smaller anion (Br-) which is expected to carry a relatively high negative charge per unit area. One would expect this type of a system t o be effectively solvated in the organic phase and extracted into it by molecules having a n exposed positively charged surface-e.g., dipolar molecules with acidic protons such as chloroform, phenols, and alcohols. If either an alcohol such as 1-pentanol o r a phenol such as p-tert-butylphenol, as were employed earlier ( I ) , were used, the differences observed above would be expected t o be even greater since these agents can solvate the ion pair much more effectively than can chloroform. Although nitrobenzene and cyclohexanone have higher solubility parameters than chloroform, they are also more nucleophilic (Le., more electron rich) and would be expected to solvate the ion pair primarily through the large protonated dextromethorphan ion where the charge is probably less concentrated. Thus solvation would be less effective than in the case where the smaller anion is solvated by chloroform. Therefore, specific solvation accounts for the data presented here, whereas neither the solubility parameter nor dielectric constant approaches do. Freiser also questions our interpretation on the basis that the data were obtained in solutions which have a high concentration of chloroform. The interpretation presented earlier does indeed depend on the assumption that the solvent mixture is nearly ideal. If one assumes that chloroform and cyclohexane form a regular solution, the following equation can be use t o calculate the excess free energy
+
GE = ( X I C ~ ~ 2 ~ 2 ) ( 6-1 82)*4142 (1) where x is mole fraction, c is molar volume, 6 is solubility parameter, and I$ refers to volume fraction (8). Using this (8) J. H. Hildebrand and R. L. Scott, “Regular Solutions,” Prentice Hall, Englewood Cliffs, N. J., 1962, p 102.
equation for a 50-50 mole fraction mixture (which should exhibit maximum nonideality), a value of 28 cal per mole of mixture is obtained for GE. An experimental heat of mixing value of 160 cal/mole of mixture has also been reported for a 50-50 mole fraction mixture of chloroform and cyclohexane (9). These values indicate that although a chloroformcyclohexane mixture is not ideal, deviations from ideality are sufficiently small that large errors in interpretation would not be expected from this source. We reiterate that although straight line correlation can be obtained using either dielectric constants or solubility parameters, interpretation of data for irregular systems in this manner goes well beyond the original intentions of regular solution theory (8). O n the other hand, specific interaction or solvation can adequately account for the observed phenomena and can provide a useful basis for selection of extraction solvents.
TAKERU HIGUCHI ARTHURMICHAELIS‘ J. HOWARD RYTTING Department of Analytical Pharmaceutical Chemistry and Pharmaceutics The University of Kansas Lawrence, Kan. 66044 RECEIVED for review June 3, 1970. Accepted November 30, 1970. Present address: Hoffmann-LaRoche, Inc., Kingsland Street, Nutley, N. J. 07110. (9) E. Baud, Bull. SOC.Clzim. Fr., 17, 339 (1915).
I AIDS FOR ANALYTICAL CHEMISTS
1
Submerged Drum Activated Platinum Voltammetric Microelectrode John T. Stock Department of Chemistry, University of Connecticut, Storm, Conn. 06268 ELECTRODE DEACTIVATION is often encountered in the anodic solid-electrode voltammetry of organic compounds ( I ) . Deposition of oxidalion products o n the anode can restrict the current and thus cause distortion o r elimination of the waves. Abrasive continuous activation of a voltammetric electrode was introduced over 20 years ago ( I , 3). Klinger ( 4 ) has reviewed continuous activation by brushing and by rubbing with a felt pad. More recently, Berge and Struebiag (5) have described voltammetry at a platinum electrode that presses lightly o n the periphery of a horizontal drum. Rotation of the drum, which dips into the solution being examined, rubs (1) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker, New York, N. Y., 1969, p 189.
(2) J. R. Baylis, H. H. Gerstein, and K. E. Damann, J. Amer. Water Works Ass., 38, 1057 (1946). (3) H. C. Marks and G. L. Bannister, ANAL.CHEM.,19,200 (1947). (4) S. Klinger, Freiberg. Forschungsh., Reihe A , 358, 5 (1965). ( 5 ) H. Berge and B. Struebing, - Fresenius 2.Anal. Chem., 234, 321
(1968).
