Anal. Chem. 1989,61, 1502-1504
1502 (13) (14)
Bruckenstein, S.: Miller, B. Acc. Chem. Res. 1977, 10, 54. Rosamilla, J. M.; Miller, 6.; Schneemeyer, L. F.; Waszczak. J. V.; 0’-
Bryan, H. M:, Jr. J . Electrochem. SOC. 1987, 134, 1863. (15) Miller, B. J . Necfrochem. SOC. 1969, 7 16, 11 17. (16) Bruckenstein. S.; Feldman, G. A. J . Electrochem. SOC.1965, 9 . 395. (17) Tlndall, G. W.: Cadle, S. H.; Bruckenstein, S. J . Am. Chem. SOC.
1969, 9 1 , 2119. (18) Shafer, M. W.; de Grott, R . A.; Plechaty, M. M.; Scilla, G. J. Physica 1988, 153-155, 836.
C
RECEIVEDfor review January 6, 1989. Accepted April 5 , 1989.
Determination of Conjugated Dienes in Gasoline by Differential Pulse Polarography Stephen J. Swarin* Analytical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090-9055
Kevin L. Perry Fuels a n d Lubricants Department, General Motors Research Laboratories, Warren, Michigan 48090-9055
A rapid, repeatable technique for the determination of conjugated dienes in gasoline has been developed. The method, which is based on the reduction of conjugated dienes at a droppingmercury electrode, is faster (15 min vs 100 min) and more precise (5.3% vs 10% relative standard deviation) than the maleic anhydride method currently used. I t is applicable to fuels containing alcohols, which cannot be analyzed by the current method. Application of this method to 25 model compounds and 37 gasolines of varylng conjugated diene contents demonstrates that it Is the method of choice for this determination.
INTRO DUCT1 0N During the past several years, some cars with multiport fuel injection systems have had driveability problems caused by deposits t h a t form at the tip of the fuel injector. These deposits were found to be fuel dependent, and extensive use was made of a wide variety of analytical techniques for the characterization of “deposit-forming’’ and “non-depositforming” fuels ( I , 2). One fuel characteristic that showed correlation with the deposit-forming tendency of base fuels (without detergent additives) was the total conjugated diene content ( 3 ) ,but further investigations were hampered by the poor repeatability and long analysis time of the analytical method used to obtain the conjugated diene content-the maleic anhydride (MA) method (4). High-resolution capillary column gas chromatography was considered for this analysis ( 5 ) but, at the current state of development, analysis times are long and resolution is insufficient for the unambiguous analysis of all t h e possible conjugated diene isomers. Therefore, a method t h a t quickly determined the total conjugated diene content of gasoline was still needed. In a recent publication, Polak e t al. (6) described a polarographic method t~ determine the conjugated diene content of “pyrolyzed” gasoline and distillation and hydrogenation refinery fractions. Although the samples analyzed in t h a t report were not numerous (four), not commercial gasolines, and of high conjugated diene content, it was decided to try to extend this technique, which is based on the well-known electrochemical reduction of conjugated dienes (7), t o commercial gasolines.
Table I. Experimental Parameters for Determination of Conjugated Dienes in Gasoline Mode: Differential Pulse Polarography -1.500 V replications: 1 -2.800 V standard curve blank subtraction: yes drop time 0.5 s tangent fit: n o scan increment 2 ml’ peak location: yes pulse height 0.050 Y derivative: n o initial E final E
EXPERIMENTAL SECTION Chemicals. Diene compounds used in this work were the best available grades from Pfaltz and Bauer (Stamford, CT) or Chem Service, Inc. (West Chester, PA). Gasoline samples were obtained from retail service stations throughout the United States and were stored at 0 “C. Dimethylformamide (DMF) was Baker Photrex grade (Phillipsburg, KJ)or Aldrich HPLC grade (Milwaukee, \VI), used as received. All other chemicals were ACS reagent grade. The supporting electrolyte was 0.02 M tetrabutylammonium iodide in DMF. Apparatus. An EG&G Princeton Applied Research (Princeton, NJ) Model 384 polarographic analyzer, Model 303A mercury drop electrode, and Model RE 0082 digital plotter were used. The experimental parameters used in this method are shown in Table I. Measurements were performed with a dropping mercury electrode (0.08 mm id.), a platinum counter electrode, and a nonaqueous reference electrode (silver wire) filled