Rapid Spectrophotometric Determination of Triethyllead, Diethyllead, and Inorganic Lead Ions, and Application to the Determination of Tetraorganolead Compounds S. R. HENDERSON and
L. J. SNYDER
Research and Development Deparfment, Ethyl Corp., Bafon Rouge
b A simple, rapid method has been developed for simultaneous determination of triethyllead, diethyllead, and inorganic lead ions by converting them into their dithizonates in chloroform and measuring the absorbances of the solution at three wave lengths. Results show a precision (twice the standard deviation of error) for any one of the three forms of lead in a of the amount mixture of &4% present at the 100-pg. level. Tetraethyllead and other tetraorganolead compounds may be determined by converting them to the ionic form and measuring the increase in ionic lead. The precision of the method thus applied to tetraorganolead compounds is f l . 3 % of the amount present. This application of the method has been successfully used in the determination of tetramethyllead, tetraethyllead, tetra-n-propyllead, tetrai-butyllead, tetra-i-amyllead, tetra-namyllead, tetravinyllead, and tetraphenyllead, individually, or in the presence of appreciable amounts of ionized lead.
N
have been developed for the determination of tetraethyllead and inorganic lead. However, methods for determination of the decomposition products of tetraorganolead compounds are few and for the most part they are lengthy and cumbersome. Calingaert (1) in extensive studies on organolead compounds isolated the various components by fractional solution techniques, converted the lead in each fraction to inorganic lead and measured the lead by conventional volumetric, gravimetric, or colorimetric procedures. Snyder, Barnes, and Tokos (12) in determining microgram quantities of tetraethyllead in air reported the formation of colored ethyllead dithizonates having absorption spectra differing sharply from that of lead dithizonate. No analytical procedure was reported for the determination of the intermediates. Cremer (3) in studies of the toxicological properties of tetraethyllead and its decomposition products used a dithizone UhlEROUS METHODS
1 172
ANALYTICAL CHEMISTRY
I, la.
colorimetric method in which the color of the dithizone freed from the triethyllead-dithizone complex by acidification was used as a measure of the triethyllead ion. Interference from inorganic lead was eliminated by complexation with EDTA. DeVries, LauwZecha, and Pellecer (4) developed a polarographic method which directly measures the triethyllead ions in the presence of tetraethyllead and hexaethyldilead. A rapid method for the simultaneous determination of triethyllead, diethyllead, and inorganic lead ions has long been needed to eliminate the lengthy and tedious separations and subsequent decompositions now required in research on the preparation and application of organolead compounds. This paper describes a method for accompli.hing this determination. I t has already proved its value and reliability as an analytical tool in organolead research previously reported by this laboratory (6,8). PRINCIPLE OF
METHOD
In an organic solvent such as benzene and with a limited reaction time, tetraethyllead reacts quantitatively with iodine to form triethyllead iodide. This is the basis for the iodimetric method for the determination of tetraethyllead (10). With bromine or chlorine in organic solvent, tetraethyllead reacts rapidly to form the corresponding diethyllead halide. In direct sunlight tetraethyllead is slowly converted to the triethyllead ion, then more slowly to the diethyllead ion, and finally to the simple lead ion. Under excessive heat tetraethyllead decomposes to form metallic lead and a variable mixture of hydrocarbon gases. This study was concerned primarily with the analytical implications of the progressive degradation of tetraethyllead stepwise to inorganic lead. These reactions may be represented by the following equations in which R signifies an organic radical and X a halogen: PbRi X z PbRs+ X RX PbRa+ Xz + PbRz+z X RX PI)Rz+* X t -,PbCZ 2 R X
+ +
-+
+
+ + + + +
The third equation might be broken down into two steps, showing the formation of the monoethyllead ion, a theoretical possibility with some supporting evidence (9, 11), although this ion was not detected experimentally by Calingaert ( I ) or during this study. If the monoethyllead ion is formed, its life span in the presence of iodide ion is extremely short, and it breaks down almost instantaneously into inorganic lead ions.
