from cup “G”.This approach permits complete separation of the two layers. Added features of this system are that sample vials instead of sample cups could be used in the secondary separator and that the organic phase can be saved for use in other analytical procedures. A comparison of the results obtained by automated extraction vs. manual extraction in volumetric flasks is presented in Table I. The data show an acceptable correlation between the two methods in the concentration range from 0 to 40 pg/l. Although the extraction efficiency of the automated method gradually decreases a t concentrations greater than 40 pgll., its efficiency is still within 90% at the 95106 pg/l. level. Because of the principles attending an automated system and barring interfering substances in the sample, extraction efficiency at the higher concentrations becomes of secondary importance since both standard and sample are treated identically and thus are affected equally by the extraction process. Studies in our laboratory have shown that exceptionally high cation concentrations do not significantly increase ion carry-over from one sample to the following sample. Samples containing an ion concentration of 100 pg/l. or greater can be analyzed by direct aspiration of the sample. Recovery experiments were conducted by adding known amounts of metal cations to water samples which were previously analyzed by the manual extraction method. The results recorded in Table I1 show a maximum difference of 2 pgll. even a t the higher concentrations.
Data regarding the precision of the automated extraction method are presented in Table 111. Water samples containing three concentrations of each metal were analyzed for cadmium, copper, iron, and lead. Each sample was analyzed thirteen times, allowing a five-minute interval between determinations. The mean and standard deviation calculated for samples containing the highest concentrations was found to be: cadmium, 41.9 f 1.18 pg/l.; copper, 36.6 f 1.26 pg/l.; iron, 41.1 f 0.95 pgll., and lead, 37.2 f 1.21 pg/l. The sensitivity for each of the cations analyzed was: cadmium, 0.2 pg/l.; copper, 0.4 pg/l.; iron, 1.0 pg/l.; lead, 1.5 pgll.; manganese, 0.4 pg/l.; nickel, 1.0 pgll.; silver, 0.4 pg/l., and zinc, 0.2 pg/l.
ACKNOWLEDGMENT The authors thank R. E. Isaacs for his support. LITERATURE CITED (1) T. T. Chao, M. J. Fishman, and J. W. Ball, Anal. Chim. Acta, 47, 189 (1989). (2) P. D. Gouiden, P. Brooksbank, and J. F. Ryan, Amer. Lab., 5 (8), 10 (1973). (3) C. R. Parker, “Water Analysis by Atomic Absorption Spectroscopy”, Varian Techtron Ry. Ltd., Springrale, Australia, 1972, pp 28, 29, 42, 43, 48-51, 57, 58, 82-65, 72, 73.
RECEIVEDfor review October
2, 1974. Accepted February
3, 1975.
Determination of the Solubility of Manganese Hydroxide and Manganese Dioxide at 25 OC by Atomic Absorption Spectrometry H. A. Swain, Jr., Chris Lee, and R. B. Rozelle Department of Chemistry, Wilkes College, Wilkes-Barre, PA 18703
A number of methods have been employed to measure the solubility product constant of Mn(0H)Z and are documented in the text. This paper contains measurements which have for the first time measured the K,, of Mn(OH)2 by atomic absorption spectrometry. Also, the solubility of MnO2 was measured as a function of pH and graphical results used to calculate the K,, for the solubility of MnOz. The MnO2 used had a surface area of 1.0 m2/g and had the rutile crystal form.
EXPERIMENTAL Fisher Scientific Company reagent grade MnS04. H20 and MnO2 were used. The distilled water used was boiled and cooled under Nz. Carbonate-free NaOH solutions were prepared and used. The experiments were carried out under N2 and necessary precautions taken to avoid CO2 contamination of samples. The Mn02 samples were washed with distilled water and dried a t 110 “C before use. A Perkin-Elmer 290 atomic absorption spectrometer was employed for manganese analysis and found to have an ultimate sensitivity of 0.02 ppm manganese. The instrument was calibrated before each analysis using MnS04. H20 solutions of known concentrations in distilled water.
