Tetraethyllead in Gasoline
A Direct-Reading Polarograph For Determination of Tetraethyllead in Gasoline E. B. OFFUTT A N D L. V. SORG Research Department, Standard Oil Company (Indiana), Sugar Creek, Mo. The successful investigation of antimony as a pilot ion in the polarographic determination of lead has made possible the construction of an instrument by which the lead concentration of a gasoline may be read directly from a calibrated dial while the dropping mercury electrode is polarized in the hydrochloric acid extract (D 526-48T) from the oil. Predetermined potentials are applied through a sequence of push-button operations. Compensation of the resulting currents is accomplished by adjustment of potentiometers using a galvanometer as a
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MBER of analytical procedures have been proposed for the polarographic determination of tetraethyllead (TEL) in gasoline ( 2 , 4 ) . These procedures involve tetraethyllead decomposition by hydrochloric acid, subsequent extraction of the resulb ing lead chloride in a manner similar to that described by Calingarrt and Gambrill(3), and analysis of the extract by means of currcnt measurements in a polarographic cell containing a dropping inrrcury electrode. Inasmuch as lead is the only metallic element present in the extract solution from the gasoline, the solution readily lends itself to polarographic analysis; the lead concentration in such extracts is generally in the range particularly suithle for this type of analysis. The method of Calingaert and Gambrill is currently the tentative htandxrd method of the American Society for Testing Materials (1). This method, commonly referred to as the A.S.T.M. method, consists of acid extraction of lead from the gasoline, gravimetric determination of the lead as lead chromate, and conversion to milliliters of tetraethyllead per gallon (3785 ml.) of gasoline. Carbonaceous material contained in the lead extract must be destroyed by wet oxidation prior to precipitation of the lead chromate. This is a time-consuming operation from the standpoint of routine analytical procedure because of the required multiple evapxation. Application of certain polarographic techniques for determination of the lead in the extract makes it unnecessary either to remove the carbonaceous material or to evaporate the solution. In general, therefore, a polarographic prosedure offers the advantage of requiring less time for a tetraethyllead determination than the A.S.T.M. gravimetric method. However, in many of the advocated procedures for polarographic analysis, a polarogram of the solution is prepared by either manual or automatic plotting of the relationships between voltage and diffusion current that result from the effect of the lead ion, and sometimes of other ions, upon a dropping mercury electrode placed in the solution. The subsequent measurement of the lead-ion diffusion current is converted to an equivalent tetraethyllead value by reference to a previously prepared standard calibration curve or chart. Such a polarographic technique requires relatively complicated and expensive equipment. I n view of the specific nature of the lead solution obtained from the tetraethyllead extraction, it has been recognized that polarograms are not required and that a relatively simple polarograph can be employed for the analysis. I n order to render the polarographic technique more suitable for routine use and to reduce it to
null instrument; the last such adjustment gives the reading of tetraethyllead concentration in milliliters per gallon. Provision has been made for rapid standardization of the instrument and repetition of the push-button sequence for check determinations. Comparison of this polarographic method with the standard A.S.T.M. method for the determination of tetraethyllead in gasoline shows that the polarographic method is a t least equivalent to the A.S.T.M. method with respect to both accuracy and reproducibili t y
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thc simplest practical form, a new polarographic instrument has been developed. This unique instrument has been designed to indicate directly-without calculations or referenre to graphs or charts-the lead content of the acid extract froin a gasoline. The analytical result, obtained after performing several einipl? operations and adjustments, is read directly from a rloul~le-scaledial
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.32
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i PESIDUAL t
VOL TS
Figure 1. Relationship between Voltage Applied to Dropping Mercury Electrode and Resulting Diffusion Currents Due to Antimony and to Three Different Concentrations of Lead
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1235
V O L U M E 22, NO. 10, O C T O B E R 1 9 5 0 calibrated for the tN0 ranges 0 to 4 and 4 to 8 ml. of tetrs~thyllend per gallon of gasoline.
lower measuring-cell chamber, which contains the quiet mercury p w l and the solution to he analyzed. The measuring cell is d e signed to facilitate measurement of successive silmnles without
S'RUiCIPLE
Operation of the new polarograph is based upon the use of antimony as a p i b t ion in the polarographic measurement of lead in the acid extract from gasoline. Maintaining the half-wave potential of the pilot ion below that of lead-as is the case with antimony-permits simplification of the design and use of the polarograph. Figure 1 presents a set of characteristic curves showing ihe relrttionship hetween voltage applied to the dropping mercury electrode and the resulting diffusion currents due to ant,imony and to three different concentrations of lead, equivalent to 1, 2, and 3 ml., respectively, of tetraethyllead per gallon of gasoline. The voltage values are those measured hetween the dropping mercury electrode and the quiet mercury pool with a hydrochloric acid supporting electrolyte. The current increase obt,ainedby raising the voltage to 0.32 volt is the diffusion current due to antimony ions, and the next current increase obtained by raising the voltage to 0.75 is due to lead ions. With this instrument, diffusion currents are not actually measured, hut rather the magnitude of the current due to lead ions is compared with the current due to added antimony ions. As the amount of antimony added to the solution is constant, the resulting ions serve aa pilot ions to which the lead may hereferred. By eomprsring the diffusion current due to the lead, whioh varies in concentration, with that due to antimony, which is constant in concentration, variations due to moderate differencesin mercnrydropping rate, cell temperature, acidity of the solution, and solution dilution are nullified. The influence on lead determinations of several variahles has been investigated. The mercury dropping rate may vary from 3 to 5 seconds per drop. Temperature change within the range of 20' to 30' C. has no measnrahle effect upon the accuracy of the analyses, No measurahle effect is evident when the acidity of the solution to he analyzed is increased or decreased by 40% from the typical value of 1.2 N . I n the find preparation of the solution to hc analyzed, exact dilution is not required. DESCRIFTION
The new direct-reading polarograph.is illustrated in Figure 2. [In accordance with the long estahlished policy of Standard Oil Company (Indiana) of making its new developments generally available, i t is expected that arrangements will he consummated in the near future for the licensing of an estahlished instrument company to manufacture and sell this type of polarograph.] The instrumentation is housed in a ca8e measuring approximately 10 inches wide. 14 inches deea m d 8 inches hieh. Two
adiustine txe'ealvanometer "zero" to the center of the ealvahomeGr se& is located on the top rear of the galvanom
