Anal. Chem. 1998, 70, 5332-5338
Computer-Controlled, Coulometric Karl Fischer System for Continuous Titration of Water Based on Zero Current Potentiometry Magnus Rosvall, Lars Lundmark, and Anders Cedergren*
Department of Analytical Chemistry, Umeå University, S-901 87 Umeå, Sweden
A flexible computer-controlled coulometric Karl Fischer system based on zero-current potentiometry is described. A negligible influence of the generating current on the potentiometric signal is indicated by the low noise level of the system. For example, the noise in the background observed at an iodine end-point concentration of 5 × 10-5 M using a current amplification factor of 181 µA (generating current)/mV (deviation between measured and preset potential) was about 5 µA (peak-to-peak). For a 1-min titration, this corresponds to a maximum error in the evaluation of the current/time integral of about (0.01 µg calculated as water. The accuracy of the system was tested using a standard water-in-methanol solution, and no significant difference was found between the predicted and experimentally found values. Trace determination of water in a mineral oil was used to illustrate the high sensitivity of the described system. Water determinations based on the Karl Fischer (KF) reaction are carried out in numerous laboratories throughout the world. During the past decade, coulometric KF titration systems have gained in popularity, and today there are many manufacturers of such equipment. The design and functioning of commercially available coulometric KF intrumentation is very similar: controlled current potentiometry with two platinum electrodes is used for end-point detection, and the microprocessor of the instrument controls the analytical procedure and displays the result in micrograms of water (or ppm). Pulsed coulometry1 is normally used and arranged so that the signal from the indicating electrode system does not coincide with the current pulses used for the electrochemical generation of one of the reactants, iodine. The controlled current potentiometric end-point system used in commercial instruments requires an end-point setting corresponding to an iodine excess concentration in the range (3-7) × 10-5 M. For trace determinations of water, when relatively large sample volumes have to be added, this high level will result in significant errors due to dilution since the titration is carried out to a predetermined end-point concentration of iodine. Moreover, if sample constituents interfere with iodine, the rate of this reaction will, of course, increase at higher concentrations of iodine. This creates a problem of overcompensation when subtracting the baseline in the evaluation of the water content of the sample, since (1) Macleod, S. K. Anal. Chem. 1991, 63, 557A-566A.
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the side reaction is normally much slower during the course of the titration as compared to the situation at the end-point. Zero-current potentiometry has not yet been used as a feedback system in commercial KF instruments, although it has been reported in the literature to function very well in KF media.2-4 However, this indicating principle has been successfully used for a long time by Dohrmann Instruments in their very popular microcoulometric techniques for the determination of sulfur, nitrogen, and halogens in petroleum products. With regard to the coulometric KF technique, the use of zero-current potentiometry was recently3,5,6 shown to offer the possibility of accurately controlling the iodine excess level of a titration in the concentration range 10-10-10-5 M, provided that the composition of the KF reagent was selected to accelerate the KF reaction. It should be pointed out that the main problem with zero-current potentiometry as a feedback system in continuous coulometric analysis arises when the impedance of the indicating electrode system is high because of the interaction between the measurement and electrolysis circuits caused by the presence of a strong electric field. In this paper, we describe and characterize a flexible computercontrolled KF coulometric system based on zero-current potentiometry. Thanks to its high stability and sensitivity, the system is shown to be well suited for trace determinations of water. EXPERIMENTAL SECTION Chemicals. Methanol (Merck, p.a.), imidazole (Fluka, puriss p.a.), sulfur dioxide (Fluka, >99.97%), and iodine (Riedel-deHae¨n, p.a.) were used as received. 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 vapor may cause headache, nausea, vomiting, and dizziness. Prolongen 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: Intensely irritating to eyes and respiratory tracts. Asbestos: Carcinogenic. Reagents. Hydranal Coulomat A (contains chloroform) from Riedel-deHae¨n was used for determination of water in the (2) Verhoef, J. C. Mechanism and Reaction Rate of the Karl Fischer Titration Reaction. Ph.D. dissertation, Free University of Amsterdam, Amsterdam, The Netherlands, 1977. (3) Cedergren, A. Anal. Chem. 1996, 68, 784-791. (4) Cedergren, A.; Luan, L. Anal. Chem. 1998, 70, 2174-2180. (5) Cedergren, A. Anal. Chem. 1996, 68, 3682-3687. (6) Cedergren, A. Anal. Chem. 1996, 68, 3679-3681. 10.1021/ac980710b CCC: $15.00
