis cell was constructed as follows: The iodinegenerating part consists of a platinum foil electrode, Type 934-0H, as an anode and platinum electrode as cathode. The platinum electrode was separated from the solution in the cell to prevent the diffusion of cathodic reduction products. The end point detection part consists of a two platinum wire electrode, Type 9381-OH.
Preparation of Solutions. The following solutions were prepared: 1 N potassium iodide and 1N stock solution of sodium thiosulfate, the later was used for the preparation of several 0003-2700/84/0356-1182$01.50/0
solutions of different concentrations ranging from 0.01 to 2 X IO4 N. Phosphate buffer (pH 7 ) was prepared from 13.61 g of potassium dihydrogen phosphate and 14.20 g of disodium hydrogen phosphate dissolved in distilled water, and the solution was diluted to 1000 mL. Procedure. A 75-mL aliquot of phosphate buffer and 25 mL of potassium iodide were added to the stoppered cell. One milliliter of the standard sodium thiosulfate solution was introduced into the cell with a 2-mL calibrated syringe. The speed of the stirrer was fixed and the measurement was made. The amount of thiosulfate expressed in millicoulombs is read from the display board and the micrograms of sulfur was calculated. Eight measurements were reported for each sample and 11samples of different concentrations of thiosulfateranging from loF2N down N were used. The results are shown in Table I. to 2 X Preparation and Measurement of Synthetic Standard. A small amount of sulfur was weighed accurately and dissolved in about 60 mL of hexane. Sodium suite, 2.5 times the molar ratio, was weighed and dissolved in about 60 mL of distilled water. The two solutions were mixed in a round-bottom flask, 500-mL capacity, fitted with a thermometer and double condenser, and the mixture was boiled under reflux for 5 h. Then the flask was cooled under the tap, and 10 mL of 40% formaldehyde and 40 mL of diluted acetic acid were added. The solution was transferred to a 250-mL volumetric flask and made up to the mark with distilled water. The determination of sodium thiosulfate formed in the synthetic standard was carried out coulometrically exactly in the same way as that used for the standard sodium thiosulfate solution. Eight measurements were reported for each sample and 14 samples containing from 2.06 X lo4 g/mL to 8.64 X lo4 g/mL of elemental sulfur in solution of phosphate buffer and potassium iodide were used, The results are shown in Table 11.
RESULTS AND DISCUSSION The conditions for the quantitative conversion of elemental sulfur in hydrocarbon to sodium thiosulfate were carefully investigated, these are temperature and the length of time required for the reflux, sulfur to sodium sulfite ratio, choice of solvent, and removal of excess sulfite. From these investigations, a novel simple procedure was developed for the quantitative conversion of free sulfur to sodium thiosulfate Na2S03 S Na2S203 (1)
+
-
This reaction is quantitative if Na2S03quantity is 2.5 in excess stoichiometrically. The reaction during coulometric titration is
+ 12
2s2032-
-
s40:-
+ 21-
The measurements were performed in a phosphate buffer of 0 1984 American Chemical Society
Anal. Chem. 1904,56, 1183-1186
1183
Table I. Summary of Results for Determination of Sodium Thiosulfate mean of sample thiosulfate 8 readings, re1 std mC/mL dev, % soln, N no. 0.41 1.0 x l o * 969.69 1 0.94 767.99 8.0 x 2 0.96 6.0 x 1 0 - ~ 576.05 3 1.60 388.74 4.0 x 10-3 4 1.83 190.69 2.0 x 10-3 5 3.02 94.65 1.0 x 10-3 6 0.75 69.22 7.0 x 7 4.19 47.02 8 5.0 x 2.69 36.86 4.0 x 10-4 9 4.25 32.02 1 0 3.5 x 1 0 - ~ 8.83 20.62 11 2.0 x 10-4
amt of Na,S,O,, g/mL found calcd 2.494 X 1.975 X 1.482 X 9.999 x 10-4 4.905 X lom4 2.435 X 1.780 X l o w 4 1.209 X 9.482 X 8.236 X 5.304 X
Table 11. Summary of the Results for the Determination of Elemental Sulfur in Synthetic Standard % error samwt of S absolute in S ple wt of pure S, no. g recovered, g error recovered 1 2 3 4 5 6 7 8 9 10 11 12 13 14
2.16 X 2.74 x l o - ) 4.02 x 10-3 4.21 X 8.53 x 10-3 9.21 X 1.205 X lo-’ 1.234 X lo-’ 1.303 X l o - * 1.381 X 10‘’ 1.421 X 1.568 X l o - ’ 1.960 X lo-’ 5.135 X lo-’
2.14 X l o p 3 2.76 x 3.97 x 10-3 4.27 X 8-72 x 10-3 9.37 X lo-’ 1.206 X lo-’ 1.198 X 10” 1.312 X 1.342 X lo-’ 1.378 X lo-’ 1.551 X lo-’ 1.955 X lo-’ 5.245 X
-2.32 X +1.56 x -5.41 x 10-5 t5.90 X t i . 9 0 x 10-4 +1.60 X +1.00 X -3.53 X +9.23 X -3.83 X -4.38 X -1.70 X -5.31 X +1.10 X
1.07 0.57 1.35 1.40 2.23 1.74 0.08
2.86 0.71 2.77 3.08 1.08 0.27 2.15
pH 7 as supporting electrolyte. The end point potential was set at the predetermined value of 8 mV and the polarizing potential was set at 150 mV. A time delay circuit was inserted into the instrument and was set at 5-5 intervals, to allow complete homogenization.
