5-substituted 6-thiopyrimidines. They wish also to thank Juliet Backshall and W, J. Watts for helpful technical assistance in the polarography. Grateful acknowledgment is made to the British Empire Cancer Campaign for financial assistance to N. G. Luthy in this work. LITERATURE CITED
( 1 ) Barker, G. R., Luthy, N. G., Chemistry and Industry 1955,983.
“Polarography,” 2nd ed., Vol. 11, Interscience, Kew York, 1952. (10) Maggiolo, A , Hitchings, G . H.,
(2) Barker, G . R., Luthy, N. G., J. Chem. SOC.1956, 917. (3) Barker, G. R., Luthy, N. G., Dhar, M. M . , Ibid., 1954,4206. ( 4 ) Buttner, Ber. 36, 2234 (1903). (5) Cavalieri, L. F., Lowy, B. A., Arch. Biochem. Biophys. 35, 83 (1952). (6) Elion, G. B., Ide, W. S., Hitchings, G. H., J . Am. Chem. SOC.68, 2137 (1946); 69,2138 (1947). (7) Heath, J. C., Nature 158,23 (1946). (8) Xarchmer, J. H., Walker, M. T., ANAL.CHEM.26, 271 (1954). (9) Kolthoff, I. M., Lingane, J. J.,
J . Am. Chem. SOC.73,4226 (1951). (11) Siggia, S., “Quantitative Organic
Analysis Via Functional Groups,’’ Wiley, Kew York, 1949. (12) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” p. 257, Van Nostrand, Iiew York, 1937.
RECEIVED for review September 17, 1956. Accepted May 14, 1957.
Anodic Polarographic Estimation of Aliphatic Sulfides in Petroleum HARRY V. DRUSHEL and JAMES F. MILLER Mellon Institute o f lndusfrial Research, Pittsburgh, f a .
b An anodic polarographic procedure is described for the estimation of aliphatic sulfides in petroleum. Stationary platinum wire electrodes are used following a special alternating current treatment prior to polarization. A solvent-electrolyte containing nitrobenzene is used, which readily dissolves petroleum samples and has a relatively high electrical conductivity. A number of pure sulfides were studied by this procedure and the effect of molecular weight upon the apparent rate of diffusion was evaluated. By fractionating the crude oil or petroleum sample into fractions of narrow molecular weight range b y molecular distillation, it was possible to use this information to estimate aliphatic SUIfides. Cyclic and noncyclic alkyl sulfides and alkyl aryl sulfides are determined.
Preliminary separation of petroleum samples by molecular distillation yielded distillates having narrow molecular weight ranges. Information from the anodic polarograms of these fractions and a knowledge of the effect of molecular weight upon the average diffusion coefficients of aliphatic sulfides made possible the estimation of aliphatic sulfide sulfur.
necessary to find a solvent-electrolyte system having a relatively high specific conductivity and good solvent action for crude oil. A 70:30 (by volume) mixture of nitrobenzene and methanol containing hydrochloric acid was found satisfactory (4). The cell resistance was further reduced by using a simplified reference electrode. A silver electrode of special shape coated with silver chloride was introduced directly into the chloride supporting electrolyte near the anode (6, 9). With this arrangement cell resistances between 150 and 350 ohms were obtained.
EXPERIMENTAL
Experimental details have been described (4). The silver-silver chloride reference electrode was formed into a helical coil about 10 mm. in diameter
Residual Current
Curve I Curve 2 Curve 3
I
studies of the polarography of organic sulfur compounds Nicholson (8) solved the fundamental differential equation for cylindrical diffusion and tested the resulting equations for the polarographic current with several aliphatic sulfides. Nicholson ( 7 ) also observed that the -iimax./cvalues varied with concentration and attributed this to a n ohmic potential effect. She found t h a t correction for this effect by the approximate equation of Delahay ( 2 ) or by using iR drop compensators ( 7 ) was adequate. I n order to apply this technique to crude oils, i t was necessary to add appropriate solvents such as benzene t o the electrolyte. When this was done, however, the Delahay correction was too large. The polarograph used in the present work was not equipped with a n iR drop compensator; therefore, it was N HER
1456
ANALYTICAL CHEMISTRY
YI
f 0
e
.-
-.-I I
1.00
I
I
I
I
I
E (Volts
I
I
I
0.60
0.80 VS.
I
I
I
C 0
Ag-MCI)
Figure 1. Successive anodic polarograms of thiacyclopentane
0.384 millimole per liter; platinum electrode No. 4
used
and 10 mm. in length. The platinum wire anode was then positioned concentrically inside this reference electrode during use.
