Use of a Wax-Impregnated Graphite Electrode in Polarography V. FRANCES GAYLOR, ANNE 1. CONRAD, and JEAN H. LANDERL Chemical and Physical Research Division, The Sfandard Oil Co. (Ohio), Cleveland 6, Ohio
b W a x impregnation of a graphite rod yields a solid electrode which is superior in several respects for polarographic measurement. Absorption of wax into the electrode pores greatly reduces the high residual currents normally found with the plain graphite rod. Sensitivity to small concentrations of polarographically active species is thus increased by a factor of 10 or more and the reproducibility of both current and potential measurements i s improved. The wax-impregnated electrode is applicable to both oxidation and reduction reactions. Satisfactory polarographic data have been obtained on oxidation waves of typical organic compounds of the phenolic, amino, and diamine types, and on reduction waves of typical inorganic species. Half-wave potentials can b e measured with a standard error of less than 0.01 volt. Diffusion currents are proportional to concentration and may be used as a basis for quantitative analysis; the standard deviation of Concurrent measurements is 2%. centrations of 10-5M or lower can b e satisfactorily measured in a stirred test solution.
S
ELECTRODES possess several major favorable attributes for certain types of polarographic work. The useful anodic range of most solid electrodes is considerably greater than that of the dropping mercury electrode; consequently, their use permits polarographic study of many oxidizable compounds to which the conventional dropping mercury electrode is not applicable. Solid electrodes can be used with stirred test solutions; the enhanced diffusion currents obtained greatly increase the sensitivity of concentration measurement. I n addition, solid electrodes are particularly adaptable for use in organic test solutions, showing none of the erratic behavior often associated with the dropping mercury electrode in such media; the latter is of particular importance in the analysis of petroleum products, which usually require anhydrous test solutions for direct analysis. These advantages have provided incentives in the search for a solid electrode which would approach the general reliability, OLID
224
ANALYTICAL CHEMISTRY
usefulness, and versatility of the dropping mercury electrode. Platinum has been the most widely used material for solid electrodes (3, 6). Satisfactory results are generally obtained when the electrode surface can be reproducibly restored after each use; this involves renewal of activity as well as removal of deposition products. Special care is necessary to prevent surface gaseous film formation which may result in anomalous effects (7‘). An electrode for which a new surface can be supplied for each run would eliminate many of these difficulties-e.g., the mercury pool electrode ( l a , I S ) and the graphite rod electrode (3, 8). However, neither of the latter two electrodes is entirely satisfactory. The mercury pool electrode possesses good sensitivity for reducible species but has a very limited anodic range because of the anodic dissolution of mercury. The graphite electrode can be used both cathodically and anodically, but its utility is limited by both poor sensitivity and, in some cases, poor reproducibility (3). Difficulties with the graphite electrode are largely attributable to the high residual currents, i,, often encountered. Sensitivity is particularly limited by the slope of the background current in the less positive potential
ranges (Figure 1). I n some cases this problem can be circumvented, because both current slope and magnitude of i, are influenced by experimental conditions. Residual currents become greater as the rate of voltage scanning is increased and as the initial potential is shifted to more negative values. Limitations imposed by the high i, values of the graphite electrode are of particular importance in the determination of compounds having half-wave potential, El/?, values more negative than +0.2 volt. The slope of the residual current in the potential range of -0.2 t o +0.2 volt is such that detection of limiting currents of the order of &a. is difficult. For example, the problem became acute in attempting to determine S,S’-di-sec-butyl-p-phenylenediamine in concentrations of 10-4M and less; Ell2was approximately +0.1 volt in the desired electrolyte and limiting currents were less than 2pa. Accurate measurement of such small current values in the presence of the high i, of the normal graphite electrode is impractical. The present study describes the development of a modified graphite electrode which avoids the difficulties described. Impregnating the pores of the graphite with a solid wax reduces residual currents to negligible values
50
1
40
io2
0
to2
+04
+06
+08
+ I IO
VOLT
Figure 1. Residual currents encountered with graphite electrodes (3, 8)
pH 5.2 acetate buffer in 1:1 alcohol; automatically recorded in a positive direction; quiet solution A . Rate of potential variation, 1.24 mv./sec.; initial potential, 0.25 volt B . Rate of potential variation, 1.21 mv./sec.; initial potential, 0.00 volt C . Rate of potential variation, 0.62 mv./sec.; initial potential, $0.25 volt
but does not affect faradayic currents to the same extent. The resulting waximpregnated graphite electrode consequently displays greatly increased sensitivity to low concentrations and considerably improved reproducibility. K i t h the proper choice of impregnating agent, it can be utilized in either aqueous or nonaqueous solutions. It is applicable to both inorganic and organic olidizable and reducible species.
