species formed is probably PtCLBr2-*, which has a much stronger absorption maximum a t 380 mp. A typical spectrophotometric curve for the back-titration is shown in Figure 3. The initial absorbance of the soluof tion used in this titration-Le., PtC16-*-was only 0.104. Table I summarizes data obtained in 24 titrations of 2.6 to 13.1 mg. of platinum using potentiometric end point detection. The average error for titrations of platinum alone was ca. *0.5%. Interference of the other platinum metals was investigated. Xeither palladium(I1) nor rhodium(II1) is reduced to any significant extent by tin(I1) under the conditions of the titration. The potentials of the appropriate palladium and rhodium couples are such (8) that reduction by tin(I1) would be expected, and again failure of this reduction must be ascribed to a slow rate of reaction. At higher current densities the gold generator electrode gets slightly plated with palladium metal, owing to some direct electrode reduction of palladium(II), and slight positive errors result. This error can be minimized by low current densities, or pregeneration
of the tin(I1) before addition of the deaerated sample. The bromide of the supporting electrolyte was oxidized by iridium(1V) to bromine, in agreement with the findings of Dwyer, McKenzie, and Nyholm (7). This bromine may be removed by boiling the acidified bromide solution containing the sample for about 15 minutes, before addition of stannic chloride. The solution is then cooled, stannic chloride is added, and the solution is titrated. This treatment was partially satisfactory, but still yielded high (1 to 2%) results, probably due to incompleteness of the iridium(1V)-bromide reaction. Table I gives data for the titration of platinum in the presence of various amounts of palladium(II), rhodium(III), and iridium(1V). Osmium and ruthenium in the higher oxidation states interfere, but are usually easily remoyed by volatilization. Among the other metals commonly found associated with platinum, copper, iron, and gold interfere. Lead, mercury, and silver probably do not. I n the presence of interfering metals the usual methods for the separation of the platinum metals (6) must be employed.
LITERATURE CITED
(1) Ayres, G. H., Meyer, A. S., ANAL. CHEM.23, 299 (1951). (2) Bard, A. J., Lingane, J. J., Anal. Chi?. Acta 20, 463 (1959). (3) Ibzd., p. 581. (4) ,Basolo, F., Pearson, R. G., “Mechanisms of Inorganic Reactions,” pp. 1928, Wiley, New York, 1958. (5) Beamish, F. E., -4naZ. Chim. Acta 20, 101 (1959). (6) Beamish, F. E., Talanta 1, 3 (1958). (7) Dwyer, F. P., McKenaie, H. A., N holm, R. S., J. Proc. Roy. SOC.N . S. d l e s 81, 216 (1947).
(8) Latimer, W. H., “Oxidation States of
the Elements and Their Potentials in Aqueous Solutions,” 2nd ed., PrenticeHall, New York, 1952. (9) Lingane, J. J., Anal. Chim. Acta 19, 394 (1958). (10) Ibid., 21,227 (1959). (11) Lingane, J. J., “Electroanalytical
Chemistry,” 2nd ed., Interscience, New York, 1958. (12) Meyer, A. S., Ayres, G. H.,J. Am. Chem. SOC.77, 2671 (1955). (13) Schlesinger, H. I., Palmateer, R. E., Ibid., 52, 4316 (1930). (14) Sweet, T. R., Zehner, J., ANAL. CHEM.30, 1713 (1958). RECEIVED for review October 12, 1959. Accepted February 2, 1960. Division of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
Polarographic Determination of Manganese in Gasoline Triethanolamine Complexes of Manganese(l1) and (111) and Lead(II) E. R. NIGHTINGALE, Jr., University of Nebraska, Lincoln 8, Neb. G. W. WlLCOX and A. D. ZIELINSKI, Research Laboratories, Ethyl Corp., Detroit 20, Mich.
b The triethanolamine complexes of manganese(l1) and (Ill) and lead(l1) have been studied polarographically, and their nature has been determined. A polarographic method for deter(methylcyclopentadienyl)manmining ganese tricarbonyl and related compounds in gasoline has been developed. The organomanganese compounds are decomposed and the manganese is determined by measuring the diffusion current of the manganese(ll1)-triethanolamine complex in alkaline solution. The feasibility of determining tetraethyllead simultaneously has been investigated.