Figure 1 by the peaks a t f0.84 volt for thiacyclopentane, show a pronounced dependence upon concentration or peak current (Figure 2 ) even after being corrected for t h e ohmic potential in t h e usual manner. For the purpose of comparing peak potentials, it is consequently preferable t o extrapolate t o infinite dilution (or zero current). The peak potentials, extrapolated t o infinite dilution, of a number of representative alkyl sulfides and alkyl aryl sulfides are listed in a previous publication (4). Even though a wide variety of aliphatic sulfides are represented, the
DISCUSSION OF METHOD
Anodic polarograms of thiacyclopentane, a typical aliphatic sulfide, are arranged in Figure 1. The described alternating current electrode treatment was applied prior to the recording of each curve. Peak currents from these and similar polarograms are reproducible to within 1%. P e a k Potentials, The peak potentials (Emax,),as exemplified in
o9a -
over-all range of peak potentials is only 0.10 volt, a requisite for analytical applications. Effect of Concentration upon P e a k Current. Nicholson (8) observed that t h e peak current divided by t h e concentration decreased with increasing concentration. Later she showed t h a t this was caused, a t least in part, by the influence of the ohmic potential and could be essentially eliminated through the use of a n iR drop compensator on the polarograph ( 7 ) . As the polarograph used for this study was not equipped with such a compensator, it was necessary to operate a t low cell resistances and correct for the effect of the ohmic potential a t increasing concentrations by extrapolation to infinite dilution or by application of the approximate equation of Delahay ( 2 ) . The value of AE, the potential interval corresponding to the ascending portion of the wave, was taken as 0.15 volt. TO make the correction, the measured peak current was multiplied by the correction factor (1 Ri,/AE)1'2 corresponding to the particular ohmic potential, Rip, involved. Effect of Temperature. From anodic polarograms of the same thiacyclopentane solution a t 20.0°, 25.0°, and 30.0" C., the corresponding peak currents (using platinum wire electrode No. 4) were -7.71 f 0.05, -8.57 =k 0.02, and -8.74 =t 0.06 *a. This represents an increase of about 1.5% per degree over the temperature range studied. Behavior of Other Sulfur Compounds. Before undertaking a discussion of the quantitative aspects of this study, it is desirable t o summarize the anodic behavior of other compounds a t the platinum microelectrode. Of the sulfur compounds studied, only the aliphatic or alkyl aryl sulfides produce well-defined waves in the solvent-electrolyte used. Aromatic sulfides, condensed thicphenes, most aliphatic and aromatic disulfides and polysulfides, and most aromatic hydrocarbons are oxidized a t potentials near or greater than the decomposition potential of the solventelectrolyte (4). Hydrogen sulfide and high molecular weight thiols do not exhibit waves but produce currents which are slightly higher than the residual current. Thiols below a molecular weight of about 150 produce waves near +0.5 volt (see Figure 3). As thiols and disulfides have not been reported to be present to any great extent in crude oil, very little interference from such compounds is expected in the analysis of crude oils or fractions in which no thermal degradation has taken place. Only in the case of heavy, dark, aromatic fractions is oxidation of polynuclear aromatic hydrocarbons likely to cause error.
+
o 2,6-Dimethyl-4-Thioheptone
5-Thiononone 0 2-Thiobutone T
b/
3-Methyl-2-Thiobutone
I
o 3-Thiobentone
0.84
I
I
I
20
0
I
I
40
- im(
I
100
80
60 M icroo rnperes)
Effect of peak current upon peak potential
Figure 2.
iR correction applied to Emax.
