Anal. Chem. 2001, 73, 5611-5615
Progress in Karl Fischer Coulometry Using Diaphragm-Free Cells Anders Cedergren* and Svante Jonsson
Department of Chemistry, Analytical Chemistry, Umeå University, S-901 87 Umeå, Sweden
Different designs of a semiopen, drainable cathode compartment of a medium-sized coulometric Karl Fischer (KF) cell for the determination of water in the range 0.1500 µg were evaluated. The main criterion for the design was to keep the resistance between the anolyte and catholyte low enough to permit the generation of currents larger than 20 mA (for an output voltage of 28 V). It was found that a good compromise between the size of this current and a minimal influence from diffusing/migrating oxidizable reduction products from the catholyte was achieved by means of an interface having a channel length and diameter of 8 and 2.1 mm, respectively (catholyte volume, ∼1 mL). To show the general applicability of the concept, the following different types of coulometric reagents suitable for nonpolar and polar samples, as well as for samples containing active carbonyl compounds, were investigated: Hydranal Coulomat A, AD, AK, AG-H (modified with chloroform, Merck), and two homemade methanolic reagents modified with 40% (v/v) chloroform and 50% (v/v) formamide, respectively. Except for Hydranal Coulomat A, the mean value of five consecutive titrations of 50 µg water did not deviate by more than 0.2% from the expected value for all reagents. Draining after every titration was sufficient to obtain accurate results, even for Coulomat A which, when used in the commercial diaphragm-free system of Metrohm, gave values which were about 10% too high. As compared to earlier reported results for diaphragm-free coulometry, the descibed modified cell represents a significant improvement, mainly because of the high accuracy achieved for all types of reagents. The current trend in Karl Fischer (KF) coulometry is toward the use of cells without a diaphragm between the anode and cathode.1,2 The different designs of such cells was recently reviewed (see ref 2). The main advantages over the conventional technique are the shorter conditioning times required (approximately 0.5 h, as compared to at least 2 h for a diaphragm cell), the absence of problems due to clogging of the membrane and, in most cases, a simpler cell construction. In addition, the cleaning procedure is simpler. The major drawback of the diaphragm-free concept is that part of the generating current causes the formation of oxidizable (with iodine) reduction products, such as thiosulfate * Corresponding author. E-mail:
[email protected]. (1) Scholz, E. Int. Lab. 1989, Oct, 46-53. (2) Nordmark, U.; Cedergren, A. Anal. Chem. 2000, 72, 172-179. 10.1021/ac010355g CCC: $20.00 Published on Web 10/05/2001
© 2001 American Chemical Society
and sulfide3 in the cathode reaction. Such compounds will simulate water in their reaction with iodine, leading to a positive error for the water content in the sample. To cope with this problem, special reagent compositions have been designed2,4,5 which, in combination with the use of a very high cathode current density, limit the positive errors to the range 0-5%. Nordmark et al.5 showed that for a coulometric cell in which the cathode is in direct contact with the working medium, the expected error for the best combination of commercially available reagent and coulometric equipment is ∼2%. The same authors6 also showed that this value could be further reduced to 0.1%, provided the following conditions were fulfilled: (i) the use of an imidazole-buffered methanolic reagent at pH 10 (pH scale in methanol) containing molar concentrations of modifiers, such as hexanol, ethylene glycol, carbon tetrachloride, or chloroform2,5; (ii) the use of pulsed current coulometry with pulse heights selected2,6 to give a cathodic current density in the interval 1000-4000 mA/cm2; (iii) the length of the pulses should be above 60% of the total pulse cycle6; and (iv) a low background drift should be established in order to minimize the error that arises when subtracting the background.6 Thus, it is possible to obtain very accurate results for the type of diaphragm-free coulometry in which the cathode is in direct contact with the anolyte. However, the general applicability of such systems has yet to be demonstated. In this respect, the diaphragmfree cell previously described by the authors7 is compatible with a wider range of KF-reagents, which is necessary for applications requiring alternative solvents to methanol, other bases than imidazole, and a greater freedom in the selection of suitable modifiers. This type of cell construction makes it possible, at any time in a sequence of titrations, to renew the catholyte by means of a Teflon plunger inside the cathodic compartment. In this way, the interference effects caused by the oxidative reduction products can be eliminated in a pretitration step. In this work, a further development of this type of diaphragmfree cell has been made by evaluating different designs of the interface between the anodic and cathodic compartments. Seven different types of coulometric reagents were tested in order to show the general applicability of the concept. The results are (3) Katoh, H.; Fujimoto, Y.; Kuwata, S. Anal. Sci. 1991, 7, 299-302. (4) Scholz, E. “Reagent, cell and method for the coulometric determination of water” U.S. Patent 5 139 955, Aug. 18, 1992. (5) Nordmark, U.; Cedergren, A. Fresenius’ J. Anal. Chem. 2000, 367, 519524. (6) Nordmark, U.; Rosvall, M.; Cedergren, A. Fresenius’ J. Anal. Chem. 2000, 368, 456-460. (7) Cedergren, A.; Jonsson, S. Anal. Chem. 1997, 69, 3100-3108.