+
+
+
PbRz+' Xz -+ PbR+' X - RX PbR+a X - 4 Pb+Z RX
+
+
In this system we can conclude that, with the exception of the monoethyllead ion which for all practical considerations is nonexistent, we have four components requiring analytical consideration: tetraethyllead, triethyllead ion, diethyllead ion, and inorganic lead ion. Since dithizone does not react with tetraethyllead, our field of analysis is narrowed to three components. Each of these three forms of lead has a distinctive chloroform-soluble dithizonate. Triethyllead ion forms a canary yellow complex, diethyllead ion an orange complex, and inorganic lead ion a I.ed complex. Absorption maxima are a t wave lengths of 435, 487, and 520 mp, respectively. By extracting the combined dithizonates and measuring the absorbances of this solution at three wave lengths it should be possible to calculate each component provided the color system of the dithizonates follows Beer's law and the absorption curves of the individual dithizonates have sufficient differentiation. EXPERIMENTAL
To calibrate a spectrophotometric method involving three colored species, pure materials were prepared and absorption curves established. Triethyllead chloride and diethyllead dichloride were prepared by classical procedures (a, 5 ) . Other triorganolead ions were obtained by treatment of the tetraorganolead compound with iodine in chloroform solution and were purified by washing with dilute hydrochloric
Table I. Calibration Constants Obtained for Various Dithizonates
IO-mm. cell 25 ml. CnCll
Absorbancelpg. Pb 424
500
540
Triethyllead 0.0045 0.0013 0.0000 0.0028 0.0118 0.0054 Diethyllead Inorganic lead 0.0020 0.0118 0.0097
L 40
WAVE LENGTH, m p
Figure 1 . zonates
Typical absorption curves for lead dithiI. II. 111.
100 pg. Pb as [PbEtJ]Dz 100 pg. Pb as [PbEtz]Dzz 100 p g . Pb as PbDzs
acid (1%). Standard solutions of their dithizonates containing approximately 100 pg. of lead per milliliter were prepared from each of these, and a similar standard was also prepared from purified lead nitrate. Figure 1 shows typical absorption curves for triethyllead, diethyllead, and inorganic lead standards. A mathematical examination of these curves revealed that greater differentiation could be obtained by selecting wave lengths at points other than the maxima. Calibrations made using absorbances measured at 424, 500, and 540 mp showed close conformity to Beer’s law. Table I shows the constants (absorbance/microgram of lead) observed for the dithizonates. These mere used in deriving the equations used in the calculations. These equations were derived as follows : Let
and c equal the absorbance ineasurenients of the sample at 424, 500, and 540 mp, respectively x, y, and 2 equal micrograms of lead (tri), lead (di), and lead (inorganic), respectively LI, tz, and t 3 equal the absorbance per microgram of lead (tri) at 424, 500, and 540 mp, respectively dl, dz, and d~ equal the absorbance per microgram of lead (di) a t 424, 500, and 540 mp, respectively 21, is, and ia equal the absorbance per microgram of lead (inorganic) at 424, 500, and 540 mp, respectively a, b,
Then a
rtl
+
++ ++
+
ydl 221 = 0.00452 0 . 0 0 2 8 ~ 0.00202 b = ~ L z ydz zit = 0.00134~ 0 . 0 1 1 8 ~ 0.01182 c 2t3 yda Zi8 = 0.0054~ 0.00972 =
+ +
+
+ +
Solving for x, q, and z in terms of a, b, and c gives the equations necessary for the final calculations:
+ + +
2 4 5 ~- 77b 4 1 ~ - 5 5 ~ 192b - 2 0 3 ~ z = 2 6 ~ 90b 194~ z y z = 216a 25b 32 2 =
y
+ +
-
+
+
(1) (2) (3) ~(4)
REAGENTS AND EQUIPMENT
Ammoniacal Buffer Solution. Dissolve 20 grams of ammonium citrate, 8.0 grams of potassium cyanide, and 48 grams of sodium sulfite in 320 ml. of distilled water. Dilute to 2 liters with concentrated ammonium hydroxide. The citrate is added as a solubilizer of cations. The cyanide complexes interfering cations. The sulfite maintains a reducing system, thereby stabilizing the dithizone which is readily destroyed by oxidants. Dithizone Solution. Dissolve 60 mg. of diphenylthiocarbazone in 1 liter of reagent grade chloroform. This solution should be protected from light (opaque container) t o avoid excessive decomposition. At room temperature this solution is normally stable for 1 month or more. Iodine Solution, 0.1N. Dissolve 13 grams of resublimed iodine crystals and 25 grams of potassium iodide in 200 ml. of water and dilute to 1 liter with water. Spectrophotometer. A Beckman Model DU spectrophotometer was used in this work. Other instruments would be equally applicable but would require individual calibration. The special absorption cells and cell compartment cover described in (7) were used. PROCEDURES
