Utes). After cooling, 4 ml. of buffer seconds in a Klett-Summerson colorimsolution and 0.2 ml. of the o-dianisidine eter using a No. 44 filter. reagent are added and the color is measured after 4 minutes with a KlettDISCUSSION Summerson colorimeter using a KO.41 filter. The ligands, ethylenediamine and ESTIMATION OF I)IBUTTLAJIIKE. A cyanide ion, which form very stable standard curve is prepared by using complexes with cobalt, could not be known concentrations from 0 to 400 y detected or estimated by either of the of dibutylamine per ml. of ethyl ether. procedures described here. This would To 1 ml. of the sample solution in a be expected for cyanide from the 15-ml. test tube is added 1 ml. of the value for the couple cobaltous chloride reagent solution. The test tube is heated in a w t e r bath [co(cs)S]-4 [Co(CN)G]--- e (60” to 80” C.) until the solvent has evaporated. After cooling, 0.2 ml. of which is +OB3 volt. The fact that t h e o-dianisidine reagent and 4 nil. of ethylenediamine does not give the rethe buffer solution are added and the action may be interpreted as meaning color is measured inimediately with a t h a t the oxidation potential for the Klett-Summerson colorimetei using a couple KO.44 filter. Results. Different concentrations [ C o ( e ~ i )*~+]~t [ C ~ ( e n ) ~ ] e of hydrogen peroxide are used in t h e is too positive for the oxidation of cobaltous chloride reagent for t h e o-dianisidine. The cobalt(I1)-ethyleneestimation of t h e dithioether and the amine. The order of addition of t h e diamine complex is, in fact, a good reredox indicator also differs in t h e t1I-o ducing agent (6). procedures. Inasmuch as complex formation has Qualitative experiments indicated an effect on oxidation potential, and t h a t excess peroxide x i s undesirable in oxidation potential is related to dissothe estimation of the dithioether, popciation or stability constant through the sibly because of a competitive osidaKernst equation, i t is possible that tion of the dithioether. information on the oxidizing effect of Ligands in Aqueous Solution. R E AGENTS. Cobaltous chloride hexacoordination compounds on redox inhydrate (0.2 gram) is dissolved in 100 dicators having different oxidation poml. of water to which 2 drops of 307, tentials may lead t o a semiquantitative hydrogen peroxide have been added. approximation of dissociation constants o-Dianisidine, 0.04% in ethyl alcohol. in certain cases. Clark and Lubs p H 2 buffer solution The scope and limitations of this (4). method with respect to ligands and ESTIMATION OF 2-DIETHYLBMINOETHmetals have not yet been determined. AKOL. A standard curve is prepared by using known concentrations of 2-diethylACKNOWLEDGMENT aminoethanol in water. T o 1 nil. of the sample solution is added 1 nil. of cobalThe authors wish to thank William tous chloride reagent, 0.2 nil. of o-dianisiD . Ludemann, Jr., and Arturo L. dine reagent, and 2 ml. of buffer soluCardenas for performing some of the tion. The color developed is read in 30
+
+++
+
spectrophotometry in connection with this investigation. LITERATURE CITED
Bailar, J. C., Jr., J . Cheni. Edirr. 21, 523 (1944). Bricker, C . E., Loeffler, L. J., ANAL. CHEU.27, 1419 (1955). Chiarottino, A, Industria chiniica 8, 32-3 (1933). Clark, W. M., “Determination of Hydrogen Ions,” 3rd ed., Williams & Wilkins, Baltimore, )Id., 1928. (5) Copley, M.J., Foster, L. S., Bailar, J. C., Jr., Chem. Revs. 30, 227 (1942). ,(6) Diehl, H., Butler, J. P., SNAI.. CHm. 27, 777 (1955). (7) Latimer, W.M., “Oxidation Potentials,” 2nd ed., pp. 2-5, PrenticeHall, New York, 1952. (8) hIoeller, J., “Inorganic Chemistry,” pp. 300-305, W’iley, Xew Tork, 1952. (9) SIorgan, G. J., Ledbury, IT., J . Chern. SOC.121, 2886 (1922). (10) Price, C. C., Roberts, R. &I,,J . Org. Chena. 12, 261 (1947). (11) Shibata, K., Watanabe, ,4,, Izoata Inst. Plant Biochena. Pub. 2,97-128 (1936). (12) Spacu, G., Macarovici, C. Gh., Bul. SOC.Stiinte Cluj 8, 245-56 (1935). (13) Tschugaeff, L., Ber. 41, 2222 (1908). (14) Tschugaeff, L., Compt. lend. 154, 33 (1912). (15) Tschugaeff, L., Kobljanski, h.,Z. anorg. Chena. 83, 8 (1913). (16) Tschugaeff, L., Subbotin, K., Ber. 43, 1200 (1910). (17) Waters, W., A., “The Chemistry of Free Radicals,” pp. 247-252, Osford University Press, London, I
~
1946.