The Mn(0H)Z solubility measurements were made under a NZ atmosphere in a flask maintained a t 25.0 f 0.2 “C by means of a constant temperature water bath. Carbonate-free NaOH was added to 1000 ppm manganese(I1) solutions which were then equilibrated for varying periods of 24-72 hours and filtered under Nz. The data showed only random variation when the equilibration times were greater than 24 hours. The pH was measured before and after filtration and, in each case, the pH dropped by about 0.5 pH unit. The MnOz solubility measurements were made under a N2 atmosphere by mixing 0.25 g of washed and dried MnO2 and 50 ml of carbonate-free distilled water. The pH was varied from 2.1 to 10.6. The washed and dried MnO2 was found to have the rutile crystal structure and to have a surface area of 1.0 m2/g, increasing from about 0.7 m2/g before washing and drying. The solutions were allowed to equilibrate for approximately l to 4 hours a t 25 f l “C. Further precision in temperature control was not merited by the relatively poor precision for the small concentrations being measured. Samples from the MnOz, as well as from the Mn(0H)z runs were filtered through 50-mp millipore filter sheets. As a check, 100-mp millipore filters were used occasionally, giving results equivalent to those from the 50-mp runs. The pH was measured before and after filtration and, if the difference in the two pH’s was greater than 0.5 pH unit, the data were ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1135
Table I. Solubility D a t a a n d Calculations for Mn(0H)z at 25.0 "C Ionic strength,
Equil. pH
Manganese concn,M
ccizt
8.84 8.89 9.46
6.5 x 10'3 3.1 x 10-3 3.6 x 10-4
0.104 0.091 0.080
a
3'
7f
0.32 0.33 0.35
KC
Kso
3.1 x 10'13 1.9 x 10-l3 3.0 x 10-13
1.0 x 10-13 0.6 x 10-13 1.0 x 10'13 20%
K,, = 9 x 10-14 Calculated using modified Debye-Huckel equation and data from J. Kielland, J. A m . Chem. Soc., 59, 1675 (1937).
Table 11. Solubility of MnOz (1 m2/g, Rutile Structure) i n Aqueous Solution at 25 "C as Determined by Atomic Absorption PH
PPm
Mole/liter
2.1 3 .O 5.4 6.9 7.0 8.4 8.7 10.2 10.6
1.3 0.9 0.5 0.1 0.1 0.3 0.02 0.1 0.2
2.4 x 10-5 1.6 x 10'5 0.9 x 10'5 0.2 x 10-5 0.2 x 10'5 0.5 x 10-5 0.04 x 10-5 0.2 x 10'5 0.4 x 10-5
0
Table 111. Comparison of Solubility Measurement K
Temp, "C
SOb
Method
Reference0
Room temp. Solubility measurements (1) 4 x 10-14 18 Conductivity (2) 1.3 x 10-14 18 Hydrogen electrode (3) Glass electrode solubility 1.3 x 10-13 25 measurement (4) Solubility measurements, corrected to zero 2 x io+ 25 ionic strength (5) Differential emf method, corrected to zero ionic 2 x 10-13 25 strength (6) Polarographic 4.5 x 10-l~ 22 method (7) Solubility from at. 9 x 10-14 25 absorption (this work) a The following references were obtained from Special Publication No. 17, The Chemical Society, London, G.B., 1964, "The Stability Constants of Metal Ion Complexes;" and are listed in the bibliography a t the end of this article. * Ksok have been rounded in this paper to 1 or 2 significant figures.
8x
rejected. The pH was varied by addition of carbonate-free NaOH solution, HCl solution, or a sodium hydroxide-borate buffer solution.
.
atomic absorption are set equal to that of manganese(II), since the equilibrium constants for the formation of Mn(OH)z(aq)tMn(OH)+(,,), Mn(OH)3-(aq) given by Butler ( 8 ) indicate negligible concentrations of these complexes present compared with that of the manganese(I1). Manganese(1V) Oxide. It is recognized that MnOz often has a large surface area and is associated with surface-catalytic activity. The surface areas of the material used in this research were found to be quite low, both before (0.7 m2/g) and after (1.0 m2/g) washing and drying, and it is not likely that the measured solubilities are significantly affected by unique surface activity in this material. These surface areas were determined by BET plots and by the B-point method, the results of the two methods agreeing within 10%. The powder X-ray analysis of the washed and dried MnOz showed it to be in the rutile crystal form. A plot of the log of the manganese concentration in MnOz saturated solution vs. pH is shown in Figure 1. The slope of the line, Alog [manganese]/ApH, is -0.2 f 0.05. Considering these results, a generalized equilibrium for MnOa(s) in water may be hypothesized: MnOL(s) + (4 - s)H' + (S -
2 ) HZO
Mi1(OH),'"-''
RESULTS AND DISCUSSION
Manganese(I1) Hydroxide. The results of solubility measurements on Mn(0H)Z and MnOz are presented in Tables I and 11. The K,, of Mn(OH)2 calculated from the results are given in Table I11 where it can be compared to values reported in the literature (1-7). Of these values, three are reported at 25 "C; these are 1.3 X (41, 2 X ( 5 ) ,2 X (6). The total manganese concentrations as determined by 1136
Figure 1. Log of manganese concentration in equilibrium with solid M n 0 2 (1 m2/g, rutile structure) vs. pH at 25 OC
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
4-X
log K,, = log [M!i(OH), log Keq
+
] - log [H']'.'
(s - 4) pH = log [Mn(OH),4-X]
A plot of log of the soluble M~I(OH),*-~concentration vs. pH should have a slope of x - 4 and an intercept equal to log Keq.The observed slope is observed to be about -0.2
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
