NOTES
Glass-Metal Composite Electrodes George G. Guilbault and G. J. Lubrano Department of Chernlstry, Louisiana State University in New Orleans. New Orleans. La. 70722
Don N. Gray Owens-lliinois Technical Center. Toledo. Ohio
Electrodes constructed of noble metals are used in amperometric, electrogravimetric, coulometric, conductometric, and potentiometric analysis. The inert properties of the noble metal based systems offset their high cost and their sometimes troublesome ductile character. In this note, we wish to report some preliminary data on an electrode system using a metal/glass composite carried as a film on a rigid inert glass carrier. Such electrodes have the advantages of durability, inertness to poisoners, and low cost.
inlay electrode was used as the counter electrode. The test electrodes used were the conventional platinum inlay electrode and the gold and palladium/gold sensors. The removal of oxygen was unnecessary since the potentials of interest were so positive that reduction of oxygen could not interfere. Pretreatment of the electrodes consisted of immersion in a dichromate-sulfuric acid cleaning solution for several seconds each morning it was to he used. The electrode was then rinsed several times with doubly distilled water and immersed in the test solution. A potential of -0.2 V us SCE was then applied until the cathodic current decayed close to zero. Then the potential was moved to +0.05 V us SCE until the current decayed close to zero.
EXPERIMENTAL
RESULTS Potentiometric Measurements. Figure 1 contains the curves of the potentiometric titration of iron(I1) with cerium(1V) using a platinum inlay electrode, platinum sensor, gold sensor, and palladium/gold sensor. The end points of the titrations calculated from the curves are 8.00, 7.96, 7.99, and 8.00 ml, respectively. The theoretical end point was calculated to be 8.00 ml. Figure 2 contains the curves of the potentiometric titrations of chloride with silver(1) using a silver billet and a silver sensor coated with AgC1. The end points were calculated from the curves to be 8.43 and 8.50 ml, respectively. The theoretical end point was 8.45 ml. These examples of potentiometric titrations show the possible usefulness of the new sensors as substitutes for the more costly billet, foil, or wire electrodes now commonly used in potentiometric measurements. Applications of these electrodes can be found in both educational and research laboratories. Cyclic Voltammetric Measurements. Figure 3 shows the background characteristics of the platinum inlay electrode, the gold sensor, and the palladium/gold sensor in 0.1M phosphate buffer, pH 6.0. The areas of the electrodes are approximately 0.2, 1.0, and 1.0 cm2, respectively. In Figure 3, A , it is seen that there is an anodic prewave that starts at about 0.2 V and a cathodic-current peak. Kolthoff and Tanaka ( I ) found that the anodic prewave is due t o oxidation of the platinum surface a t potentials that depend on the p H of the solution. The cathodic-current peak is due to the reduction of the oxide film formed on the surface. An excellent discussion of oxide formation has been given by Adams ( 2 ) . Figure 3, B, shows that the background using the gold sensor also contains a small anodic wave preceding oxygen evolution when the polarogram is recorded in the anodic
Reagents a n d Solutions. Analytical reagent grade chemicals were used without further purification. Doubly distilled water was used throughout. Apparatus. Potentiometric measurements were made with a Keithley Model 610B multirange electrometer equipped with a Keithley Model 6107 electrode adapter. Cyclic voltammetric measurements were made with a Heath Model EUA-19-2 polarograph module, Heath Model EUW-19A operational amplifier system, and a Houston Instrument Model HR-97 X-Y recorder. The conventional electrodes used were Beckman 39273 platinum inlay electrodes and Heckman 39261 silver billet electrodes. The new type of sensors was prepared from a chemically pure, low alkali glass cut to sections l/E-inch thick, 1 inch wide, and 4 inches long. Onto this substrate was screened a formulation composed of a noble metal powder and pure glass hinder of low alkali content and of a fine particle size mixed in a fugitive binder. The screen printed substrate was then fired in a furnace at a sufficiently high temperature to remove the fugitive and mature the glass binder, 880 "C for 1 hr. The ratio of