T
introduction of (methylcyclopentadieny1)manganese tricarbonyl as an antiknock compound for internal combustion engines (2) has necessitated the development in these laboratories of methods for the assay of organomanganese compounds and the determinaHE
tion of manganese in gasoline. This paper reports a procedure for the determination of manganese in gasoline which employs the equipment used for the polarographic determination of tetraethyllead. The manganese compound is decomposed by ultraviolet light and the manganese is extracted by refluxing with hydrochloric acid. The manganese is determined polarographically as the manganese(II1)-triethanolamine complex. Tetraethyllead is decomposed and lead is extracted simultaneously with the manganese, and may be determined polarographically in an aliquot of the acid extract. The nianganese(I1) and (111) and lead(I1) complexes with triethanolamine are characterized, and the formation of a manganese(II1)-peroxide complex is confirmed (6). REAGENTS AND APPARATUS
c. P. chemicals and distilled water were used to prepare all of the reagent
solutions. Triethanolamine (TEA) 98% (Matheson Coleman & Bell KO. 2885) was used to develop the analytical procedure, although “regular” (80y0 minimum) triethanolamine is satisfactory for routine analyses. The triethanolamine can be dispensed COIIvenientl from a 5-ml. hypoderniic syringe ztted with a 13-gage needle. Current-potential curves were recorded a t 25.0’ =t 0.1’ C. using a Sargent Model XX Polarograph. A Hume-Harris saturated calomel reference electrode (S.C.E.) was used throughout, and all potentials are rrferred to the S.C.E. PROCEDURE
Pipet a 50-ml. sample of gasoline containing 0.1 t o 0.7 gram of manganese per gallon into an ASTM tetraethyllead extraction apparatus ( I ) . Add 50 ml. of heavy petroleum distillate and 50 nil. of concentrated hydrochloric acid. Estract the gasoline as described in ASThl rocedure D 526-56 ( I ) . During the rst stage of the estraction, irradiate
x
VOL. 32, N O . 6, MAY 1960
625
I
0 -I
! ! -
I
I-
2
-08
-04
LOG
00
[TE4]
EXPERIMENTAL RESULTS AND DISCUSSION
Triethanolamine Complexes of Manganese and Lead. When man-
ganese(I1) is oxidized in alkaline triethanolamine solution, a green manganese(II1)-triethanolamine complex is formed ( 4 ) . Although the manganese(II1)-triethanolamine complex has been discussed in several polarographic studies (3,6,6,Q), the manganese complexes have not been adequately characterized. 626
ANALYTICAL CHEMISTRY
POTENTIAL,
I
rr.SCE
-0 e
1
-1 2
Figure 2. Current-potential curve for solution containing 2mM manganese in 0.6M triethanolamine and 0.6M sodium hydroxide after air oxidation
Figure 1. Variation of with triethanolamine concentration for reduction of lead(ll) in 0.54M sodium hydroxide
the sample for about 25 minutes with a 275-watt ultraviolet lamp (sun lamp) placed as close to the extraction vessel as possible. Evaporate the aqueous extract to about 20 ml. Add 5 ml. of triethanolamine and 1 drop of 0.1% thymol blue solution. Seutralize to p H 9 with sodium hydroxide, and add an excess of 3 ml. of 9Jf sodium hydroxide, stirring well to dissolve any nianganesr (11) hydroside precipitated by a local excess of the base. Add 50 to 100 mg. of trilead tetroxide (PbaOc) and boil the solution until all the lead oxide is dissolved. Add 2 ml. of 2-If sodium sulfite. Transfer the solution to a 50ml. volumetric flask and dilute to volume. The final solution contains 0.6M triethanolamine and 0.5431 sodium hydroxide. Measure the diffusion current a t -0.7 volt us. S.C.E., and determine the manganese by comparison with standard aqueous samples. Tetraethyllead may be determined in a n aliquot of the same solution. Extract the sample as indicated above. Add 2 ml. of 0.1% grlatin solution t o the acid extract and dilute the sample to 250 mi. Evaporate a 200-ml. aliquot of this solution to 20 ml. and determine the manganese in this aliquot as described above. In the remaining portion of the solution, measure the diffusion current for the aquolead(I1) at -0.6 volt, and determine the lead l y comparison with standard samples.