a t 40" F. under nitrogen to prevent deterioration. All electrolytes used were C.P. chemicals. The aqueous stock buffer solution was 1.OM in sodium acetate, acidified with acetic acid; the apparent pH, on dilution with an equal volume of alcohol, was 5.2. Commercial tank nitrogen was purified and conditioned by passage through alkaline pyrogallate, water, and a portion of the test solution. DEVELOPMENT OF ELECTRODE
APPARATUS
Current-potential curves were automatically recorded with a Sargent Model XXI polarograph. A saturated calomel electrode (S.C.E.), used as an external reference electrode, was connected through a n agar bridge to the 25-m1.capacity polarographic cell. The latter was jacketed with provision for circulation of water, controlled a t 25" f 0.2" C. Test solutions were agitated by means of a constant speed (300 r.p.m.) two-bladed stirrer ( 2 ) . Cell resistances n ere measured with a conductivity bridge (Industrial Instruments Co., Model RC-BC). All potentials are reported with reference to the saturated calomel electrode and are corrected for iR drops if necessary. Impregnated electrodes were prepared from 0.25-inch special graphite spectroscopic electrodes (rational Carbon Co.). Spectrographic Grade A graphite rods (0.24-inch) of density grades U-2, U-1, and U-F4 (United Carbon Products Co.), and pencil leads (Eagle and Orloff, and Eberhard and Faber) were also tested as electrodes. The outer surface of each electrode, both normal and waximpregnated graphite rods, was coated with Seal-All (Allen Products Corp.) and allowed to dry. A short piece of rubber tubing containing mercury served as a contact between the electrode and the lead to the polarograph. The lower end of the rod, inserted into the test solution, served as the electrode surface; a new surface was exposed for each run hy sawing or grinding off the end. REAGENTS
Catechol, hydroquinone oxalic acid, p-phenylenediamine, and quinone (Castman Kodak white label grade); S,S'-di-sec - butyl - p - phenylenediamine (Tennessee Eastman commercial grade); phenol (Baker c . P . , purified by distillation) ; resorcinol (Eastman Kodak practical grade) ; and N-n-butyl-p-aminophenol (complimentary sample from E. I. du Pont de Nemours &- Co.). Hydrogen peroxide (Merck & Co. Superoxol, purity assayed by chemical analysis) ; lead nitrate (Baker c.P.); and silver nitrate (Baker and Adamson reagent grade). Ceresin wax (hiamaroneck Chemical Corp., 165-170" F. melting point); castor wax (Baker Castor Oil Co.); and opal wax (E. I du Pont de Nemours & Co.). Isopropvl alcohol (Fisher C.P. grade). The dibutylphenylenediamine, used as a standard test substance, was stored
Several possible methods for altering the i, characteristics of the normal graphite electrode were investigated. Both increasing the graphite density and decreasing the apparent electrode area served to decrease i,; however, as the id values decreased by about the same ratio, no net gain in sensitivity resulted. Altering the electrode shape resulted in more effective improvement. A cone-shaped electrode surface, obtained by grinding a graphite rod in a pencil sharpener, was compared to a flattipped rod. A large reduction in i, was obtained and sensitivity, evaluated from the ratio of id/i,, was increased approximately tenfold. A lead pencil electrode (8) subsequently produced an id/& ratio which was 15 times that obtained from the flat-surfaced graphite rod. The superiority of the pencil electrode \+-asfinally traced to composi-
Table I.