RECEIVED for review June 15, 1956. Accepted September 14, 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 27, 1956.
Isotopic Analysis of Tetramethyllead G. L. BATE‘, D. S. MILLER, and J. L. KULP lamont Geological Observatory, Columbia University, Palisades, N.
b The method of isotopic assay of lead by the analysis of tetramethyllead has been studied. The average reproducibility for duplicate runs is 0.6,0.4,0.4, and 0.2% for lead-204, -206, -207, and -208, respectively. Within these limits thereis no evidence of fractionation or contamination during chemical preparation. Two 6-inch mass spectrometers were used with slightly differing sources. The lead spectra in both the Pbf and Pb(CH&+ regions were analyzed. Within the experimental 1 Present address, Department of Physics, iJ-heaton College, Kheatori, Ill.
84
ANALYTICAL CHEMISTRY
Y
errors the analyses from both mass spectrometers were in agreement. An interlaboratory comparison shows agreement generally to within 0.5% for the heavier isotopes, and to about 1 for lead-204.
yo
T
HE USE of tetramethyl vapor for the isotopic determination of lead has been reported by Dibeler and Mohler (3) and Collins, Farquhar, and Russell ( 2 ) . The great convenience and accuracy of this method for macroscopic lead samples have led to its adoption in this laboratory for geologic
studies involving large numbers of samples. Many geological interpretations, including age determinations by the lead method, need an accurate isotopic determination in order to be meaningful. This paper reports experinients on the reproducibility of the method using different spectrometer tubes, sources, and spectra. Interlaboratory comparisons \vere also made. MASS SPECTROMETER
Two mass spectrometers were employed in the course of this work, both of the direction-focusing, &inch sector
type following the original design of K e r ( 7 ) . Instrument I was of glass-metal construction with a source similar to that of Palmer and Aitken (9)and gave a mass resolution of better than 1 part in 220. The exit slit of the source was made 0.075 mm. wide, and the entrance slit a t the collector 0.3 mm. wide, in order to give maximum signal with optimum resolution. K i t h these slit configurations the typical ion current a t mass 208 \\as of the order of 10-11 ampere. This instrument was used to analyze the lead ion spectrum from tetramethyllead. Instrument 11) a n all-metal tube of better alignment and n i t h a slightly different ion source than t h a t used in Instrument I, gave a mass resolution of a t least 1 part in 300. Instrument I1 had more than adequate resolution for both lead ion and trimethyllead ion deter mination. Instruments I and I1 both used a vibrating reed electrometer (Applied Physics Corp., Model 30) for amplification of the ion current received a t the collector, and the final signals nere displayed on a Leeds & Korthrup Speedomax recorder. K i t h Instrument I a 101l-ohm input resistor 1\88 used with the vibrating reed electrometer. The resultant time constant was larger than desired and necessitated a relatively slow scanning rate in order to allow the full signal to be developed a t the peak. Because of the improvements in resolution and sensitivity in Instrument 11, 11 hich permitted the use of the trimethyl spectrum, an input resistoi of 1010 ohms nas found to be satisfactory and peirnitted more rapid scanning. K i t h pioper shielding of the ion collector, the noise level n a s on the order of 0.05 mv. The short-term drift of the particular vibrating reed electrometer employed was less than 0.004 mv. (per second) nhich was entirely adequate. For both instruments a glass viscous f l o ~leak arid a metal manifold with metal diaphragm valves were employed for sample introduction. This system provided for an efficient transfer of gas, IF ith the result t h a t typical analyses u ere made a t a n indicated tube pressure of 1 to 4 X lO-’nirn. of mercury. The full details of the electronics and gas handling systems are given by Bate and Kulp ( I ) . CHEMICAL TECHNIQUES
I n order t o introduce the lead into the mass spectrometer in gaseous form, it mas necessary to synthesize tetramethyllead [Pb(CH&] from the mineral lead samples. The general technique has been dewribed by Collins, Farquhar, and Russell ( 2 ) and consists briefly of: preparation of pure lead chloride from the lead minerals; Grignard reaction with lead chloride; hydrolysis of excess Grignard reagent; separation of ether solution of tetramethyl from water solution; and separation of tetramethyl from ether by fiactional distillation, completed by evaporation of excess ether under reduced pressure, until the volume of
