lost. Thus, more liberal plateau detection parameters were necessary to obtain results with any degree of certainty. Multiplexed Sampling of Multiple Electrode Inputs. Computer-controlled monitoring was evaluated for the case where the potentiometer was operated in the multiplexed mode. DUMB was used to monitor a standard addition Fanalysis at the 10-3M concentration range. The F- monitoring electrode was attached in parallel to the five inputs, but each electrode input was assigned a different bucking potential. The data handling routines for each electrode could be assigned different values of TW to force asynchronous operation. The output of the potentiometer under multiplexed operation is shown in Figure 7. When each electrode input is computer-selected, the DAC bucking voltage is applied first, and then the proper relay is closed. The noise spikes at 4 msec after the different potential changes result from one relay opening while another is closing. The time delay, q,in the data acquisition part of the program, (see Figure 4) is sufficient to reject this spike and allow the amplifier to settle on the new value. A storage scope display is shown in Figure 8 for one step in the standard addition experiment observed at the five different electrode inputs. The results for a typical run, with TW = 10 min for all 5 electrodes, are presented in Table 111. The results are essentially identical for all electrodes and demonstrate that the multiple electrode data acquisition and data handling algorithm does not degrade the results. When differing values of T W were used for each electrode, analysis results varied, as would be expected. But the important observation was that the program does indeed operate asynchronously. This is important for future applica-
tions where different electrodes in different parts of the flow system will be monitored simultaneously.
CONCLUSIONS The original objectives of this work have been met in that a well-defined instrumental and procedural approach has been developed which is suitable for computerized monitoring of multiple ion-selective electrodes in a flowing system. Because optimized conditions were selected for the studies reported here, the results can be used as a “benchmark” for comparison with future studies involving less ideal electrode and/or flow systems. Along these lines it is obvious that the flow system must be modified to eliminate unwanted mixing (albeit at the expense of increased noise), and that more capable data interpretation programming must be developed to allow automated data sampling for less ideal electrode systems. However, the foundation has been laid here for further meaningful applications of computerized electrode monitoring systems.
ACKNOWLEDGMENT The authors wish to acknowledge the assistance of Greg Ridder in developing the mathematical approach and Don Evans in the multiplexer design. The assistance of M. J. Brand of the Technicon Corp. is also gratefully acknowledged. RECEIVEDfor review May 28, 1974. Accepted August 28, 1974. This work supported in part by National Science Foundation Grant GP-21111, and by the Office of Naval Research under Contract No. N00014-67-A-0026-0021.
Investigation of Reference Electrodes for Complexation Studies in Cells without Liquid Junction Kathleen M. Stelting‘ and Stanley E. Manahan Depadment of Chemistry, University of Missouri-Columbia, Columbia, Mo. 6520 1
,
In order to eliminate errors from liquid junction potentials, fluoride and perchlorate ion-selective electrodes were used as reference electrodes for the potentiometric determination of formation constants of acetonitrile and allyl alcohol complexes of Ag(l). Solvent effects at these electrodes are greater than liquid junction potential effects at a conventional calomel reference electrode. Although liquid junction and solvent effects upon potential cannot be separated rigorously, corrections for these potentials were approximated using a glass sodium-selective electrode.
