Anal. Chem. 1983, 55,516-522
516
chloromethane, 67-66-3; 2-iodopropane, 75-30-9; 2-methyl-4heptanone, 626-33-5;3-ethyl-1,2-dimethylbenzene, 933-98-2.
LITERATURE CITED (1) Hiatt, M. H. Anal. Chem. 1981, 53, 1541-1543. (2) Fed. Regist. 1979,44 (No. 233),69532-69552. (3) Easley, D. M.; Kleopfer, R. D.;Carasea, A. M. J . Assuc. Off. Anal. Chem. 1981, 64 (3),653-656. (4) Biazevich, J., US. Environmental Protectlon Agency, Region IO, Manchester, WA, personal comrnunlcation, 1980.
(5) "Handbook of Chemistry and Physics", 53rd ed.; CRC Press: Cleveland, OH, 1972;pp D147-DI48. (6) United States Envlronmental Protection Agency. "Sampling and Analysis Procedures for Screenlng of Fish for Priority Pollutants", U S . Environmental Monitoring and Support Laboratory-Cincinnati, Cincinnati, OH, Aug 1977.
RECEIVED for review May 19, 1982. Accepted November 1, 1982.
Production and Certification of Sulfur in Gas Oil Reference Materials Alan S. Lindsey and Peter J. Wagstaffe" Community Bureau of Reference, BCR, Commission of the European Communities, Rue de /a Loi 200, B- 1049 Brussels, Belgium
The development by the Communlty Bureau of Reference (BCR) of four gas oils certlfled for thelr sulfur content Is reported. The materials are shown to be stable wlth respect to time and temperature and to be of adequate homogenelty. The collaborative certificatlon campaign involving 14 European laboratories and five independent methods (Wickbold, bomb, and Schonlger flask combustion methods, X-ray fluorescence and inductlvely coupled plasma (ICP) emlsslon spectrometry) is described wlth a dlscussion of sources of errors In these procedures. Finally, the results of the measurements are presented and it ls shown that, after technlcal and statistical evaluatlon, sufficiently good agreement could be achieved to allow cerllflcation. RMs 104, 105, 106, and 107 were certifled as containing percentage mass fractions of sulfur of 0.091 f 0.002, 0.363 f 0.010, 0.502 f 0.008, and 1.040 f 0.015, respectively. The uncertaintles shown represent the 95% confidence limlts.
In the countries of the European Communities, the maximum permissible levels of sulfur in gas oils destined for combustion are fixed by an EEC Directive ( I ) , the aim of which is to reduce atmospheric sulfur pollution. These limits have given rise to a need for a series of reference gas oils containing determined amounts of sulfur which can be used in checking the analytical method applied and for calibrating the apparatus used in the analysis. The most suitable reference materials for this purpose were considered to be a series of gas oils spanning a range of sulfur content from 0.1 to 1.0 mass fraction % sulfur. Therefore, the Community Bureau of Reference of the Commission of the European Communities initiated in 1977 a project for the production and certification of petroleum oils containing sulfur at suitable levels. When the present work was started the only relevant reference materials available were two residual fuel oils from the National Bureau of Standards (NBS) containing approximately 0.2 and 1.0 mass fraction % sulfur. Recently the NBS has announced the availability of reference materials consisting of five residual fuel oils (nominal % S: 0.2,0.7, 1.0, 2.0,and 5.0)and one distillate (diesel) fuel oil (nominal % S: 0.1) (2).Likewise in 1979 the Service des Materiaux de Reference (SMR), Paris, reported the certification of four residual fuel oils containing sulfur a t nominal levels of 0.5,