Comparison of Impregnating Agents (l0-4M dibutylphenylenediamine in pH 5.2 acetate buffer in 1 : 1 alcohol-water;
Xational Carbon graphite electrodes; potential variation of 1.24 mv./sec.; quiet solution) Impregnating Material idiir Xone 0 1 Saran resin 1 3 Sylon 2 0 Silicone resin 2 0 Lemon wax 2 5 Ceramid wax 3 7 Silicone 200 (12,500 viscosity) 6 . 5 Silicone 200 (350 viscositvi 8 7 Opal wax 11.3 Ceresin wax 12.2 Castor wax 20.0
Table II.
tion; pencil leads usually contain small amounts of n-axy solids as binding agents. The effects of saturating graphite elcctrodes lvith various materials were then evaluated quantitatively. Saturation was attained by soaking the graphite rod in the liquefied impregnating agent, which was then allowed to solidify in the pores of the graphite a t a suitable temperature. All substances tested had some effect on i, and on the id/irratio (Table I). Opal, ceresin, and castor waxes were the most effective; residual currents of electrodes impregnated nith any of these mere negligibly low in magnitude and had very flat slopes. Furthermore, the anodic wave of 10-4M dibutylphenylenediamine was well defined and easily measured (Figure 2). Saturation of the graphite was most efficient when the hot graphite rods were completely immersed in melted 11 ax. After an initial soaking period, longer immersion in castor wax had little effect on polarographic properties (Table 11); i, continued to decrease slightly, but the dibutylphenylenediamine wave was relatively independent of the impregnation time. Similiar results were obtained with opal and ceresin waxes. As the desirable effect of wax impregnation probably results, a t least in part, from displacement of gases from the graphite pores, impregnation is probably a simple absorption process, requiring only a short period of soaking to displace entrapped air. Choice of an impregnating agcnt requires consideration of solubility in the test media to be used. Opal, ceresin, and castor waxes are all insoluble in water, alcohol, and acetone. Opal and castor waxes are also insoluble or only slightly soluble in ethyl acetate and paraffinic and aromatic hydrocarbons, in which ceresin is soluble. The cement used for insulating the outer surfaces of the waxy rods, Seal-All, is insoluble in all the solvents mentioned except acetone and ethyl acetate. PROCEDURES USED IN APPLICATION EVALUATION
The indicator electrode was prepared by soaking a 0.25-inch graphite rod
Effect of Time of Immersion on Wax-Impregnated Electrodes buffer in 1: 1 alcohol-vater; potential variation of 1.24 mv./sec.; stirred solution)
( 10-451 dibutylphenylenediaminein pH 5.2 acetate
Time of Immersion in Hot Castor Wax, Min. 45 60 90 a
a = 0 . 0 5 6 / ( E ~ a-
El,*, Volt + O . 177 0.188 0.183 E1,4)( a t
pa.
~ d ,
5.72 5.60 5.67
an
0.6 0.5 0.5
zr,
pa.
0 52 0.22 0.18
25" C.).