© 1998 American Chemical Society Published on Web 11/12/1998
a
b
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homemade water-in-methanol standard (see below). All other experiments, except for the determinations of water in oil, were carried out using a very rapidly reacting5 reagent with the following initial concentrations: 5 M imidazole, 0.10 M iodine, 0.9 M sulfur dioxide, 30% (v/v) chloroform in methanol. The compositions of the different reagents used for trace determination of water in mineral oil are given in the caption to Figure 2. Instrumentation. The main control unit in this system is a Pentium 166-MHz computer and Lab PC+. Some of the control functions in the system were moved outside so that a potential can be maintained even if the computer is not running. The control unit is a simple titrator, which is described in detail below. All functions, information, and data processing are combined in a single LabVIEW program under an optional Windows operating system. To isolate the computer from the control unit, isolation amplifiers were used. Lab PC+ IO Card from National Instruments and Its Configuration in This System. The Lab PC+ contains a 12-bit successive approximation A/D converter with adjustable gain (eight lines), two 12-bit D/A converters, 24 digital IO lines using TTL logic, and three 16-bit counter/timer channels for timing. The digital and analog grounds are separated. The card has several different input configurations. NRSE input configuration was used in this setup. We connected the (AISENSE/AIGNG) input to analog ground (AGND) for a more stable signal. The data sampling rate was 1 kHz. Ten samples were then collected, and their mean values represent one sample in the program. Description of the Program. To ensure easy usage, all parameters and controls are collected on the same screen, see Figure 1a. This includes a real-time graph of the current passing through the cell, icell, and the redox potential between the indicating and reference electrodes. Pop-up menus are used only in the current peak evaluation and in the save and load functions. The different parameters available to initiate the system, and hence those values that the user needs to type in, are the cell volume, Vcell, and set point, Eset (corresponding to a certain iodine concentration in the end-point of the titration). There is also a special section where more advanced adjustments to the system can be made. This is called “Configure the system”, and here the user can adjust the amplification of the signal before A/D converter on the Lab PC+ card, the number of samples per second, and the set point change in calibration. After the above parameters are adjusted, the titrator can be switched on. When the user presses the button “Titrate to start”, the titrator receives the “set point” value. To follow how the current and the redox potential of the cell vary with time, the user needs to press “Start measuring”. This program also has a special function for determination of the calibration curve which describes the relationship between the concentration of iodine (M) and the potential E (V) of the indicating electrode versus the reference (E ) const. + (RT/2F) ln [iodine]) in the KF cell. We can write this relationship in a simple form as follows (C ) [iodine]):
∆C + 1 ) 10∆E/(RT/2F) C1
(1)
When the user presses “Start calibration”, the set point rises by the value at “set point change in calibration” (normally 10 [mV]). The titrator now generates more current (2I- f I2 + 2e-) through the cell, and after a current peak a new potential is reached (corresponding to C1 + ∆C). The program now needs the concentration difference as indicated in (1) to calculate the calibration curve, and therefore it jumps into the integration subroutine. In this subroutine (SubVI), the user can set the baseline. The area from the baseline to zero is then subtracted from the peak area before returning to the main program. This “base area” is shaded when shown on the screen. How this is done in a general LabVIEW program is shown in Figure 1b. This type of baseline correction should be useful for signal evaluation in other analytical techniques such as chromatography and spectroscopy. The analytical tool for integration comes from an analytical package that is linked to the LabVIEW language (bought separately). When the routine returns to the main program, the calibration curve is shown in the lower right-hand corner of the display. Data-log-file and experiment-log-file are saved after integration in two different directories. Old data-log-files can also be loaded. This enables the user to reintegrate old data-log-files. With the built-in editing tools in LabVIEW, it is easy to check peak height for kinetic calculations and to zoom in integration start and stop points. When these functions