1.0, 2.0, and 4.0 mass fraction % sulfur ( 3 ) . The materials prepared by the BCR and whose development is now reported are considered to be especially applicable for use as analytical reference materials for the accurate determination of the total sulfur content in gas oils regulated by the EEC Directive ( I ) . The certification program of four gas oils of nominal sulfur levels 0.1, 0.3, 0.5, and 1.0 mass fraction % proceeded in four main stages: (1) an initial 12 month stability study of the candidate materials stored in glass ampules sealed under nitrogen a t 25 "C and 40 "C; (2) main batch ampuling and homogeneity control; (3) a collaborative measurement excercise involving 14 laboratories and five independent methods (Wickbold, bomb, and Schoniger flask combustion methods, X-ray fluorescence, and inductively coupled plasma emission spectrometry) (A sixth method, combustion microcoulometry, which had been successfully applied to heavy fuel oils, was found to give results of very poor precision for these materials and was judged inapplicable because of the higher volatility of the gas oils.); (4) technical and statistical evaluation of the results and calculation of the certified values. EXPERIMENTAL SECTION Candidate Reference Materials. The four gas oils examined as potential reference materials for the analysis of sulfur in oil were straight run distillate products containing no unsaturates and had nominal sulfur contents of 0.1,0.3,0.5, and 1.0% by mass. The oils varied in color from water-white to deep straw, flowed readily, and had densities of less than 1.0 g/cm3 and approximate flashpoint values of 66 O C (150 O F ) . Stability Study of the Candidate Reference Materials. For the initial stability study 50 ampules of each batch of oil were filled. The ampules were manufactured from neutral amber glass and were sealed under nitrogen. Full details of the ampuling method and conditions have been given elsewhere ( 4 ) . Six ampules of each batch were stored in a constant temperature cabinet at 25 f 2 "C in the dark, and a further six ampules of each batch were stored at 40 f 2 "C in the dark. At the end of every second month one ampule from each batch of oil stored at the two temperatures was taken at random and analyzed for sulfur by the Wickbold method. Six determinations were made on the oil from each ampule. Main Batch Ampuling of Gas Oils and Homogeneity Checks. The homogeneity of the four gas oil samples was checked prior to ampuling and again during ampuling. The bulk homogeneity check was made by taking "dip" samples of the oil from the bulk containers at each of six levels, starting
0003-2700/83/0355-05 16$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMIISTRY, VOL. 55, NO. 3, MARCH 1983
517
Table I. Publishled Standard Procedures for Sulfur Determination method
ASTM
1. Wickbold
2. 3. 4. 5. a
XRF bomb Schbniger ICP
D 2622 D 129
CEN
IP
EN41
243
standards applied DIN EN41
other
NF
BS
T60 142 (Norme EN 41)
5379
T60 109
4454
51 450 (Blatt 3 ) 61 242
(5h ( 6 )
51 400 (Teil 3)
NEN 3276a (71, ( 8 )
Dutch Standard for Schoniger combustion method issued by the Nederlands Norrnalisatie Instituut.
from the bottom layer. The samples were then analyzed by the wavelength dispersive XRF method. Ampuling of thie gas oils in units of 25 g mass was carried out in a similar way to that employed for the stability trials samples, and as previously the air in the ampule was replaced by oxygen-free nitrogen before sealing. For the purpose of checking the maintenance of homogeneity during ampuling, six ampules were withdrawn from each batch (five from batch 104),taking the first and last and the intermediate ones randomly to cover operational discontinuities (e.g., refXlling of reservoir). The total of 23 withdrawn ampules were analyzed by the wavelength dispersive XRF method for determination of the sulfur contents. Design of Certification Exercise. The objective of the certification exercise was to establish the most accurate values for the sulfur concentration with an acceptably low uncertainty. Five well-established and independent methods, as indicated in Table I, were applied b y the participating laboratories in order to eliminate systematic method and individual laboratory errors. For each method that EL participating laboratory was to apply, two ampules were taken at random from the bulk stock of ampules of each RM (no. 1.04,105,1106,107). The certification requirement was that six measurements of the sulfur content should be made by each method nominated by the participating laboratory such that three measurements were made on the oil from each ampule but that all six measurements were not made within a single day.
