Rapid Polarographic Determination of Tetraethyllead in Gasoline WALTER HUBlS and RALPH 0.'CLARK Gulf Research & Development Co., Pittsburgh 30, Pa. The Cellosolve-hydrogen chloride procedure of Hansen and coworlrers for decomposition of tetraethyllead and the sirnultancous extraction of lead from gasoline containing tetraethyllead is more amenable to routine testing than other procedures, but is subject to error with certain samples when the lead is determined polarographically. This error was essentially eliminated by a simple modification of the original procedure.
M
OST accepted polarographic methods for determining tetraethyllead in gasoline have a common disadvantage: the time required for analvsi9. The method of Hansen, Parks, and Lykken (S), however, in which the tetraethyllead is decomposed in anhydrous Cellosolve containing hydrogen chloride, circumvents the most time-consuming operation of many methods -Le., the extraction of the lead from the gasoline. For this reason a study of the Cellosolve-hydrogen chloride method was initiated t o confirm its applicability to a variety of gasoline types. Almost immediately a number of difficulties were encountered: Residual current of the electrolyte could not be measured with the required degree of reliability; although a satisfactory calibration curve could be established for concentrations of 0.5 t o 2.0 ml. of tetraethyllead per gallon, there was not a linear relationship between diffusion current and concentration outside these limits; the half-wave potential shifted gradually to more negative values with increasing concentrations of tetraethyllead; and the results on some gasolines differed as much as 15% from those obtained by ASTM method D526, a recognized referee method of test ( 1 ) . 4 s the potential advantage of the procedure could not be disregarded, it was decided to attempt to overcome its undesirable features. Subsequent studies demonstrated that if the procedure was modified slightly, the objectionable features could be almost entirely eliminated. If the Cellosolve was cooled in an ice bath during acidification with anhydrous hydrogen chloride, the precision with which the residual current could be measured was improved significantly. Similarly, the addition of a small amount of gelatin solution improved the reliability with which both the residual and diffusion currents could be measured. Finally, if water was added to the Cellosolve-hydrogen chloride following decomposition of the tetraethyllead, certain interferences encountered with aged, cracked gasolines could be eliminated. REAGENTS
chloride electrolyte and 3 ml. of the leaded gasoline into a 50ml. borosilicate glass volumetric flask. Add, from a measuring pipet, 0.1 ml. of the gelatin solution. Immerse the bulb of the unstoppered flask in a water bath maintained a t a temperature 2' C. and heat for 30 minutes. Remove the flask and, of 95' while hot, add 10 ml. of distilled water from a pipet and swirl vigorously. (For calibration purposes, substitute 3 ml. of leadfree gasoline or iso-octane for the leaded gasoline sample and 10 ml. of the diluted lead nitrate solutions for the distilled water.) Stopper the flask and mix thoroughly, Cool to room temperature and transfer a portion of the aqueous Cellosolve layer to a polarographic cell, exercising care to avoid contamination of the solution with the hydrocarbon. Purge the solution with nitrogen, thermostat the cell, and obtain the polarogram trace in the usual fashion between 0.2 and 0.6 volt us. a mercury anode. Measure the height of the polarographic wave, deducting the residual current obtained in the manner described, but substituting 3 ml. of lead-free gasoline or iso-octane for the leaded gasoline sample. From the previously prepared calibration curve, determine the apparent concentration of tetraethyllead in the sample. Convert this figure to milliliters of tetraethyllead per U. S. gallon at 60' F. by applying the appropriate temperature corrections (1, a).
*
DISCUSSION
During the early stages of the investigation, the Cellosolve was purified by refluxing it with solid ferrous sulfate, folloived by distillation. Later experiments demonstrated that although smaller residual currents could be obtained with Cellosolve puri-
Table I.