Potentiometric determination of stability constants for complexes with organic ligands is complicated because changes in solvent composition accompany ligand addition. This is most serious for weak complexes, which require relatively high organic concentrations for significant complexation. Under these conditions, measured potential shifts may reflect not only complexation effects, but also changes in activity coefficient or liquid junction potentials. If the latter effects constitute a significant portion of the total shift observed, inaccurate stability constants result. 2118
Weak silver(1) complexes of acetonitrile in water have been reported from potentiometric studies ( 2 - 3 ) . Allyl alcohol complexes have been determined by other methods (4-6). In both cases, there is considerable disagreement concerning maximum ligand numbers and stability constants. Calculations using the extended Debye-Huckel equation predict negligible activity coefficient changes for transfer of silver(1) ion from aqueous to partially nonaqueous solutions containing acetonitrile or allyl alcohol ( 7 ) . Although liquid junction effects in partially nonaqueous solutions have not been widely investigated, junction potentials on the order of 100-200 mV are not uncommon for nonPresent address, Department of Chemistry, California State University, Fresno, Calif. 93710. (1) F. Pawelka, Z. Elekfrochem., 30, 180 (1924). (2) F. Koch, J. Chem. SOC.(London),2053 (1930). 13) . . S . Manahan and R. Iwamoto, J. Electroanal. Chem. lnferfacial Hecfrochem., 14, 213 (1967). (4) W. Krueger and W. Treumann, Proc. N. Dak. Acad. Sci., 8 , 38 (1954). ( 5 ) R. Keefer, L. Andrews. and R. Kepner, J. Amer. Chem. SOC.,71, 3906 f1949). (6) S. Winstein and H.Lucas, J. Amer. Chem. SOC.,60, 836 (1938). (7) K. Stelting, Ph.D. thesis, Univ. of Missouri, Columbia, 1973, pp 29-38
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
aqueous solvent-aqueous reference electrode interfaces (8). Therefore, the major source of error in earlier determination of these complexes was presumably liquid junction effects. Recent studies in aqueous solution have successfully used ion-selective electrodes as references to eliminate liquid junction effects (9). Changes in liquid junction potential with variations in pH at constant ionic strength have also been evaluated for conventional reference electrodes, using ion-selective reference electrodes ( I O ) . This paper examines the feasibility of using ion-selective reference electrodes to eliminate liquid junction effects during potentiometric complexation studies in partially nonaqueous media of variable composition. The performance of solid-state and liquid membrane ion-selective electrodes is evaluated as a function of solvent composition in partially nonaqueous solutions. The relative contribution of liquid junction and solvent effects at ion-selective reference electrodes to total measured potential shifts for silver(1) complexes of acetonitrile and allyl alcohol is reported.
EXPERIMENTAL Eastman Chromatoquality Reagent acetonitrile was used without further purification. Aldrich Analyzed allyl alcohol was distilled prior to use. Sodium perchlorate for ionic strength adjustment was prepared by neutralization of reagent grade perchloric acid. All other chemicals were reagent grade. Solutions were prepared in deionized water and stored in polyethylene containers. Potential measurements were made with a Corning Model 12 expanded scale pH meter. Solutions contained in glass vessels connected to a circulating constant temperature bath (25.00 & 0.05 O C ) were magnetically stirred during measurement. Silver wire and Orion solid-state silver/sulfide (No. 94016) indicating electrodes were used. Measurements in systems with liquid junction employed a conventional aqueous calomel reference electrode with 4.OM NaCl filling solution and internal agar bridge. Orion solidstate fluoride (No. 94-096) and liquid membrane perchlorate (No. 92-81) electrodes were used as references in cells without liquid junction. The ionic strength-controlling electrolyte was used directly to poise the perchlorate reference electrode; for measurements us. the fluoride elect,rode, a constant trace concentration (-10-4M) of NaF was added. A Beckman No. 39137 sodium-sensitive glass electrode was used for evaluation of solvent effects on the ion-selective reference electrodes. Stability constants for the silver(1) complexes were determined by potentiometric titration of 10-3F Ag(1) solutions in 0.1M NaC104. Titrant solutions, also 10-3F in Ag(1) and 0.1M in NaC104, contained 12-13M acetonitrile or allyl alcohol. During the titration procedure, organic concentrations varied from 0.02 to approximately 4M. Solvent effects a t various electrodes were evaluated by similar titration procedures in solutions containing no silver(1) ion. Experiments were conducted in which the medium was changed from a higher to a lower organic concentration and were found to yield identical shifts in potential. Potential equilibration time was the same (several seconds) a t all organic concentrations. Fluoride electiode response in partially nonaqueous solutions was evaluated by addition of small volumes of 10-4-10-*M NaF titrants in 0.1M NaC104 a t fixed organic concentrations to 50.00 ml of 0.1M NaC104 a t the same organic concentration. Perchlorate electrode response was evaluated similarly, using low5, and 10-'M NaC104 titrants. No electrolyte was added to control ionic strength during evaluation of response to perchlorate ion.