VOL. 29, NO. 2, FEBRUARY 1957
225
(special spectroscopic grade, National Carbon Co.) in molten opal wax at 100" C. for 2 hours. The wax-impregnated rod was then withdrawn and allowed to cool and harden a t room temperature. The rod surface was coated with a thin layer of Seal-All and allowed to dry. Approximately 0.25 inch was then removed from each end of the rod. The lower end, to be used as the electrode surface, was abraded lightly with a fine (1/0). grade of sandpaper. A new graphite surface was exposed and lightly sanded before each run. Organic Substances. Test solutions of organic compounds were prepared by mixing a n aliquot of a n alcoholic solution of t h e sample with 2.5 ml. of t h e aqueous acetate buffer stock solution in a 50-ml. volumetric flask. The final volume was adjusted with isopropyl alcohol; 25 ml. of this solution was placed in the nitrogen stream conditioner bubbler and 25 ml. in the polarographic cell. Kitrogen was passed through the cell solution for 15 minutes, during which time anodic pretreatment of the electrode was conducted ( 3 ) . This consisted of a 10-minute application of a constant potential equal to 90% of the final potential to be applied in the electrolysis, followed by equilibration for 5 minutes at the initial potential. After the nitrogen stream was transferred to the cell atmosphere, the current-potential curve was recorded in a positive direction. At completion of t h e run, the polarographic leads were disconnected and the cell resistance was measured. Inorganic Substances. For t h e study of inorganic compounds, t h e electrodes were preconditioned cathodically in the absence of t h e reducible species. Twenty-five-milliliter portions of t h e aqueous base solution were placed in t h e cell and nitrogen stream bubbler. The stirrer was started and nitrogen was passed through the test solution for 5 minutes. During this time, a potential equal to 90% of the final potential to be applied was impressed across the cell for 2 minutes, followed by equilibration a t the initial potential for 3 minutes. An aliquot of a n aqueous stock solution containing the sample was added to the cell during the final minute of this treatment. After transferring the nitrogen stream to the cell atmosphere, the cathodic wave was recorded in a negative direction; the cell resistance mas then measured. Polarograms were corrected for iR drop whenever necessary. I n most cases the cell resistance did not exceed 700 ohms and the iR correction was less than 0.01 volt. Ellz and i d values were determined by geometrical methods. DISCUSSION
OF
EXPERIMENTAL RESULTS
Behavior of the wax-impregnated electrode is similar in many respects to that of the graphite electrode. I n aqueous 0.1M potassium chloride solution, the useful anodic range of the electrode extends to about + l . O volt. 226
ANALYTICAL CHEMISTRY
Table 111.
Application of Wax-Impregnated Graphite Electrode to Typical Organic Compounds (pH 5.2 acetate buffer solution in 1 : 1 alcohol-water; opal wax-impregnated electrode; potential variation of 1.24 mv./sec. ; quiet solution)
Literature Values Experimental Values Method EI;2,Y Em, id Ref. electrode volt volt pa. ab ( 1 4 ) D. h1. E. +0.268 +0.275 4.7 1.4 (11j Platinum Destructive 0.652 5 . 8 1.1
Compound at 0.5m.W C,oncn. Catechol Resorcinol Phenolc
(1)
Ec
(3)
Graphite
(4) Platinum
Butyl-p-aminophenol p-Phenylenediamine
(1)
Dibutyl-p-pheny1eiedi:iminc
Oxalic acidd
(11 )
Quinone Hydroquinone Quinhydrone
oxidation 0.677 0.74 0.65
0.711
0.170 0.212 0.149 0.530 Platinum Destructive S o anodic osidation E, 0 . 1 5 1 ~ ~0 . 1 1 3 0.151 0.178 E, 0.151 0.145 Eo
E,
0.234
3.2
1.0
2.1 4.0 2.9 0.5
3.5 1.2 0.8 1.6
6.0 3.2 8.8
1.1 1.2 1.1
wave
a Published potentials converted to E , 2 values st pH 5 . 2 by the expression: El;%= Eo - 0.059 pH (at 25" C.). (at 25" C.I. CY = 0.056/(&/, c Experimental values for phenol obtained using ceresin was-impregnated graphite electrode. Oxalic acid polarographed i n 0.531 H2SO4.