Determination of Tetraethyllead in Gasoline
Gasoline Sample Motor I
Motor I1
Motor I11
Motor I V
Motor V
N o t o r VI
Cellosolve-Hydrogen Chloride Electrolyte. Pass anhydrous hydrogen chloride into Cellosolve (Carbide & Carbon Chemicals Co.) maintained at a temperature of 0" to 5' c. until the solution is approximately l.LV, as determined by diluting a portion with water and titrating with a standard base. Store a t a temperature of 0" C. JThen not in use. Standard Lead Nitrate Solutions. Transfer 1.338 grams of dry lead nitrate into a 1-liter flask, dissolve in lead-free distilled xater, and dilute to volume with distilled water. Prepare additional stocks containing 1.014 and 6.690 grams of lead nitrate per liter. Dilute a 10-ml. aliquot of each stock to exactly 100 ml. for calibration purposes. Ten milliliters of these diluted solution; contain an amount of lead equivalent to that present in 3 ml. of gasoline a t 60" F. having tetraethyllead contents of 1.00, 3.00, and 5.00 ml. per gallon, respectively. Gelatin Solution, 0 . 2 7 ~ . Dissolve 0.20 gram of gelatin in 100 ml. of hot distilled water.
Motor VI1
Motor VI11
Special motor
Aviation I
Aviation I 1
Aviation I11
PROCEDURE
Measure the temperature of the sample to the nearest 1' F. and by means of pipets transfer 10 ml. of Cellosolve-hydrogen
1009
TEL Content, 1\11. per Gallon ASTRl D 526 Polarographic Difference 3.11 3.15 3.12 3.16 A T . . 3.12 3.16 +0.04 1.98 1.99 1.91 1.96 Av. 1 . 9 5 1.98 +0.03 2.89 2.87 2.82 2.87 Av. 2.86 2.87 +0.01 2.79 2.83 2.78 2.85 Av. 2.79 2.84 +0.05 2.17 2.25 2.18 2.26 Av. 2 . 18 2.26 Jr0.08 1.90 1.96 1.89 1.97 Av. 1 . 9 0 1.97 40.07 1.89 1.86 1.92 1.88 Av. 1 , 9 1 1.87 -0.04 1.76 1.76 1.76 1.80 Av. 1 . 7 6 1.78 +0.02 3.04 3.11 3.02 3.08 Av. 3 . 0 3 3.10 f0.07 4 23 4.24 4.22 4.22 .Iv. 4 . 2 3 4.23 0.00 0 51 0.52 0 51 0.50 AT. 0.51 0,5l 0.00 4.11 4.19 1.16 4.17 .% 4I .15. 4.18 f0.03
ANALYTICAL CHEMISTRY
1010 fied in this fashion, the improvement was not sufficient t o warrant the additional time required. If the Cellosolve is received in metal cans, it should be transferred t o glass containers to prevent contamination by the container. .4n electrolyte 1.5AVwith respect to hydrogen chloride was chosen in preference t o the I S solution employed by Hansen and coworkers. This choice was based on the fact that slightly low results occasionally obtained with the weaker acid solution seemed to be eliminated by increasing the acidity of the solution. Where a large number of samples are t o be processed in a relatively short period of time, it has been found desirable to prepare the Cellosolve-hydrogen chloride solution 4 N with respect t o the hydrogen chloride and store at 0’ C. As the supporting electrolyte is required, a portion is withdrawn and diluted with cool Cellosolve t o 1.5N. A 4AVstock solution prepared in this fashion is stable for about 3 months: beyond this time a slight decrease in the normality of the solution may be observed, but otherryise there should be no apparent change. Hansen, Parks, and Lykken ( 3 ) pointed out that their procedure gave erroneous results with some samples of gasolinee which had been blended with cracked material that had been stored for a period of time, and suggested that interference from peroxide compounds might be responsible. This hypothesis was confirmed, a t least in part, by substituting iso-octane containing known amounts of organic hydroperoxides or peroxides. During the present investigation it was found that such reduction waves could he eliminated entirely if water was added t o the Cellosolve-hydrogen chloride mixture following heating, and the aqueous layer employed for analytical purposes. Employing the procedure described, tests on samples of gasoline containing known amounts of tetraethyllead t o which various peroxide compounds had been added substantiated the desirability of adding water, and demonstrated that quantitative recovery of the tetraethyllead as lead could be assured. Subsequent evperiments proved that this