RESULTS AND DISCUSSION Plots of potential shift us. -log[ligand] constructed from titration data obtained in cells with liquid junction gave smooth curves. Limiting slopes at ca. 4 M ligand indicated maximum ligand numbers of 2 for silver(1)-acetonitrile and 1 for silver(1)-allyl alcohol complexes. Results were virtually identical for either silver wire or silver/sulfide indicating electrodes. (8) J. Prue, Lab. Pract., 325 (1955). (9) S. Manahan, Anal. Chem., 42, 128 (1970). (10) E. Baumann, J. Electroanal. Chem. Interfacial Electrochem., 34, 238 (1972).
0
-120
L
-
1
12
08
04
00
-04
-log C C H 3 C N I
Figure 1. Complexation of silver(1)by acetonitrile 0 Silver/sulfide vs. fluoride. A Silver/sulfide vs. 4 M calomel
~
Table I. Stability Constants for Silver(1)-Acetonitrile Complexesa Electrode system
31
32
Silver wire-4, OA' calomel Sil~er/sulfide-4.0,\~calomel Silver/sulfide-perchlorate S ilve r/sulf ide - f luor ide
2.6 2.6 2.3 2. a
4.4 5.2 2.4 1.6
a Calculated without consideration of changes in liquid junction potential or changes in potential of reference ion-selective electrode.
Silver(1)-Acetonitrile Complexes in Cells without Liquid Junction. Plots obtained for silver(1)-acetonitrile complexes in cells without liquid junction are illustrated in Figure 1 for silver/sulfide us. fluoride electrodes. Significantly lower shifts a t high ligand concentration are evident (cf. Figure 1, silverhulfide us. 4M calomel). Complexation plots obtained with fluoride reference electrodes were distorted above 1.OM acetonitrile and did not attain a limiting slope. Assuming the existence of two complexes, stability constants calculated by non-linear least squares refinement of preliminary p, values were p1 = 2.8 and p2 = 1.6. As anticipated from the general shape on the complexation plots, agreement between calculated shifts and data points was poor (residuals CQ. 10 mV). Perchlorate reference electrodes gave data qualitatively similar to those obtained in cells with liquid junction, although maximum potential shifts were approximately 10 mV less than observed with conventional reference electrodes. Limiting ligand numbers of 2 were obtained; least squares analysis gave /31 = 2.3 and p2 = 2.4. Stability constants for the silver(1)-acetonitrile complexes, calculated from the data obtained with various electrode systems, are summarized in Table I. Generally, p1 values are independent of the electrode system used to monitor the complexation reaction. Values calculated for @z are a function of the electrode system, presumably because liquid junction effects ( A E L J ) or solvent effects at the ion-selective reference electrodes (Ma) become appreciable a t higher acetonitrile concentrations. Effect of Solvent on Electrodes in Aqueous-Acetonitrile Mixtures. Simultaneous potential measurements were made during the titration at a silver/sulfide measuring electrode using both fluoride and perchlorate reference electrodes and switching between the two reference electrodes. At 3.75M acetonitrile, the potential of the measuring electrode us. the perchlorate electrode was 21 mV more negative than the corresponding potential us. the fluoride electrode. For the difference in solvent effects a t the two
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 14, D E C E M B E R 1974
2119
1 0
t
80
I
E 9 t40
" -40
-1
i
e
.