". $
02
BASE SOLUTION
0
-02
-01
+01
0 V0
+02
+03
1 +04
LT
Figure 2. Application of wax-impregnated graphite electrode
pH 5 . 2 acetate buffer in 1: 1 alcohol-water; ceresin
wax-impregnated graphite electrode; potential variation of 1.24 niv./sec.; quiet solution
Cathodically, the limiting potential imposed by hydrogen overvoltage is about - 1.3 volts. The polarization rate-Le., the rate of potential increase -has significant effects on both El/z and idvalues-e.g., a change in polarization rate from 0.62 to 1.24 mv. per second increased id values for phenol by about 70% and caused positive shifts of 0.03 volt in EUZ. Diffusion currents measured with the impregnated electrode were considerably lower than those observed with an untreated rod, probably because of
partial iyax coating of the available surface. Other effects may also be attributable to the latter effect. The Ellz observed may depend upon the type of wax used-e.g., Eliz values of +0.07 and f0.15 volt were measured for dibutylphenylenediamine with ceresin and opal wax-impregnated electrodes, respectively. Compounds such as hydroquinone and phenol produced double anodic waves when the active surface of a ceresin-impregnated electrode was not pretreated with sandpaper; light abrasion of the surface restored
*he normal single waves. The double wave usually consisted of a well-defined small wave, followed by a poorly defined drawn-out large wave; the normal single wave occurred at the potential of the small wave but was much higher. Oxidation-Reduction of Organic Compounds. Several types of organic compounds produced well-defined anodic waves at t h e wax-impregnated electrode. Sharp waves with flat limiting plateaus were generally observed. Maxima were sometimes encountered, b u t these mere usually small and occurred only ivith compounds oxidized at less positive potentials. Experimentally obtained EL values are generally in good agreement with those calculated from data reported in the literature (Table 111). Two of the compounds studied, resorcinol and oxalic acid, were reported to undergo destructive oxidation a t a platinum electrode (11); the former gave a well-defined wave a t the wax-impregnated electrode, but the latter gave no anodic wave. Quinhydrone gave a single unbroken wave, whose Ell*, 0.145 volt, agreed with the value of +0.151 volt calculated from the standard potential. E1rqvalues for quinone and hydroquinone were displaced from the latter value, but the continuous wave obtained for quinhydrone was evidence of reversible behavior a t the wax-impregnated electrode. Data on the compounds studied illustrate the usefulness of the polarographic technique for studying structural differences and relative oxidation stability characteristics. For example, the experimental difference of 0.38 volt in Ellz values for catechol and resorcinol (Table 111) can be attributed to the relative stability levels of the ortho and meta positions of the two hydroxy groups. Similarly, insertion of two butyl groups into phenylenediamine lowers El,*by 0.06 volt. No attempt has been made to assign the anodic n-aves of the compounds studied to definite chemical reaction. However, I d / c ratios of hydroquinone, phenol, butylaminophenol, phenylenediamine, and dibutylphenylenediamine were all similar, indicating that the same number of electrons-probably hvowere probably involved in each electrode reaction. Diffusion currents of catechol and resorcinol were approximately twice that of phenol, a n indication that oxidation of a dihydroxy aromatic compound requires twice as many electrons as that of phenol. The large id observed for the cathodic wave of quinone agrees with the quinhydrone pattern. Diffusion currents are proportional to concentration and can therefore be utilized for quantitative determination. A method for the direct determination
+
Application of Wax-Impregnated Graphite Electrode to Inorganic Species (Opal wax-impregnated electrode; potential variation of 1.24 mv./sec.. stirred solution) C'oncn. of Active Species, EI12, Ldi k', Medium Jf T'olt ptt. i d / C x 106 Pb++ 0 . l M RC1 1 . 6 x 10-4 -0.61 46.1 0,20 Table IV.
Ag
,
-
0 2
0 1M KSOI 0 05M KCl
1 . 0 x 10-4 1 . 0 x 10-5 1 o x 10-4 1 o x 10-5 Air saturated
-0.60 -0.63 +o 21 +O 13 -0 41 -0 79
31.3 3 7 15 0 1 4 68 45
0.31 0 37 0 13 0 14
Table V. Reproducibility of Measurements Made with Wax-Impregnated Graphite Electrode Quiet Solution Stirred Solution Compound EIIP, volt i d l pa. Eli*, volt id, pz. Dib~it\.lpheiiJ;lenediamine"
AV
Standard deviation Lead nitrateb
+0 0 0 0 0
070 068 064 063 066
+ 0 066 0 00.3
Av Standard deviation
0 0 0 0 0
83 78 81 i9
80
0 80 2
+o
0 0 0 0 0 0 0 +O 0 -0 -0 -0 -0 -0 -0 0
116 117 122 118 116 117 116 123 118 003 612 598 582 637 637 613 024
,
4 40 3 75 4 OD 4 42 4 48 4 58 4 11 5 21 4 42 10% 43 5 44 1 46 2 50 4 46 5 46 1 6%
10-4M in pH 5.2 acetate buffer in 1: 1 alcohol-water; ceresin was-impregnated elec-