are collected in one program, the chemist receives a good support for further development and understanding of the Karl Fischer reaction. Titrator Layout and Function. An overview of the titrator circuit is shown in Figure 2. Because of the high output impedance of the indicating electrode system, the titrator has a FET input follower (D). The potential after D is also monitored by the computer and is collected as the true redox potential. At E, the signal from the computer (negative) is divided 10 times to ensure higher resolution. The two signals are combined in a two-stage summation circuit (F). To isolate the indicating system from the generating system, we used an isolation amplifier (ISO 122P). To phase shift the signal by 180° and correct for offset error, the circuit described in H was used. When the set-point value is lower than the actual redox potential, the signal from H is negative. To prevent back-titration, an “ideal diode” circuit was used (I). The resulting potential is then fed into the positive input of a potential-to-current converter (J). To collect a true value of current through the KF cell, the potential over the 100 Ω ( 0.1% resistor was sampled. The setpoint and the other two potentials collected by the computer were isolated by three isolation amplifiers (AD210). Their connections as followers are shown in Figure 3. Two different types of coulometric cells (only schematically shown in the figure) were used, and these have been described
Figure 1. (a) Main screen layout. The set-point was first set to 78 mV. The user then pressed “Start calibration”, and a current peak was generated as the program raises the set-point to 88 mV. When the peak was integrated, the calibration curve could be calculated according to eq 1. A small amount of methanol (sample) was then added to the cell. (b) The position of the two cursors, shown in “Front Panel”, gives the x and y positions to the program (Block Diagram) using attribute nodes (from XY graph). The positions are then printed out in the XY graph, which now represents a plot. The plot adjustments are then made at plot 0 in the “Front Panel” (Fill Baseline > zero). The base block can then be calculated in the program. 5334 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
Figure 2. Titrator and its connections to the surrounding system. The generating system is isolated from the indicating system by the isolation amplifier ISO 122P. C1 (47 µF) is a capacitor that takes away some of the distortions from the indicating electrode.
Figure 3. Isolation amplifier connected as follower. The AD210 generates some high-frequency noise that is reduced by an RC filter (νg ) 15 Hz).
earlier.3,7 One of these, a microcell,3 consisted of three chambers (each filled with 5 mL of reagent): one for the platinum auxiliary electrode, one for injection of sample through a silicone rubber septum, and one for the platinum reference electrode. The middle cell compartment contained the platinum generating electrode (placed quite close to the auxiliary electrode cell compartment) and a single 0.1 cm2 platinum electrode. The indicating electrode was placed as far away from the anode as possible in order to minimize the influence from the generating current field. Electrolytic contacts were made using asbestos-filled liquid junctions. The reference as well as the auxiliary cell compartments contained an excess of iodine, 1-5 mM, which corresponds to a transparent KF reagent solution. By keeping an excess of iodine in the cathode compartment, the influence of oxidizable reduction (7) Cedergren, A.; Jonsson S. Anal. Chem. 1997, 69, 3100-3108.
products (diffusing or migrating into the working compartment) can be completely eliminated. The other cell used in this work for determination of water in oils was a diaphragm-free cell.7 This device was used for direct coulometric determinations as well as for stripping/continuous coulometry. Preparation of the Water in Methanol Standard. Dry methanol was filled up to the mark of a 100-mL measuring cylinder. The concentration of water in this solution was determined coulometrically to be 0.0069% (v/v) (standard deviation 0.0001%) using a calibrated 10 µL Hamilton syringe (standard deviation 0.012 µL). The coulometric reagent used for these determinations was the Hydranal Coulomat A. A 201.4-mg portion of water was then added to the measuring cylinder, which resulted in a volume change from 100.0 to 100.15 mL, as calculated theoretically on the basis of density data. The resulting concenAnalytical Chemistry, Vol. 70, No. 24, December 15, 1998
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Figure 4. Titrations at different gain levels. Ten microliters of dry methanol was added to the microcell. Gain 2 (46 µA/mV, 1 Hz) gives 0.469 µg of water; gain 3 (91 µA/mV, 1 Hz) gives 0.488 µg of water; Gain 4 (181 µA/mV, 10 Hz) gives 0.481 µg of water; and gain 5 (453 µA/mV, 10 Hz) gives 0.487 µg of water. This gives an average of 0.481 of µg water, with a standard deviation of 0.008 µg.