0
z e
-80
-
o
i> n
12
08
04
00
-04
w-
- l o g CCH3CNI
-40
-80
1 1 30
Figure 2. Effect of acetonitrile on reference electrodes 0 4 M Calomel vs. sodium glass electrode. X Perchlorate vs. sodium glass electrode A Fluoride vs. sodium glass electrode. Sodium glass electrode: Beckman No. 39137
electrodes, therefore, AEa,~- AE.,clo4- = -21 mV, which means that in going from aqueous solution to 3.75M acetonitrile, the potential of the fluoride electrode shifts negative 21 mV more than does the perchlorate electrode. The sums of liquid junction potential effects a t the calomel electrode and solvent effects a t fluoride and perchlorate electrodes were determined by addition of acetonitrile to solutions 0.1M in NaC104 and 1 X 10-4M in NaF and measuring the potentials of the calomel-perchlorate and calomel-fluoride electrode systems. At 3.75M acetonitrile, values of -36.6 mV and -15.1 mV were obtained for (@a,F- ~ L J and ) (Ua,c104-- U L JThese ) . data did not provide information necessary for conversion of measured potential shifts to AE,,, the shift in potential due exclusively to complexation, because liquid junction and solvent effects upon potential could not be separated. The separation of the effects of liquid junction and solvent upon potential cannot be made in a rigorous fashion. One experimental approach to separating these two effects, however, is based upon evidence that the e.m.f. of glass electrodes which are not extensively hydrated (e.g., Beckman No. 39137) is largely independent of solvent composition for a variety of water and nonaqueous solvents (1113).Determination of solvent-related effects in the present study utilizes this property, assuming the effect of allyl alcohol and acetonitrile on the glass electrode to be negligible below 4M organic. This allows liquid junction and solvent effects a t ion-selective reference electrodes to be estimated during addition of acetonitrile to 0.1M NaC104 solutions by assigning total shifts measured for fluoride, perchlorate, or calomel us. sodium glass electrodes to U a , F - , &Ta,c1o4-, or ULJ, respectively. The results of such determinations, expressed as Ua or U L J us. acetonitrile composition in the mixtures, are presented in Figure 2. Appreciable liquid junction and solvent effects are evident above 1.OM acetonitrile. Although increased acetonitrile concentrations affect both ion-selective electrodes, a t 3.75M acetonitrile, the effect a t the fluoride electrode ( A E a , ~ = - -40.8 mV) is approximately double that observed a t the perchlorate electrode (AEa,c104-= - 18.8 mV). Solvent-related effects are least a t the 4M calomel electrode (ULJ = -4.9 mV). Data obtained by the above procedure over the range of acetonitrile concentrations employed for the complexation study were used to correct total shifts measured by various The . resulting plots of electrode systems for AEa or U L J AEcx us. -log[CH&N] gave identical smooth curves, independent of electrode system. Using experimentally evaluated slopes for the ion-selective reference electrodes, limit(11) G. Rechnitz and s. Zarnochnick, Talanta, 11 979 (1964). (12) R Lanier, J. Phys. Chern., 69, 2697 (1965). (13) Advan. Anal. Chern. lnstrurn., 4, 302 (1965).
2120
40
50
-1ogCF-]
Figure 3. Fluoride electrode response in 4.OM acetonitrile Fluoride ion-selective electrode vs. 4 M calomel. Slope = -59.14 mV/decade
Table 11. Response of Fluoride and Perchlorate Electrodes in CH3CN Solutions CH3CN,
Slope (mV/decade)
I
Fluoride
Perchlorate
0.000 1.000 2.000 4.000
-59.11 - 59.51 - 59.33 - 59.04
- 58.97
... ...