with 20’3 LiCl/saturated AgCl in methanol. Procedure. Ten milliliters of supporting electrolyte was pipetted into the Model 303A cell and the system was purged with argon for 300 s. A blank run was performed by using the experimental parameters in Table I. This run was stored in the memory of the Model 384 polarographic analyzer. Then a gasoline sample (100 pL) or standard solution (50-200 p L ) was injected into the supporting electrolyte with stirring. The solution was purged for 10 s, and a sample run was performed using the experimental parameters in Table I. The data were then printed on the digital plotter with the blank run subtracted from the sample run. Reference Method. The diene values of the gasoline samples were determined by the maleic anhydride (MA) method ( 4 ) . The diene value is defined by the MA method as the number of grams of iodine equivalent to the amount of maleic anhydride that reacts with 100 g of sample. RESULTS AND DISCUSSION Electrochemical Reduction of Conjugated Dienes. Conjugated dienes undergo a two-electron reduction at a
0003-2700/89/0361-1502$01.50/0‘C 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 14, JULY 15, 1989
1503
Table 11. Peak Currents for Dienes at -2.7 Va conjugated dienes
current,b nA/rg
1,3-butadiene, 2,3-dimethyl1,3-butadiene, 2-methyl1,2-butadiene, 3-methyP 1,3-pentadiene IJ-pentadiene, 3-methyl1,3-pentadiene, 2,4-dimethyl1,3-hexadiene 2,4-hexadiene 2,4-hexadiene, 2,5-dimethyl1,3-octadiene 1,3,7-octatriene
2.04 3.52 2.84 1.60 3.60 3.81 1.86 3.11 0.40 3.17 1.10
average
2.66
isolated dienes
current, nA/rg
1,4-pentadiene l,l-pentadiene, 2-methyl1,4-pentadiene, 3-methyll,4-hexadiene 1,5-hexadiene 1,5-hexadiene, 2-methyl1,5-hexadiene, 3-methyl1,5-hexadi,ene,2,5-dimethyl1,5-heptadiene 1,6-heptadiene l,4-octadiene 1,7-octadiene 1,6-octadiene, 5,7-dimethyl1,9-decadiene average
0.09 0.31 0.37 0.20 0.05 0.24 0.38 0.42 0.29 0.34 0.34 0.16 0.24 0.30 0.27
1,2-Butadiene, 3-methyl, a Vs nonaqueous Ag electrode. 250-pg sample in 10 mL of electrolyte. * nA/pg = nanoamperes per microgram. is a cumulated diene-the two double bonds are on the same central carbon atom.
8.0
6.0
I
c
i
i
3.0
1
A
IV
'T
I d / \L.-d / I
2.0
U
00
I
-1.6
.1 8
.2 0 -2 2 -24 Electrode Potential, Volts
-2 6
-2.8
Figure 1. DPP curves for (A) supporting electrolyte and (B) 250 Mg of 2,4-hexadiene in supporting electrolyte. Curve B has been blank corrected.
dropping mercury electrode in the presence of a proton donor according t o eq 1. Isolated double bonds, such as those in monoolefins (i.e., RCH=CHR) and nonconjugated dienes (i.e., RCH=CHCH2CH2CH=CHR) do not react electrochemically.
RlCH=CHCH=CHR2
+ 2e- + 2H+
-
RICH2CH2CH=CHR2 (1) R1 and R2 m a y be the same and m a y be H For analytical purposes, the current generated at the electrode by this electron transfer and measured by the polarographic analyzer is directly related t~ the conjugated diene content of a sample. Figure 1 is a differential pulse polarography (DPP) curve for a typical conjugated diene, 2,4hexadiene. Also shown is the DPP curve for the blank (supporting electrolyte), which has been subtracted from the curve for 2,4-hexadiene. T h e DPP peak which occurs at approximately -2.7 V (vs the nonaqueous silver reference electrode) is due to the reduction of 2,4-hexadiene as shown in eq 1. T h e area or height of this peak is a measure of the amount of 2,Chexadiene in the supporting electrolyte solution. Table 11contains the DPP data for 25 dienes, 11 containing conjugated double bonds and 14 containing isolated double bonds. Allowing for the variable purity of the samples obtained, the conjugated dienes exhibit, on average, about 10 times the peak current of the isolated dienes at -2.7 V. One exception to this trend is 2,5-dimethyl-2,4-hexadiene, which
A
00
~
I
I
I
I
I
I
1504
ANALYTICAL CHEMISTRY, VOL. 61, NO. 14, JULY 15, 1989 35
30
,
, / 3 -
-
/;;
'
I
Table IV. Effect of Oxygenates on Conjugated Diene Measurements
sample description no. 30 no. 30 + 10% methanol no. 30 + 10% ethanol no. 30
+ 10% methyl tert-butyl ether
DPP currents,a diene value nA/mg 1.1 >3.0 >3.0
1.0
26.0 26.8 26.2 26.2
"Average of at least three determinations of peak current at -2.7 V. Currents are corrected for 10% dilution bv oxvnenates. 0
04
12
08
16
2
DIENE VALUE
Figure 3. Correlation between diene value, measured by the MA method, and the peak current at -2.7 V, measured by DPP.