metal-to-glass binder used in preparation of these electrodes was 95:5. The vehicle for printing the electrode configurations was 15% (w/v) ethyl cellulose thickener in a solvent system of butylcarboxyl acetate/isoamyl salicylate ( 3 : l v / v ) . The metal layer was screened as a 3/4inch strip along the 4-inch length of glass substrate. The electrodes were immersed 0.2 inch in test solutions and electrical contact was made by means of an alligator clip to a portion of the electrode which remained out of the solution. All measurements were made at 30 "C using a 150-ml cell consisting of a double-walled glass vessel. Water thermostated a t 30 f 0.05 "C was circulated in the space between the walls. Potentiometric Measurements. The titration of 80 ml of 0.01M NaCl with 0.094iM 4gN03 was followed potentiometrically using a SCE with a double junction containing 1M KN03 and either a silver billet electrode or a silver sensor plated with silver chloride. The electrodes were plated by placing a pair in saturated potassium chloride, connecting one electrode to the positive pole of a 1%-V dry cell and the other electrode to the negative pole, reversing the current polarity to clean and recoat the desired electrode. and then after allowing the desired electrode to be coated sufficiently, disconnecting the battery. Cyclic Voltammetric Measurements. All cyclic polarograms were recorded in unstirred solutions with a sweep rate of 5 volts per minute. The potential is swept first in the anodic direction and then in the cathodic direction. A saturated calomel electrode was used as the reference electrode and a conventional platinum
( 1 ) I . M . Kolthoff and N. Tanaka,Anai Chem.. 26, 642 (1954). (2) R. Adams, 'Electrochemistry of Solid Electrodes." Marcel Dekker. New York, N.Y., 1969, pp 187-209.
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1973
2255
1200
> E
5 80C v) m
> w
1
I
I
8.0 Figure 1. Titration of -Platinum
9.0
Volume C e
(IT) added,
- gold sensor,
,,
(ml)
iron(l1)with cerium ( I V )
inlay electrode. - - - - - - - - platinum sensor,
-.
.. . . . . palladium/gold sensor
400
300 >
E
-
W V (I)
y1
> w
200
IO0
I
1
25
Volume
Figure 2.
Titration of NaCl with
8.5
80 Ag'
J
95
ml
AgN03
-Silver billet electrode. - - _ - - - - - silver
sensot
direction. There is also a cathodic dissolution pattern when the potential is scanned in the cathodic direction. This behavior is similar to that found by Bauman and Shain (3)for conventional gold electrodes. (3) F Bauman and I Shain Anal Chem 29, 303 (1957)
2256
added,
90
The background characteristics of the palladium/gold sensor shown in Figure 3, C, are similar to those of the gold sensor with respect to the cathodic dissolution peak and anodic wave, but the residual current is much higher in both the anodic and cathodic scans and the peaks are smaller.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, N O V E M B E R 1973
c
C 0, L L
3 V
I2
Figure 3. pH 6 0
0.8
0.4 0 E vs SCE, V
-0.4
I
-0.8
Background characteristics in 0.1M phosphate buffer,
\ A ) Platinum inlay electrode, ( B ) gold sensor, (C) palladium/gold sen-
sor Anodic currents below horizontal, cathodic above
.6
.4
.2
E vs. SCE, Figure 5. Cyclic polarograms of 1 . 0 0.1M phosphate buffer, p H 6.0
0
T2
V X
10-3M K4Fe(CN)6in
( A ) Platinum inlay electrode. ( 6 ) gold sensor; ( C ) palladium’gold sensor. Anodic currents below horizontal, cathodic above
If the anodic potential limit of the cyclic polarogram is restricted to potentials less anodic than that needed for electrode oxidation, the cathodic current peak can be reduced and even eliminated from the cathodic sweep. As shown in Figure 4, when this is done, the residual currents of the gold and platinum electrodes are brought to negligible values. The gold sensor is very promising, giving a low linear cyclic residual current from 0.0 to 0.8 V. Cyclic polarograms of 1.0 x 10-3M K4Fe(CN)s in 0.1M phosphate buffer, p H 6.0 obtained with these three electrodes gave similar curves as is shown in Figure 5 , The peak current/geometrical electrode area was about the same for the platinum inlay electrode and the palladium/ gold sensor (approximately 85 pA/cm2). The value for the gold sensor was about twice that of the others (approximately 185 wA/cm2). No estimate of the “roughness” of these electrodes was made. The peaks obtained using the palladium/gold sensor were more rounded than the others because of the large residual current.