sample was down to about 1 ml. The sample is then p u t in a d r y ice bath, evacuated with a high vacuum pump for approximately 20 minutes, and introduced into the spectrometer. K i t h approximately 0.03 gram of lead chloride, the final yield of tetramethyl was of the order of a few hundredths milliliter by volume, sufficient for several runs on the mass spectrometer. The possibility of appreciable lead contamiiiation from the chemical reagents employed was ruled out on the basis of the following considerations. Only ieagents of the highest purity commercially available were used and the possible lead content specified was very small compared to the amount of lead in the sample. Radiogenic leads were processed which shon-ed no evidence of common lead. A lead iodide salt was divided into tn-o portions, one of which n a s converted into lead chloride for the Grignard reaction and, because the Grignard reaction will also take place with other halides, the remaining iodide portion was utilized directly for the Grignard reaction. The lead isotopic compositions of the two tetramethyls thus prepared v-ere identical within limits of experimental error, and it was therefore concluded that contamination from inorganic reagents, the most likely source of contamination, )vas negligible.
It was found t h a t heavy organic iesidues were present in the tetramethyl sample, which could contribute to ion signals in both the lead ion and trimethyl ion spectra. These impurities exhibited low vapor pressures relative to tetramethyllead and were a source of trouble only if the sample container was heated. Although fractionation occurring in the chemical procedure has been studied by Ducheylard, Lazard, and Roth (4), further tests were conducted to detect any fractionation effects in the chemical procedure. I n the last two steps of ether removal by fractional distillation and by evaporation, the effect of isotopic fractionation would favor removal of the lighter isotopes and thereby increase the percentage composition of the heavier isotopes in the residual sample. For the typical sample, the fractional distillation reduced the volume of the ether solution by a factor of about 4 and the final evaporation involved a volume reduction by a factor of about 100. Although the predicted fractionation a t the boiling temperature of the mixture based on the mass differences of the tetramethyllead-204, tetramethyllead - 206, tetramethyllead207, and tetramethyllead-208 niolecules is small, two evperiinental tests were conducted for final proof. These tests simply consisted of repeating the fractionation and evaporation process two times, after adding a volume of
ether equal to t h a t just removed. Any effect originally present would have been multiplied by about a factor of 3, but, although the tests were repeated several times on two different samples, no change from the original isotopic composition could be detected. The final purification step r a s checked for fractionation and none was detected. It is believed, therefore, that errors in the data due to contamination or isotopic fractionation of the chemical process are within the experimental error reported. BACKGROUND SPECTRA
The use of mercury diffusion pumps eliminated the general background due to organic vapor but necessitated slight correction at mass 204 for niercury vapor present. After extended use of the tetramethyllead, a common lead background was consistently present but, because the 208 peak height (most abundant) \\-as on the order of 1;iOO of the signal obtained during a normal run, the background could be disiegarded. A substantial background was rarely observed in the trimethyl ion spectrum, The mass spectrometer tubes were baked for about 15 minutes between samples to dissipate surface charge accumulated within the tube and to eliminate any memory effect from the preceding sample. The pumping speed of the vacuum system n-as such that on sample removal the peak heights were decreased by a factor of several hundred within a few minutes. METHODS
OF
CALCULATION