modification did not introduce undesirable side effects from unsaturated hydrocarbons or other substances which are considered normal components of commercial gasolines. Moreover, the addition of water seemed to improve the consistency with which diffusion and residual currents could be measured. While this increased precision may be reflected in several ways, it is believed that by elimination of the hydrocarbon phase from the test solution the capillary tip is maintained in a more reproducible state, which assures a more consistent drop time. The failure of the half-wave potential to remain independent of the tetraethyllead concentration in the original Cellosolve procPdure has likewise been eliminated by the proposed modification. .4 large number of leaded gasoline samples have been analyzed and compared with the results obtained by the referee method of test (1, 2 ) . A portion of these data, covering a period of over 2 years (Table I), illustrate. the applicability of the modified Cellod v e procedure to petroleum testing. ITith the possible exception of gasoline samples I’ and TTI,the agreement of results by the two methods is extremely good. At the time this work was done, it was thought that the polarographic results for samples V and VI were in error; however, now nearly 3 years later, there is some ieason to doubt whether or not this conclusion is warranted. Recently it was found desirable to revise ASTM method D 526 ( I ) , because it gave known low iesults in some samples. The two Gamples in question may have been of a similar type. LITERATURE CITED
(1) din. Soc. Testing Materials, “Standards on Petroleum Products and Lubricants.” I S T l I Designation D 526-48T, S o r e m b e r
1950. ( 2 ) Ibid., Designation D 52B -5XT. November 1953. ( 3 ) Hansen, K. A . , Parks. T. D.. and Lykken. L., ANAL.CHEJI.. 22, 1232-3 (1950). RECEIVEDfor review .4ugust 30. 19.54,
Accepted December 30, 1954
Quantitative Spectrographic Analysis of Rare Earth Elements Determination of Holmium, Erbium, Yttrium, and Terbium in Dysprosium, Determination of Yttrium, Dysprosium, and Erbium in Holmium, and Determination of Yttrium, Dysprosium, Holmium, Thulium, and Ytterbium in Erbium VELMER A. FASSEL, BEVERLY QUINNEY, LAIRD C. KROTZ, and CARL F. LENTZ institute for
Atomic Research and Department o f Chemistry, lowa State College, Ames, lowa
Emission spectrometric methods are described for quantitative determination of rare earths commonly associated with purified dysprosium, holmium, and erbium. The concentration range from the detection limit up to 1% is covered by these methods. The procedures are based on direct current carbon arc excitation of rare earth oxide-graphite mixtures. The unique similarity in excitation behavior among many of the rare earths provides a high degree of internal standardization of variables inherent in direct current carbon arc excitation. An unusual example of self reversal in the Ho 3456.00 A . line is noted.
I
N PREVIOUS papers of the series (3, 4 ) , emission spectrometric methods for the quantitative determination of the rare earths commonly found as impurities in purified lanthanum, cerium, praseodymium, neodymium, and samarium were described. The present article is on the extension of the same basic method to the determination of the other rare earths commonly associated with purified dysprosium, holmium, and erbium. -4s in the pro-
cedures discussed previously, the methods described cover the concentration ranges from the detection limits up to about 1%. Impurity concentrations above 1% can usually be determined by spectrophotometric measurements (8, 10, 11). 4PP4R4TUS
The spectrograph, e\ternal optical system, excitation source, electrode assembly, photographic processing, and microphotometer employed in this investigation have been described ( 4 ) . EXCITATIOU CONDITIONS
The considerations involved in the selection of direct current carbon arc excitation of the samples in the form of rare earth oyide-graphite mixtures have been given in detail (2-4). Briefly stated, these factors were as follon s: the desirability of exciting the samples under conditions requiring a minimum of preliminary sample preparation, hence the choice t o employ the oxide form as obtained from niost flactionation procedures; the convenience and sensitivitj- of the direct current carbon arc for exciting the spectra of refractory oxides; the enhanced stability