L
c L
U 3
4 100 ,pA
i
,
I
1
I
I2
08
04
0
E v s SCE,
v
1 @ I
Figure 4. Background characteristics with limits in 0 1M phosphate buffer pH 6 0
restricted potential
( A ) Platinum inlay electrode ( B ) gold sensor (C) palladium goid sensor Anodic currents below horizontal cathodic above
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, N O V E M B E R 1973
2257
Because of t h e difficulty a n d cost of making a conventional gold electrode, m a n y workers resort t o using some form of p l a t i n u m electrode, even though a gold electrode would be more a d e q u a t e . T h e examples of potentiometric and voltammetric measurements given here indicate t h e possible usefulness of t h e new type of sensors. T h e y c a n perform m a n y of t h e tasks of conventional electrodes a n d can be constructed of m a n y different types of metals or
mixtures of metals while requiring m u c h less m e t a l per electrode than was previously possible. Furthermore, because t h e outer surface is glass r a t h e r t h a n m e t a l , t h e y are m u c h less susceptible to poisoning t h a n conventional m e t a l electrodes. Received for review M a r c h 5 , 1973. Accepted M a y 7 , 1973. One of u s (G.J.L.) acknowledges t h e financial support of Owens-Illinois in t h e form of a graduate fellowship.
Determination of Organic Peroxides in Low Concentration by a Biamperometric Method F. C. Montgomery, R. W. Larson, and W. H. Richardson' Department of Chemistry. California State University. San Diego, Calif. 927 75
Various chemical methods have been developed for t h e determination of organic peroxides ( I , 2) b u t t h e iodometric method is by f a r t h e most widely used. Typically, acetic acid is used a s t h e proton source in t h e reaction of t h e peroxide with iodide t o give iodine or triiodide. T h e use of acetic acid is perfectly suitable for t h e determination of peroxides in high concentration; however, it is u n satisfactory for determination of peroxides in low concentration because of large blanks. T o circumvent t h e problem of large blanks encountered with acetic acid, Hartm a n a n d White (3) used citric acid as t h e proton source in a mixture of t e r t - b u t y l alcohol a n d carbon tetrachloride. By t h i s procedure, t h e liberated iodine was t i t r a t e d with sodium thiosulfate solution t o a visual e n d point with t h e aid of starch indicator. It became necessary for us t o develop a n analytical procedure for the determination of small q u a n t i t i e s of 1,2dioxetanes (four-membered cyclic peroxides). Although the method of H a r t m a n a n d W h i t e held promise, t h e visual end-point determination would prove too insensitive for our purposes. We report here a biamperometric procedure with sodium thiosulfate a n d potassium iodate a s tit r a n t s a n d with t h e peroxide decomposition carried o u t in a tert-butyl alcohol solution containing potassium iodide a n d citric acid. T h e method is suitable for conveniently measuring low concentrations of peroxides a n d should prove useful in kinetic studies or where hazards or n a t u r a l occurrence dictate analyses of small a m o u n t s of peroxides.