Residuol Current I-Oodecomthiol
I
I
1.00
Figure 3.
1-Dodecanethiol 1-Butanethiol
I
I I I 0.80 E (Volts vs. As-AgCl) I
I
(
I
I
/
0.60
Anodic polarograms of thiols
Concn. (Mmoles/Liter) 4.58 6.18
VOL. 29, NO. 10, OCTOBER 1957
0
1457
Figure 4.
Variation of --imsx./c values with molecular weight of some sulfides
1. 2-Thiabutane (API) 2. Thiacvclooentane (API) 3. 3-Thiipeitane (API) ’ 4. 2-Thiapentane (API) 5. 3-Methyl-2-thiabutane (API) 6. Thiacydohexane (API). 7. Thiacyclobutane (API) 8. 1-(p-Methylpheny1)-1-thiaethane (Eastman) 9. 5-Thianonane (Eastman)
Platinum electrode No. 2 used 10. 2,6-Dimethyl-4thiaheptane(Eastman) 11. 2,2,4,4Tetramethyl-3-thiapentane (Eastman) 12. 1-(p-Methylpheny1)-1-thiapropane(Eastman) 13. 2,8-Dimethyl-5-thianonane(Eastman) 14. 6-Thiaundecane (Eastman) 15. 7-Thiatridecane (Eastman) 16. 1,3-Diphenyl-2-thiapropane(Eastman) 17. 8-Thiapentadecane (Eastman) 18. 9-Thiaheptadecane (API)
Figure 5.
Estimated average effect of molecular weight upon apparent diffusion coefficients of dialkyl and alkyl aryl sulfides
1458
ANALYTICAL CHEMISTRY
Number of Electrons. The electrode reaction .is assumed to be the oxidation of the sulfide to the sulfoxide which would require 2 electrons. The number of electrons transferred has been determined by coulometry a t controlled potentials using a modified Lingane potentiostat and current recording equipment. The solventelectrolyte system described for the polarographic procedure was used for these experiments. Using such sulfides as 5-thianonane, 2,6-dimethyl-4-thiaheptane, 7-thiatridecaneJ 1,3-diphenyl2-thiapropane, and 1-(pmethylpheny1)1-thiaethane, n-values of 1.7 to 2.2 electrons were obtained. Similar results rrere obtained by Kicholson (,?) for 3-thiapentane. Infrared spectra of some of these products of electrochemical oxidation showed that the principal product was the sulfoxide with only a few per cent of sulfone impurity. As the number of electrons transferred is independent of the molecular weight, it was conceived that coulometry a t controlled potential could be used for the determination of aliphatic sulfides. Such a method is currently under study. Convection voltammetry has also been considered but the narrow usable voltage range makes it impractical. Equations (Calibration of Electrodes). I n order t o describe the quantitative aspects of this study, it is necessary to present some basic equations. By neglecting a rather small deviation from linearity of a function involved in the equation for the current with To, D, n, and a, the following equation for the peak current a t cylindrical electrodes is obtained:
1 and 2 for reversible processes are used to calculate apparent diffusion coefficients, D , for the sulfides. Solutions of potassium ferricyanide in 0.2M aqueous potassium chloride were used to check the measured area of a number of platinum wire and platinum sheet electrodes. Both sides of the platinum sheet electrodes were exposed and edge effects were neglected. It mas observed that the peak currents decreased upon successive polarization, but this decrease could be eliminated by treating the electrode for several minutes with hot concentrated nitric acid after each polarization. The value of D used was that given by Kolthoff and Lingane (6) (0.89 X sq. cm. per second). Because of the low cell resistances no
Table I.
correction for ohmic potential was necessary. Results are given in Table I for several different electrodes. The differences between the measured and calculated areas are within the experimental errors involved in the measurements. The pronounced decrease in the peak current upon repeated polarization is observed for electrodes 8 and 9, which were not cleaned with nitric acid between runs. Evaluation of Apparent Sulfide Diffusion Coefficients. The apparent diffusion coefficient for thiacyclopentane has been determined using the equation for both cylindrical and linear diffusion. Equation 1 for cylindrical diffusion a t 25.0" C. and a
Standardization of Platinum Electrodes with Potassium Ferricyanide in 0.2M Potassium Chloride
Concentration Platiniim Electrode Number 2 (Wire)
rat Cm. 0.0314
5 (Wire)
0.0311
10 (Wire)
0.0203
11 (Wire)
0.0583
8 (Sheet)
...
