from Figure 1. Thus, x = 3.8 and the intercept is about -4.4 so that K,, would be about 4 X Based on the above generalized equilibrium, therefore, with K,, = 4 X IOd5, it appears that a Mn(OH)4 or related complex predominates over the pH range studied. It may be that a series of manganese( IV) hydroxide complexes are in equilibrium with solid MnOz, and that the average number of OH- ions complexed is about 4.
ACKNOWLEDGMENT Jeffrey Dann of G.T.E. Sylvania, Towanda, PA, laboratories provided the X-ray data and J. M. Fetsko, Center for Surface Coating Research, Lehigh University, Bethlehem, PA, the surface area measurements.
LITERATURE CITED
(4) (5)
(6) (7) (8)
W. Hertz, 2.Anorg, Chem., 22, 279 (1900). 0. Sackur and E. Fritzmann, 2.Electrochem., 15, 842 (1909). H. T. S. Britton, J. Chem. Soc., 127, 2110 (1925). Y. Oka, J. Chem. SOC.Jap., 59, 971 (1938). R. K . Fox, D. F. Swinehart, and A. B. Garrett, J. Am. Chem. Soc., 63, 1779 11941). R. Nasanen: 2.Phys. Chem., 191, 54 (1942). P. N. Kovaienko, Zh. Neorg. Khim.. 1, 1717 (1956). J. N. Butler, "Ionic Equilibrium-A Mathematical Approach," AddisonWesley, Reading, MA, 1964, p 287.
RECEIVEDfor review April 11, 1974. Accepted February 3, 1975. This research was carried out under EPA contracts 14OlOGSI and 801-236. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Determination of Lead in Gasoline by Atomic Absorption Spectrometry Using a Total Consumption Burner L. L. McCorriston and R. K. Ritchie Gulf Oil Canada Limited, Research and Development Department, 2489 North Sheridan Way, Sheridan Park, Ontario
Until recently, analytical methods for lead in gasoline covered a concentration range that was appropriate to regular and premium gasolines (1-5 g Pb/USG). However, other grades of gasoline with lower lead levels are now being produced to ensure a satisfactory catalytic converter life on 1975 model-year cars, and to satisfy various government environmental regulations. The introduction of lowlead (10.5 g Pb/USG) and no-lead (10.05 g Pb/USG) gasolines has resulted in a reassessment of existing methods and the development of more sensitive methods for lead in gasoline. Atomic absorption, because of its combination of sensitivity, precision, and speed, is the most attractive approach for the laboratory determination of lead in gasoline, but the analysis is not straightforward. Problems encountered when using premix burners include slow response, memory effects, different responses for the various lead alkyls, and different responses for organic and inorganic standards (1-6). Some authors (1-3) claim that these problems can be overcome by a combination of solvent selection (isooctane), careful attention to operating parameters (notably a lean flame), and calibration with lead alkyls. Others report that constant aspiration and washout times are necessary to compensate for slow response and memory effects ( 4 ) and that burner height is important ( 5 ) . However, atomic absorption methods involving chemical pretreatment to convert lead alkyls to lead iodide and calibration with inorganic standards (6, 7) have recently gained favor, culminating in their adoption as standard procedures by ASTM (D3237-73) and the Canadian Government ( 3 ) . In the following, we report an atomic absorption method for the determination of lead in no-lead and low-lead gasolines that requires no chemical pretreatment and no special care in spectrometer operation. The method employs a total consumption burner, an isooctane-acetone solvent (81, and calibration with lead alkyls. It is derived from one that has been used successfully for many years in our labo-
ratory for the determination of lead in regular and premium gasolines.
