Anal. Chem. 1995,67, 999-1004
Coulometric Study of Recovery Rates for Karl Fischer Titration of Water in Aldehydes and Ketones Using Rapidly Reacting Methanolic and 2-Methoxyethanolic Reagents Christina Oradd* and Anders Cedergren Department of Analytical Chemistry, UmeA University, S-901 87 UmeA, Sweden
Factors which influence the recovery of water with use of rapidly reacting imidazole-based methanolic or 2-methoxyethanolic Karl Fischer reagents were investigated coulometrically. Using cyclohexanoneand benzaldehyde as model substances, it was found that, in addition to the well-known problems associated with low recoveries of water due to the bisu%te addition reaction, errors may also be caused by side reactions involving iodine. Such interferences are largest at the end point of a titration, where an excess of iodine is present, and will lead to an overcompensation when correcting the results for the endpoint drift. The extent of the sum of all the side reactions was found to decrease with the concentration of iodide for all types of KF reagent investigated. The high reaction rates for the imidazole-based KF reagents, despite the high concentration of iodide, make it possible to use a very low end-point concentration of iodine. In this way, side reactions involving iodine can be further suppressed. However, for titrations carried out in 2-methoxyethanolic reagents, a relatively high concentration of iodine must be present at the end point in order to decrease the time needed for the reversal of the bisulfite addition reaction. For small amounts of water, Le., less than 30 pg, a reagent based on methanol can be recommended, since the titrations are rapid and the recovery was found to be 100% for both cyclohexanone and benzaldehyde. For large amounts of water or slow titrations, the recoveries tend to decrease for all reagent mixtures investigated. One reason for this seems to be related to a change in stoichiometry for the KF reaction involved in the reversal of the bisulfite addition reaction.
(m
The determination of water in ketones and aldehydes using the KF titration technique is complicated by the fact that at least two different side-reactions occur simultaneously. In the ketal/ acetal formation reactions, the ketone/aldehyde reacts with the alcohol in the KF reagent, thus producing water according to reaction 1,
0003-2700/95/0367-0999$9.00/0 0 1995 American Chemical Society
while the bisulfite addition reaction consumes water according to reactions 2 and 3, SO2 + H2O + B H HSOj'
+HE*
(2a)
SO2 + H20 + 2B H SO3'- + 2 HE+
R'
,C=O + SO^^- + 2 HB+
(2b)
R, ,SO3HE H
c
'R
R/
+B 'OH
where B stands for the base used in the KF reagent. Depending on the pH of the reagent, the bisulfite ion in reaction 3 may be exchanged for the sulfite ion.' In order to avoid reaction 1, the methanol in the classical KF reagent has been replaced by other solvents or solvent However, Scholz has shown5that the bisulfite addition reaction is much more pronounced in nonalcoholic reagents and leads to low recoveries. The more favorable conditions obtained in the presence of methanol can be explained by the fact that methanol and sulfur dioxide react to form methylsulfurous acid? which lowers the concentration of free sulfur dioxide in the reagent. Verhoef et al.'j determined the dissociation constant for the equilibrium 4 to be in the presence of 0.5 M sodium iodide.
CH,OH
+ SO,
-
H++ S0,CH3-
(4)
This means that almost all sulfur dioxide is present in the form of methyl sulfite when pH > 6 in the reagent. 2-Methoxyethanol also forms an alkyl sulfurous acid with sulfur dioxide but probably to a lesser extent, as indicated by the results reported by S c h ~ l z . ~ The effect of using different bases, like pyridine, diethanolamine, and imidazole, has also been in~estigated.~ Imidazole seems to be the best choice for reagents based on 2-methoxyethanolor (1) Stewart, T.D.; Donnally, L. H. 1.Am. Chem. Sac. 1932,54, 3559-3569 (2) Fischer, F.; Schiene, R Z Chem. (Leipzig) 1964,4, 69-70. (3) &ova, V. A; Sherman, F. B.; L'vov, k M. Bull. Acad. Sci. USSR, Diu. Chem. Sci. (Engl. Transl.) 1967,2477-2479. (4) Mitsubishi Chemical Industries. German Patent Application DE 30 40 474, 1980. (5) Scholz, E. Anal. Chem. 1985,57,2965-2971. (6) Verhoef, J. C.; Bahrendrecht, E.]. Electroanal. Chem. 1976,71,305-315. (7)Scholz, E. Karl Fischer Titration-Defemination of Water-Chemical Laborat o y Practice; Springer: New York, 1984.
