Anal. Chem. 1996, 68, 3679-3681
Comparison between Bipotentiometric and True Potentiometric End-Point Detection Using Rapidly Reacting Karl Fischer Reagents Anders Cedergren
Department of Analytical Chemistry, Umeå University, S-901 87 Umeå, Sweden
Bipotentiometry was compared with true potentiometry (zero current) as end-point indicating systems for the Karl Fischer (KF) titration. Using a coulometric microcell equipped with both systems, it was shown that the latter technique gives a significantly more precise end-point location. In addition, for rapidly reacting KF reagents, the true potentiometric method offers the possibility of accurately controlling the iodine excess of a titration in the concentration range 10-10-10-5 M, which is not possible when the bipotentiometric technique is used. In contrast to this, the true potentiometric end-point system was found to be relatively little influenced by the electric field created by the generating electrode system in coulometric analysis. Numerous end-point detection techniques have been proposed for the Karl Fischer (KF) titration, including visual,1 spectrophotometric,2 true potentiometric (zero current),3,4 biamperometric,5 and bipotentiometric (according to IUPAC it should be named controlled current potentiometry with two electrodes).6 The two last-mentioned methods are those most commonly used in commercial KF instruments.7 No study has yet been reported in the literature in which a comparison of the above-mentioned electrometric techniques has been performed. Such an investigation is not straightforward, since the conditions for the end-point detection are influenced by a number of parameters, such as the mode of titration e.g., volumetric or coulometric, titration cell design, reagent delivery rate, the kinetics of the reaction between water and the KF components, response times for the indicating electrode system at different iodine levels, the extent of background drift as caused by moisture diffusion, and different types of side reactions. All these complications were recognized long ago by Lingane,8 who stated that the accuracy and precision of most detection techniques are usually governed by the chemical and thermodynamic characteristics of the titration system rather than by limitations of the measuring technique itself. (1) Kolthoff, J. M.; Elving, P. J. Treatise on Analytical Chemistry; Vol. 1, Part II, Sec. A, p. 69; Interscience: New York, 1971. (2) Connor, K. R.; Higuchi, T. Chem.-Anal. 1959, 48, 91-93. (3) Cedergren, A. Talanta 1974, 21, 553-563. (4) Verhoef, J. C.; Bahrendrecht, E. J. Electroanal. Chem. 1976, 71, 305-315. (5) Mitchell, J.; Smith, D. M. Aquametry; Interscience: New York, 1948. (6) Wu ¨ nsch, G.; Scho¨ffski, K. Fresenius J. Anal. Chem. 1991, 340, 691-695. (7) Scholz, E. Karl-Fischer TitrationsDetermination of WatersChemical Laboratory Practice; Springer: New York, 1984. (8) Lingane, J. J. Electroanalytical Chemistry, 2nd ed.; Interscience: New York, 1958. S0003-2700(96)00047-9 CCC: $12.00
© 1996 American Chemical Society
True potentiometry using a platinum electrode was early considered to be a very accurate sensing system for iodine in KF media.3,4 Using the slowly reacting standard pyridine reagent [with a rate constant about (1-3) × 103 M-2 s-1], it was shown that the end-point error at 10-4 M iodine in the end point was significantly lower for the true potentiometric technique as compared to the biamperometric one. Recently, it was found that9 the rate of the KF reaction can be greatly speeded up by selecting a very high free concentration of a baselike imidazole. An even more favorable situation arises when solvents like (30%) chloroform and dichloromethane are mixed with such reagents.10 The fast reaction rate of such reagents makes it possible to obtain a rapid reaction even at such low iodine levels as 10-10 M. This fact creates a somewhat new situation for the KF reaction since it opens up the possibility of controlling the KF titration on a much lower level than has hitherto been possible. In the present paper, a critical study of the potentiometric techniques has therefore been carried out using a very rapidly reacting reagent in a coulometric cell in which bipotentiometric as well as true potentiometric detection can be studied simultaneously. EXPERIMENTAL SECTION Chemicals. Methanol (p.a.) was from Merck. Iodine (p.a.) was from Riedel-deHae¨n. Imidazole (puriss p.a.) and sulfur dioxide (>99.97%) were from Fluka. Chloroform (p.a.) was from Prolabo. Safety Considerations. Methanol: highly flammable, toxic by inhalation, in contact with skin, and if swallowed. Chloroform: inhalation and