Table 11. Summarized Results of Sulfur Determination of Stored S-in-Oil Candidate Reference Materials
candidate RM laboratory batch 104 batch 105 batch 106 batch 1 0 7
%
%
0.090 0.101
0.362 0.363 0.514 0.502 1.049
0.993
Uncertainty shown as
i
i t i i i i i i
0.002a 0.002 0.005 0.016 0.007
0.006 0.009 0.027
0.090
i
0.002
0.101
i i
0.003
0.365 0.005 0.362 i 0.014 0.515 ~t 0.008 0.501 i 0.005 1.047 i 0.010 0.986 i 0.019
standard deviation. -
Table 111. Bulk Hoimogeneity Check Mean Values of the Percentage Mass Fraction of Sulfur batch nu mean
RESULTS AND DISCUSSION Stability Studly. As the first stage in the production arid certification of suitable sulfur in oil reference materials, the stability and constancy of sulfur content of the four gas oils when stored in sealed glass ampules at temperatures of 25 "C and 40 "C over a period of 12 calendar months were studied. At the time of ampuling the sulfur contents of the gas oils were measured by the Wickbold method (BS5379: 1976 EN 42) to confirm that the levels were close to the nominal values required. It was envisaged that instability of the gas oils with respect to nonconstancy of the sulfur content might arise through precipitation of the sulfur component as, or occluded in, some solid phase (e.g., wax, polymeric product, oxygenated product, etc.), through adsorption on or chemical reaction with the glass walls of the ampule, or through conversion to some volatile component (e.g., H2S)which was lost when the ampule was opened. All the oils remained clear during storage and no sediment or precipitated solid waEi visible in the ampules when withdrawn for analysis. The mean results of the two monthly sulfur determinations of samples stored for the 112-month period of gas oils measured a t the two laboratories by the Wickbold method are shown in Table IT. Statistical analysis of the total results of the sulfur detesminations led to the conclusions that the measured values showed no significant changes with time and that the measurements a t the two temperatures studied showed no significant temperature effect. I t was noted that there appeared to be some systematic differences between the means of the two laboratories, but the,se do not influence the conclusions regarding stability From the results it followed that the gas
I I1 I I1 I I1 I I1
mean values or' sulfur determination stored at stored at 25 "C 40 "C S content, S content, mass fraction mass fraction
Sb
104 13 0.090 0.001
105 13
0.370 0.001
a n = number of mleasurements. tion.
106 12 0.511 0.002
107 12 1.069
0.003
s = standard devia-
Table IV. Homogeneity Check during Ampuling Mean Values of the Percentage Mass Fraction of Sulfur batch
104
na
10 0.085 0.001
mean Sb a
105 12 0.363 0.003
n = number of measurements.
106 12 0.512 0.001
107 12 1.075 0.002
s = standard devia-
tion. oils were of adequate stability to serve as reference materials. Homogeneity Checks. The results of the analyses of the bulk samples which CHONCC31
CP
I
,OOi
' f't-; 1
0 350
0 075r
I LAB
XSF
1
II 0100
ACI(@OLD/
cc
1 15891314
,
1 , , , , , I I57108 2 6
, , , I , I347
I I
I2
Flgure 1. Bar graphs showing mean measured values and 95% confidence limits for RM 104.
measurements by these methods were excluded from the final evaluation of the certified value. The ICP results exhibited an outlying mean, possibly arising from viscosity effects, and therefore were rejected. Measurements by the Wickbold and XRF methods provided by Laboratory 1 were rejected on the basis of their outlying variances and means which indicated a lower level of precision than expected for these methods. RM 105 and RM 106: Measurements by the bomb and Schoniger methods provided by Laboratory 11were rejected on the basis of their poor precision values which fell outside those expected for the methods. RM 107: Measurements by the Schoniger method provided by Laboratory 7 were rejected on the basis of the outlying mean value (see Figure 5). Measurements by the bomb and Schoniger methods provided by Laboratory 11were rejected on the basis of the outlying variances and low precision indicated. Statistical Evaluation of Results. The basis of the statistical treatment was as follows: (1)The data received were arranged in sets where each set represented the results of each individual method used in a laboratory. (2) The mean and standard deviation were calculated for each set of data. First the Cochran test was applied to detect sets which were outliers or stragglers of variance. Variances were judged to be outliers if they had a less than 1% probability of belonging to the parent population and, correspondingly, to be stragglers if less than a 5% probability. Variance outliers were only
1
1
1J
i
1'
LAB
158913'4
1578lC
20
1734
I2
Figure 2. Bar graphs showing mean measured values and 95% confidence limits for RM 105.
4 1
0 480
I'!
-
1
LAB
I589314
157810
26
347
I2
Flgure 3. Bar graphs showing mean measured values and 95% confidence limits for RM 106.