I
I -0.4
The current-potential curve for the reduction of manganese(II1) in alkaline triethanolamine solution exhibits two waves (3). The first corresponds to the reversible 1-electron reduction of the manganese(II1)-triethanolamine complex to form the manganese(I1)-triethanolamine complex. -4t 25’ C., a plot of E us. log ( i d - i ) / igives a straight line with a slope of 0.060 volt as compared to the theoretical value of 0.0591 volt. I n agreement with previous investigators, the half-wave potential for the manganese(I1)-(111) couple was -0.500 volt. This potential is independent of triethanolamine concentrations from 0.05 to l.5M and is not affected appreciably by changes in sodium hydroxide concentration. The second current-potential curve corresponds to the reduction of the manganese(I1)-triethanolamine complex to form manganese metal. I n a strongly basic solution, the wave approaches that of a reversible 2-electron couple. I n a less basic solution or in a solution containing large concentrations of sodium ions, the reduction wave is distorted by hydrogen discharge or the reduction of sodium ion. The half-wave potential of the manganese(I1)-(0) couple in 0.6M triethanolamine and 0.54M sodium hydroxide is - 1.7 volts. B y a method of continuous variations, the empirical composition of the manganese (11)-triethanolamine complex has been determined to be Mn(TEA)2+2 (6). Since the half-wave potential of the manganese (111)-(11) couple is independent of the concentration of triethanolamine, the manganese(I1) and (111) complexes have the same empirical formulas, and the formula for the manganese(II1) complex is given as ~‘~I(TEA)~+~. I n alkaline triethanolamine solution, the lead(I1)-triethanolamine complex is
reduced to form lead amalgam. The reduction yields well-defined currentpotential curves, but the reaction does not proceed reversibly and becomes less reversible as the concentration of triethanolamine is increased. I n 0.54M sodium hydroxide, the slope of the plot of E us. log ( i d - i)/i increases from 0.058 volt for solutions containing 0.075.44 triethanolamine to 0.093 volt for 1.2M triethanolamine. The theoretical value for this plot is 0.030 volt. The half-nave potential in 0.6.M triethanolamine and 0.54.V sodium hydroxide is - 1.02 volts. For a reversible reaction of this type, the slope of the plot of half-wave potential us. the logarithm of the triethanolamine concentration permits the determination of the number, p , of triethanolamine molecules bound per lead ion. Figure 1 illustrates the variation in half-wave potential with the logarithm of the concentration of triethanolamine. The decrease in the absolute magnitude of the slope with increasing triethanolamine concentration parallels the decreasing reversibility. Since p cannot decrease with increasing triethanolamine concentration, this variation is attributed to the decreasing reversibility of the electrode reaction and undoubtedly arises from a change in the ratedetermining reaction. Under these conditions, the number of triethanolamine molecules associated with the lead ion cannot be accurately evaluated. However, the slope a t higher triethanolamine concentrations is -0.065 volt, from which p is estimated as 2. The diffusion current constant, I , for the reduction of the complexes of manganese(II1) and lead(I1) has been evaluated as a function of the triethanolamine and sodium hydroxide concentrations, These data, together with the absolute viscosities, 9, of the original solutions, are presented in Table I. For the dropping mercury electrode used in this work the values of m2’3 P a were 1.971 mg.2’3 see. 1’8 a t -0.70 volt and