T o obtain the isotopic abundances from the relative ion currents various corrections have to be made. 1. Correction for mercury-204 appearing at mass 204 in the lead spectrum 2. Correction for the hydride formation in both the lead ion and trimethyllead ion spectrum 3. Correction for hydrogen loss in the trimethyllead spectrum and 4. Correction for the carbon-13-carbon-12 effect +
Before corrections 2, 3, and 4 are applied, the signals are compensated for a voltage multiplier circuit in the output of the vibrating reed. All the resistor ratios used were measured a i t h a precision potentiometer and it is believed that they are known to better than 0.1%. The procedure is then to reduce the relative ion currents to a percentage composition, after n hich the appropriate hydrogen corrections are made. The abundance of the hydride is pressure- and temperature-dependent. However, these variables are not known to change appreciably during the course of a n analysis, and the hydride component n-as in fact never observed to change VOL. 29, NO. 1, JANUARY 1957
85
significantly during a run and was remarkably constant from day to day for long periods of time. The hydride abundance is estimated from the ratio of the peak height a t mass number 209, due to the lead-208 hydride, to the peak height a t mass 208. I n the lead ion spectrum the hydride factor was found to be very sensitive to the electron-accelerating voltage, which was variable over a range of about 90 volts. The isotopic composition of a sample was shown to be independent of the electron-accelerating voltage, E,, and hence independent of the hydride abundance. It was found that the ion current increased with electron accelerating voltage to a mayimum a t about 40 volts, and then decreased on further increase of electronaccelerating voltage. Higher values of the electron-accelerating voltage gave rise to unstable source conditions and most of the analyses mere therefore made with E , = 40 volts. For this value of E,, the efficiency of hydride formation with lead ions was approximately 10%. For the lead ion spectrum the true abundances of the isotopes are calculated from the peak heights observed on the recorder chart in the following manner: Let 204’, 206’, 207‘, 208’, and 209’ represent the observed peak heights of these isotopes (204’ excludes mercury204) and let 204, 206, 207, and 208 represent the corresponding true abundances. Both sets of numbers express percentage composition. The true hydride factor may then be written as: C = 209’/208 = Pbm*H+/Pb*J*+
From the nature of the hydride formation, the following equations can be written as: 204’ = 204 - C(204) = (1
- (3204
(1)
206‘
=
206 - C(206)
207’
=
207 - C(207) C(206) = (1 - C)207 C(206) (3) 208 - C(208) C(207) (1 - C)208 C(201) (4)
208’ =
=
(1 - C)206 (2)
+ + + + n
L
Now writing k = the foregoing (1- (3’ equations become : ~
207 208
204 = (1
+ k)204’
(5)
206
4- k ) 206’
(6)
=
(I
+ k ) 207’ - k(206) (1 + k ) 208’ - k(207) (1
(7) (8)
Equations 6, 7 , and 8 are actually three nonlinear equations in the unknowns 206, 207, and 208, for which the exact analytical solution becomes somewhat cumbersome. This difficulty may be avoided and the roots may be obtained with the desired accuracy by 86
ANALYTICAL CHEMISTRY
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Figure 1.
Reproducibility of instrument II
using successive approximations. For the first approximation k‘ = 209’/208’, and corresponding values of 206’, 207’, and 208’ are calculated. The next approximation of k is IC” = 209’/ (208-209’) and corresponding values of 206” and 207” and 208’’ are computed. This is then followed with IC”’ = 209‘/ (208”-209’), etc., until the required precision is obtained. Usually the closure is so rapid that two approximations suffice. For a given analysis, 204’, 206’, 207’, 208’, and 209’ are actually the averages of percentage compositions of the individual scan comprising the analysis. For each average ion abundance an average deviation is computed. The average deviations of the true composition will have an added uncertainty due to the error in the 209‘ peak. The effect of the hydride error was calculated for each analysis, but invariably the contribution to the error in the true composition was found to be small-usually negligible. The relative error in 209’ never exceeded 1% and the resulting uncertainty contributed to the true abundances of the lead isotopes rarely approached 0.1%.