Table 111. Repeatability of Peak Current Measurement sample
peak current"
SDb
RSD'
no. 4 no. 19 no. 29
3.99 15.72 20.19 27.41
0.20 0.97 0.38 2.06
5.4 6.2 1.9
no. 33
7.5
Average Relative Standard Deviation = 5.3%
"Average of six determinations of peak current at -2.7 V, nA/ mg. SD, standard deviation. RSD, relative standard deviation, percent. Therefore this D P P peak was further investigated. Figure 3 shows the diene values and D P P peak currents (at -2.7 V) for 37 gasolines which exhibited increasing fuel injector plugging tendencies with increasing sample number. The data correlate with a coefficient of determination (R2)of 0.947. Thus it appears that both techniques are measuring the conjugated diene content of the sample. A key factor in the D P P technique is the capability of the microprocessor-based analyzer to subtract the current due to the blank (supporting electrolyte) from that due to the fuel sample (Figure 1). This capability is reflected in the repeatability of the measurement of D P P current a t -2.7 V which was evaluated by performing six determinations on each of four gasoline samples (Table 111). The average repeatability was 5.3% (relative standard deviation) which is certainly acceptable for this type of measurement, and which is better than the reported repeatability of the MA method, which is the larger of 10% or 0.1 unit. Since it appears that the D P P current a t -2.7 V is a selective and sensitive measure of conjugated dienes in gasoline, it is possible to calculate the actual conjugated diene content of gasolines by using the average current value for known conjugated dienes from Table I1 and the D P P currents for 37 gasolines analyzed by DPP. The conjugated diene content calculated in this way ranges from 0.12 to 1.2% which is in agreement with approximate values determined by high-
resolution capillary column gas chromatography. The gas chromatography values are approximate because not all conjugated dienes are identified and quantified by this technique. A serious shortcoming of the MA method that the authors have noted is the dramatically high diene values obtained for fuel samples containing methanol or ethanol. This is caused by the esterification reaction between maleic anhydride and primary alcohols. However, the D P P method should not be affected by oxygenates in gasoline since they are not electrochemically active. Indeed, our results on fuels containing conjugated dienes that were spiked with methanol, ethanol, or methyl tert-butyl ether showed that the oxygenates had no effect on the D P P currents within the repeatability of the method (Table IV). Finally, the analysis time for a set of three samples using the MA method is about 8 h, of which about 4 h require direct operator involvement for adding reagents, extracting, and titrating. In contrast, the D P P method would require about 40 min (all operator time) to run a set of three samples.
CONCLUSIONS The electrochemical reduction of conjugated dienes in gasoline can be measured by DPP, and the currents obtained correlate with the diene value of the fuel as measured by the MA method. The DPP method is faster and more repeatable that the MA method, and it can provide reliable results on fuels containing primary alcohols which cannot be analyzed by the MA method. LITERATURE CITED Benson, J. D.; Yaccarino, P. A. SOC.Automot. Eng. Tech. Pap. Ser. No. 861533. Taniguchi, B.; Pryla, R.; Parsons, G.; Hockman, S.; Voss, D. SOC.Automot. Eng. Tech. Pap. Ser. No. 861534. Hilden, D. L. SOC.Automot. Eng. Tech. Pap. Ser. No. 881642. Diene Value by Maleic Acid Anhydride Addition Reaction, UOP Method 326-82;Universal Oil Products: Des Plaines. IL, 1982. Johansen, N. G.;Ettre, L. S.;Miller, R. R. J. Chromatogr. 1983, 256, 393-417. Polak, J.; Janacek, L.; Volke, J. Analyst 1986, 7 7 7 , 1207-1211. Zuman, P.; Kolthoff, I.M. Progress in Polarography; Interscience: New York, 1962; Vol. 11, Chapter XXXII. Young, D. C.; Vouros. P.: Decosta, B.; Holick, M. F. Anal. Chem. 1987, 5 9 , 1954-1957.
RECEIVED for review February 21, 1989. Accepted April 18, 1989.