rejected if they clearly indicated that the repeatability was worse than usually expected for the method used. Next, the Dixon test was used to detect outlying means using the same probability criteria described above for the Cochran test. Outlying means were only rejected after careful technical consideration. A careful comparison of the means of the individual methods showed absence of detectable systematic differences.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
and the 95% confidence interval is given by
S MASS FRACTION
L WCKBOLD 1
I I IO0
1
1
X i F
BOMB / S C i O N C E ? I
8:P
Sln
m f t;P
1
i r
where t is the value of the Student t distribution for a number of degrees of freedom v = p - 1 and a = 0.05 level of significance, and s, is thie standard deviation of the distribution of the mean xi. Two further statistical parameters were calculated which are of importance to the user who wishes to compare his results with those of the laboratories whose measurements were ,accepted for calculation of the certified values, viz.: sB = [,he between-laboratory (or more precisely, the between set) standard deviation, and sw = the within-laboratory (or set) standard deviation, which were calculated from the populatiion of accepted individual results. The results are shown in Figures 1-4 in the form OT bar graphs, grouped by analytical methods. The extremities of the bars respresent the 95% confidence limits of each of the means which, in turn, are indicated by the midpoints of the bars. Except for RM 104, the bar charts only indicate accepted values. For RM 104 the results for bomb and Schoniger combustion and for ICP are given to show that, even though these methods were excluded on technical grounds for the low sulfur concentration, their collective means gives excellent support to the certified value as shown by the following comparison:
1
i J
1 LAB15891314
521
1578'0
I34
26
I2
Figure 4. Bar graphs showing mean measured values and 9 5 % confidence limits for RM 107.
Two further tests were applied before calculation of the certified values: the means of the remaining sets were shown to be normally distributed according to the KolmogorovSmirnov-Lilliefors test, finally, the F test, when applied to the laboratory means and their within and between laboratory variances, indicated values which were too high to allow the individual results to be regarded as independent estimates of the quantity t o be certified. Consequently, t,he certified values and the uncertainty were derived from the means and not from the individual results, according to the following formulas: the certified value is the arithmetic mean of the p laboratory mean results, i.e.
S mass fraction, R;
-
RM 104
mean
methods accepted for certification (XRF and Wickbold) all methods (XRF, Wickbold, bomb, Schoniger, and ICP)
0.0914
~0.0018
0.0915
i0.00313
95%CI,
-
BII W14 I I2 X 7 i l l 5 4 W8 VVI B 2 8 3 W5
SI
W131 X I
B 6 1x8
m , , ,
x5 Iw9
1.1 2
W I4 W13 I I2
6
I
c
'50
'46
I
I
'54
I
I
,60
t
B6
2
1571
I '
Bll 54
x7 I I2 W14 X 5 8 3 W13 W8 B 2 X I W9 V V 5 X 8 I X l O l W l l S l
1
.36
32
I
,40
,
t *
y s I I I l l .44
6
,083
'
087
'
091
'
095
'100
Figure 5. Histograms of mean values of individual sets for RMs 104-107 related to method and laboratory: ordinate, number of results; each abscissa, mass fraction % ,S; (t)outlier of variance, ( * ) outlier of mean; code: (W) Wickbold, (X) XRF, (B) bomb, ( S ) Schoniger, (I) ICP.
Anal. Chem. 1983, 55, 522-527
522
Table XI. Summary of Evaluated Results and Certified Values for the Sulfur Content of Reference Materials No. 104,105,106, a n d 107 RM104 0.0914 0.0022 0.0021
M Sm SB
sw
95% CL P N
0.0014 i.0.0018
9 56
mass fraction of sulfur, % RM105 RM106 RM107 0.3631 0.0191 0.0185 0.0091 +0.0095 18
0.5022 0.0164 0.0153 0.0079 +0.0082 18
122
122
1.0396 0.0283 0.0265 0.0150 +0.0146 17 116
Certified Values: Mass fraction of sdfur % RM RM RM RM
104 105 106 107
0.091 + 0.002 0.363 + 0.010 0.502 5 0.008
1.040 + 0.015
Figure 5 shows the distribution of all the individual means, including those eventually rejected (the mean value X1 and S11 for RM 104 fall outside the scale-see Table VII). In this figure, each individual mean is coded to identify the associated method and laboratory from which it is possible to judge the absence of systematic method effects. Table XI provides a summary of the final evaluated results for each reference material and gives the certified value, the standard deviation of the accepted means (sm), the standard deviations between laboratories (sg) and within laboratories (sw)obtained by ANOVA calculations, the 95% confidence limits (95% CL), and the number of sets of results (p) utilized and the total number of individual results (N). The certified value represents the numerically rounded values of the grand mean and the 95% confidence limits.