Analytical Chemistry, Vol. 67, No. 5,March 1, 1995 999
methanol since the titration times are short and the ketal and acetal reactions are suppressed due to the higher pH in these
solution^.^ Commercial KF reagents are mostly based on methanol or 2-methoxyethanol, and the KF reaction, for a methanolic reagent, should be written as follows according to Scholz:
&,I3- + H,O
+ CH,SO,(HB) + 2B
-
Table I Composition of Reagents Prepared in 2-Methoxyethanol
A
B C
CH,SO,(HB)
+ 2(HB)I
(5)
where B is a suitable base. Cedergren8found that the reaction is of f i s t order with respect to iodine, water, and sulfur dioxide in a methanol/pyridine reagent, and thus the rate equation can be expressed as
D E F
H I
J
+
where (-d[I2, Is-lldt) is the background consumption of iodine (drift). During the titration of water in samples containing active carbonyl compounds, water will be tied up in the bisulfite addition reaction according to eqs 2 and 3. The rates of these reactions will of course be highest at the start of the titration when the concentration of water in the titration cell is at its maximum. Reactions 2 and 3 are known to be reversible, so provided that the concentration of water at the end point of the titration is low enough, the back reaction should be complete. In this way, the water earlier tied up will be liberated again and can thus be found in the titration. This was recently showdl to be true for a pyridine/methanol reagent provided that the rate of water formation according to reaction l was kept low and a relatively large concentration of iodine was present at the end point of the titration. For this type of slowly reacting reagent, the use of a high iodine concentration at the end point is needed to promote the reversibility of the bisulfite addition reaction and to minimize errors due to untitrated water which remains at the end point of the titration.ll (8) Cedergren, A Tuluntu 1974,21, 265-271.
(9)Orldd, C.; Cedergren, A Anal. Chem. 1994,66,2603-2607. (10) Verhoef, J. C.; Bahrendrecht, E.]. Electronnul. Chem. 1977,75, 705-717. (11) Cedergren, A; Oriidd, C. Anal. Chem. 1994,66,2010-2016. 1000 Analytical Chemistry, Vol. 67, No. 5,March 1, 1995
1 1.5 2 1.5 1.5 1.5
[Id
0
(M)
0.5 0.5 0.5 0.5 0.9 0.9
0.25 0.25 0.25 0.25 0.05 0.05
[KII
0
0.5 0.4
[I-ltot 0
[SOZleff 0
0.5 0.5 0.5 1.0 0.1 0.5
0.37 0.36 0.37 0.26 0.87 0.86
Table 2. Compositlon of Reagents Prepared in Methanol
G
Recently, the rate of the Karl Fischer reaction in imidazolebuffered methanolic reagents was systematically investigated by the authors? It was shown that the reaction rate constant, k3, in the reagent in the pH range 7-9.4 increases with the square of the concentration of free imidazole (the nonprotonated form) according to k3 = [(274 k 4) x 102[Imke12 (112 f 7) x lo2] L2 molV s-]. This means that the reaction rate can be speeded up by a proper choice of the concentrations of imidazole and sulfur dioxide in the reagent. As compared to a pyridine-buffered methanolic reagent, where k3 is in the range (1-3) x lo3 M2 s-1,6,8-10 imidazole reagents that react more than 50 times faster can easily be prepared? An increased reaction rate will lead to a lower concentration of water at the end point of the titration. Provided that no side reactions involving iodine occur, the steadystate concentration of water can be expressed as follows:
[sod
reagent
K L
M N
1 1.5 2 1.5 1.5 1.5 1.5 1.5
0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9
0.25 0.25 0.25 0.25 0.05 0.05 0.05 0.05
0.5" 0.4 0.4" 0 9
0.5 0.5 0.5 1.0 0.1 0.5 0.5 1.0
0.31 0.36 0.38 0.28 0.87 0.86 0.85 0.84
Sodium iodide was used instead of potassium iodide.