trode. b
1.6 X 10-4M in O . 1 M KC1 solution: opal wax-impregnated electrode.
of N,N' - di - sec - butyl - p - phenylenediamine in gasoline, based on this technique, has been developed (2). Reduction of Inorganic Species. Applicability of t h e wax-impregnated electrode t o t h e polarography of reducible inorganic species was also investigated. T h e cathodic waves for silver and lead, representative of mono- and divalent ions, were studied in stirred test solution. EllZfor lead (Table IV) was 0.2 volt more negative than that measured with the dropping mercury electrode and 0.1 volt more negative than that reported for a stationary platinum electrode (6). The Ellz of silver (Table IV) was also approximately 0.1 volt more negative than that measured with a stationary platinum electrode (6). A tenfold reduction of silver ion concentration resulted in a negative shift of 0.08 volt in Ell2,in only fair agreement with the theoretical shift of 0.059 volt which has been verified experimentally for a platinum electrode (10). Diffusion currents of lead were expected to be about twice those of silver if the electrode reactions involved two and one electrons, respectively; ob-
served i d values confirm this relation (Table IV). Diffusion currents are proportional to the metal ion concentrations over limited concentration ranges. A major advantage of the m-ax-impregnated electrode for the determination of metal ions is its sensitivity to low concentrations Jvhen used in stirred solution. Excellent polarograms were obtained for 10-5~M solutions of silver and lead. In view of the relatively high current levels involved, the technique could probably be extended to the measurement of concentrations as low as 10-6M. Two cathodic waves were found for dissolved oxygen; maxima were not observed. The appearance of t a o w v e s is analogous to the behavior of oxygen a t the dropping mercury electrode. However, Elizof the first lvave, -0.41 volt, was more negative than that observed at the dropping mercury electrode while the second wave appeared a t potentials less negative than with the dropping mercury electrode (Table IV). The mechanism of the electrode reactions was not determined. Since the total id for a n air-saturated VOL. 29, NO. 2 , FEBRUARY 1957
227
solution of 0.05M potassium chloride was 113 pa., the impregnated electrode, used in stirred solution, should be especially applicable to the determination of very small amounts of dissolved ouygen. This sensitivity to dissolved oxygen emphasizes the need for efficient deoxygenating techniques to eliminate interference with other cathodic waves. Reproducibility of Measurements. I n unstirred solution, t h e reproducibility of measurements made with t h e impregnated electrode is excellent. Replicate determinations on a 10-4M solution of dibutylphenylenediamine showed standard errors for a single run of 0.003 volt for Eli* and 2% for id (Table V). The precision of these measurements is equal to and in some cases better than results reported for other solid electrodes (5-6,8-10, 12, I S ) . Stirring the test solution resulted in positive displacement of Ellz but had no adverse effect on its reproducibility; the standard error in measuring id was, however, increased t o 10%. The magnitude of id was greater by a factor
F., Conrad, -4. L., 27, 310 Landerl, J. H., ANAL.CHE~LI.
Gaylor, V.
of 5.5 when stirring was used. The increased sensitivity resulting from stirring will often justify the reduced measurement accuracy. Similarly, the 6% error in measuring id for the cathodic lead wave (Table V) will usually be satisfactory for determining the low concentrations to which the technique is applicable. The reproducibility of Eli2 values for the lead wave \ m s very poor; in general, the method is not recommended for accurate measurement of cathodic potentials.
(1955); 29, 228 (1957).
Gaylor, V. F., Elving, P. J Conrad, A. L., Ibid., 25, 1078 (1953). Hedenburg, J. F., Freiser, H., Ibid., 25, 1355 (1953).
Kolthoff, I. M., Jordan, J., Heyndrickx, A,, Ibid., 25, 884 (1953). Kolthoff, I. M.:,Lin me, J. J., “Polarography, 2 n 8 ed., Interscience. New York. 1952. Kolthoff,’I. M ., Tanaka, X,, AKAL. CHEI) I . 26, 632 (1954). Lord, S. S., Jr., Rogers, L. B., Zbid., 26, 284 (1954).
