Substitute Method for the Karl Fischer Titration: Gas Chromatographic Determination of Water in Ketonic Solvents by Use of the Method of Standard Addition John C. MacDonald' and Charles A. Brady Polymer Industries, Inc., Viaduct Road, Springdale, CT 06907
Polyurethanes are copolymers of polyalcohols and polyisocyanates. Synthesis of these copolymers, whether a t the research, development, or production level, requires analysis of solvents for interfering species. Active hydrogens react with isocyanates and the most common interference is ubiquitous water which might be low on a weight basis but unacceptable on an equivalance basis; because of quantity of solvent in a polymer formula, water content of the solvent is frequently a limiting factor in the production of a quality polyurethane. The common solvents in urethane technology are ethyl acetate, acetone, methyl ethyl ketone, and toluene. The standard method for the determination of water, the Karl Fischer titration, is routinely used with ethyl acetate and toluene. Ketones, however, interfere in the Karl Fischer method, and water in acetone or methyl ethyl ketone is usually determined using gas chromatography with thermal conductivity detection. The ordinary procedure is the calculation of area percent with no standardization or the use of standards in a matrix different from the analyte. Both methods were here undesirable and the method of standard addition was combined with gas chromatography to yield a technique that is superior to those previously available. T h e feasibility of the method was demonstrated by analysis of ethyl acetate using the method and comparing results with those from the standard Karl Fischer titration. The results agreed, and the method is now routinely used in this laboratory for the determination of water in solvents to be used for synthesis of polyurethanes.
EXPERIMENTAL Apparatus. A Hewlett-Packard Research Chromatograph Model 7520A (thermal conductivity detector a t 200 " C and 150 mA; 6-ft X '/E-inch stainless steel column packed with Chromosorb 104 (Johns-Manville), 80/100 mesh; injection port temperature: 165 " C ; column temperature: 150 " C for methyl ethyl ketone and acetone, 140 " C for ethyl acetate; helium carrier gas flow: 40 ml/ min.), a Hewlett-Packard integrator, Model 337OB (settings: noise suppression, maximum; u p slope sensitivity, 0.03 mV/min; down slope sensitivity, 0.03 mV/min; base-line reset delay, zero; area threshold, 10; front shoulder control, off; rear shoulder control, 1000 mV), and a Photovolt Corp. Aquatest I1 for Karl Fischer titrations were used. Procedure. Dry a 4-oz screw cap glass bottle a t 150 " C . Weigh into t h e bottle an amount of water corresponding approximately t o t h a t expected in a 50-gram sample of solvent. Weigh into the bottle 50 gram of t h e solvent t o be analyzed a n d mix. Inject 10 pl of the solvent into the gas chromatograph using the above experimental conditions and determine t h e area of t h e water peak. Inject 10 GI of solvent with added water and determine the new area of t h e water peak. Repeat injections until precision acceptable t o t h e analyst is obtained. Calculate weight percent of water in the solvent using:
Permanent address, Department of Chemistry, Fairfield University, Fairfield, CT 06430. To whom correspondence should be addressed.
where A,,I, is t h e mean area of t h e water peak before standard a d dition, AStd is t h e mean area of the water peak after standard addition, Wsolv is t h e weight of solvent before standard addition, Wstd is W,,I, x, and x is t h e weight of water used for standard addition; g is calculated from Equation 1 and is t h e weight of water originally present in Wsolv; knowing g and Wsola,calculate t h e weight percent of water in t h e solvent:
+
RESULTS AND DISCUSSION The rationale for the selection of Chromosorb 104, a porous polymer, as column packing has been presented previously ( I ) . Basically, a porous polymer packing was desired since polar molecules were to be separated; this particular phase has a large difference in retention indices for ketones and alcohols and, under the above conditions, gives base-line separations of water, acetone, ethyl acetate, methyl ethyl ketone, and toluene. The absolute retention times of 1-pl injections, of these solvents using Chromosorb 104 (6-ft X I/a-in., 80/100 mesh) a t 150 "C and a carrier gas flow rate of 40 ml/min are presented in Table I (the column temperature of 140 O C for analysis of ethyl acetate is necessary for base-line separation of water and ethanol, where the latter is a common contaminant in ethyl acetate). The method was applied to reagent grade ethyl acetate, acetone, and methyl ethyl ketone; the results are presented in Table 11. The Karl Fischer titrations of the reagent ethyl acetate in Table I1 using the Aquatest 11, were done concomitantly with the gas chromatographic analyses. This Karl Fischer method is a coulometric determination using a volume of sample and yields results in weight/volume percent. Division of the result by the density of the solvent yields the weight/weight percents in Table 11. The precision of water determinations using the Aquatest I1 was initially determined by multiple analyses of reagent ethyl acetate. In our hands, the result of six determinations was a mean of 0.0195% water with a standard deviation of 0.0011%, and this is consistent with the previous analysts ( 2 ) who developed the Aquatest 11. From the data of Table 11, the results for the gas chromatographic determination of water in a sample of ethyl acetate (mean, 0.0242% and standard deviation 0.0004%) are comparable to the concomitant results using the accepted standard method of Karl Fischer (mean, 0.0241%). This method was also applied to ketonic reagents that cannot be analyzed by Karl Fischer titration. Acetone and methyl ethyl ketone were analyzed for water and these results are also in Table 11. Of greater interest from the standpoint of method development is the additional data from the analyses of methyl ethyl ketone that is presented in Table 111; these data suggested the final experimental procedure given above. Alternate injections of the methyl ethyl ketone with and without added standard water were made and the corresponding areas of water peaks (in mV-seconds), area perANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
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Table I. Absolute Retention Times on Chromosorb 104Q Air Water Acetone Ethyl acetate Methyl ethyl ketone Toluene a See text for conditions.