1.938 1~g.2’3sec.l/B a t -1.2 volts in 0.6M triethanolamine and 0.54;M sodium hydroxide. The viscosity of triethanolamine a t 20’ C. is 8.27 poises ( 7 ) . Small variations in either triethanolamine or sodium hydroxide concentrations affect the magnitude of the diffusion current appreciably. If the change in viscosity is responsible for the variation in diffusion current with variations of triethanolamine or sodium hydroxide concentrations, a combination of the IlkoviE and Stokes-Einstein equations predicts that the product 12q should be constant. The relative constancy of 12q for both the manganese and lead currents in Table I is evidence that the viscosity of the solution is predominant in determining the magnitude of the diffusion current. The diffusion coefficient for the manganese(II1) complex in 0.6.M triethanolamine and 0.54M sodium hydroxide is calculated to be 4.24 X sq. cm. per second. This value compares favorably with t h a t reported by Issa et aL(3). The previous polarographic procedures for manganese using triethanolamine involved the preparation of the manganese(II1)-triethanolamine complex by air oxidation of the manganese (11) complex (3, 6, 8, 9). All attempts in these laboratories to achieve rapid quantitative air oxidation a t room temperature have been unsuccessful because the last stage of the oxidation reaction is slow and an interfering manganese(II1)-peroxide complex is formed (6). I n recent studies, Issa and coworkers (3) have observed t h a t the time required to obtain complete oxidation of the manganese(I1)-triethanolamine complex was proportional to the concentration of manganese and dependent upon the concentrations of triethanolamine and sodium hydroxide. The more concentrated solutions are not airoxidized quantitatively unless they stand for a t least 24 hours. Contrary to the work by Issa, the hydroperoxide formed by the air oxidation of manganese(I1) was found to interfere with the determination. The difficulties associated with achieving rapid quantitative oxidation are due primarily to the interference of the manganese(II1)peroxide complex, the details of which have been described (6). Figure 2 illustrates the current-potential curve obtained for a solution containing 2mM manganese in 0.6M triethanolamine and 0.6M sodium hydroxide immediately following air oxidation. I n contrast with the dark green color characteristic of the manganese(II1)triethanolamine complex, this solution exhibited an olive green color resulting from the presence of the red manganese peroxide complex. The cathodic wave arising from the anodic mercury dissolution is due to the reduction of the manganese(II1)-peroxide complex prior
Table
I.
Diffusion Current Constants and Viscosity of Solutions
TEA, Moles/ Liter
NaOH, Moles/ Liter
0.075 0.30 0.60 0.91 1.21 0.60 0.60 0.60 0.60
0.54 0.54 0.54 0.54 0.54 0.054 0.18 0.54 1.08
Viscosity, 7, Poise
Manganese(111)-TEA I I =7
0,0105 0.0120 0,0139 0,0168 0.0202 0,0127 0.0132 0,0139 0,0176
1.49 1.34 1.25 1.11 1.04 1.31 1.31 L 25 1.14
Sample IA IC
Determination
Tetraethyllead, Ml./Gal. Present Found
0.248 0.255 0.246 0.563 0.574
3.09
0.251
0.253 .
3.06
0 608
0.273 0.256 0.563
0.602
3.10
...
