Anal. Chem. 1986, 58,2954-2961
2954
The s k p response of the mass spectrometer equipped with the probe with samples contained in relatively small volumes suggests that it may be of utility for other applications such as combined HPLC/mass spectrometry. Although this was demonstrated by Ito et al. ( 4 ) ,the higher flow rates used in the present work may provide much greater general utility for such an interface. Although flow rates of about 5 pL/min can be used with microbore HPLC, the device could also be of use for large-bore applications where stream splitting is acceptable.
LITERATURE CITED (1) Barber, M; Bordoli. R. S.; Sedgwick, R. D.; Tyler, A. N. J. Cbem. SOC Cbem. Commun. I981, 325-327. (2) Stroh, J. G.; Cook, J. C.; Milberg, R. M.; Brayton, L.; Kihara, T.; Huang, 2.; Rhinehart. K. L. Anal. Chem. 1985, 5 7 , 985-991. .)
(3) Dobberstein, P.; Karte. E.; Meyerhoff, G.: Pesch, R. Int. J. Mass Spectrom. Ion Pbys. 1983, 4 6 , 185-188. (4) Ito, Y.; Takeuchi, T.; Ishi. D.; Goto, M. J. Cbromatogr. 1985, 346, 16 1- 166. (5) Biakeiy, C. R.; Carmody. J. J.: Vestal, M. L. Anal. Cbem. 1984, 56, 1236-1239. (6) Pilosov, D.; Kim, H. Y.; Dykes, D. F.; Vestal, M. L. Anal. Chem. 1984, 5 6 , 1236-1239. (7) Arpino, P. J.; Bounine. V. P.; Dedieu, M.; Guiochon, G. J. Cbromatogr. 1983, 271 43-48, (8)Covey, T.: Henion, J. D. Anal. Cbem. 1983, 55, 2275-2279. (9) Martin, S. A.; Costeiio, C. E.; Biemann, K. Anal. Cbem. 1982 5 4 , 2362-2368. (10) Arpino, P. J.; Krien, P.; Vajta, S.: Devant, G. J. Chromatogr. 1981, 203,117-130. ~
RECEIVED for review March 24, 1986. Accepted July 25, 1986. Support for this work by NSF Grant PCM-8404230 and NIH Grant RR-01720 is gratefully acknowledged.
Fundamental Factors in the Polarographic Measurement of Ion Transfer at the Aqueous/Organic Solution Interface Sorin Kihara,* Mitsuko Suzuki, Kohji Maeda, Kaoru Ogura, Shigeo Umetani, and Masakazu Matsui T h e Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan
Zenko Yoshida Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
I n order to elaborate on polarography for Ion transfer at the aqueouslorganlc solutlon Interface as a quantitatlve method in analytical chemistry, fundamental factors In polarography were reviewed systematkally. A polarographic cell and a clrcUn for the compensation of the ohmk drop were proposed. The characterlstksof the aqueous solution dropplng electrode were summarlzed In cormedlon wlth the slze of the capillary and kinds of both supporting electrolytes and olgank solvents. The potential windows of the residual currents and the halfwave potentlals for Ion transfers were Investigated by use of various supportlng electrolytes and many organlc solvents whose dlelectric constants are between 5 and 36. The limiting currents were proportlonal to the concentratlon of such monovalent ions as Cs', tetramethylammonium Ion, CIO,-, IO,-, I-, and Br-, Reo,-, and BF,- In the concentratlon range M In aqueous solution when 1,2-dlbetween and chloroethane was used as the organic solution.
The voltammetry for the ion transfer at the interface of two immiscible electrolyte solutions, VITIES, has become recognized as a powerful method in understanding the dynamic feature of the ion transfer because of the unmatched advantage that the transfer free energy and the amount of ions transferred can be measured simultaneously ( 1 , 2). The ion transfer at aqueous (w) and organic (org) solution interfaces has been reported in connection with alkaline metal (3-5), alkaline-earth metal (6, 7),tetraalkylammonium (8,9), and heavy halide and oxyacid (10, 11) ions using VITIES without or with such strong complexing agents as ionophores in the organic phase.
Since most of these ions are not easily reduced or oxidized in solutions, ordinary redox voltammetry cannot be applied and the electrochemical determination of them had been limited to potentiometry with ion selective electrodes, ISE. Potentiometry, however, is not appropriate for precise determination because the potential in the method depends merely on the logarithm of the concentration of the objective ion, Cion. Therefore, VITIES, in which the limiting current is proportional to C,,, (B), is expected to be an epoch-making electroanalytical method for the precise determination of ions. Among many methodological investigations on VITIES (for review, see, e.g., ref 12 and 13), polarography, PITIES, with the electrolyte solution dropping electrode, EDE, proposed by Koryta et al. (8, 14) is considered to be most promising to get the quantitative and reproducible results, because the continuous renewal of the solution drop greatly decreases the contamination of the electrode surface which is one of the difficulties in VITIES. In the present paper, fundamental factors concerning the measurement of the ion transfer by PITIES are investigated in behalf of the wide application of polarography to the field of analytical chemistry. The characteristics of the currentscan polarography employed in this work were described previously ( 15).