Analytical Chemistry, Vol. 73, No. 22, November 15, 2001 5611
Figure 1. (a) Schematic diagram over the diaphragm-free cell construction: (1) reference electrode, polymer-coated (fluorinated ethylene propylene) platinum, 0.5 cm free metal in contact with a Karl Fischer medium containing ∼0.05 M iodine excess; (2) indicating electrode, same as 1; (3) cathode, same as 1, area 0.08 cm2.; (4) platinum wire in contact with the platinum gauze working electrode; (5) Teflon plunger; (6) Teflon top; (7) silicone rubber septum; (8) channel in which a Teflon tube is inserted and connected to a drying tube containing silica gel or magnesium perchlorate (Dehydrite); (9) O-ring; (10) normal liquid level; (11) Vycor glass; (12) magnetic bar; (13) titration vessel; (14) platinum gauze. (b) Lower part of the cathode compartment (“the interface”).
compared to those obtained using the best commercial equipment and reagents for diaphragm-free coulometry. EXPERIMENTAL SECTION Chemicals. Methanol (p.a.), iodine (resubl.), chloroform (p.a.) and formamide (p.a.) were from Merck. Hydranal Coulomat A (contains chloroform), Coulomat AK (for ketones; contains 2-methoxyethanol), Coulomat AD, Coulomat AG-H, and Hydranal Water Standard (5.0 mg/mL of xylene/butanol) were from RiedeldeHae¨n. Merck coulometric reagent no. 1.09257, imidazole (puriss p.a.), and sulfur dioxide (>99.97%) were from Fluka. Safety Considerations. Methanol: highly flammable; toxic by inhalation, in contact with skin, or 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. Chloroform is an animal carcinogen. Imidazole: harmful by inhalation, in contact with skin, and if swallowed. Iodine: is a poison and may be fatal if swallowed; avoid 5612 Analytical Chemistry, Vol. 73, No. 22, November 15, 2001
contact with eyes, skin, or clothing, do not breathe the vapor, and use adequate ventilation. Sulfur dioxide: intensely irritating to eyes and respitory tract. 2-Methoxyethanol: causes severe irritation; combustible; harmful if inhalated or absorbed through the skin; overexposure may cause male or female reproductive disorders; exceptional health and contact hazards. Reagents. Except for Hydranal Coulomat AG-H, which was mixed with chloroform 30/70 (v/v), the commercial reagents were used as delivered from the manufacturers. One of the homemade reagents, reagent A, was prepared by dissolving 0.1 M iodine, 0.6 M sulfur dioxide, and 6.5 M imidazole in methanol containing 40% (v/v) chloroform. The other, reagent B, was prepared by dissolving 0.1 M iodine, 0.6 M sulfur dioxide, and 1.4 M imidazole in methanol containing 50% (v/v) formamide. These reagents were decolored with water immediately after preparation so that only a slight excess of iodine remained. The procedure for preparing these reagents was described earlier.8 (8) Cedergren, A. Anal. Chem. 1996, 68, 3679-3681.
Table 1. Results for 5 Consecutivea Additions of 50 µg of Water into Different Designs of the Drainable Diaphragm-Free Cell (Figure 1) reagents cathode construction config. no.