IIA
IIC
I
0,107 0.105 0.111 0.114 0.132 0.103 0.114 0.111 0.118
of Manganese and Tetraethyllead in Gasoline
Manganese, G./Gal. Present Found 0.248
3.20 2.96 2.82 2.61 2.56 2.85 2.95 2.82 2.59
Determination of Manganese. The decomposition of the organomanganese compound in gasolines is effected by irradiation with ultraviolet light for about 25 minutes during the extraction procedure. During the development of the procedure, the extracted gasoline residues were analyzed for manganese and lead, using a flame photometer with an oxyhydrogen flame (1.2). For iso-octane and themore volatileaviation fuel stocks, no traces of lead or manganese could be detected in the gasoline residues. For less volatile motor fuels, , particularly those containing appreciable amounts of olefinic components, small concentrations of manganese on the order of 1 mg. per gallon may remain unextracted. A second extraction procedure utilizing mixed hydrochloric and nitric acids has been investigated to obtain complete extraction within 10 minutes. This procedure is not satisfactory for olefinic base stocks, however, because appreciable quantities of organic matter are extracted from the gasolines and interfere in the subsequent polarographic determinations. I n attempting to effect the rapid quantitative oxidation of manganese(I1) without formation of the peroxide complex, a large variety of oxidizing agents has been investigated. With the exception of the trilead tetroxide (PbBOJ recommended in the present procedure, only oxygen effected the oxidation with-
to the reduction of the triethanolamine complex. Upon standing, the peroxide complex slowly decomposes, and the reduction current for this complex decreases to the normal residual current, approximately 0 pa. Although the composition of the peroxide complex has not been determined, the diffusion coefficient evidently does not differ greatly from the diffusion coefficient of the triethanolamine coqplex, because the absolute magnitude of the manganese diffusion current a t -0.7 volt remains approximately constant after the peroxide complex has decomposed. Riha and Serak (IO)have investigated the polarographic behavior of mixtures of manganese(II1) and iron(II1) in alkaline triethanolamine solutions and have observed that, in the presence of iron(III), hydroperoxide does not interfere with the manganese determination. However, the reduction of iron(II1) in freshly prepared solutions containing hydroperoxide results in a catalytic iron(II1)-peroxide wave, the height of which is approximately proportional to the manganese and hence the peroxide concentration. The failure of previous investigators to identify the manganese (111)-peroxide complex may lie in the fact that the peroxide complex cannot be distinguished polarographically from hydrogen peroxide, because hydrogen peroxide itself is also reduced a t potentials more positive than that a t which mercury is oxidized.
Table II.
0.0233 0.0215 0.0217 0.0207 0,0218 0.0218 0.0227 0.0217 0.0229
Lead(11)-TEA I 1%
0 * 552
...
3.18
3.14 3.10 3.11 3.09 3.10 3.14 3.10 3.12 3.12 3.28 3.26 3.12
VOL. 32, NO. 6, MAY 1960
627
out formation of the peroxide complex. The details of these investigations on the nature of the oxidation process and a n alternative procedure for oxidation using oxygen have been described (6). I n the present procedure, the concentration of excess sodium hydroxide in the supporting electrolyte must be controlled within rather close limits because the diffusion coefficient of the manganese(II1)-TEA complex changes appreciably with the concentration of excess base. The oxidation of manganese(I1) with PbsOl is rapid and complete for solutions as concentrated as 10mM. I n addition, the PbsO, is a n effective catalyst for the decomposition of hydroperoxide in alkaline solution ( I I ) , and thus prevents the formation of the manganese (111)-peroxide complex, The excess PbsOc is easily dissolved in the boiling solution to form the lead(I1)triethanolamine complex. This complex is reduced a t potentials more negative than that for the manganese(II1)triethanolamine complex and does not interfere with the determination. The sodium sulfite removes any excess oxygen from the solution and is also effective in decomposing any remaining hydroperoxide. If the sulfite is omitted and the oxygen removed by bubbling with nitrogen, the determinations will be less precise. Contrary to the work by Issa et al., the concentration of sodium sulfite does not appear to affect the magnitude of the diffusion current. No evidence was obtained for the instability of the manganese (11)-triethanolamine complex or for partial precipitation of