ingestion are harmful and may be fatal. Inhalation of vapors may cause headache, nausea, vomiting, and dizziness. Prolonged skin contact may result in dermatitis. Liquid is readily absorbed through the skin. Imidazole: harmful by inhalation, in contact with skin, and if swallowed. Sulfur dioxide: intensily irritating to eyes and respiratory tract. Asbestos: carcinogenic. Reagents. The properties of the reagent used have recently been described.10 It was prepared by dissolving 5 M imidazole, 0.9 M sulfur dioxide, 30% (v/v) chloroform, and 0.1 M iodine in methanol. Instrumentation. The coulometric cell used has been described before.8 It was constructed of poly(methylpentene), TPX Mitsui Petrochemical Industries Ltd., and consisted of three chambers, one for the platinum auxiliary electrode, one for the injection of sample through a silicone rubber septum, and one for the platinum reference electrode (used for the true potentio(9) Cedergren, A. Anal. Chem. 1996, 68, 784-791. (10) Cedergren, A. Anal. Chem. 1996, 68, 3682-3687.
Analytical Chemistry, Vol. 68, No. 20, October 15, 1996 3679
metric system). The middle cell compartment contained the platinum generating electrode (placed quite close to the auxiliary electrode cell compartment), the bipotentiometric (two identical 0.1 cm2 platinum wire electrodes) electrode system, and a single 0.1 cm2 platinum electrode used for the true potentiometric measurements. The two indicating electrode systems were placed as far away from the generating electrode as possible in order to minimize the influence from the generating current field. Electrolytic contacts were made using asbestos-filled liquid junctions. The generating and the true potentiometric indicating electrodes were connected to an LKB 16300 coulometric analyzer. This instrument contains a high-impedance voltmeter which measures the voltage between the indicator and reference electrodes. This voltage is compared with a preset potential, and any deviation is linearly amplified and used to control the current through the generating electrode system. The way in which the end point is approached could be adjusted with the gain of the instrument so that it decreased rapidly at first and then asymptotically toward the background current. The current time integral can be followed on a display down to 10-11 equiv, which corresponds to 0.0901 ng of water. The temperature was normally within the interval 21° ( 1.5 °C. The potential between the indicating and reference electrode as well as the polarization voltage of the bipotentiometric system could be followed accurately by a Fluke 45 dual-display multimeter connected to a computer in which the data collection program Fluke QS 45 was available. The constant current mode of a second LKB coulometric titrator was used to control the polarization current delivered to the bipotentiometric system. Procedure for Preparation of the Coulometric Cell. The excess of iodine in the prepared reagent was reduced to a transparent solution (∼10-3 M iodine) by carefully adding water with a Hamilton syringe, and 4.3 mL of this reagent was then transferred to each of the cell compartments. A suitably strong water-in-methanol solution was then carefully added to the working compartment with a 10 µL Hamilton syringe until a suitable iodine level was achieved, e.g., 10-5 mol/L. A stable reference electrode system (drift less than 0.1 mV h-1) was established simply by adding ∼1 µL of water to the reference cell compartment (∼4.3 mL). The redox equilibrium attained probably involves a waterbuffered equilibrium between an iodine complex (Im2I+), hypoiodite, and iodate.10 The cell was then turned upside down in order to remove any moisture from the walls. The titrator was switched on, and the drift value was noted. Normally 5-10 min was needed to obtain a stable drift value in the range 0.1-0.3 µg/min. When this value had been reached, the calibration curve, i.e., the relationship between the redox potential and the concentration of iodine in the working cell compartment was obtained by incremental generation of iodine, normally the interval 10-52 × 10-4 M, in combination with the the measurement of the electrode potential which typically needed only a few seconds to equilibrate. The slope of the calibration curve as obtained by regression analysis based on five electrode potential readings did not normally deviate by more than a few hundredths of a millivolt from the theoretical value. It should be pointed out that no correction for the drift was made for this calibration procedure. However, the error will be very small since the time for the whole procedure was typically