EXPERIMENTAL SECTION Chemicals. In order to prepare crystalviolet tetraphenylborate, CV+.TPhB-, and tetraphenylarsonium dipicrylaminate, TPhAs+.DPA-,which are the supporting electrolytes in the organic medium, a methanol solution of CV+-CI-or TPhAsC.C1-was mixed with a methanol solution of Na+.TPhE- or Na+.DPA-, respectively. After filtration, the precipitate of CV+.TPhB-was dissolved with 1,2-dichloroethane,DCE, and then recrystallized by pouring the DCE solution into methanol. The precipitate of TPhAs'BDPA-
0003-2700/86/0358-2954$01SO/O0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 * 2955 RE2
,_----.
0 Flgure 1. Polarographic cells for ion transfer at aqueouslorganic solution interface: (a) for use when d , < d,, (b) for use when d, > d,: (1) reservoir for aqueous solution (aqueous reservoir). (2) glass capillary, (3)aqueous soluliin drop (ASO), (4) organic solution, (5) Teflon cylinder with small bore (Tefloncapillary). (6) Teflon tip; RE1, RE2, electmde for potential dfterence [REI, silver-silver chloride electrode; RE2, see tea];
CEI. CE2. eleclrode for current. was dissolved with acetonitrile and then recrystallized by adding water to the acetonitrile solution. The recrystallization was re-
I .
1
0
0
equivalent to CI- and evaporating the water in the supernatant under reduced pressure. Magnesium tetraphenylhorate was prepared according to the procedure of Ueno et al. (16). All other reagents Were of reagent grade and used without further purification. The organic solvents were shaken with water for 30 min to get the partition equilibrium between aqueous and organic solutions prior to the preparation of their electrolyte solutions. Apparatus. A potentiometer, a potentiostat/galvanostat,a identical with function Eenerator. and an x.y recorder used those mentioned in the previous paper (17). All measurement8 were carried out at 25 1 "C.
*
RESULTS AND DISCUSSION In the following, we define the transfer of a cation from aqueous to organic solution or an anion from organic to aqueous solution as the anodic reaction and an anion from aqueous to organic solution or a cation from organic to aqueous solution as the cathodic reaction. Fundamental Research on the Apparatus. Polarographic Cells. For the recording of polarograms of the ion transfer at the aqueous/organic interface, a &I1 has been used that was proposed by Samec et al. (8)and is illustrated in Figure la. The cell is applicable to the measurement when the specific gravity of the organic solution, d, is larger than that of the aqueous solution, d,. The aqueous electrolyte solution, w, containing supporting electrolyte, was forced
upward dropwise into the organic solution, org, containing sunnortine electrolvte. through a canillarv made of Teflon
ment of the potential difference a t the aqueous/organic interface, and two counter electrodes, CE1 and CE2, are used for the measurement or the control of the current flow through the interface. The tip of RE2 should he close to the aqueous solution drop and, usually, should not apart more than-1 mm from the surface of the drop when the drop is atits maximum, When d, was larger than dorg,we found that such a cell as that illustrated in Figure l b was most feasible from a practical viewpoint. In order to bold the tip of the fluorocarbon resin capillary in the organic solution, the aqueous overflow was kept a t a level above that of the tip in the cell. The cell of Figure l a is also applicable to the measurement even when d, > d forcing the organic dropwise upward into T the aqueous solution (organic solution dropping electrode, OSDE). In this case, a glass (e.g., Pyrex) capillary should be employed instead of one made of Teflon, because most organics used in the present work adhere to Teflon and, hence, i t is difficult to get the solution drop when a capillary made of Teflon is used. Because OSDE has such a disadvantage with the resistance between RE1 and RE2. R(REl-RE2). being extremely large compared with that of ASDE due low solubility and/or the weak dissociation of the supporting electrolytes in the organic phase in the Teflon capillary, the ASDEs in Figure 1 are used in the following. Ohmic Drop between RE1 a n d RE2 a n d Its Compensation. It is impossible to observe the ion transfer at the
to
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I. Characteristics of Aqueous Solution Drop Electrode
Teflon capillary diameter of length, bore, mm mm 1.4, 1.4, 1.45 LOo 0.8, 0.0, 1.0, 1.0,
1.0, 1.0,
1.0, 1.0, 1.0,
supporting electrolyte
aqueous reservoir height, cm
w(MgSO,), M
M
3.0 2.0
165 165
1.0 1.0
1.0 1.o
165 165 165 165
1.0
0.1" 0.1" 0.1" 0.10 0.1" 0.1" 0.1"
1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0
82 42 82 82 82 82 82
1.0 1.0 1.0 1.0 1.0 0.1 1.o 0.1 1.0 1.0
flow
org(CV+-TPhB-), R(REl-RE2):
0.1" 0.1" 0.01"
0.01" O.l* 0.1c
XlOO R
5.0 2.3 2.0 3.4
rate,e mLd
drop time,es
volume of drop: mL
0.020,
6.2
0.12,
0.020,
6.3 6.2 5.1 4.0 1.6
0.12, 0.12, 0.10, 0.08,
0.0202
0.0203 0.020,
4.8 11.1 3.4 3.4
0.020, o.0103 0.005; 0.0154
19.7 7.7 25.0 3.3
0.O1O8 0.0144
6.8
0.010,
0.010,
10.7 19.2 3.7 10.8