- 59.32
ing ligand numbers of 2 were calculated from the limiting slopes of the plots. Stability constants evaluated from corrected data were: & = 2.6, (32 = 6.0. Relative contributions of AE, and AELJ to total measured shift in potential a t maximum ligand concentration were calculated to be: AELJ = 2.5%, = 34% A E a , c 1 0 4 - = 16%. Thus, while the use of ion-selective reference electrodes eliminates liquid junction effects in water-acetonitrile mixtures, it introduces larger errors due to solvent effects on the ion-selective reference electrodes. Correction of data for Sa and AELJ effects does not significantly alter limiting slopes of the complexation plots ( i e . , ligand numbers remain 2 ) . However, ignoring liquid junction effects yields low B2 values. Evaluation of the response of fluoride and perchlorate electrodes a t various acetonitrile concentrations indicated Nernstian behavior to approximately the same p F and pCl04 as in aqueous solution. A typical response plot, for fluoride us. 4M calomel electrodes in 4M acetonitrile is given in Figure 3. Response characteristics for both electrodes a t various acetonitrile concentrations are summarized in Table 11. These data indicate that the shifts in potential observed as a function of increasing acetonitrile concentration do not result from non-Nernstian behavior. Very few data concerning the effect of nonaqueous solvents on ion-selective electrode potentials are available for comparison. However, the shifts observed a t the fluoride electrode during this study are in the same general direction as those observed by Lingane (14) for fluoride us. SCE in 060 vol % ethanol. The shift in fluoride electrode potential in going from water to 4M acetonitrile can be explained by lower solubility of the lanthanum fluoride membrane (15). Silver(1)-Allyl Alcohol Complexes in Cells without Liquid Junction. Complexation plots for the silver(& allyl alcohol system in 0.1M NaC104, obtained with fluoride and perchlorate reference electrodes, showed signifi(14) J. Lingane. Anal. Chern., 40, 935 (1968). (15) K. M. Stelting and S. E. Manahan, Anal. Chern., 46, 594 (1974).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
cantly lower (ca. 28 mV) maximum shifts than those obtained in systems with liquid junction. The plots were qualitatively similar to those obtained in solutions containing acetonitrile. For fluoride reference electrodes, limiting slopes were not attained, because of distortion a t higher alcohol concentrations. Limiting ligand numbers of 0.8 were calculated from slopes of complexation plots obtained with perchlorate reference electrodes. Assuming the existence of one complex, stability constants calculated from data obtained with fluoride and perchlorate reference electrodes were 12.5 and 13.6, respectively (cf. /31 from cells with liquid junction = 15.4 f 0.2). Effect of Solvent on Electrodes in Aqueous-Allyl Alcohol Mixtures. Simultaneous complexation measurements with silver/sulfide us. fluoride or perchlorate electrodes indicated comparable allyl alcohol effects at the two ion-selective reference electrodes. Measurements for fluoride and perchlorate us. 4M calomel electrodes gave values of approximately -25 mV and -23 mV a t 3.2M allyl alcoat the fluoride and perchlorate elechol for (U, ULJ) trodes, respectively. Solvent effects calculated from potentials measured us. the sodium glass electrode a t 3.2M allyl alcohol were: s a , F - = -29.0 mV and ~ . , c l o , - = -25.8 mV. Liquid junction effects were considerably smaller (ULJ = -10.0 mV), although they were approximately double those observed in solutions containing acetonitrile. on allyl alcohol concenThe dependence of A E , and ULJ tration in the mixtures is given in Figure 4. Correction of raw complexation data for U , and AELJ yielded AE,, us. -log[allyl alcohol] plots with limiting slopes of 1.4. Because ligand numbers significantly greater than 1 were obtained, the possibility of two complexes was considered during least squares analysis of data. This procedure confirmed the existence of two complexes with stability constants as follows: /31 = 14.3, /32 = 4.0 (maximum residuals = 0.5 mV). The relative contributions of 1E, and SI.J to maximum potential shifts observed were: SLJ = 8.6%, U,J- = 25%, and AE.,clo,- = 22%. These data indicate that, as in water-acetonitrile mixtures, solvent effects a t the ion-selective reference electrodes were more serious than liquid junction effects. However, in the case of waterallyl alcohol mixtures, both effects were considerable. In this case, correction of raw data gave significantly larger ligand numbers, changing the number of complexes detected from one to two. These data show that ignoring the liquid junction effect in allyl alcohol-containing solutions causes the second complex to be overlooked entirely. As response studies for the fluoride and perchlorate electrodes showed Nernstian behavior over the range of allyl alcohol concentrations employed for the complexation study, the L E , effects observed in allyl alcohol solutions presumably have the same origin as those seen in acetonitrile solutions. These experiments show that it is possible to use ion-selective reference electrodes to eliminate liquid junction effects in partially nonaqueous solvents of variable composition. However, the effect of solvent on the ion-selective reference electrodes may be greater than liquid junction effects. Under these circumstances. the use of ion-selective
+
1 1
w