Coulomat A
dimensions (mm)b a b c
max currentc (mA)
mean recovery (%)
Coulomat AD std (%)
DId (µg)
max current (mA)
mean recovery (%)
std (%)
DId (µg)
1
5
0
1.8
11
104.6 1.7 baseline:e 0.90 f 1.10 µg/min
110
10
101.4 0.3 baseline: 0.83 f 0.83 µg/min
42
2
10
0
1.5
27
103.0 1.4 baseline: 1.16 f 1.32 µg/min
83
20
100.7 0.3 baseline: 0.98 f 1.00 µg/min
43
3
10
3
1.5
15
100.7 0.8 baseline: 0.49 f 0.54 µg/min
62
14
99.8 0.2 baseline: 0.60 f 0.60 µg/min
26
4
10
3
1.8
18
100.7 0.5 baseline: 0.97 f 0.95 µg/min
91
19
100.2 0.2 baseline: 0.75 f 0.75 µg/min
68
5
10
3
2.1
22
100.6 0.3 baseline: 1.00 f 1.00 µg/min
72
22
100.2 0.2 baseline: 1.02 f 1.02 µg/min
82
a Each titration was allowed to proceed 5-6 min. b The meaning of these letters is explained in Figure 1. On the basis of results for 10 V output voltage, the expected maximum current for 28 V is calculated. d DI means the first draining after a sequence of 5 titrations (after about 30 min). e This means the baseline drift before and after the additions of 5 samples, each containing 50 µg of water.
Instrumentation. A schematic description of the diaphragmfree coulometric cell is given in Figure 1a,b. The components and the function of the cell are very similar to those descibed before in this journal.7 The titration vessel and the auxiliary electrode compartment were made from polymethylpentene (PTX, Mitsui Petrochemical Industries Ltd.). Draining of the auxiliary electrode compartment was carried out by pushing the Teflon plunger down and up. When the bottom of the Teflon plunger passes the hole on the upward stroke, the liquid level inside the tube falls to the same level as that of the liquid in the main compartment, because the normal liquid level is below the hole.The draining products (oxidative reduction products plus trace amounts of water) are determined in a normal titration, and the result is denoted D1 in Table 1. The generating and indicating electrodes (zero-current potentiometry) were connected to the computer-controlled coulometric titrator, as recently described.9 For accurate determinations of the water concentration of the Hydranal standard, a threecompartment coulometric cell (with diaphragm) containing Hydranal Coulomat A was used as described before.9 Procedure for Preparation of the Cell. The reference electrode compartment was filled with ∼0.3 mL of a KF medium, normally Hydranal Coulomat A or AD, to which iodine was added to obtain an excess of about 0.05 M. Refreshing this solution was necessary only after several weeks, which means that the change in the composition of this solution was negligible during the time for a titration. A spent KF reagent containing an excess of water can also be used as the electrolyte in the reference electrode compartment, as was discussed in a previous paper.10 Normally, 18 mL of the reagent, containing a small excess of iodine, was transferred to the cell followed by careful shaking in order to remove all of the moisture from the inner surfaces of the cell. Detailed procedures for starting up a titration, including the selection of end-point potential, amplification factor, etc., are given in refs 7 and 9. For the experiments with Hydranal Coulomat AK, (9) Rosvall, M.; Lundmark, L.; Cedergren, A. Anal. Chem. 1998, 70, 53325338. (10) Cedergren, A.; Luan, L. Anal. Chem. 1998, 70, 2174-2180.
the end-point concentration of iodine was set to 30 µM because of the slow reaction rate in this reagent, but for all others, 6-8 µM iodine excess was sufficiently large for a rapid attainment of the end-point potential. RESULTS AND DISCUSSION The performance of the diaphragm-free cell described in this paper is illustrated in Figure 2 and Table 1 by the results from five consecutive titrations of 50 µg water using the commercial coulometric reagents Hydranal Coulomat A and AD. The deviation from 100% recovery reflects the positive error caused by diffusion/ migration of the oxidative reduction products into the anolyte. It should be mentioned that the errors reported earlier2,5,7 for these reagents, when used in cells without any barrier between the generating electrodes, were 10-35% and 2-10%, respectively, depending on the cathodic current density. For comparison, the results from a previous work7 are given in Figure 1 and Table 1 for a design identical to that denoted configuration 1. In some separate experiments, the degree of interference obtained with the plunger totally lowered into the cell (using the same experimental conditions as those of Table 1) was determined for the most troublesome reagent, Hydranal Coulomat A. The relative errors were found to be in the range +22 to +17%, depending on the maximum current used (10 mA and 6 mA, respectively). The rising tendency for all curves in Figure 2 with the number of titrations reflects the increased rate of diffusion/migration due to the buildup of larger and larger concentrations of reduction products inside the cathodic compartment. The difference between the results obtained with the cell constructions denoted configurations 1 and 2 can be explained by the larger dilution of the reduction products in the latter, which is larger as a result of the larger catholyte volume. By considering the differences in dimensions for all interfaces, it is evident that the length of the channel is of utmost importance. A channel length of 8 mm (configurations 3, 4, and 5) was found to be suitable, because the diameter of the channel could then be increased from 1.5 to 2.1 mm without significantly affecting the accuracy, while at the same time, the Analytical Chemistry, Vol. 73, No. 22, November 15, 2001
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Figure 2. Recoveries for five consecutive titrations of 50 µg of water using Hydranal Coulomat A and AD using different interfaces, the dimensions of which are given in Table 1.