3.5 8.0 2.4
0.03, 0.10, 0.10, 0.05, 0.10, 0.05,
0.08, 0.02,
Nitrobenzene organic solvent. 1,2-Dichloroethane organic solvent. Chloroform organic solvent. At drop maximum. e At open circuit. {Because of the low viscosity of the aqueous solution containing 0.1 M MgSO,, the flow-rate at the glass capillary is faster than that of the aaueous solution containing 1 M MESO,. objective aqueous/organic interface when the resistances of phases or interfaces other than the objective interface, Rother, are larger than that of the objective interface, Robj.Even when Rotheris less than Robj,the precise compensation of the ohmic, iR, drop in current-scan polarography (15)or the rigid feedback of the iR drop in potential-scan polarography ( 4 ) is not possible when Rotheris large. The iR drop in ASDE is controlled by the conductivity of the aqueous and/or organic solution as well as the size of the bore of the fluorocarbon resin capillary (15). When such solvents containing enough supporting electrolyte as nitrobenzene, NB, or DCE are employed as the organic phase, the iR drop is mainly attributable to the electric resistance of the aqueous solution in the fluorocarbon resin capillary. Table I summarizes R(RE1-RES) observed with ASDE and NB as org under various conditions. By use of a fluorocarbon resin capillary with a bore of 0.3 mm diameter X 3 mm height, which was proposed by Samec et al. (8),and aqueous solution containing 1 M MgS04 as supporting electrolytes for ASDE, R(RE1-RE2) is as large as 8.5 kR. By use of a capillary with a bore of 1 mm diameter X 1mm height, the resistance is 340 R and the iR drop caused by the resistance with a current of 40 PA, which is comparable to the limiting current usually M of monovalent ion, is about 14 mV observed with 5 X and is negligibly small for the analytical purpose. In a previous paper (15), we proposed a simple circuit to compensate for the iR drop in current-scan polarography, which is feasible for the measurement when R(RE1-RE2) is less than ca. 1 kQ. When the resistance is larger than ca. 1 kR, however, the electric field produced by the circuit interferes with the potential measurement. In the present work, therefore, we adopted an improved circuit composed of two isolation amplifiers (Tokyo Musenkikai Co., Ltd., Model LX0402) and shown in Figure 2. With this circuit it is possible to compensate R(RE1-RE2) as large as more than 10 kR. Characteristics of the Aqueous Solution Drop, ASD. Not only the size of the fluorocarbon resin capillary but also the concentration of the supporting electrolyte in aqueous and/or organic solution affects the characteristics of ASD as listed in Table I. Since the surface tension of a solution is the function of the concentration of the electrolyte in the solution, the drop time, t , and, hence, the surface area of the aqueous solution drop depends on the concentration of the supporting electrolyte in aqueous and/or organic solution (mainly on that in aqueous solution where the concentration is much larger than that in organic solution. The characteristics of the aqueous solution drop is also
lsola tion CEI
R~=R(RE~-REz)
Figure 2. Circuit for iR drop compensation: R,, resistance equivalent to that between RE1 and RE2.
affected with the variant of organic solution, as seen in Table I, which may be connected with viscosities of the organic. The viscosities of NB, DCE, and chloroform (in the absence of electrolyte) are 2.03,0.84, and 0.56 cP, respectively, at 20 "C. Although t greatly depends on the kind of organic, the flow rate of the aqueous solution, m, is practically unchanged with the variety of organic when the height of the aqueous reservoir, h,, is constant. With TPhAs+.DPA- as the supporting electrolyte in the organic instead of CV+-TPhB-,the characteristics of the solution drop are hardly varied. Supporting Electrolytes and the Potential Window of Residual Currents. The supporting electrolytes in aqueous or organic phases should have an appropriate solubility as well as a large dissociation constant in each phase. And, also, they should be composed of ions that are stable in one phase and hardly transfer to the other, because the final rise and the final descent in the residual polarogram are determined by the transfer of supporting electrolyte ions at the aqueous/organic interface. Residual currents were investigated with ASD containing 1 M MgS04as the supporting electrolyte and with NB or DCE containing various supporting electrolytes. Potential windows, P,, of residual currents at the aqueous/NB interface are summarized in Table 11. Here, we define the potential difference between the potential where the residual current is 20 wA,AV(I = 20 PA), and where the residual current is -20 wA, AV(1 = -20 wA), as P,. As for the standard potential, TPhE, in the table, the detailed explanation will be given later. The wide P, can be attained when supporting electrolytes in the organic phase composed of monovalent ions of large ionic radii as CV+, TPhAs+,TPhB-, and DPA- are employed. These ions are hardly transferred from organic to aqueous phase. Among supporting electrolytes, which are prepared from salts both stable and commercially available, CV+.TPhB-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