9 (Sheet)
...
=
1.00 millimole per liter. Area, Sq. Cm.
E,,,., Volt 09. Ag-AgC1 0.12 0.12 0.12 0.12 0.11 0.11 0.12 0.12 0.11 0.11 0.12 0.12 0.12 0.11 0.11 0.11
pa.
9.36 9.16 9.81 9.55 10.16 10.14 5.09
5.01
13.18
13.18 10.67 9.55 9.47 30.45 28.25 27.35
From equation 0.179 0.175 0.187 0.182 0.194 0.194 0.092
0,090 0,267 0.267 0.234 0,210b 0.2056 0.669 0 620b 0 . ti03h
Measured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
181 181
181 181 202 202 096 096 271 271 234 234 234 704 704 704
Corrected for residual current,. Run without recleaning in nitric acid.
[;P 0235 ( E )'/' + 0.3061
(1)
where n, F , R, and T retain their usual electrochemical significance, A i$ the area of the electrode, D is the diffusion coefficient, C" is the bulk concentration of the electroactive species in the solution, a! is the polarization rate, and TO is the radius of the cylindrical (platinum wire) electrode. I n the limiting case, as ro +m , the equation for linear diffusion results.
For anodic Iyaves the current, i,,,., is preceded by a minus sign. These equations are derived for reversible electrode processes. There is evidence that the sulfide oxidation is irreversible ( 5 ) (from the shape of the waves and the absence of a nearby sulfoxide reduction wave). Because an analytical expression for the polarographic wave of a n irreversible process a t a cylindrical electrode would be hopelessly complicated, Equations
Table II. Typical Data Taken from Anodic Polarograms of Thiacyclopentane for Evaluation of D
Measured Platinum Elec- Area, Sq. trode Number Cm. 4 (Wire, TO = 0.138 0.0314 cm.)
Concn., Mmoles/ Liter 3.216 3.216 0.4288 0.4288 5 (Wire, TO = 0,202 2.144 0.0311 cm.) 2.144 0.4288 0.4288 8 (sheet) 0.234 3.216 3.216 0.4288 0,4288 10 (Wire, ra = 0.0961 0.425 0.0203 cm.) 0.425 11 (Wire, TO = 0,271 0.425 0.0583 cm.) 0.425 0.425 a Corrected for residual current.
--iomx., Pa.
61.8 61.3 8.90 8.74 59.9 60.5 12.86 12.88 91.0 91.4 13.56 13.76 6.62 6.56 15.63 15.97 16.24
Cell --kSx.Resist(Corr.), ance, pa. Ohms 66.6 380 65.8 380 8.99 350 8.83 350 63.4 300 64.0 300 13.03 300 13.05 300 102.1 430 102.6 430 13.82 430 14.02 430 6.65 200 6.59 200 15.78 140 16.10 140 16.39 140
VOL. 29, NO. 10, OCTOBER 1957
D,Sq.
Cm./Sec. X 108 9.36 9.55 9.09 9.49 .
9.24 9.61 9.98 8.76
1459
-
C I
I
Fraction 4.
Fraction 3.
.- -301
i-
-40
ol - - -
I
Figure 6.