the electrode and carries past it a film of the solution. The applications reported are to well known inorganic ions that d o not normally cause electrode deactivation. These workers (6) later extended the principle to the graphite electrode, but gave no indication of the attainable degree of activation. Stock (7) has shown that stable anodic waves of 7-hydroxy-1, 2,3,4-tetrahydroisoquinoline(HTQ) and of related compounds can be obtained at the Berge and Struebing platinum electrode system. I n alkaline solutions, these compounds strongly deactivate the rotating platinum electrode (RPE). A horizontal drum requires a cell of awkward shape, with side openings for the driving shaft. A compact unit, the submerged drum activated platinum electrode (SDAPE), has been developed. The cell for the SDAPE is merely a 250-ml beaker. Berge and Struebing (5) d o not specify the (6) Ibid., 247,12 (1969). (7) J . T.Stock, Microchem. J., in press.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
289
I D (
,---. (9 I
+
s---?
Figure 1. Submerged drum activated platinum electrode (a) front view (b)view in direction of arrow (c)electrode tip (d) glass drum and adaptor
1, j ,,:I I
0
0
---'
_ _ _ L _ _ _ _ _ -
0.5
1.0
POTEYTIAL,V
Figure 2. Current-voltage curves at RPE and at wood drum SDAPE. Curve A , 0.5 mM tyramine in 0.1M NaHC03, RPE, 1st run; B, as A , 2nd run; C, as A , but SDAPE, 1st run; D,as C, 3rd run; E, 0.5 mM K4Fe(CN), in 0.01M HC104, SDAPE; F, as E, but RPE exact nature of their drum material. Accordingly, drums with various surfaces were examined. I n view of possible use of media that attack or soften organic construction materials, glass drums and electrode mountings were devised. EXPERIMENTAL
Construction of the SDAPE. The entire assembly (Figure 1) is built o n a frontless wood box of approximate dimensions 10 X 10 X 10 cm. The box has a rod A for mounting o n a ring stand. The Lucite sheet back of the box carries motor switch B and binding post C. A short length of subminiature flexible lead connects C to the electrode contact wire. The 120 rpm synchronous motor D (Hurst Model DA) is mounted so that its spindle is coaxial with the 6-mm glass rod shaft 290
0
50
100
CONCEVTRATION,,pM
Figure 3. Current-concentration curves at wood drum SDAPE. 4- HTQ in 0.1M NaHCOa, E = f0.70 V; -0Tyramine in 0.1M NaHC03, E = f0.70 V; -@Coclaurine in 0.01M HC104, E = f1.00 V
of the 25-mm diameter activating drum E. This shaft turns in a bearing of thin brass tubing, the ends of which have been spun inward to give free running but without shake. A short length of rubber tubing couples the spindle to the shaft. Sleeves of rubber tubing secure the 6-mm diameter electrode tube F within its brass tubing holder. This permits the easy adjustment of the position of the electrode upon the drum. The electrode holder and the screwed load rod G are soldered to one half of a small cabinet hinge. Thumbnuts o n this rod, which projects from a n opening in the side of the box, allow the pressure of the electrode on the drum to
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2 , FEBRUARY 1971
_
Compound Coclaurineb
11
iii iv 0 . 1 M NaHC03
HTQc
I
ii ...
111
Isocorypallinec
0 . 1 M NaHC03
iv i 11
Tyraminec
K aFe(CN)6b
_
_
_
_
~ ~
~ ~
~
Table I. Current-Time Relationships in 0.5mM Solutions Total current, pA Medium Drum 0 2 4 6 1.37 1.36 1.38 0.01MHClO4 i 1.40
0 . 1 M NaHC03
0.01M HClOa
iii iv i 11 ... 111 iv 1 11
0.31 0.47 2.94 1.30 0.18 0.17 2.42 1.33 0.48 0.44 2.50 1.35 0.20 0.22 2.53 1.48 1.09 1.26 2.95
iii iv a Excluding initial current surge. Zero time taken as 10 sec after circuit closure.
0.13
...
2.92 1.31 0.07 0.06 2.40 1.34 0.09 0.08 2.45 1.36 0.12 0.08 2.55 1.39 1.06 1.27 2.85
0.09 0.40 2.90 1.32 0.04 0.04 2.45 1.33 0.05 0.03 2.44 1.35 0.04 0.06 2.45 1.41 1.05 1.27 2.86
8
~10 mina
1.36
1.37
0.05 0.35 2.89 1.30
0.31 2.90 1.30
0.29 2.86 1.31
...
...
... ...
2.45 1.33
2.42 1.32
2.41 1.32
...
.
.
... ...
...
I
...
,..