2957
Table 11. Potential Window at Aqueous (w)/NB Interface with Various Supporting Electrolytes in w
sumortina electrolvte in NB
1 M MgS04
1 M MgSO, 1 MHCl
+ + 0.1 M HCl + 0.01 M HCl + 1 M LiCl + 0.1 M LiCl + 1 M NaCl + 1 M NaF + 1 M NaBr + 1 M KC1 + 1 M NaN03 + 0.5 M Na2S04 + 0.1 M NaSCN + 0.1 M LiClO,
P, (V vs. TPhE)
TBA+*TPhBCV+*TPhBTPhAs+-DPATPhAs+*C104' TPhAs+-SCNTEA+.TPhB-
-0.21 -0.39 -0.34 -0.34 -0.34 -0.13
to +0.27 to +0.27
0.05 M TPhAs+*DPA-
-0.18 -0.25 -0.29 -0.18
t o t0.14
0.05 M 0.05 M 0.05 M 0.05 M 0.05 M 0.05 M
to +0.28 to +0.07
i
AV / V vs TPhE
to +0.04
to +0.27
to t0.18 to +0.22 to + o x -0.25 to +0.20 -0.18 to +OM -0.34 to +0.13 -0.15 to t0.13 -0.18 to to.08 -0.05 to t0.13 -0.34 to +0.13 -0.06 to +0.19 -0.02 to t0.20
realizes the widest P, when NB or DCE is employed as organic. Though TPhAs+.TPhB- is supposed to be one of the ideal supporting electrolytes, the solubility of this salt is not enough for the supporting electrolyte, e.g., less than 2 X lo-', 2X M a t 25 "C in NB, DCE, and chloroform, and respectively. In the following experiments, 0.05 M of CV+TPhB- or TPhAs+-DPA-is added to the organic solution as the supporting electrolyte. In order to prepare CV+.TPhB- and TPhAs+-DPA-, we examined several procedures such as the extraction of precipitates of salts, which had been produced in water from chloride salts of cations and sodium salts of anions, into the solvent under investigation, or the recrystallization after dissolution of the salts precipitated in water with various organic solvents. Among them, procedures described in the Experimental Section are most promised from the viewpoints of purity, recovery, and time required for the preparation. These salts are stable enough even at room temperature in the dark when solids but are not so stable when they are dissolved in such organics as NB or DCE. These organic solutions after standing for 2 days at room temperature (ca. 20 "C) gave fairly large residual currents, but, when they were stored in a refrigerator a t 4 "C, they were stable for at least for 7 days. Table I1 includes P, observed with aqueous solution containing various salts in addition to 1 M MgS04 and NB containing 0.05 M TPhAs+.DPA-. The salts composed of mono(or di-)valent ions of relatively large ionic radii in aqueous solution make P, narrow, especially when their concentrations are high. Roughly speaking, the above mentioned effect of supporting electrolytes on P, holds when DCE or chloroform is used as organic instead of NB. The pH-buffered solutions are often necessary to get an objective ion of an appropriate chemical state. By use of aqueous solution that contained pH-buffering salts and DCE containing 0.05 M CV+.TPhB-, residual polarograms a t the aqueous/DCE interface were recorded as illustrated in Figure 3.
As the pH-buffering salts are those of weak dissociation and, therefore, the resistance of their aqueous solutions are fairly large, the final rise and the final descent of the polarograms were considerably retarded, when the measurement was done without the iR drop compensation (see, polarogram 1' in
Figure 3. Residual currents with various pH-buffering agents in aqueous solution. Supporting electrolyte: in aqueous solution, (1) 0.05 M sodium acetate + 0.05 M acetic acid + 1 M Li,SO, (pH 4.57), (1') as (1) but without Li,SO, (pH 4.42), (2) 0.2 M citric acid 0.25 M LiOH 1 M Li,SO, (pH 3.60),(3)0.045 M NaH,PO, 0.0275 M Na,B,O, 1 M Li,SO, (pH 7.92), (4) 0.1 M potassium hydrogen phthalate 0.027 M LiOH 1 M Li,S04 (pH 4.41); in DCE, 0.05 M CV'STPhB-. Scan rate was 0.5 FA'S-'. ASD drop time was 6.3 s. Flow rate was 0.013 mL.s-'.
+ +
+
+
+
+
Figure 3). The presence of 1 M Li2S04(or 1 M Na2S04)in addition to the buffering salts in aqueous solution made the iR drop small (polarogram 1)maintaining the buffering ability, though pH values are somewhat changed by the presence of Li2S0, (or Na2S04)due to the change of the ionic strength. Here, MgS04 cannot be used in alkaline solutions because of precipitation of its hydroxide. It is seen from Figure 3 that pH-buffering salts composed of such monovalent ions of small ionic radii as acetate or alkaline metal ions and such di- or trivalent ions as sulfate or phosphate ions are feasible for the PITIES from the viewpoint of P,. Reference Electrodes. The potential difference at the aqueous/organic interface, AV, is measured with two reference electrodes, RE1 and RE2, set in aqueous and organic solutions, respectively, close to the interface. The silver-silver chloride electrode, which has been most frequently used in the works of this field, is employed as RE1 also in the present work. The potential of RE2 should be correlative to the ion transfer free energy, AG,,, since A V at the interface under investigation, which is connected with AG,, by eq 1 ( 3 ) ,is measured referring to RE2, where, z and F a r e the charge of the ion and the Faraday constant, respectively.