Marplle,_. T. L., Rogers, L. B., Zbid., 25, 1351 (1953).
Rogers, L. B., Miller, H. H., Goodrich. R. B., Stehney, A. F., Zbid.,
ACKNOWLEDGMENT
The authors gratefully acknowledge the help and encouragement received from Philip J. Elving throughout this study. The work was performed in the laboratories of The Standard Oil Co. (Ohio) and permission to publish is appreciated. LITERATURE CITED
( 1 ) Fieser, L. F., J. A m . Chem. SOC.5 2 , 5204 (1930).
21, 777 (1949).
Skobets, E. M., Atamanenko, N. N., Zavodskaya Lab. 15, 1291 (1949).
Streuli, C. A., Cooke, W. D., ANAL. CHEM.25. 1691 (1953).
Zbid., 26, 963 (1954). ’ Vlcek, A. K., Mansfeld, V., Krkoskova, D., Collegium 1943, 245.
RECEIVED for review December 23, 1955. -4ccepted October 4, 1956. Pittsburgh Conference on Analytical Chemistry and hpplied Spectroscopy, March 1955.
Polarographic Determination of Antioxidants in Gasoline V. FRANCES GAYLOR, ANNE L. CONRAD, and JEAN H. LANDERL Chemical and Physical Research Division, The Standard O i l Co. (Ohio), Cleveland 6, Ohio
A , polarographic method for the direct determination of antioxidants in gasoline i s based upon measurement of the oxidation waves produced at a wax-impregnated graphite electrode. Analysis i s conducted directly on an alcoholic solution of the sample, thus eliminating time-consuming separation steps. The method has been applied to the determination of two commercial antioxidants, N,N’-di-sec-butyl-pphenylenediamine and N-N-butyl-parninophenol. A single sample can be analyzed in approximately 30 to 45 minutes. Standard deviation of the method i s approximately 9%. Concentrations as low us 2 p.p.m. in the gasoline can be determined.
P
methods for the determination of antioxidants in gasoline have mainly consisted of colorimetric procedures, which usually require the preliminary extraction of the antioxidant so that color can be developed and measured in an aqueous solution-e.g., use of the Folia-Denis UBLISHED
228
ANALYTICAL CHEMISTRY
reagent after alkaline extraction (6), of hydrogen peroxide after acidic extraction (6), and of tungstophosphoric acid after acidic extraction (7). The only direct determination of antioxidants in gasoline described is based on ultraviolet absorption (3) ; the difference in absorption between the base gasoline and the inhibited sample is taken as a measure of the antioxidant content. Unfortunately, in many refineries a representative uninhibited base gasoline is difficult to obtain. Considerable error may be associated with the separation step required for colorimetric procedures. Gasoline dyes or other colored or color-producing materials may be extracted along with the antioxidant. I n addition, the antioxidants tend to be unstable and to drconipose during the extraction procedure. The present paper describes a polarographic procedure which can be applied directly to the gasoline sample and does not require an uninhibited base gasoline for comparison. The method is based on the oxidation waves produced by
gasoline antioxidants a t a suitable indicator electrode. The wax-impregnated graphite rod ( I ) is sufficiently sensitive to determine the very low concentrations involved, is suitable for use in organic solvents, and has the anodic potential range involved. EQUIPMENT
Current-potential curves were automatically recorded a t a speed of 1.24 mv. per second, using a Sargent Model XXI polarograph. The wax-impregnated indicator electrode (1) was prepared from a 0.25-inch graphite rod (special spectroscopic grade, Il’ational Carbon Co.). -4 convenient length (6 to 12 inches) of graphite rod was immersed in melted opal wax (E. I. du Pont de Nemours & Co.) or castor wax (Baker Castor Oil Co.), and allowed to stand for 2 hours a t 100” C. The waximpregnated rod was then withdrawn and allowed to cool to room temperature in a vertical position. The outer surfaces of the rod were covered with a n insulating layer of Seal-All (Allen Products Corp.) and allowed to dry. Approximately 0.25 inch of graphite