The aim of the work presented in this paper was to investigate the conditions for the determination of water in active carbonyl compounds using reagents based on imidazole/methanol or imidazole/2-methoxyethanol.Because of the lack of information in the literature, it was of special interest to investigate the influence of the concentrations of imidazole, sulfur dioxide, and iodide in different combinations of such reagents. EXPERIMENTAL SECTION
Chemicals. Methanol (pa.), sodium iodide (p.a.), propyl acetate (s), benzaldehyde (zur syntese), cyclohexanone (puriss), and acetic acid @.a,)were from Merck. 2-Methoxyethanol (puriss p.a.), imidazole (puriss p.a.), and sulfur dioxide (>99.97%)were from Fluka. Iodine (p.a.), potassium iodide (p.a.), imidazole hydroiodide, and Hydranal Coulomat AK were from RiedeldeHaen. Methanol, 2-methoxyethanol, benzaldehyde, and cyclohexanone were dried with 3-A molecular sieves before use. The potassium iodide and sodium iodide were dried at 105 "C for several hours and were then cooled and stored in a desiccator. Safety Considerations. (i) Methanol highly flammable; toxic by inhalation, in contact with skin, and if swallowed. (ii) 2-Methoxyethanol: flammable; harmful by inhalation, in contact with skin and if swallowed; irritating to respiratory system; may cause birth defects. (iii) Imidazole: harmful by inhalation, in contact with skin, and if swallowed. (iv) Sulfur dioxide: intensely irritating to eyes and respiratory tract. (v) Asbestos: carcinogenic. Reagents. Reagents containing 2-methoxyethanol were prepared according to Table 1and reagents containing methanol were prepared according to Table 2. Coulometric determination of the effective concentration of sulfur dioxide, [SO2],ff(i.e., the sum of all SN species obtained from the added SOz), was performed iodometrically according to the procedure described by Cedergren et using an LKB 16300 coulometric analyzer. The titration medium consisted of an aqueous solution of 0.4% KI and 0.68% acetic acid.
Table 3. Influence of the Reaction Rate Constant and the Concentratlon of lodlne In the End Point on Calculated Corrections (in pg of water) for a Delta Drift Equal to 1 pg H2O/mln in a Reagent Containing 0.37 Y
so2 calcd corm k3
(M-'
S-')
[I21
(MI
103 103 104
10-4 10-5 10-4
104
10-5
@g of HzO)
0.45 4.5 0.05 0.45
Instrumentation. The cell used for the coulometric KF titrations was constructed of poly(methy1pentene) Crpx Mitsui Petrochemical Industries Ltd.) and is, in principle, of the same type as that described earlier.I3 It consisted of three chambers, one for the auxiliary electrode, one for the injection of sample through a silicon rubber septum with both generating and indicating electrodes, and one for the reference electrode. Electrolytic contacts were made using asbestos-filled liquid junctions. All electrodes were manufactured from platinum and connected to an LKE3 coulometric analyzer. The instrument contains a high-impedancevoltmeter which measures the voltage between the indicator and the reference electrodes. This voltage is compared with a preset potential, and any deviation is amplilied and used to control the current through the generating electrode system. The way in which the end point was 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 cell was thermostated at 25 i 0.5 "C. Procedure. Each cell compartment was filled with 4.3 mL of reagent. The cell was then inverted in order to remove any moisture from the surfaces of the cell compartments. The iodine in the compartments containing the reference and auxiliary electrodeswas reduced by addition of water with a 10-pLHamilton syringe until the color was light brown and the reagent was transparent enough to make the electrodes visible, while water was added to the working compartment until the iodine concentration was ca. 10-5 mol/L. The end-point potential, corresponding to approximately (2-3) x 10-5 mol/L iodine, was set on the analyzer, and the excess of water was titrated before a calibration curve was made. The calibration curve is then used to set the end-point potential corresponding to the desired iodine level. Titration Procedures. First the end-point potential corresponding to the desired iodine level was set M when nothing else is mentioned) and the background drift recorded. In all coulometric titrations, the sample was added with a 10- or 50-pL Hamilton syringe, and the value read on the integrator after 10 min of titration was corrected for drift on the basis of the mean value of the drift during the last 4 min. Recovery Experiments in the Presence of Cyclohexanone or Benzaldehyde (Tables 4-7). As shown in Figure 1,when 50 pL of dried (less than 0.005%of water) cyclohexanone or benzaldehyde was added (A), an increased drift OD) was noted. Next @), a known amount of water was added (30 pg) and titrated. Thereafter, the titrator was tumed off at the same time as another 30 pg of water was added (C). An interval of 6 min elapsed between addition of the sample and start of the titration @). (12) Cedergren, A; Wikby, A; Bergner, IC Anal. Chem. 1975,47, 100-106. (13) Cedergren, k Talanta 1974,21, 367-375.