A V = -AG,,/zF
(1)
To realize a stable and reproducible potential a t RE2, ion selective electrodes, ISE, of liquid membrane type of which liquid/liquid interfaces are completely depolarized by an ion of fairly high concentration have been investigated and, for practical use, such ISEs as TBA+ ( 4 , 9, 18) and tetramethylammonium ( 5 , 1 9 ) ,TMA', ISEs have been proposed as RE2. In the present work, we studied the feasibility of ISE given as eq 2 and 3, denoted as TPhAsE and TPhBE, respectively, because we consider that the potentials of these ISEs can easily be related to the standard transfer free energy under the extrathermodynamic assumption mentioned later. In the cell of eq 2 or 3, the same organic solvent as that for the organic phase (org) is used as the organic solution in RE2, org(RE2). When these ISEs are employed as RE2, the interference in the measurement of the polarogram as well as the deviation of the potential of RE2 caused by the oozing of org into org(RE2) or vice versa may be minimized since org(RE2) is composed of the same solvent and the same electrolyte as those in org. The potential of electric cells of eq 2 and 3 can be expressed as eq 4 and 5 , respectively, where, AV, Aworgp,
2958
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14,DECEMBER 1986
Ag-AgC1 1 M LiCl
(SSE) 1 [aqueous] I [aqueous] Ag-AgC1 1 M LiCl
'
I 0.05 M LiCl 10.025 M I 1 M MgSO, I (TPhAs+)2. 1 0.05 M I
LiCl 1M MgS0,
0.05 M TPhAs+. SO>DPA' 1 M MgSO, 1 [organic, 1 [aqueous] I org(RE2)I
'
10.05 M
0.025 M
I
Mg2+. 1 (TPhB-), (SSE) 1 M MgS04 [aqueous] 1 [aqueous] l[aqueous]
1
I
1
0.05 M TPhAs'. DPA[organic, orgl
(2)
0.05 M
1 CV+. 1
I
CV+. TPhBTPhB[organic, [organic, org(REZ)] 0 5 1
(3)
-4-
-0.2
--
A5
+
02
AV/ V vs TPhE
04
I i
RE2
AV
+ const = AWor,(a(TPhAs+)=
AWorg(Po (TPhAs+)
AV
-
( R T / F ) In
+ const = Aw,,,p(TPhB-)
aTPhAst,w/aTPhAs+,org
(4)
=
Aworg(ao(TPhB-)+ (RT/F) In aTPhB-,w/aTPhB-,org (5) AWorgpo,and a,* are the electrical potential difference, the Galvani potential difference, the standard Galvani potential difference at the aqueous/organic interaface and the activity of i'. The const is independent of the nature of iz and usually small ( 3 ) . In this connection, (TPhAs+)2.S0,2- and Mg2+(TPhB-), are adopted as the salts in the aqueous phases, since such divalent ions as SO,,- or Mg2+ hardly transfer from aqueous to organic and, hence, these counterions may not affect the potential expressed by eq 4 or 5. The actual feature of RE2 is illustrated in Figure 1. The potentials of TPhAsE and TPhBE are reproducible and the deviation of the potential was confirmed to be less than 3 mV by the five repetitions. Potentials of these reference electrodes are stable and the deviation after standing for 1 day at room temperature (ca. 20 "C) or after standing for 1 week in a refrigerator at 4 "C was less than 5 or less than 7 mV, respectively. The potentials in the present work are referred to the standard potential denoted as TPhE, which corresponds to AGO,, being zero and is determined based on the extrathermodynamic assumption that AG",, values of TPhAs+ and TPhB- across a liquid/liquid interface are equal (20-22). The assumption can be expressed as eq 6 considering eq 1. In
Aw,,,po(TPhAs+) in eq 4 = -AW,,rgqo(TPhB-)in eq 5 (6) order to get TPhE, the potential difference between TPhAsE and TPhBE was measured connecting these electrodes through the organic solution. Correcting the potential difference for activity coefficients of TPhAs' and TPhB- (17), the difference between AWorgpo (TPhAs') and A"orgqo (TPhB-) was estimated and the mean of the difference was determined to be TPhE. Organic Solvents for PITIES. The aqueous solution has been employed without exception as one of two immiscible solutions for VITIES. As the other solvent, NB and DCE, which dissolve and dissociate supporting electrolytes well, have been most frequently used (1,2). Hundhammer and Solomon reported the use of acetophenone (11) and the mixture of chlorobenzene and NB (23). In the present work, various organic solvents of dielectric constants, e, between 5 and 36 were investigated in addition to the above-mentioned solvents. Most of solvents studied gave wide P, when 0.05 M TPhAs+-DPA-or CV+.TPhB- was used a s the supporting electrolyte, as summarized in Table 111, and, hence, these solvents are considered to be capable for the observation of the ion transfer at the aqueous/organic interface by the PITIES or the VITIES. To record the polarograms using solvents indicated by e in Table 111, the cell
-
Figure 4. Current-scan polarogram for transfer of CIOc at aqueous/DCE interface: (1) cathodic current for CI0,- (aqueous DCE), 5 X lo-, M LiCIO, 4- 1 M MgSO, in aqueous solution: 0.05 M CV+aqueous); 1 M TPhB- in DCE; (2) anodic current for CI0,- (DCE MgSO, in aqueous solution, 5 X M CV+CIO,0.05 M CV+. TPhB- in DCE. Scan rate was 0.5 PAS'. ASD drop time was 6.3 s. Flow rate was 0.013 mL-s-'.