EXPERIMENTAL Apparatus and Operating Conditions. The spectrometer used was a Jarrell-Ash model 82-516 with multipass optics. The burner was a Jarrell-Ash high-efficiency total consumption (HETCO) burner operating on air/hydrogen. Total consumption burners are also supplied by Instrumentation Laboratories for use on their spectrometers. Lamps were Westinghouse WL23146 Pb-Cu-Zn-Cd multielement and Varian 2N258 hydrogen lamps, both used a t 283.3 nm. Calibrants and Diluents. Tetraethyl lead (TEL), tetramethyl lead (TML), and mixed lead alkyl (MLA) concentrates were obtained from Ethyl Corporation and E. I. DuPont de Nemours, Inc. The concentrates should be handled in a suitable fumehood since lead alkyls are extremely toxic. Lead-free base was prepared from gasoline base stocks, all of which were essentially lead-free as received. Analysis of the leadfree base by a colorimetric dithizone method showed that it contained less than 0.0003 g Pb/USG (79 ppb w/v). The isooctane (2,2,4-trimethyl pentane)-acetone solvent is 1/1. Standard and Calibration Solutions. Regular/premium standard solutions covering the 1-10 g Pb/USG concentration range are prepared by diluting the TEL concentrate with isooctane. The lead concentrations of these solutions are determined by the standard ASTM D526 gravimetric procedure. No-lead and low-lead standard solutions are prepared by diluting the regular/premium standards with the lead-free base, as shown in Figure 1. Calibration solutions for regular/premium, low-lead, and nolead gasolines are prepared by diluting the appropriate standards 3/250, 5/100, and 25/50, respectively, with isooctane-acetone as shown in Figure 1. Blanks for low-lead and no-lead gasolines are prepared by diluting lead-free base in the same manner. Isooctaneacetone serves as a blank for regular/premium gasolines. Procedure. Regular/premium, low-lead, and no-lead gasolines are diluted 3/250, 5/100, and 25/50, respectively, with isooctaneacetone, then run along with blanks and calibration solutions. Calibration curves are linear. The absorbance for a gasoline containing 0.05 g Pb/USG is typically 0.21. The lead contents of regular/premium and low-lead gasolines are read directly from the calibration curves. The lead contents of ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1137
R E G U L 4 R l PREMIUM S T 4 N D I R D S I T O I O G P B l U S G IN ISOOCT4NE
L O W - L E A D STANOARDS
0 1-1.0 G P B I U S G LE4D-FREE B4SE
1 R E G U L 4 R I PREMIUM
LOW
-
1
I
0.30
N O - L E 4 0 STANDARDS 0.012-0.12
1101100 D I L U T I O N W I T H
I
1 0 PBIUSG
I31250 DILUTION WITH LE4D-FREE
BASE
I
1 LEAD
N O - LEAD
0.20..
C 4 L l B R A T l O N SOLUTION
C A L l B R 4 T l O N SOLUTION
CALlBR4TlON SOLUTION
131150 D I L U T I O N WITH
(51100 D I L U T I O N WITH
125150 DILUTION W I T H
ISOOCTANE-ACETONE 1
ISOOCTANE-ACETONE )
ISOOCTANE-ACETONE
Y
I
U
z
4
Figure 1. Standard and calibration solutions
m e 0 v)
m
no-lead gasolines are determined from the calibration curve using absorbances which have been corrected for background variations. The background correction is obtained for sample and blank using the hydrogen continuum lamp at 283.3nm or by use of a nearby non-absorbingline. Comparative Methods. Chemical (ASTM D526), X-ray fluorescence (ASTM D2599), and a colorimetric dithizone method were used to obtain an estimate of the accuracy of the atomic absorption method.
0.10..