Determination of Water in Benzaldehyde or Cyclohexanone (Table 8). When 10 pL of sample was added, the titration was started immediately. Kinetics of the KF Reaction. The rate constant of the Karl Fischer reaction, k3, was determined by following the decrease of iodine concentration as a function of time after addition of waterag A calibration curve of the redox potential versus iodine concentration was established by generatingiodine and registering the potential at several points. The concentration of iodine was calculated from the Nemst equation, and the slope obtained did not differ by more than 0.5%from the theoretical value. A suitable iodine concentration (10-5-10-4 mol/L) was selected, and a known amount of water (1-9 pg) was added. The change in potential was then measured every second during 1-2 min by a Fluka 45 multimeter connected to a computer. Thereafter, the titration was started in order to determine exactly the amount of water added. Finally, the concentration of the sum of s u l f u r 0 oxides from the added sulfur dioxide in the reagent was determined coulometrically in a separate cell. RESULTS AND DISCUSSION
Reduction of the Quotient DD&. The increase in the endpoint drift when samples containing active carbonyl compounds were titrated (see reaction 1) leads to an increase in the concentration of untitrated water. The correction that has to be made can be calculated from eq 7 (assuming no change in concentration of iodine), giving
where DD is the difference in drift before and after the addition of sample. As can be seen in Table 3, for a given concentration of sulfur dioxide in the reagent, the correction can be brought to a minimum by choosing a rapidly reacting reagent in combination with the use of a high-end-point concentration of iodine. In order to keep the corrections due to the increase in the concentration of water below 0.1 pg of water, the quotient DD/k3 should be less than 5 x lo-" M3 for the following set of parameters: 4.3 pL of reagent, 0.37 M sulfur dioxide, and low4M iodine at the end point. At an iodine level 10 times lower than this, the quotient should thus be less than 5 x 10-l2 M3 to meet this requirement. Reversibility of the Bisulfite Addition Reaction. One of our main goals was to find a reagent composition which gives a rapid back-reaction of the bisulfite addition reaction and thus allows short titration times. In order to study the completeness of the back-reaction for different compositions of 2-methoxyethanolic and methanolic reagents, we carried out experiments of the type shown in Figure 1. The results from these titrations are given in Tables 4-7. The two recoveries given for the experiments including a delay before the start of the titration can be used to judge the completeness of the back-reaction. The left-hand recoveries are background compensated using the drift value obtained after 10 min of titration, while the values in brackets are compensated using the drift value measured just before the sample addition (at point C in Figure 1). If the two recoveries are identical, the back-reactionis complete within 10 min. The greater the discrepency between the two recoveries, the greater is the quantity of water that is still tied up in the bisulfite addition reaction at the end of the measurement. It should be emphasized Analytical Chemistty, Vol. 67, No. 5,March 1, 1995
1001
Table 4. 2.Methoxyethmol Reagent. Contalnlng Different Concentntlon8 of Imlduole
cyclohexanone reagent [Iml (M) k3 A B C a
s-l)
2.0
DD
recovery (%) DD DD/k3 (M3) 0 minu 6 minu (ug of HzO min-l)
recovery (%) DD/k3 (M3) 0 minu 6 minu
0.42 1.05 1.80
6.1 x lo-" 2.3 x 2.1 x lo-"
8.5 x 1.1 x lo-'* 0.7 x
(ug of HzO min-')
1.5 x 103 9.8 x 103 18.6 x 103
1.0 1.5
benzaldehyde
81 (84) 45 (61) 19 (41)
98 99 92
0.05 0.05 0.04
88 98 100
45 (83) 91 (97) 98 (98)
The start of the titration was delayed 0 and 6 min, respectively.
Table 5. Methanol Reagents Contalnlng Different Concentrations of Imidazole
cyclohexanone reagent [Iml (M) G H I a
k3
s-')
DD
(ug of HzO min-'1
3.8 x 103 30 x lo3 50 103
1.0 1.5 2.0
230 4.79 6.19
recovery (%) DDD3 (M3) 0 minu 6 minu 3.5 x lo-" 2.7 x lo-"
97 97
benzaldehyde recovery (%) DD (ug of HzO min-l) DD/k3(M3) 0 minu 6 minu
7 (6)