-+
illustrated in Figure l b was used because d, > dorg. Incidentally, the final rise and the final descent of the polarogram a t the aqueous/chloroform interface were conspicuously retarded when the polarogram was recorded without iR drop compensation, since the resistance of the chloroform solution is rather large. Mixtures of two organic solvents, miscible and immiscible with water, are also feasible as organic phase as suggested by P, listed in Table 111. Ion T r a n s f e r Polarograms at Aqueous/Organic Interfaces. The polarogram for the reversible ion transfer at the w/org interface can be expressed (3, 5 ) as eq 7 ,
neglecting const in eq 4 or 5 , where, z , Diz,,,ui2,, and i are the charge of the ion i', the diffusion coefficient of i' in a phase, the activity coefficient of i' in a phase and the instantaneous current, respectively. The anodic and the cathodic limiting currents, ila and iL, are given as eq 8 and 9, respectively, using
The ion transfer polarogram of Clod-from aqueous to DCE and that from DCE to aqueous are realized as curves 1 and 2, respectively, in Figure 4,as examples. The limiting currents, il, of both polarograms were proportional to the square root of the height of the aqueous reservoir, h,, as well as to the concentration of ClO,- in aqueous or DCE in the range between 10-5and M or 5 X and M, respectively. The logarithmic analysis, log (il - i)/i vs AV, of these polarograms showed straight lines with slopes 58 A 3 mV. Facts mentioned above suggest that the transfer of C104- between aqueous and DCE is reversible and controlled by the diffusion
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f
Figure 5. Current-scan polarogram for transfer of tetrapropylammonium ion from aqueous to DCE: 5 X lo-, M tetrapropylammonium bromide 1 M MgSO, in aqueous phase: 0.05 M CV+. TPhB- in WE. Scan rate 0.5 pA.s-'. ASD drop time was 6.3s. Flow rate was 0 013 mL-s-'.
+
of C104 in aqueous and/or DCE. Polarograms for most ions listed in Table I11 show reversible character, too, irrespective of the kind of organic. In Table IV, I , for the ion transfer of (210,-a t interfaces of aqueous and various organic solutions are summarized with the characteristics of the ASD. The concentration of C104in both aqueous and organic phases is 5 X M. Since the values of ilc/m2/3t1'6are nearly a t constant, even when the organic is varied, the transfer of C10, from aqueous to organic is considered to be controlled by the diffusion of the ion in the aqueous phases. The values of ila/m2/Jt1/6differ with different organic. One reason for the difference may be attributable to the dependency of D,i,orgon the viscosity o f the organic, 7 . Even when D,i,orgare corrected for 7 taking the Stokes-Einstein relation (24),D 0: l / q , into account, however, the difference cannot be explained. The other reason may be the dependence of D,l,orgon the activity of the ion (24)which is usually small in such organic solution of small dielectric constant as DCE and chloroform. Polarograms for the transfer of various ions (5 X lo-, M in aqueous solution) from aqueous to various organic phases were investigated and the AV,,? observed are summarized in Table 111. Polarograms for cations of which LV,,,lie at less positive potentials and those for anions of which hV1,? lie at less negative potentials than the potential of the point of zero charge, PZC, were somewhat irregular, which is attributable to the change of the surface charge of the ASD at the PZC (25-28). Therefore, 1 V l , 2 for these cations include some uncertainties. A typical example of the irregular polarogram is realized in Figure 5. Limiting currents, I , , of polargrams whose LV,,, are indicated by the superscript d were confirmed to be proportional t o the concentration of the ions in the aqueous phase in the range between 5 X 10-5 and M. Transfer energies, A G O , , , of Clod-, IO4-, CIO1-, I-, Br-. BF,-, SCN-, Cs+, and TMA+ at the aqueous/NB interface calculated from AV1 using eq 1 and 10 agree well with AGO,, obtained based on partition data and/or solubility data (29),when AVl are corrected for D and u. For most of the monovalent anions of which AVl are listed in Table 111, there exists a linear relation between AV1 observed with an organic and the inverse of the thermochemical ionic radii. The details of the relation will be discussed separately (17). In this connection, it is noteworthy that the transfer of hydrogen ion from aqueous to organic is facilitated and AVl of the ion is found at a less positive potential when DPA is employed as the anion of the supporting electrolyte in org, suggesting a fairly strong ion-pair formation between DPAand proton in the organic phase. Speciation of Ions by PITIES. Since l V 1 depends on the charge and the radius of the ion, PITIES is considered to be useful for the speciation of ions. As an example of the speciation. polarograms for the transfer of hydrogen phthalate
Av/v
VS
TPhE
Figure 6. Current-scan polarogram for transfer of hydrogen phthalate ion from aqueous to DCE: in the aqueous phase (5 X lo-, M KHC8H,04 1 M Li,SO, +) (1) H,SO, (pH 0.92), (2)H,S04 (pH 1.93),(3) 2 X lo-' M citric acid M Na,HPO, (pH 2.92), (4)1.6 X lo-* M citric acid 8 X M Na,HPO, (pH 3.38),(5) 3.3 X lo-' M acetic 2.2 X acid M sodium acetate (pH 3.90),(6)8.8 X M 6X KH'PO, M Na,B,O, (pH 5.51),(7) 6.2 X M KH,PO, 1.9 X M NapB,O, (pH 6.50),(8)4.5 X M KH,PO, 2.8 X M Na,B,O, (pH 7.46),(9)5 X M Na,B,O, (pH 8.90):in DCE, 0.05M CV'STPhB-. Scan rate was 0.5 PA'S-'. ASD drop time was 6.3 s. Flow rate was 0.013 mL-s-'. Broken line indicates residual current.