0.24 Minutes 1.65 2.65 3.27 4.52 6.65
Area of water
Area of water
Feaatka lX a r100 ea (percent)
*iter Ccentration a l c u l a t e d conof Before
Aver age 0.0239 0.0239 0.0244 0.0241 0.111 0.110
0.0576 0.0604 0.0625 0.0620
cents of water peaks, and analytical results using standard addition are listed in Table 111. Data of ten sets of 10-pl injections using four different solutions of methyl ethyl ketone and added water are presented. The mean area of the water peak in neat methyl ethyl ketone is 1193 mV-seconds with a standard deviation of 57 mV-seconds; the mean area percent of the water peak in neat methyl ethyl ketone is 0.0758% with a standard deviation of 0.0036%. The mean water concentration using the standard addition method is 0.0600% with a standard deviation of 0.0028% for the 10 determinations. The difference between mean area percent (0.0758%) and mean water concentration using standard addition (0.0600%) denotes the necessity of using standards when accurate results are required. From the standpoint of method development, it is useful to focus on the data of standard addition for the three most deviant results: 0.0542%, 0.0640%, 0.0639%. The greater deviations from the mean water concentration for these three analyses result from less accurate determinations of the water peak of the neat methyl ethyl ketone than occurred for the remaining seven samples. For example, the area percents of the water peaks in methyl ethyl ketone with added water standard were consistent for all ten analyses: solution 1, 0.404, 0.406, 0.402, 0.407; solution 2, 0.664,0.667; solution 3, 0.751. 0.747; solution 4, 0.232, 0.236. The area percents of the water peaks in neat methyl ethyl ketone, however, were less consistent and greater deviations from the calculated mean water concentration thereby resulted. For solution 1, the 0.0542% water is calculated where the area percent of neat solvent is 0.0698 vs. the more consistent area percents, 0.0748,0.0745,0.0748; for solution 3, the 0.0640% water is calculated where the area percent of neat solvent is 0.0806 and the second water peak area is 0.0783%; for solution 4, the 0.0639% water is calculated where the area percent of neat solvent is 0.0823 and the 948
eak,
(IllV-SeCOnZS)
Table 11. Analytical Results of Percent Water in Solvents Ethyl acetate A. Gas chromatography Sample 1 0.0239, 0.0238 Sample 2 0.0243, 0.0235 Sample 3 0.0236, 0.0253 B. Karl Fischer 0.0243, 0.0239 Acetone A. Gas chromatography Sample 1 0.110, 0.111 Sample 2 0.110, 0.110 Methyl ethyl ketone A. Gas chromatography Sample 1 0.0542, 0.0582 0.0581, 0.0600 Sample 2 0.0605, 0.0603 Sample 3 0.0640, 0.0610 Sample 4 0.0601, 0.0639
Table 111. Experimental D a t a of Methyl Ethyl Ketone Analysis for Water
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
Solution 1 1100 Solvent: 53.3034 g 1175 Added water: 0.1391 g 1185 1192 1178 Solution 2 Solvent: 54.4599 g 1180 Added water: 0.2586 g Solution 3 1261 Solvent: 55.0280 g 1198 Added water: 0.2957 g Solution 4 1147 Solvent: 76.9723 g 1309 Added water: 0.1003 g a
Aitqi addition
Before
addition
water, %:
6377
0.0698
0.404
0.0542
6427
0.0748
0.406
0.0582
6488 6356 10380
0.0745 0.0748 0.0753
0.402 0.407 0.664
0.0581 0.0600 0.0605
10420
0.0755
0.667
0.0603
11770
0.0806
0.751
0.0640
11690
0.0783
0.747
0.0610
3627
0.0737
0.232
0.0601
3736
0.0823
0.236
0.0639
Calculated using the method of standard addition.
second water peak area is 0.0737%. The mean area percent of water in neat methyl ethyl ketone for the seven most precise analyses is 0.0750% with a standard deviation of 0.0008, and this mean is many standard deviations from the deviant area percents of 0.0698, 0.0806, and 0.0823. Using the seven most precise sets of data, calculations using standard addition give a mean water concentration of 0.0597% with a standard deviation of 0.0012, and the relative standard deviation is then 2%. The factor that is most limiting in the application of the method of standard addition using gas chromatography t o determine low water concentrations is then the ability to obtain a water peak area Asolv, in the neat solvent, such that the area used in Equation 1 truly represents the amount of water present in neat solvent. The consistency of water peak area, As& following standard addition, indicates this measurement makes minimal contribution t o the overall variance of the method. The analyst using this method must decide upon the number of injections of neat solvent necessary for adequate precision in measurements of area for that analysis; the higher the concentration of water in the solvent, the less will be the number of injections required. Finally, the method should be considered not only for the determination of water in the ketonic solvents of the urethane industry, but also for the determination of water in other systems containing those interferences (free halogens, aldehydes, most acids, oxidizing agents, reducing agents) such that the Karl Fischer titration cannot be used. LITERATURE CITED (1)J. C. MacDonaldand C. A. Brady, Am. Lab., 6 (lo),1 1 (1974). (2) H. Dahms, D. M. Seltzer. and G. B. Levy, 22nd Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, OH, March 197 1.
RECEIVEDfor review September 3, 1974. Accepted January 6, 1975.