The foregoing hydride correction assumes that the hydride efficiency obtained for lead-208 applies t o the remaining lead isotopes. A possible systematic effect violating this assumption would consist of a mass discrimination in hydride formation. If such an effect did exist, the hydride factor for lead-204 would differ to the greatest extent from that obtained by lead-208. HoLYever, even for lead-204, the greatest possible effect on k , assuming a functional dependence as the square root of the ratio of the masses, mould not exceed 1%. A systematic error in the hydride factor of this order of magnitude would introduce an uncertainty in the true abundances well n.ithin the experimental error reported. I n the trimethyl mass spectrum region (mass numbers 248 through 254) the hydride effect is present but to a lesser extent than in the case of the lead ion spectrum, and no mercury-204 correction is needed in the trimethyl spectrum. However, additional corrections must be made in the trimethyl lead ion spectrum for the carbon-13-carbon-12 effect and for the loss of a hydrogen atom. The analytical expressions for the
trimethyllead ion spectrum may be developed as follows. Let 249', 251', 252', 253', and 254' represent the observed signals a t these mass numbers. Also, designate by 249, 251, 252, and 253 the true abundances of lead-204, lead-206, lead-207, and lead-208, respectively, appearing a t the indicated mass numbers in the trimethyl spectrum. The carbon-13-carbon-12 ratio and hydride formation have the same effect of increasing the observed signal a t mass number (m l),a t the expense of the signal a t mass number m. Let p therefore represent the fraction of the true signal of mass number m appearing a t mass number (m 1) due to the combined effects of hydride formation and carbon-13 appearance. Similarly, let q represent the fraction of the true signal a t mass number na lost to mass number ( m - 1) to the loss of one hydrogen atom. h'eglecting second-order effects, the observed spectrum may be analytically expressed as follon s:
+
I. Comparison of Spectra (EC I1 standard) Spectrum Instrument 206 204 PbC 1.338 25.12 I1 Table
Date 1-9-56 1-16-56
3-12-50 4-9-56
Pb-
Pb(CH3) '
+
- ~ ( 2 4 9 )- q(2.19)
249' = 249
=
249(1 - p - 9 )
+ +
251' 251 - ~ ( 2 5 1 )- p(25l) p(252) q(252) = 251 ( 1 - p - 4)
(9) (10)
252' = 252 - ~ ( 2 5 2 )- q(252) f p(251) rj(253) = 252(1 - p - q ) ~ ( 2 5 1 ) q(253) ill)
+
253'
+
+
253 - ~ ( 2 5 3 )~ ( 2 5 3 ) p(252) = 253(1-- p - p)-+ ~ ( 2 5 2 ) (12) 204' = ~ ( 2 0 3 )
+
Writing r
=
p
r + q and s = 1__ - r'
Equations 9 through 12 may be implicitly solved for 2-29>251, 252, and 253:
+ s ) 249' 251 = (1 + 251' - q (1 + 252 252 = (1 + 252' p(l + 251 - p (1 + s j 253 253 = (1 + s ) 253 - p ( l + 252 249
=
S)
(1
S)
S)
21.16 21.21
52.20 52.06
I1
1.341 1.347
25.23 25.29
21,19 21,24
52.24 52.11
I1
1 333 1 33';
25 25 25 26
21.25 21 23
52.1; 52.16
I1
1.342 1,340
25.16 25,28
21.30 21.20
52.20 52.18
I1
1.347 1.351
25 27 25.39
21.22 21.18
52 16 52. 08
1.341 10.004
25.26 i 0.06
21.22
52.17
=t 0 03
=IC0 . 0 6
25.22 21.24 Sverage of 10 runs
52,22
Av. 12-1-53
through 12-24-53
Pb+
Table II.
I
1.34
Comparison of Analyses of Nier Samples
Inst. Great Bear Lake
Galena
Spec.
Sier No. 1 Lamont 37 I PbC Lamont 124N I1 Botha Broken Hill, Galena Nier 10. 2 N .S.\I-. Lamont 137 I Pb+ Lamont 120N I1 Both" Joplin, 110. Galena I1 Xier S o . 8 Lamont 74 I Pb+ Lamont 114K I1 Botha Casapalca, Bournonite Sier No. 16 Peru Lamont 120 I Pbf Lamont 117N I1 B o t P Casaoalca. Galena Kier KO.17 Pefu ' Lamont 121 I Pb+ Lamont 119K I1 Both" a Results obtained from average of Pb+ and PblCH:)?'