.oo
polarization rate of 0.00333 volt per second reduces t o : -imax,/c = (14.58 X lo4) ( A ) ( I N 2 )
(3)
Values of -iimax./c, corrected by Delahay's approximate equation, A , and ro mere substituted and the equation was solved for Dl12 by the quadratic formula. Equation 2 for linear diffusion at 25.0' C. reduces to the simple relationship --irnax./~ = (4.48
x 104) ~ 0 1 ' 2
(4)
which is easily solved for D112and hence, also for D. I n these cases and in subsequent discussion, D represents the apparent rather than the true diffusion coefficient. Table I1 presents typical data obtained for the evaluation of D for thiacyclopentane with several different electrodes. The average value of D for thiacyclopentane from data obtained with all of the electrodes was 9.0 X 10-6 sq. cm. per second for the more concentrated solutions and 9.4 X 10-6 sq. em. per second for the dilute solutions. The low results calculated from the data of more concentrated solutions indicate that the approximate correction of Delahay does not account for all of the variation of -imax./c with concentration. These values of D above may be used for the secondary standardization of 1460
ANALYTICAL CHEMISTRY
I-
Recorded Polarogram Residual Current
----.Corrected for I
.oo
1.00
I
I
I
1
0.80
I
0.60
E (Volts vs Ag - A g C l )
Sample Concn., G./Liter 2,243 2.316 2.449 2.461
1
+ 0.306)
-40
Typical anodic polarograms of a series of fractions from Sample A
2 3 4
D1'2
I
I
Fraction
('T
i-
-80
0.80
1.00
F r a c t i o n I.
-30t/
t
P
-40
Fraction 2 .
-601
-30/
t
t
Platinum Electrode No. 4 5
4 4
platinum electrodes with thiacyclopentane in other laboratories where the average D us. molecular weight data described below are used in the analysis of samples. However, the measured dimensions of the electrode will usually suffice. Variation of Apparent Diffusion Coefficient with Molecular Weight. I n order t o analyze a sample of known average molecular weight, i t was necessary t o determine t h e effect of molecular weight upon t h e apparent diffusion coefficient. This was done by finding the average value of -imax,/c
over several different concentrations for each sulfide a t only one platinum wire electrode. As no correction for ohmic potential was made in the case of the samples, no correction was made in calculating the average value of -imax/c for each of the sulfides studied. A plot of the -i,,, /c values vs. molecular weight for a single electrode is shown in Figure 4. The curve was drawn to represent the estimated average effect of molecular weight. From this curve and the measured dimensions of the electrode, the corresponding relationship of D us. molecular weight
Results of Analysis of Sample LS-2-2, Fraction 3 with a Number of Different Cylindrical Electrodes (Molecular weight = 259. Concentration of sample = 1.750 grams/liter)
Table 111.
Platinum Cell Electrode Resistance, Number Ohms 2 150 2 150 4 180 4 180 5 150 10 180 10 180 11 150 11 150
0
*
( -imsx./C)std..
Earnax.,
-i*nlax.,
pa.Liters/Mmole 13.67 13.67 10,43 10 43 15.23 7~45 7.45 20,02 20.02
yt. %.
Aliphatic pa. Sulfide Sulfur 2.05 15.30 1.93 14.40 1.00 10.84 1.90 1.77 10.10 1.00 * 1.88 1.00 15.66 1.84 7.50 0.99 1.75 7.11 0.99 1.78 19.45 1.00 1.61 1.00 17.60 Av. 1.83 i 0.09 For comparison, iodine complex method yielded 1.26 S= 0.00 Corrected for ohmic otential. Corrected for residuz current. Volts us. Ag-AgC1 1.01 1.01
was derived (Figure 5 ) . The curve in Figure 5 then supplies the basic information for the estimation of (-?kax./C)std from Equation 3 for any cylindrical electrode of known dimensions. Calculations. T h e aliphatic sulfide sulfur content of a sample was calculated by means of t h e following equation: Weight
aliphatic sulfide sulfur = ( - i m s x , ) (0.032) (100) ( -imax./C)std.
(c)
Table IV.
Comparison of Results from Anodic Polarographic and Iodine Complex Methods
Weight
Sample Fraction 1 Fraction 2 Fraction 3 Fraction 4 Residue Fract,ion 1 Fraction 2 Fraction 3 Fraction 4 Residue
A
(5)
B
where -Lax. is the peak current observed after subtraction of the residual current, c is the concentration of the sample in the solvent-electrolyte in is a grams per liter, and (-&x./C)std constant (described above) derived from to molecular the relationship of -&./c weight and t o the dimensions of the electrode under the conditions described for the procedure.