Determination of Triethyllead, Diethyllead, and Lead Ions. Place 50 ml. of water, 20 ml. of ammoniacal buffer solution, and 25 ml. of dithizone
solution in a special absorption cell ( 7 ) . Shake the contents of the cell vigorously until equilibrium is attained. A slight turbidity which is sometimes observed can usually be removed by further shaking. Measure the absorbance of the contents of the cell at 424,
500, and 540 mp. Record these readings as the sample blank. n’rap the cell with an opaque cloth to protect the light-sensitive dithizonates and add an aliquot of the sample containing approximately 100 pg. of lead and a n acid content not t o exceed the equivalent of 2 ml. of 10% nitric acid. Shake the cell vigorously until the ionized lead compounds have been completely extracted into the chloroform phase as the dithizonates. Again measure the absorbance of the solution a t 424, 500, and 540 mp. Correct the latter readings by subtracting the respective blanks, Calculate the triethyllead, diethyllead, and inorganic lead in the sample using the special equations derived earlier. The algebraic sum of the three forms of lead is equal to the total ionized lead in the sample. The results given in Table I1 show the accuracy and precision of this method for the determination of ionic forms of lead in a series of synthetic samples containing varying amounts of triethyllead, diethyllead, and inorganic lead ions. Determination of Tetraethyllead. Tetraethyllead does not react with dithizone t o form a complex. Analyses of samples by the described method will, therefore, give values for only the ionized lead present in the sample. If two equal portions of the sample are analyzed, one by the above method and the other nith a n added oxidation step by which the tetraethyllead is converted t o ionic lead, the difference in the two analyses will be equal to the tetraethyllead originally present. A simple, satisfactory oxidation procedure was readily available from a standard method (10)-the iodimetric determination of tetraethyllead. A few tests readily confirmed the applicability of this technique. A slight excess of iodine in chloroform converted the tetraethyllead to ionic lead almost instantaneously. Predominately triethyllead ions were formed. Bromine in chloroform was equally reactive, forming predominately diethyllead ions. The recommended procedure is as follows :
Introduce 50 ml. of water, 5 ml. of chloroform, 5 drops of 0.1N iodine solution, and 20 ml. of ammoniacal buffer solution into one absorption cell. Shake well to reduce the iodine VOL. 33, NO. 9, AUGUST 1961
1173
Table II.
-
Analysis0 of Samples Containing Known Amounts
Added Pb+t
Total
50.0 0.0 107.0 50.0 0.0 50.0 50.0 25.0 0.0
107.0 95.0 107.0 152.5 148.0 141.0 160.0 173.0 182.0
Tri
Di
0.0 0.0 0.0 45.5 91.0 91.0 91.0 91.0 182.0
57.0 95.0 0.0 57.0 57.0 0.0 19.0 57.0 0.0
of Triethyllead Chloride, Diethyllead Chloride, and Lead Nitrate
Found Tri 0.6 0.0
0.1 46.3 91.7 92.9 92.7 92.5 182.0
Di 54.4 95.2 -0.3 52.8 55.8 1.0 17.3 56.6 0.0
Error Pb +2
Total-
Tri
Di
Pb+’
Total
49.2
104.2 95.2 107.1 151.9 148.1 143.5 158.7 176.5 182.0
+0.6 0.0 +0.1 +0.8 $0.7 $1.9 +1.7 $1.5 0.0
-2.6 +0.2 -0.3 -4.2 -1.2 $1.0 -1.7 -0.4 0.0
-0.8
$0.3 +2.8 $0.6 -0.5 -1.3 f2.4 0.0
-2.8 +0.2 $0.1 -0.6 $0.1 +2.4 -1.3 $3.5 0.0
-1.0
+0.3
$0.1
f1.6
11.4
f1.9
0.0
107.3 52.8 0.6 49.5 48.7 27.4 0.0
Mean error $ 0 . 8 Std. dev. of error f 0 . 7 a
0.0
All data expressed in microgram of lead.
Table 111.
Compound Tetrameth llead Tetraethy lLad Tetra-n-propyllead Tetra-i-butyllead Tetra-i-amyllead Tetra-n-amyllead Tetravinyllead Tetraphenyllead
Analyses of High Purity Tetraorganolead Compounds
Weight Per Cent Lead Gravimetric Theoretical Iodimetric sulfate Dfthizone 77.50 64.06 54.59 47.58 42.14 42,14 65.70 40.19
76.40 63.61 54.70 47.28 41.95 41.06
... ...
solution. This cell serves as a sample blank. Into a second absorption cell introduce 50 ml. of water, 5 ml. of chloroform, and 5 drops of 0.1N iodine solution. To each cell add an aliquot of the sample containing approximately 200 pg. of lead as tetraethyllead. Shake the second cell for 5 seconds, immediately add 20 mi. of ammoniacal buffer solution, and mix the contents to reduce the excess iodine. To each cell add 20 ml. of dithizone solution and shake vigorously until equilibrium is attained. Measure the absorbance of each of 424, 500-, and 540-mp wave lengths. Subtract the readings of the first cell from the corresponding readings of the second cell and calculate the micrograms of lead in the tetraethyllead by use of Equation 4.