+
+
+
+
+
+
+
ion from pH-buffered aqueous solutions to DCE are shown in Figure 6. At pH more than 10 or less than 2, hydrogen phthalate changes its chemical form to phthalate ion or phthalic acid, respectively, which hardly transfers from aqueous to organic phases and, hence, does not give polarograms. Here, pK,, and pK,, of phthalic acid at 20 OC are 2.95 and 5.41 (30),respectively. Figure 6 obviously demonstrates that the relation between pH and the chemical species in aqueous phase can be obtained by PITIES. It has been demonstrated in the present paper that transfers of various ions at interfaces of aqueous and various organics can be observed by PITIES and the il of polarograms are proportional to the concentration of ions. The relative standard deviations in the 5 repeated measurements of the limiting currents for the transfer of IO4-, Clod-,Reo,-, C103-, BF,-, I-, SCN-, CN-, and TMA' from aqueous to DCE were 0.9 and 5.1% when the concentrations of these ions in the aqueous phase were 5 X and 5 X M, respectively, and CV+.TPhB- was used as the supporting electrolyte in DCE. In conclusion, since even transfer polarograms of such divalent ions as Ca2+and Cu2+can be recorded if such strong complexing agents as ionophors coexisted in the organic phase (6), which we have not described in the present paper, wide application of PITIES in the field of analytical chemistry is expected.
ACKNOWLEDGMENT The authors wish to thank T. Fujinaga for his helpful discussion. LITERATURE CITED (1) Koryta, J.; Vanjkek, P. Advances in Electrochemistry and ElectrochemicalEngineering; Wlley: New York, 1981; Vol. 12: p 113. (2) Koryta. J. ion-Selecfive Nectrode Rev. 1983. 5 , 131
Anal. (?hem. 1986, 58,2961-2964 Koryta, J ; Vanqsek, P.; Biezina, M. J . Nectraanal. Chem. 1977, 75, 211. Samec, 2.; MareEek, V.; Weber, J. J . Elechoanal. Chem. 1979, 700, 841. Kihara, S.:Yoshida, 2 . Talanta 1984, 3 7 , 789. MareEek, V. Nectrochemical Detectors ; Plenum Press; New York, 1984; p 141. Samec, 2.; Homolka, D.; MareEek, V. J . Electroanal. Chem. 1982, 735,265. Samec, 2.: MareEek, V.; Weber, J.; Homolka, D. J . Electroanal. Chem. 1979, 9 9 , 385. Koczorowski, 2.; Geblewicz, G. J . Electroanal. Chem. 1982, 139, 177. Hundhammer, 6 . ;Solomon, T. J . Nectroanal. Chem. 1983, 157, 19. Solomon, T.: Alemu, H.; Hundhammer, B. J . Electroanal. Chem. 1984, 169, 303. Koryta, J. Anal. Chlm. Acta 1984, 159, 1. Vangsek, P.; Buck, R. P., J . Electroanal. Chem. 1984, 163, 1. Koryta, J.: Vanqsek, P.; Biezina, M. J . Elechoanal. Chem. 1978, 6 7 , 263. Klhara, S.;Yoshida, 2.;Fujinaga, T. BunsekiKagaku, 1982, 3 1 , E297. Ueno, K.; Saitoh, M.: Tamaoki, K. Bunseki Kagaku, 1968, 17, 1548. Kihara, S.; Suzuki, M.; Maeda. K.; Ogura, K.; Matsui, M. J . Electroanal. Chem., in press. Senda, M.; Kakutani, T.; Osakai, T. Denkl Kagaku oyobi Kogyo Butsurl Kaaaku 1981. 49. 322. (19) Fujkaga, T.; Kihara, S.; Yoshida, 2. BunsekiKagaku 1982, 3 1 , E301.