...
2.46 1.37
2.44 1.35
2.46 1.35
...
.. ...
... ...
2.47 1.40 1.06 1.30 2.85
2.50 1.41 1.05 1.28 2.83
2.51 1.40 1.04 1.29 2.82
...
E = +1.00 V . c E = $0.70 V.
be adjusted. A pressure of 5.1 g was used in the present work. The electrode can be held clear of the drum by slightly lifting G before pushing in pin H. The junction tube for the saturated calomel electrode (SCE) salt bridge (8) [not shown in view (u)] is held by a spring clip I o n the bottom plate of the box. Berge and Struebing (5) used a hook-shaped wire electrode that was covered with cement. This was ground to expose a small area of platinum that made contact with the drum. Although construction is then less easy, a covering of glass may replace the cement. The exposed area of platinum (and hence the sensitivity) of a hook-type electrode obviously depends upon the extent of grinding. The electrode used in the present work is merely a short length of 0.5-mm diameter platinum wire that is sealed into the tip of the electrode tube. The projecting end is then snipped off and is then lightly rubbed o n a fine carborundum stone until flush with the glass, as shown enlarged a t (c). Contact with the inner end of the platinum is made by means of a short column of mercury into which dips a length of bare copper wire. Drums used were of (i) hardwood. twice sprayed with Krylon No 1301 Crystal Clear Acrylic Coating (Borden Chemical Co.), (ii) polyethylene, (iii) smooth glass, and (iv) lightly sandblasted glass. Polyethylene adaptors were used to mount the beakerlike drums (iii) and (iv) as shown at (6). The axial-type R P E (9) had the wire cut down until nearly flush with the glass and was rotated a t 600 rpm. All runs were made in air-containing solutions at 24 t o 27 “C. All potentials are with respect to the SCE. Current-voltage curves, recorded by means of a Beckman Electroscan, with 0.24 volt/min scan rate and 0.5-sec damping, were not corrected for iR drop.
With 0.01M HC1O4 in the cell, the approximate total resistances (including S C E and salt bridge) under operating conditions were 1550 ohm and 5450 ohm with the RPE and the SDAPE, respectively. Curve A , Figure 2 shows the small peaked wave given by tyramine in 0.1M N a H C 0 3 at the
freshly-cleaned RPE. A second run made without cleaning the electrode gave curve B, which differs but little from the residual current curve. A larger wave that remained essentially unchanged in subsequent runs was obtained at the wood drum SDAPE (curves C and 0).Curves E and F, obtained in the same solution of KaFe(CN), in 0.01M HC104, show that the sensitivity of this R P E is actually greater than that of the SDAPE. However, the R P E and the SDAPE have approximately the same discrimination (signal current to residual current ratio). HTQ, 6-hydroxy-7-methoxy-N-methyl-l,2,3,4tetrahydroisoquinoline (isocorypalline), and 7-hydroxy-l(4-hydroxybenzyl)-6-methoxy-l,2,3,4-tetrahydr o i s o q uino line (coclaurine) in 0.1M N a H C 0 3 showed behavior similar to that of tyramine. R P E deactivation was less pronounced in acid media. SDAPE runs with these compounds using drums (ii) and (iii) gave small waves that decreased in size when the runs were repeated. The waves obtained with drum (iv) were somewhat larger than with drum (i). Residual currents were also greater than with drum (i). Linear current-concentration curves (Figure 3) and currents that are essentially independent of time (Table I) are indicative of the stability of response of the SDAPE. The large and stable K4Fe(CW6currents obtained with drums (ii) and (iii) contrast sharply with the currents of the organic compounds given with these drums. It appears that the polished surfaces of these drums have very little electrode activating power. Although the periphery of the hardwood drum has a smooth appearance, the spraying operation apparently produces tiny surface irregularities that result in very good activating properties. To avoid grooving of the periphery, the electrode was raised by about 2 mm after each hour of total running time and was resprayed after 10 hours of use. Drum (iv) gave the largest waves, but also the largest residual currents. The change in sensitivity with change in electrode positioning was also greatest with this drum. However, it can be used in media that attack drum (i).
(8) J. T. Stock, “Amperometric Titrations,” Wiley-Interscience, New York, N. Y.,1965, p 150. (9) Ibid.,p 102.
RECEIVED for review August 14, 1970. Accepted September 28, 1970. This work was partially supported by N S F Grant GP-13142.
RESULTS AND DISCUSSION
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