0.01
0.02 9
RESULTS AND DISCUSSION The repeatability data given in Table I demonstrate the excellent precision of the method. Duplicates, a t the 95% confidence level, should not differ by more than 0.002, 0.019 and 0.073 g/USG for no-lead, low-lead, and regular/ premium gasolines, respectively. In particular, 0.002 g/USG for no-lead gasolines compares favorably with the value of 0.005 g/USG reported in ASTM D3237-73. Table I1 compares our atomic absorption results with exchange averages and with colorimetric and X-ray results. The exchange samples originated from the Canadian cooperative fuel exchange and the ASTM fuel exchange. Of the eight non-exchange samples, 5 were obtained from various company service stations in the Toronto-Buffalo area. The agreement between results is very good. Assuming that the exchange averages and the other analyses are the best estimates of the true values, we obtain mean errors of +0.0003, -0.004, and -0.006 g/USG, and average absolute deviations of 0.0005,0.009, and 0.03 g/USG for no-lead, low-lead, and regular/premium gasolines, respectively. These deviations, being less than 2% of typical or specification levels, are insignificant and probably well within the experimental error of the other methods. Also, in exchange samples, our deviation from the average is well within the standard deviation of all exchange results.
0.03
0.04
0.05
Pb/USG
Figure 2. Response of lead alkyls with total consumption burner ( 0 )TEL. (A)TML, (m) MLA
The effect of the various lead alkyls was investigated using blends of TEL, TML, and MLA in gasoline. Figure 2 shows that the same response is obtained for all lead alkyls a t no-lead levels when using the total consumption burner with isooctane-acetone as solvent. Similar results were obtained at higher lead levels. On the other hand, the use of a premix burner with an air-acetylene flame resulted in errors similar to those previously reported ( I , 6); for example, blends containing 2.35 g Pb/USG gave responses equivalent to 2.64, 4.10, and 5.60 g Pb/USG for MLA 500 (=50% methyl groups), MLA 750 ( ~ 7 5 %methyl groups), and TML, respectively. Maximum signal is reached almost immediately with the total consumption burner, and there are no memory effects. This contrasts with the unsatisfactory lead alkyl re.sponse characteristics of the premix burner observed by us and others ( I , 4-6). The effects of olefinic, aromatic, paraffinic, and high (13 lb) and low (7 lb) Reid vapor pressure (ASTM D323) gasoline base stocks were investigated using blends of TEL in
Table I. Repeatability D a t a Sample
1138
Replicate lead results, g/USG
Std dev
1 2 3 4 5
0.0214 0.0307 0.0517 0.0527 0.0807
0.0223 0.0306 0.0528 0.0520 0.0787
0.0223 0.0308 0.0538 0.0527 0.0787
0.0213 0.0307 0.0520 0.0522 0.0797
6 7 8
0.204 0.480 0.497
0.212 0.487 0.488
0.202 0,492 0.496
0.206 0.498 0.506
9 10 11 12 13 14
1.87 2.38 1.97 1.89 1.54 1.08
1.90 2.36 1.95 1.85 1.52 1.12
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7,
0.0211 0.0517 0.0522
0 .OOO?
0.007
0.02 1
JUNE 1975
0.06
these base stocks. Using the paraffinic blend no-lead calibration for the olefinic, aromatic, and the high- and lowvapor-pressure gasolines resulted in mean errors of 0.0001, 0.0020, 0.0010, and 0.0009 g/USG, respectively. Application of the background correction resulted in corresponding mean errors of 0.0002, 0,0001, 0.0004, and 0.0007 g/USG. Therefore variations in base stock composition have only a small effect for no-lead gasolines. However, the small background correction is significant at refineries where the acceptable lead level is usually 0.01 g/USG, and a t service stations when the lead level is near the specification of 0.05 g/USG maximum. Otherwise the background correction can be omitted for most no-lead gasoline monitoring applications. The effect of base stock composition is insignificant for low-lead and regular/premium gasolines because of the greater dilutions involved, and no background correction is therefore required. A simple dilution is, of course, more rapid than chemical pretreatment methods such as ASTM D3237-73 and is to be preferred for that reason. Including calibration and background correction, the analysis times are 30 min for one no-lead gasoline sample, and 2% hr for 20. The corresponding times for low-lead and regular/premium gasolines are 20 min and 2 hr. Flameless atomic absorption (9, 10) and more complex chemical pretreatment (11) methods have also been reported recently for the analysis of lead in gasoline. However, flame atomic absorption methods adequately cover the concentration range of current interest and are to be preferred because of their better precision and speed. In summary, the atomic absorption method described here is a simple dilution procedure that is not affected by differences in lead alkyls or gasoline base stock composition. Neither chemical pretreatment nor unusually careful attention to operating parameters is required. The method is rapid and has very good precision and accuracy.