204
206
207
208
1 48 2 3 . 5 9 22 66 1 . 4 5 4 23 61 22 5q 1 453 23 39 22 50
52 3 52 34 52 65
1 47 23 04 22 66 1 445 23 49 22 49 1 452 23 48 22 52
52 2 52 56 52 54
1 27 27 47 1 247 27 25 1 239 27 24
51 3 51 60 51 -59
20 01 19 90 19 93
1 36 25.48 21 0 8 52 07 1 336 2 5 . 1 9 21 06 52 41 1 333 25 23 20 91 52 52 1 85 25 42 21 13 52 11 1 333 2 5 . 2 2 20 98 52 47 1 332 25 20 21 02 52 44 spectra
(13) (1.1)
8)
8)
25.39
+
2-8-56
208
1.337
Pb(CH3)3+
Pb+ Pb (CH,), PbPb(CH,), Pb' Pb (CH1)a+
207
(15) (16)
4 s with the lead ion spectrum, Equations 13 through 16 may be solved by repeated approximations. q was found to have a value consistently in the neighborhood of O.75y0. A rough estimate for q may be made from the signal a t mass number 248. A better value can be calculated from the signal appearing at mass number 250, but even this value is accurate to only a few per cent. However, variations in y up to 10% have a negligible effect on the isotopic composition. The lead-207 composition is the most sensitive to changes in q-e.g., a 30010 change in y affects lead-207 by 0.50/0, with a negligihle effect on the remaining isotopes. The value of p was fairly constant a t 3.2%. If the carbon-13-carbon-12 ratio is taken as 0.011, then the trimethyl hydride effect amounts to effectively nothing. This value is in
the range of 0.33% reported by Dibeler and Mohler and that of 0.087c reported by Collins, Farquhar, and Russell. The different values 0.33, 0.08, and approximately 0.0% are due presumably to differences in temperatures of the sources, in geometric configurations of the electron beam, and in conditions of secondary emission. RESULTS
The experiments that have been conducted may be broken down as to: (1) reproducibility, (2) comparison of lead ion spectra results on two different sources and spectrometers, (3) comparison of lead ion and trimethyllead ion analyses on several samples, and (4) interlaboratory calibration. A reference standard sample of tetramethyllead was analyzed nearly 40 times on Instrument I wit.h the lead ion spectrum. The average deviation of an analysis from the mean was about 0.2y0 for the lead-208 and nearly o.5y0 for lead-204. These variations
were usually within the standard deviation of each individual analysis. The greatest deviation for all samples from the mean was about 0.5% for the lead208 and 20/, for lead-204. A series of measurements with another standard on Instrument I1 using the trimethyllead ion spectrum is shown graphically in Figure 1. The reproducibility of this system is better, giving an average deviation of 0.4, 0.2, 0.2, and O.l%, respectively, for lead-204, lead-206, lead-207, and lead-208. Table I shows the comparison of the two instruments and the two spectra. Table I1 gives the comparative analyses on identical samples of initial lead iodide salt by Nier (6, 8) and by trimethyllead ion in this laboratory. The intercalibration with Harwell and Toronto (5) is shown in Table 111. DISCUSSION
The results of Table I show that within the experimental error there is no difference in the isotopic abundances VOL. 29, NO. 1 , JANUARY 1957
a
87
Table 111.
Toronto Harm-ell Lamont 1955 Lamont 1956 Toronto HarTyell Lamont
lntercalibration
(U. S. Geological Survey standard) 204 206 207 208 1.44(0) 23.69 22,54 52.33 1.45(5) 23.64 22.61 52.30 1.44(4) 23.50 22,55 52.52 1.44(5) 23.56 22.50 52.50 Pb(CH3)a Gas Average of 5 runs PbClz; PbIz Solid Average of 7 runs Pb(CH3)d Gas 1955 Average of 30 runs 1956 Average of 10 runs
Stieff of the United States Geological Survey for provision of the lead standard for interlaboratory calibration. Elizabeth Hodges, William Knox, Vanfred Gwinner, Barbara Wolf, and Kalter R. Eckelmann deserve credit for assistance in various phases of the work. Suggestions given by Paul Gast have been very helpful. LITERATURE CITED
(1) Bate, G. L., Kulp, J. L., “Variations
as measured by Instruments I and 11. It is also evident that identical results can be obtained for lead ion or trimethyllead ion. The trimethyllead ion spectrum is preferred, because the reproducibility is slightly better. More important, if radiogenic leads are being analyzed, the hydride correction must be assumed from the preceding and following common lead analyses. Since the corrections on the trimethyllead ion spectrum are much less than for the lead ion spectrum, i t is much less sensitive to any changes in source conditions. The results in Table I1 shoJV that the analyses of Nier (6, 8) have a syetematically higher lead-204 content (average of 1.6%) and consequently lower lead-208 (average of 0.6%). Although it is difficult to determine which assays are closer to the absolute values, the results presented above represent different sources and tubes, and suggest
that the lead iodide vapor technique used by Kier was subject to systematic discrimination. An experiment is in progress at this laboratory to measure the discrimination directly in the Lamont tetramethyllead analyses by accurately mixing lead-208-free lead with monazite lead which has a high lead-208-lead-206 ratio. The results of Table I11 on a n identical sample show that different laboratories using the tetramethyllead and solid techniques agree to within about tn-ice the errors obtained by Lamont using tetramethyllead with different techniques. This is considered excellent agreement for the present, but i t appears possible to reduce the error by factors of 2 to 3 with current equipment, ACKNOWLEDGMENT