tration of water in the standard was then calculated to be 0.2080% (v/v). To minimize the risk of contamination from ambient humidity, the samples were taken well below the surface inside the cylinder. The estimated relative error obtained when injecting 10µL of this standard solution was less than 0.2%. RESULTS AND DISCUSSION General Characteristics of the System. To test the stability of the system, titrations were carried out using different amplification factors at an iodine end-point concentration of 5 × 10-5 M, which is typical for commercial systems. Gain 5 (453 µA/mV) is the highest amplification shown in Figure 4 because higher gains resulted in overtitrations. It should be mentioned that a very rapidly reacting reagent was used for these experiments and that longer titration times as well as more unstable conditions will prevail when a slow-reacting reagent (like the pyridine-buffered traditional one) is used. A comparison of the reaction times for different reagent compositions and end-point concentrations is given in ref 5. The time for the titrations shown in Figure 4 along with the observed noise levels are represented in Figure 5. By multiplying the titration time and the value of the noise for different gains, we can see that the maximum error will decrease from 0.02 µg (calculated as water) for gain 2 (46 µA/mV) to 0.004 µg or somewhat less at the higher gains. This means that, for high-precision determinations of water, a separate investigation should be carried out in order to establish the optimum gain. Errors in Calibration. At steady state, when the titrator only generates a current (i) corresponding to the diffusion of water into the cell, d[I2]/dt is zero, and we can calculate3 the concentration of unreacted water as
[unreacted H2O] )
(i/nF)(1000/v) k[I2][SO2]eff
(2)
where k is the rate constant for the KF reaction, v is the volume 5336 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
Figure 5. Noise and titration time obtained for different gains. The dotted line represents earlier obtained results for the same experimental conditions. Table 1. Estimated Amounts of Unreacted Water at Different Iodine Levels, Assuming a Background of 0.5 µg of H2O/min unreacted waterc [I2]a (M) 10-6 10-5 10-4
c
[SO2]effb (M)
Kb/(M-2 s-1)
in µM
in µg
0.85 0.85 0.85
1.8 × 5.4 × 104 0.8 × 104
0.605 0.202 0.136
0.054 0.018 0.012
105
a Iodine level at steady state. b Values taken from ref 5 (reagent I). 5 mL of reagent (composition, see Experimental Section).
in cm3 of the cell, and [SO2]eff means the effective molar concentration of sulfur dioxide. To give an example of how this unreacted water can cause problems when the calibration is carried out at low iodine levels, we can make the following assumptions: suppose that the background drift is 0.5 µg of H2O/ min (typical background value was 0.1-0.2 µg of H2O/min for the cell used in this work), and let the Nernst slope be 29.3 mV/ decade. Using formula 1, mentioned above, and results from ref 5 for this reagent, we get the values in Table 1. Let us assume that the iodine level is 10-6 M and that we increase the potential
Figure 6. Signals obtained for small amounts of water injected into the microcell using 5 × 10-7 M iodine excess concentration at the endpoint.
so that we get a 10-5 M iodine level. Because there is less unreacted water at the iodine level 10-5 M, the titrator has to generate more iodine, and hence ∫i(t) dt becomes larger. This corresponds to ∆C ) [(10-5 - 10-6) + 0.4 × 10-6] M. When these values are put into formula 1, we get
[(10-5 - 10-6) + 0.4 × 10-6] -6
10
+ 1 ) 10∆E/29.3
(3)
which gives ∆E ) 29.8 mV. This means that the calibration curve obtained is displaced by 0.5 mV in comparison with the correct one. If we instead make this calibration at one decade higher iodine level, the error will be negligibly small. Test of the Accuracy of the Overall System. The preparation of water standard was carried out according to the method described in the Experimental Section. Five titrations were carried out at an iodine end-point level of 5 × 10-6 M using the commercially available Coulomat A. This reagent was selected for this investigation because it has earlier been found to give a stoichiometric KF reaction in our coulometric determinations. A 10.15 µL portion of this solution should contain 21.11 µg of water, with a maximum error of 0.2%. The results from the experiments were 21.179, 21.129, 21.164, 21.111, and 21.137 µg. Based on these results, the mean recovery value was 100.1%, with a standard deviation of 0.1%, which means that there is no significant difference between the calculated and the measured values. Trace Determinations. The results given in Figure 6 show that stable conditions for water determinations prevail even at very low iodine end-point levels. For the example shown in Figure 6,