Table 11. Comparison of Results with Other Methods and Exchange D a t a Lead Concentration, g / G a This Work
Colorimekic
0.008 0.0201 0.0270 0.0422
0.007 0.0200 0.02 64 0.0426
0.310 0.420 0.426 0.435 0.483 0.490 0.730 0.800
X-Ray
Fxchan e Aveb f
stan% dev.
0.31 0.426 0.423 0.435 0.461
* 0.05
0.431 0.433 0.433 0.485 0.51 f 0.09 0.74 0.82
1.32 1.36 f 0.11 1.84 1.80 i 0.12 1.90 1.96 i 0.13 1.91 1.93 2.45 2.42 2.49 2.52 0.09 2 -93 2.93 f 0.13 4.59 4.56 i 0.34 g/USG except for Canadian Cooperative Fuel Exchange data for which units are g/IG. Canadian cooperative fuel exchange results include standard deviation data. Other results are from the ASTM fuel exchange. Results were obtained in 1972 and 1973.
*
LITERATURE CITED
(3) Report EPS 1-AP-73-3, Air Pollution Control Directorate, Environmental Protection Service, Ottawa, Canada, March 1973. (4) R. A. Mostyn and A. F. Cunningharn. J. lnst. Petrol., 53, 101 (1967). (5) R. M. Dagnall and T. S . West, Talanta, 11, 1553 (1964). (6) M. Kashiki, S. Yarnazoe. and S . Oshirna. Anal. Chim. Acta, 53, 95 (1971). (7) Du Pont Petroleum Laboratory Test Method M112-71. E. i. Du Pont de Nernours & Co., Inc.. Wilrnington, DE 19898. (8) H. W. Wilson, Anal. Chem., 38, 920 (1966). (9) M. P. Bratzel and C. L. Chakrabarti, Anal. Chim. Acta, 61, 25 (1972). (10) M. Kashiki, S.Yarnazoe, N. Ikeda, and S . Oshirna. Anal. Lett., 7 ( l ) ,53 (1974). (11) K. Campbell and J. M. Palmer, J. lnst. Petrol., 58, 193 (1972).
(1) D. J. Trent, Atom. Absorp. News/., 4, 348 (1965). (2) N. Ouickert, A. Zdrojewski, and L. Dubois. Sci. Total Environ., 1, 309 (1972).
RECEIVEDfor review November 18, 1974. Accepted February 27,1975.
ACKNOWLEDGMENT We thank J. E. Coffey, P. L. Hettinga, D. Kulawic, and W. M. Meston for experimental assistance, and T. Johnson for statistical discussions.
Spect rophoto metric and Gas-Liquid Chromatographic Determination of Amitriptyline Horace E. Hamilton,’ Jack E. Wallace,’ and Kenneth Blum2 The University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78284
Amitriptyline, 10,11-dihydro-5H-5-(3-dimethylaminopropylidene)dibenzo(a,d]cycloheptene,and its monomethylamino analog nortriptyline are psychotherapeutic agents whose therapeutic value for the management of depresDepartment of Pathology. Department of Pharmacology.
sions has been well established. The ultraviolet absorption spectrum of amitriptyline is nonspecific and difficult to distinguish in biologic extracts from the background absorption contributed by normal biologic constituents. A number of spectrophotometric methods for the analysis of amitriptyline in biologic specimens have been described ( 1 - 5 ) ; however, these have generally lacked sufficient sensiANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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