Sensitivity, Accuracy, and Precision of the Coulometric Measurements. Table I shows that the results were excellent for millicoulomb values down to 30 mC. The relative
2.482 X 1.985 X 1.489 X 9.928 x 1 0 - ~ 4.964 X 2.482 X 1.737 X 1.241 X 9.928 X l o - ’ 8.687 X 4.964 X
absolute error + i sx -9.45 x -7.19 X +7.18 x -5.87 X -4.72 X +4.36 X -3.15 X -4.46 X -4.50 X t3.40 X
10-5 lo-‘ lo-‘ 10”
lom6 lo-‘
lo-‘
%
error 0.49 0.47 0.48 0.72 1.18 1.90 2.51 2.53 4.49 5.18 6.85
wt of s, glmL 3.22 x 2.55 x 10-4 1.91 x 10-4 1.26 X 6.33 x 10-5 3.14 x 10-5 2.30 x 10-5 1.56 x i o + 1.22 x 10-5 1-06 x 10-5 6.85 X
standard deviation for the reading was less than 4.5% and the percentage error of the results was about 5% for a ming/ 1mL of sample injection. imum amount of sulfur of 1x For reading below 30 mC the relative standard deviation became higher and the percentage error was 6.8%. At these values the amount of sulfur determined was 6.85 X lo4 g/mL. Table I1 shows that numerous conversions of elemental sulfur in hexane were carried out successfully and the free sulfur recovered was in good agreement with the theoretical g to 5.135 amount. The sulfur used ranged from 2.16 X X g represented by 8.64 X lo* g to 2.05 X lo4 g of elemental sulfur/l mL of sample in 100 mL of supporting electrolyte. The results show a percentage error ranging from 0.27% to 2.77%. The relative standard deviation did not exceed 5%. Registry No. S, 7704-34-9.
LITERATURE CITED (1)
Drushel, H. V.; Miller, J. F.; Hubis, W.; Clark, R. 0. Anal. Chlm. Acta
1956, 75, 394. (2) Kashikl, M.; Ishida, K. Bull. Chem. SOC.Jpn. 1967, 4 0 , 97. Chem. Abstr. 1967,66, 78007g. (3) Agrawal, E. E.; Rohatgl, H. S.;Guiati, I. E. J . Inst. Pet. 1973, 5 9 , 133. (4) Ahmad, L. A.; Abdou, I. K.; Mahmoud, 6. H. J . Prakt. Chem. 1966, 38(1-2),1-8. Chem. Abstr. 1968, 6 9 , 1 1 8 9 8 ~ .
RECEIVED for review July 22,1983. Accepted January 30,1984.
CORRESPONDENCE Second Derivative Solid Surface Luminescence Analysis of Two-Component Liquid Chromatography Fractions Sir: The use of conventional luminescence techniques for qualitative analysis of mixtures, where the components have similar spectral characteristics, can be difficult because the spectra usually have broad bands. Second derivative spectroscopy is a good way of determining the number of peaks and their positions in a spectrum because it can narrow bandwidths and increase the relative contributions of minor bands (1-4). The appearance of more than one component in a liquid chromatographic band can pose several problems when attempting to identify the components. In this paper, a simple 0003-2700/84/0356-1183$01.50/0
luminescence method is described in which second derivative solid surface phosphorescence and fluorescencewere used for the qualitative analysis of two components in high-performance liquid chromatography (HPLC) fractions. The method was applicable to solvents from both normal and reversedphase HPLC systems. In addition, the approach demonstrates how fluorescence and phosphorescence can be combined at room temperature for qualitative analysis.
EXPERIMENTAL SECTION Chromatography. The liquid chromatograph used was a 0 1984 American Chemical Society