we selected an iodine concentration of about 5 × 10-7 M. The first peak gives a clear indication that the system is capable of detecting water amounts well under under 0.1 µg. As expected at this iodine level, the noise is larger, about 4 µA (peak-to-peak), as compared to 1 µA for the same gain (10 µA/mV) at 5 × 10-5 M. The maximum error that this noise can generate is (1.9 ng of water, and at the higher iodine concentration (0.5 ng of water, assuming that the titration times are 10 s for both. The reason for the higher noise at a lower iodine level is the longer response time of the indicating electrode system at the lower iodine level in combination with the slower reaction rate of the main KF reaction. If a theoretical calculation is carried out, assuming a constant iodine level of 5 × 10-7 M and using the reaction rate constant from ref 5, the reaction time9 t99% f t99.9% becomes about 20-30 s. Calculations are thus in good agreement with the experimental value, which is about 25 s. The theoretical (derivation of Nernst equation) slope of the titration curve in the point 5 × 10-7 M iodine corresponds to about 280 mV/µg of water for the 5-mL cell volume used in the experiments shown in Figure 6. Applications. One very important application of the described coulometric system concerns trace determinations of water in oil products. As was recently pointed out in this journal,8 problems arise when the oil is not completely dissolved in the titration medium. If this is not the case, the oil phase is capable of binding or sequestering a portion of the water so that some moisture is (8) Margolis, S. Anal. Chem. 1995, 67, 4239-4246. (9) This calculation is based on the following formula: t ) ln([H2O]o/[H2O]t)/ (k[I2][SO2]eff), where [H2O]o/[H2O]t is either 99 or 99.9%. [H2O]o is here the excess of water before titration.
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Table 2. Determination of Water in a Mineral Base Oil (Statoil TO8) water found (µg/g) method stripping/continuous coulometrya coulometryb stripping/continuous coulometry (SO2-free reagent)c coulometry (SO2-free reagent)d
mean value
SD
no. of dtns
10.4 10.0
0.2 2.0
5 5
60%) in combination with the use of small sample volumes has to be chosen.8 Commercial instrumentations do not tolerate chloroform concentrations larger than 50% because of breakdown of the voltage across the indicating electrodes. However, large concentrations of such modifiers do not constitute a problem when using the described system based on true potentiometry. The high sensitivity of the coulometric detector is demonstrated by the results given in Table 2 for a mineral base oil. For both methods, a recently described diaphragm-free cell7 was used. (10) Cedergren, A.; Lundstro¨m, M. Anal. Chem. 1997, 69, 4051-4055.
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It should be pointed out that the amount of sample used in the direct coulometric method is at least 20 times less than that normally required for such a low water content in oils. The very low standard deviation obtained with the concept stripping/ continuous coulometry10 can be explained by the relative large sample volume which can be used in this technique. As can also be seen in the Table 2, the blank value obtained by means of a reagent without any sulfur dioxide (i.e., no reaction with water takes place) is very low when using the stripping/continuous coulometric technique. CONCLUSIONS A fundamental advantage inherent with the described coulometric system as compared to commercially available instrumentations is that the titration can be controlled at a very low endpoint concentration of iodine. As was demonstrated earlier,7 this option makes it possible to mininize the effects from interfering iodine-consuming sample constituents. Moreover, the high sensitivity makes it possible to use small sample volumes even for trace water determinations. For example, a volume of 0.1 mL is large enough to detect 1 ppm of water in 5 mL of titration medium, which should be compared to the need of at least 5 mL (in 50 mL of titration medium) using a commercial equipment.1 This means that the dilution of the sample will be larger when using the described coulometric system, and this will, in principle, lead to less influence from interfering side reactions. This system is a prototype which can be improved, and at present the following circuit improvements are recommended (Figure 2): use only one inverting amplifier as gain adjuster (F), and use the isolation amplifier AD210 instead of ISO 122P. If these adjustments are made, one can reduce the circuit layout by taking away the second step on F and the circuit (H). Received for review July 2, 1998. Accepted September 30, 1998. AC980710B