Greater sensitivity can be obtained by a reduction in the volume of dithizone solution. However, the calculations must be adjusted accordingly. APPLICATIONS
Laboratory tests show that the dithizonates of trimethyllead, triphenyllead, and trii-amyllead ions give absorptian curves identical with that of triethyllead dithizonate. Since triorganolead ions are the principal prod-
1 174
ANALYTICAL CHEMISTRY
76.20 63.44 54.43 47.41 41.83 41.42 65.24 40.01
% Error
$0. 26 -0.44 $0.83 -0.13 $0.74 -1.13 -0.06 0.00 Mean error $0.01 Std. dev. of error &0.63%
76.40 63.16 54.88 47.35 42.14 40.95 65.20 40.01
ucts of the reaction of iodine with tetraorganolead compounds in chloroform solution, the method was extended to the determination of tetraorganolead compounds other than tetraethyllead. The results summarized in Table I11 show excellent agreement between the dithizone, the gravimetric sulfate, and the iodimetric methods. No results are given for the determination of tetravinyllead and tetraphenyllead by the iodimetric method because the reaction in each of these cases was not stoichiometric to the triorganolead stage. The results by the gravimetric sulfate method were used as the basis for computing the errors of the dithizone method. Theoretical values for total lead are given to indicate the relative purity of the sample. For the accurate determination of the individual diorganolead ions other than the ethyl or methyl homologs, the present calibration may not be applicable. The absorption curve of diphenyllead dithizonate differed significantly from that of the diethyllead dithizonate. New calibration data are definitely needed for the accurate analysis of a system containing diphenyllead ions. For the analysis of
systems containing other diorganolead ions, further confirmation of the calibration data would be advisable. The rapidity of the dithizone technique makes it useful in the determination of yields in the preparation of a number of tetraorganolead compounds. It is especially appropriate for the determination of tetravinyllead and tetraphenyllead where the iodimetric method is inapplicable and for other preparations containing materials that interfere with the iodimetric method. The dithizone method was particularly useful during a study of reaction rates of organolead compounds with decontaminants. In storage stablity testa of antiknock blends under various conditions and with various additives, this method has replaced the much longer and less accurate ammonia extraction procedure. Gross amounts of tetraorganolead compounds do not interfere in the determination of trace amounts of ionic lead. An application has been made to the determination of tetraorganolead in leaded fuels. Possible interferences due to dyes and colored additives need further study to properly assess this use. The calibration constants of Table I should be applicable to any Beckman Model DU spectrophotometer. Their accuracy with respect to other instruments can be checked by using a standard of pure tetraethyllead. If this shows appreciable variation, it would be necessary to prepare new calibration curves. DISCUSSION
In the determination of inorganic lead by dithizone techniques, precision values vary considerably with the analyst. However, with a proper understanding of the variables in this
determination a high degree of precision is readily achieved. It is not the intent of this paper to discuss the variables affecting precision of dithizone techniques. Under the proposed method, conditions are held rigidly constant with regard to pH, relative volumes of aqueous to chloroform phases, possible sources of contamination, evaporation of solvent, blank corrections, and temperature. Making the determination in a single cell (7) eliminates a host of indeterminate errors, thereby greatly enhancing the precision. Errors in the recommended procedure arise from errors in measurement of chloroform, dithizone solution, and sample aliquot, from incomplete extraction of the ionic lead due to insufficient shaking, and from the presence of possible interferences. Errors in volume measurements of chloroform, dithiaone, and the sample can be minimized by proper calibration of glassware and care in delivery. Sample aliquots in this study were measured and delivered from a 0.5ml. capillary pipet calibrated in 0.025ml. divisions. Microsyringes with extremely fine hypodermic needles were also used for measurement of samples requiring aliquots of 0.05 ml. or less. Errors from incomplete extraction will disappear as the analyst gains experience. An additional shaking period and second series of readings can readily determine whether quantitative extraction has been achieved. Protection from light during this shaking period is essential if triorganolead ions are present. Otherwise, absorbance readings a t 500- and 540-mp wave lengths will steadily rise. The triorganolead