ACKNOWLEDGMENT We wish to thank the members of the following laboratories who contributed measurement data for use in the certification program. Those laboratories marked with an asterisk also
contributed to the stability and homogeneity studies: *British Petroleum Research Centre, Sunbury-on-Thames, U.K.; *Bundesanstalt fur Materialprufung, Berlin, Federal Republic of Germany; Centre de Recherches ELF-ERAP, Solaize, France; Centre National de la Recherche Scientifique, Service Central d'Analyse, Vernaison, France; Hoofd Analytisch Laboratorium, Estel Hoogovens BV, Ijmuiden, The Netherlands; Institute for Industrial Research and Standards, Dublin, Ireland; Institut Francais du Petrole, Rueil-Malmaison, Paris, France; Laboratoire National d'Essais, Paris, France; Mobil Oil Company Ltd., Research and Technical Service Laboratory, Stanford-le-Hope, U.K.; National Institute for Testing and Verification, Copenhagen, Denmark; *National Physical Laboratory, Teddington, U.K.; Shell Nederland Raffinaderij BV, Rotterdam, The Netherlands; Shell Research Ltd., Thornton Research Centre, Chester, U.K.; State Laboratory, Dublin, Ireland. It should be noted that the above alphabetical listing of participating laboratories does not correspond with the numerical listing given in Tables VII-x.
LITERATURE CITED (1) EEC (1975) Council Directive 75/716/EEC "On the Approximation of the Laws of the Member States Relating to the Sulphur Content of Certain Liquid Fuels"; Official Journal of the European Communities, No. L 307122 (27.11.75). (2) NBS Standard Reference Materials 1619-1624: Sulfur in Residual and Distillate Fuel Oil; National Bureau of Standards, Washington, DC; Information Sheet issued Autumn 1981. (3) Service des Materiaux de Reference, Paris, Report SMR/D/88/79. (4) Lindsey, A. S.; Coiinet, E.; Haemers, L.; Wagstaffe, P. J. "The Certification of Four Gas Oils for Sulphur Content", Commission of the European Communities Report EUR 7815 EN (1982). (5) Tertian, R. Chim. Anal. (Peris) 1060, 57, 525. (6) Toft, R. W. J . Inst. Pet. 1070, 5 6 , 269. (7) Kirkbright, G. F.; Ward, A. F.; West, T. S. Anal. Chim. Acta 1072, 62,
241-251.
(8) Eiiebracht, S. R.; Fairless, C. M.; Manahan, S. E. Anal. Chem. 1078,
50, 1849. (9) Lee, J.; Pritchard, M. W. Spectrochim. Acta, Part 8 1081, 368, 591-594. (10) Broekaert, J. A. C.; Leis, F.; Laqua, K. Talanta 1081, 28, 745-752.
RECEIVED for review August 11,1982. Accepted October 20, 1982.
Differential Pulse Polarography for a First-Order Catalytic Process Myung-Hoon Kim and Ronald L. Blrke" Department of Chemistry, The City College, City University of New York, New York, New York 10031
A theoretical expresslon for the current-time-potential relationship in dlfferentiel pulse polarography for a first-order catalytic process is derived, and properties of the expression are examlned. Ti4+/NaCI0, and TI4+/NH20Hcataiytlc redox systems were used to test the validity of the theory and homogeneous rate constants ( k ' ) were evaluated from the peak currents. A simplex optimization method was also employed to flt the current-potentlal data to the theoretical current expression in order to determine k' and the reversible halfwave potentlal, El,;, simultaneously. The rate constants found are 6.1 X l o 4 s-l M-' for the chlorate oxldant and 4.6 X 10' 8-' M-' for the hydroxylamine oxidant and these values are In good agreement wlth reported values determined by the dc and ac polarographic methods.
Recently we have applied a theoretically obtained expres0003-2700/83/0355-0522$01.50/0
sion for the pulse current for differential pulse polarography (DPP) to the case of a simple charge transfer process coupled to diffusion ( I ) . In that investigation we showed how DPP could be used to diagnose the kinetic nature of simple charge transfer processes and to evaluate the kinetic parameters of such processes. We used the method of Ferrier and Schroeder ( 2 ) to obtain this pulse current expression, and the present investigation is an extention of the same approach to the case of a first-order catalytic electrode process for a reversible charge transfer reaction. In an earlier paper we considered the effect of drop expansion on the reversible DPP currentpotential-time expression (@, and it was found to have a negligible effect on the reversible pulse current as long as the pulse duration, 6, was much shorter than the time in the drop life when the pulse is applied, 7. Thus, for the present treatment we have only considered the pulse current at a stationary electrode, and with the condition 6