the manganese from the alkaline triethanolamine solutions. Because the lead(I1)-triethanolamine complex is reduced at potentials more negative than those for the manganese (111)-triethanolamine complex, the feasibility of determining the lead as the lead-triethanolamine complex has been investigated. For iso-octane solutions and aqueous standards, the procedure is effective and yields accurate results. However, the extraction of most motor fuels introduces appreciable organic matter into the aqueous phase. The subsequent reduction of this organic matter interferes with the determination of lead as the lead(I1)-triethanolamine complex, and this procedure is less accurate than other methods. For these reasons, it is recommended that the diffusion current of the aquolead(I1) be determined directly in the acid extract a t -0.6 volt. A variety of gasoline base stocks has been used in determining the precision and accuracy of the present procedure. Typical results for the manganese and lead determinations are presented in Table 11. The base stock used in samples IA and I C contained 5,553 olefins and that used in I I A and I I C contained 41% olefins. The standard deviation from the amount present was 5.2% for manganese and 1.6% for lead. The uncertainty in these results is introduced primarily during the extraction of the motor fuel stocks. For pure isooctane or aqueous standards, the accuracy of the present method compares favorably with that of other polaro-
graphic methods, and average deviations do not exceed *0.5Q/o, ACKNOWLEDGMENT
The assistance of H. R. Xeal, Ethyl Corp., in determining the viscosities of the solutions is gratefully acknowledged. LITERATURE CITED
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “ASTM Standards on
Petroleum Products and Lubricants,”
1958. (2) Ethyl Corp., Detroit, Mich., “Kew Antiknock Compound Based on Manganese,” 1957. (3) Issa, I. M., Issa, R. M., Hewaidy, I. F., Omar, E. E., Anal. Chim. Acta 17,434 (1957). (4) Jaffe, E., Ann. chim. appl. 22, 737 (1932). (5) Mojeis, J Proceedings International Polarograpi’ic Con em Prague, 1st Congress, 1951, Pt. r p . 638. (6) Nightingale, E. R., Jr., ANAL.CHEM. 31, 146 (1959). (7) Noll, A., Bolz, F., Papier-Fa&. 33, Tech. Ted, 193 (1935). ( 8 ) Novak, J. V. A., Kuta, J., Riha, J., Chem. l k t y 47,649 (1953). (9) Pleva, M., Ibid., 49,262 (1955). (10) Riha, J., Serak, L., Ibid., 49, 32 (1955); Collection CzechosIov. Chem. Cornmuns. 20, 640 (1955). (11) Schumb, W. C., Satterfield, C. W.,
Wentworth, R. L., “Hydrogen Peroxide,” p. 480, Reinhold, New York,
1955. (12) Smith, G. W., Palmby, A. K., ANAL. CHEM.31,1798 (1959).
RECEIVEDfor review October 7, 1959. Accepted January 22, 1960. Work done in Ethyl Corp. Research Laboratories.
pH Measurement at Low Temperatures Using Modified Calomel and Glass Electrodes LAMBERTHUS VAN DEN BERG Division of Applied Biology, National Research Council, Ottawa, Canada
b The addition of polyalcohols (or some of their derivatives) to the saturated potassium chloride solution of calomel electrodes, and the enlargement of the glass membrane of glass electrodes, together with replacement of the inner buffer solution with mercury, permitted the measurement of pH down to -30’ C. Changes in liquid junction potential of calomel electrodes in acidic, alkaline, and colloidal media were substantially reduced without detriment to pH measurements in dilute aqueous solutions. pH was 30’ C. measured in solutions down to and in finely crushed frozen material down to -20’ C. Preliminary results
-
628
ANALYTICAL CHEMISTRY
of pH measurements of frozen salt solutions, milk, and meat are presented.
C
electrodes are unsuitable for p H measurement directly in frozen material because the potassium chloride solution present in reference electrodes solidifies a t -10.7’ C. and the buffer solution in glass electrodes solidifies at even higher temperatures. I n addition, the resistance of glass electrodes increases rapidly with decreasing temperature to the point where accurate measurements are precluded. Measurement of the p H of frozen material has, therefore, been ONVENTIONAL
possible only by the elaborate method of separating the unfrozen liquid from the ice and measuring the p H a t or above oo c. (3,7,9,IO). This paper describes the development of modified reference and glass electrodes suitable for p H measurement a t subfreezing and ordinary temperatures. APPARATUS AND PROCEDURE
Preparation of Modified Calomel and Glass Electrodes. Commercially ayailahle calomel electrodes with fibertype junctions were modified by replacing the potassium chloride solution with aqueous solutions of organic