70Aliphatic Sulfide
Sulfur Anodic Sulfur Weight“ polarography* 6.42 190 2.79 3~ 0.10 7.67 22 1 2.97 i 0.09 7.10 283 2.60 i 0.06 5.59 332 2.08 zt 0.14 Insoluble in solvent-electrolyte 5.43 185 1.51 i 0.09 6.45 245 1.73 zt 0.12 6.04 330 1.74 zt 0.07 5.23 400 1.14 i 0.06 Insoluble in solvent-electrolyte
yo Total Molecular
Fraction 1 3.94 217 1.54 i 0.02 Fraction 2 5.34 299 1.73 =t0.16 Fraction 3 5.79 367 1.90 It 0.08 Fraction 4 5.43 457 1 . 4 2 =!= 0.02 Residue Only partially soluble in solvent-electrolyte Determined by ebullioscopic procedure using benzene (1). Average of values from several electrodes. C
a
*
Iodine complex 3.10 3.05 2.71 2.15 1.71 1.87 1.65 1.43 0.91 1.16 1.36 1.49
RESULTS OF ANALYSIS OF SAMPLES
complex method (3) in Table IV. The precision, using different electrodes, was of the order of 5%. The precision for the same electrode, however, was of the order of 2 or 3% for a petroleum sample (Figure 7). Peak currents for a pure sulfide were reproducible to within 1%. The results in Table IV obtained by the two methods are remarkably parallel. I n the A and B series of fractions the values agree in most cases to within 5%. Because the C series of fractions is more aromatic, the slightly higher results by the anodic polarographic
A number of molecular distillates from some sulfur concentrates were analyzed for aliphatic sulfides (including alkyl aryl sulfides) by the proposed procedure. Several representative polarograms are shown in Figure 6. Definite maxima were observed, especially for fractions 1 and 2 which have rather low molecular weights. Some typical results of the analysis of one sample with a number of different electrodes are shown in Table 111. The results of the analysis of a series of samples by this method are compared with the results obtained by the iodine
Residual Current For Curve I
method might be interpreted as indicating the presenceof mixed sulfides such as alkyl aryl and dibenzyl sulfides which are not completely included in the iodine complex method or the presence of certain easily oxidized polynuclear aromatic hydrocarbons. I n fact, anodic polarograms of asphaltic fractions from crude oil suggest the presence of easily oxidized interfering material. ACKNOWLEDGMENT
The authors wish to thank the Multiple Fellowship on Petroleum sustained at Rlellon Institute by the Gulf Research and Development Co., which supported this work, for permission to publish this material. They also wish to thank R. T. Wendland who supplied the samples and molecular weight data, and Dorothy Taylor who analyzed the samples by the spectrophotometric iodine complex method.
I
LITERATURE CITED
I
I.oo
I
I
I
I
(
,
0.80 E (Volts
vS.
I
I
,
,
0.60
Ag-AgCI 1
Figure 7. Successive anodic polarograms of Sample LS-2-2,Fraction 3
1.750 grams per liter; platinum electrode No. 11 used
(1) Barr, TI7. E., Anhorn, V. J., Instruments 20, 342 (1947). (2) Delahay, P., “New Instrumental Methods in Electrochemistry,” 132-5, Interscience, New Yo%; 1954. Drushel, H. V., Miller, J. F., ASAL. CHEM.27, 495 (1955). Drushel, H. V., Miller, J. F., Anal. Chim. Acta 15, 389 (1956). Kolthoff, I. 11.. Linerane. J. J.. “Polarography,’” V o r 1,’ p. 52; Interscience. S e w Pork. 1952. Ibad., p. 359. Nicholson, hI. M., i l x . 4 ~ . CHEM. 27,1364 (1955). Nicholson, 34. bl., J. Am. Chem. SOC. 76, 2539 (1954) Tomassi, W., Jodzewicz, W.,Sojecki, W., Horoszewicz, M., PrzemysE. Chem. 9,560 (1953). RECEIVED for review November 29, 1956. Accepted May 22, 1957. VOL. 29,
NO. 10,
OCTOBER 1957
1461