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(20) Grunwald, E.; Baughman, G.; Kohnstam, G. J . Am. Chem. SOC.1960, 82, 5801. (21) Paker, J. Chem. Rev. 1989, 6 9 , 1. (22) Popovych, 0.CRCCrit. Rev. Anal. Chem. 1970, 1 , 7 3 . (23) Solomon, T.: Aiemu, H. J . Electroanal. Chem. 1984, 769, 311. (24) Bockris, J. O'M.; Reddy, A . K. N. Modern Electrochemistry; Plenum Press; New York, 1970. (25) Samec. 2.: MareEek, V.; Homolka. D. J . Electroanal. Chem. 1985, 187, 31. (26) Osakai, T.: Kakutani, T.; Senda, M. Bull. Chem. SOC.Jpn. 1985, 58, 2626. (27) Girault, H. H. J.; Schiffrin, D. J. J . Electroanal. Chem. 1984, 770, 127. (28) Maeda. K.; Kihara, S.:Suzuki, M.; Ogura, K.: Matsui, M., to be submitted for publication in J . Electroanal. Chem. (29) Marcus, Y. Pure Appl. Chem. 1983, 5 5 , 977. (30) Sillen, L. G.: Martell, A. E. Stability Constants of Metal-Ion Complexes : The Chemical Society: London, 197 1.
RECEIVED for review June 6, 1986. Accepted July 30, 1986. This work was partly supported by a Grant in Aid for Scientific Research (59470025) from the Ministry of Education, Science and Culture of Japan and partly by a grant from the Nissan Science Foundation.
Determination of Electrochemical Heterogeneous Electron-Transfer Reaction Rates from Steady-State Measurements at Ultramicroelectrodes Andrea Russell,' K a r i Repka,' Timothy Dibble,' J a m a l Ghoroghchian,' J e r r y J. Smith,' M a r t i n Fleischmann,* Charles H.Pitt,3a n d Stanley Pons*'
Department of Chemistry and Department of Metallurgical Engineering, University of Utah, Salt Lake City, U t a h 84112, and Department of Chemistry, T h e University, Southampton, Hampshire SO9 5NH, England
I n this work, we demonstrate the use of very thin ring ultramlcroelectrodes In measurlng the heterogeneous rate constants of several fast one-ektron-transfer reactions under steadydate dlffudon condmons. Under these condltlons, the theoretlcal and Instrumental efforts are greatly slmpilfled compared to those requlred for relaxation technlques.
The determination of the heterogeneous rate constants of electrode reactions has hitherto been carried out by the use of steady-state or transient techniques using conventional electrodes (characteristic dimensions 1 mm). The upper limit of the rate constants which can be measured by using steady-state methods is -0.05 cm s-l, a limit that is set by the maximum rate of convection diffusion to appropriately shaped electrodes such as rotating disks or tubular sections. Transient (relaxation) methods, which involve the perturbation of systems initially at equilibrium or in a steady state and the measurement of the resulting response, are able to reach rate constants as high as 10 cm s-l in view of the high rates of non-steady-state mass transfer to the electrode surface. The accuracy of all of these transient techniques is however limited at short times by the low impedance of the double layer which appears in parallel across the equivalent circuit of the electrode processes; furthermore, all of these techniques require relatively sophisticated and costly instrumentation. The mini-
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'University of U t a h , Department of Chemistry.
T h e University, Southampton. University of U t a h , Department of Metallurgical Engineering.
mum configuration will almost always include a potentiostat with a fast rise time and high voltage compliance, a signal generator, and high speed transient recording devices. Recent advances in the construction of ultramicroelectrodes (1-18) have opened up an alternative route to the determination of the kinetics of fast electrode processes. A high rate of steady-state diffusion is rapidly established in the spherical or quasi-spherical diffusion fields surrounding the microstructures (the mass transfer coefficient is of order k , = D / a cm s-l, where a (cm) is the characteristic dimension of the microstructure and D (cm2s-l) is the diffusion coefficient). The rate parameters of fast electrode reactions therefore become readily accessible by means of steady-state or quasi-steady-state measurements (e.g., by the use of slow linear sweep voltammetry). An important feature of these measurements is that they are not limited to the region close to equilibrium (the entire polarization curve can be determined) and that the accuracy is not limited by the double layer capacitance. Moreover, only relatively simple instrumentation is required, viz., a linear sweep generator, a current follower, and an appropriate recorder. A variety of ultramicroelectrode structures have been proposed and developed, including single hemispherical droplets and collections of droplets (8),dispersions of spherical particles (9),disks ( l o ) ,and thin rings (17, 18). In this paper we report measurements of the rates of a range of redox processes using the latter structure which has a number of advantages: the characteristic dimension (the thickness of the ring) can be made very small so that high rates of mass transfer are easily achieved; at the same time current levels are determined by the radius of the ring and are therefore
0003-2700/86/0358-2961$01.50/00 1986 American Chemical Society