dithizonates are converted to inorganic lead dithizonates more or less rapidly, depending upon the specific organic radical present and upon the amount of light exposure. Interferences as a source of error are serious only in special applications of the method. Consideration should be given to the possible presence of organic compounds of metals other than lead, and to the presence of colored components. Organic compounds of zinc, mercury, thallium, tin, and bismuth may interfere where the simple metal ions would not. Unlike inorganic mercury, the ethylmercury ion is not complexed by gross amounts of cyanide. Organomanganese hm produced an interference, while manganous ion has not. If the system contains organometallic compounds of metals forming colored dithizonates, the possible interference should be investigated. Dyes and other colored materials are obvious sources of error which are likely to be encountered in gasoline or antiknock fluid samples. No interference from this source is observed in the procedure for tetraorganolead unless iodine affects the color strength of the dye. A direct interference from colored components is encountered in the procedure for the determination of ionic forms of lead. In both procedures such color interferences can be removed by applying a correction obtained by conducting the analysis with chloroform replacing the dithizone solution. Interference from decomposed or partially decomposed dithizone, although negligible in the determination of inorganic lead, is more serious when triorganolead dithizonates are being
measured a t 424 mfi. When th; decomposed dithizone reaches a concentration sufficient to give a blank absorbance of greater than 0.20, it ,would be advisable to prepare a fresh solution of dithizone. ACKNOWLE~OMENT
We thank C. F. Yancey, W. E. Becker, and E. C. Juenge for their efforts in supplying ,the organolead compounds used in this study. LITERATURE CITED.
(1) Calingaert, G., D kstrs, F. J., ShaDiro. H.. J . Am. 8&m. SOC. 67. 190 (1945). ' (2) Calingaert, G., Shapiro, H., Dykstra,
F. J., Hess, L., Ibid., 70, 3902 (1948). (3) Cremer, J. E., Brit. J. Ind. Med. 16, 191 (1959). (4) DeVriea, J. E., Lauw-Zecha, A., Pellecer, A., ANAL. CHEM.31, 1995 f 19.59l. \--_-
(5) G i l L n , H.,Robinson, J. D., J . Am. Chem. Soe. 5 2 , 1975 (1930). (6) Griffing, M. E.,Roeek, A., Snyder, L. J.. Henderson. S. R.. ANAL. CHEM. 29, 190 (1957). ' (7) Henderson, S. R., Snyder, L. J., Ibid., 31, 2113 (1959). (8) Juenge, E. C., Cook, S. E., J . Am. Chem. SOC.81, 3578 (1959). (9) Lesbre, M.,Cumpt. rend. 204, 1822 (1937). (10) Newman, L.,Philip, J. F., Jensen, A. R., IND.ENQ. CHEM.,ANAL. ED. 19, 451 (1947). (11) Panov, E. M.,Kocheskhov, K. A., Proc. Acad. Sci. U.S.S.R. 85, 1037 (19.521. (12) Snyder, L. J., Barnes, W. R., Tokos, J. V., ANAL.CHEM.20, 772 (1948). l-._-
RECEIVED for review November 3, 1960. Accepted Februa 6, 1961. Eighth Annual Analytical%hemistry Conference, Detroit, October 1960.
A N e w Field Method for the Determination of Organolead Compounds in Air LOUIS J. SNYDER
and S. R. HENDERSON
Research and Development Department, Ethyl Corp., Baton Rouge, la.
b A simplified field method permits fhe rapid determination of several organolead compounds in air. It is superior to methods previously described for the determination of tetraethyllead in air. The method utilizes as a scrubber a small disposable glass tube containing iodine crystals supported by two glass wool plugs. Lead is removed from the scrubber by an acidic potassium iodide soluiion and is measured by a colorimetric dithizone method. The scrubber is also effective in collecting some organometallic
compounds of metals other than lead. Data are presented to show relative reaction rates and inlermediate products obtained from reacting tetraorganolead compounds wifh aqueous iodine in conveniional scrubbers.
I
research on various organolead compounds, the need for monitoring atmospheric lead frequently arises. Unlike inorganic lead, tetraorganolead compounds are present in the atmosN
phere as a vapor. The organolead vapor must be reacted in the scrubbing device and then the reaction products removed from the air stream by either scrubbing or filtering. Obviously, if the reaction rates of the organolead compounds with the scrubber solution are not of the proper magnitude, appreciable lead losses may occur in collecting and analyzing air samples. For the determination of tetraethyllead, suitable methods are available (1, 8). However, these methods were inadequate when applied to the de'
VOL 33, NO. 9, AUGUST 1961
0
1175