Figure 3. Comparison between results (mean values of five determinations) obtained for different reagents using the diaphragm-free system of Metrohm (unfilled bars) and the home-built cell (shaded bars), configuration 5. The sample size was 50 µg for the home-built instrument and 250 µg for the Metrohm.
maximum generating current was increased by about 50%. Considering the results obtained with configuration 5, which is the recommended design, it is evident that a draining step is required after every titration when using the Hydranal Coulomat A, but at least five titrations can be run in sequence without this requirement when using Coulomat AD. According to the results summarized in Table 1, the cell constructions corresponding to configurations 3, 4, and 5 also exhibit better precision, as compared to determinations based on configurations 1 and 2. The reason for this seems to be related to the better baseline stability. The lower than expected maximum current for configuration 1 is partly due to the use of smaller electrode areas and partly due to the longer distance between the working and auxiliary electrodes 5614
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in the cell previously described.7 It should be emphasized that all results reported in this paper were obtained by continuous coulometry, which means that the cathodic current density is low during the last part of the titration, and especially at the end-point. Under these condition, a large fraction of the cathodic current gives rise to the formation of oxidizable reduction products. If, instead, pulsed current coulometry (2, 5) is used, the buildup of these interfering agents will be much less, because with this technique, a very high cathodic current density is maintained near, as well as at, the end-point. As a consequence of this, a somewhat better accuracy is, thus, to be expected when using this type of coulometry. It should also be mentioned that the value of DI in the Table includes significant amounts of water being introduced
by the mechanical movement of the Teflon plunger, probably because of a removal of surface adsorbed water. A summary of all results obtained by means of cell configuration 5 for seven different types of reagents is given in Figure 3. These were selected in order to show the general applicability of the described cell, because these reagents cover most of the types of samples, from nonpolar (those modified with chloroform) to polar (formamide), as well as those containing active carbonyl compounds, such as ketones (Coulomat AK). For comparison, results obtained under optimum conditions5 for the diaphragm-free system of Metrohm are given for the same reagents. Although the results obtained with the home-built system are based on 50-µg water samples, a relevant comparison can still be made with the Metrohm instrumentation (250 µg of water was added in this case), because the former represents the mean of five determinations (i.e., a total of 250 µg). It should be mentioned that no significant change in the baseline was observed during these titrations. CONCLUSIONS As shown in this paper, it is possible to achieve the same degree of accuracy for diaphragm-free coulometry as for the conventional coulometric technique, independent of the nature of the coulometric reagent. This can be achieved by means of an (11) Cedergren, A. Anal. Chem. 1996, 68, 3679-3681.
interface at the bottom of the cathode compartment having a channel length and diameter of 8 and 2.1 mm, respectively. The main drawback to the described concept is that the generating current is restricted to ∼20 mA (corresponding to a titration rate of 112 µg water/min). This means that the practical working range is between 0.1 and 500 µg water. About 10 to 20 times larger currents are used in the Metrohm system, which means that the working range for this instument also covers the milligram range. On the other hand, the smallest amount that can be determined with the Metrohm instrument, 1 µg, is ∼10 times larger than that for the diaphragm-free system described in this work. This difference is due in part to the much larger volume of the Metrohm cell and in part to the better stability (as compared to the bipotentiometric technique used by Metrohm) of the zerocurrent potentiometric end-point technique11 used in the diaphragmfree cell described. ACKNOWLEDGMENT The authors thank Dr. Michael Sharp for the linguistic revision of the paper. Received for review March 28, 2001. Accepted August 22, 2001. AC010355G
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