EXPERIMENTAL Apparatus. The circuit diagram (Figure 1) for the biamperometric titration ( 4 ) apparatus is a modification of that given by Potter and White (5, 6). The electrodes (A, A) are placed in the solution to be titrated, which is contained in an open 100-ml beaker and stirred magnetically. To whom correspondence should be addressed. Peroxides," Vol. 0 ,D. Swern, Ed., Wiley-lnterscience, New Y o r k , N . Y . , 1971, p 579. ( 2 ) R. M . Johnson a n d I . W. Siddiqi, "The Determination of Organic Peroxides," Pergamon Press, New Y o r k , N . Y . . 1970. (3) L. Hartman a n d M. D. L. White, Anal Chem., 24, 527 (1952). (4) J. T. Stock, "Arnperornetric Titrations,"Interscience Publishers, New ( 1 ) R. D. Mair and R. T . Hall, "Organic
Y o r k . N . Y . , 1965, C h a p . 4 and 5. ( 5 ) E C . Potter and J. F. White, J. Appl. Chem. (London). 7, 309 (1957), ( 6 ) Ref. 4 , p 582. 2258
Reagents. tert-Butyl hydroperoxide (Lucidol) was purified by azeotropic separation and distillation ( 7 ) . Benzoyl peroxide (Lucidol), lauroyl peroxide (Lucidol), tert-butyl alcohol (MC/B, reagent), acetic acid (DuPont, reagent), benzene (MC/B, reagent), citric acid (MC/B, reagent), potassium iodide (MC/B, reagent), sodium carbonate MC/B, reagent), potassium iodate (MC/B), reagent), and sodium thiosulfate pentahydrate (MC/B, reagent) were used as supplied. Doubly distilled and boiled water was used to prepare the titrants. The citric acid solution was prepared by adding 75 ml of tertbutyl alcohol to 15 grams of citric acid, and the mixture was heated with stirring for about 10 min to dissolve the acid. A potassium iodide solution was prepared from 10 grams of potassium iodide, 0.5 gram of sodium carbonate, and 10 ml of doubly distilled water. Both solutions were purged with nitrogen by means of a fritted glass filter-stick. Weighed amounts of the peroxides were diluted in benzene to known concentrations. The sodium thiosulfate titrant solution ( c a 1.8 X 10-4N) was obtained by diluting 1.8 ml of 0.1N sodium thiosulfate solution to 1.0 1. with boiled doubly distilled water and 50 ml of tert-butyl alcohol. The potassium iodate titrant solution (1.05 X 10-4M) was prepared mmol) to 500 ml of solution by dissolving 11.2 mg (5.23 X with boiled doubly distilled water. Benzene solutions of approximately 0.50M (1.ON) tert-butyl hydroperoxide, benzoyl peroxide, and lauroyl peroxide were prepared for analysis by a conventional method. These solutions were diluted to between approximately 1.1 X 10-4 to 1.1 X lO-3M for analysis by the biamperometric method. Procedure. The sodium thiosulfate solution (ca 1.8 X 10-4N) was standardized in the following manner. A 100-ml beaker, containing a magnetic stirring bar, was charged by means of pipets with 5.0 ml of citric acid solution and then 0.5 ml of potassium iodide solution. This solution was diluted with 15 ml of boiled doubly distilled water and the electrodes (A, A) were placed in the solution and energized. The sample was titrated with standard iodate solution until current flow was detected by the pH meter. The amount of added iodate solution was recorded and then 2.00 ml of sodium thiosulfate solution was added. The current flow decreased and the solution was again titrated with iodate solution until current flow was again detected. The amount of added iodate solution was recorded and this procedure was repeated for additions of 3.00, 4.00, 5.00, 6.00, and 7.00 ml of thiosulfate solution. Prior to the initial titration, 1.00 ml of benzene was added to simulate titrations of the peroxide solutions. The results of the standardization titration are given in Figure 2. The normality of the thiosulfate solution is calculated from Figure 2 with the aid of the titration reactions (Equations 1 and Z), where ( 7 ) P. D . Bartlett and R. R. Hiatt, J. Amer. Chem. Soc.. 80, 1398 (1958).
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