The authors are grateful to the late
J. P. Marble, who furnished the original lead salts used by Xier, and to L. R.
in the Isotopic Composition of Common Lead and the History of the Crust of the Earth,” Columbia University doctoral thesis, 1955. (2) Collins, C. B., Farquhar, R. AI., Russell, R. D., Bull. Geol. SOC.Amer. 65, 1-22 (1954). (3) Dibeler, V. H., Mohler, F. L., J . Research Natl. Bur. Standards 47, p. 337-42 (1951). (4) Ducheylard, G., Lazard, B., Roth, E., J . chim. phys. 50, No. 10, 497 119531. ( 5 ) Fiquhar, R. RI., Palmer, G. H., Sitken, K. L., h‘ature 172,860 (1953). (6) Sier, -4.O., J . A m . Chem. SOC.60,1571 (1938). ( 7 ) Sier, A. O., Rev. Sei. Znstr. 18, 398 11947). (8) Sier, A. O., Thompson, R . IT-,Rlurphey, B. F., Phys. Rev. 60, 112 (1941). (9) Palmer, G. H., Aitken, K. L., J . Scz. Instr. 30, 314 (1953). RECEIVED for review July 25, 1956. Accepted September 22, 1956. Lamont Geological Observatory Contribution 213. Research initiated under tenure of S a tional Science Foundation predoctoral fellowship. Kork supported by the Research Division, U. S. Atomic Energy Commission, under Contract AT(30-1)1114.
Determination of Copper in Fuel Oil and Other Petroleum Products DAVID
M. ZALL, RUTH E. McMICHAEL, and D. W. FISHER
Naval Engineering Experiment Station, Annapolis,
b An accurate and simple spectrophotometric method for the determination of copper in petroleum products uses neocuproine (2,9-dimethyl-l,10phenanthroline) for the development of the copper-neocuproine complex, which has a maximum absorbance a t 450 mp. The method is rapid and direct and requires no ashing of the sample. No pH adjustment or extraction is necessary. The results compare favorably with those of other more tedious and time-consuming methods.
D
stored in copper-base alloy tanks gradually dissolves small amounts of copper. This proc88
IESEL FUEL
0
ANALYTICAL CHEMISTRY
Md.
ess is accelerated when water finds its way into the same storage tank. Extremely small amounts of copper are harmless and present no corrosion problem (8),but when the copper content becomes considerable, it causes heavy deposits in the precombustion chamber of one type of Diesel engine. The determination of copper in the fuel, therefore, becomes important. Although a number of methods for the determination of copper are available, they either lack specificity or are subject to interferences of one kind or another (4, 7 , 10, 14, 16). The neocuproine method proposed by Gahler (6), however, eliminates most inter-
ferences and simplifies the determination of copper. The use of neocuproine also has other advantages that are apparent n-hen applied to the determination of copper in fuel oil and other petroleum products. I n its application to the analysis of Diesel fuel or other petroleum products, the determination resolves itself into two parts: treatment and solution of the sample, and the spectrophotometric evaluation of the copper content. I n the treatment and solution of samples of organic materials, the three accepted procedures usually followed are wet ashing, dry ashing, or acid extraction, whether the final determination is made
