I NOTES Determination of Water in Nylon with Karl Fischer Reagent Using Fluorinated Alcohols as Solvents Gene E. Kellum and J. D. Barger G u y Research and Development Company, Merriam, Kan. 66202 THEKARLFISCHER REAGENT (KFR) method of water determination has been applied t o nylon for several years. Various sample-handling techniques have been developed t o counter the insolubility of this polymer in commonly used KFR solvents. Samples have been heated in a nitrogen stream to vaporize and sweep the water into a solvent absorption cell prior to the K F R titration ( I ) . A procedure where heating of the sample in a closed syringe allowed collection and injection of the water into a KFR cell was reported ( 2 ) . A pre-extraction of small pieces of nylon was employed to remove the water before the entire sample was titrated with K F R ( 3 ) . Cresol has been reported as a solvent for nylon, allowing a solution to be obtained for the K F R titration (4). In our experience it has been difficult t o apply most of the methods listed because of the extreme insolubility of the present-day higher molecular weight nylons. Solutions of these polymers are generally not stable and extractions are too time consuming to be useful. A rapid, direct titration procedure was developed in our laboratory to determine the water content of nylon with commercially available K F R and a n easily constructed bipotentiometric end point detection apparatus. The nylon sample was dissolved in hexafluoroisopropanol (HFIP) or trifluoroethanol (TFE) and titrated to a point where the potential response was constant for 30 seconds. The method allowed titration of water in different types of nylon, and the samples were stable over extended time intervals. The fluoroalcohols were not only advantageous because of their solvent properties, but could also be easily dried and maintained in a dry condition for extended periods of time. EXPERIMENTAL
Apparatus. The end-point detection system was the one commonly related to the built-in constant current source of many modern p H meters. An L&N Model 7401 p H meter was set up to measure the bipotentiometric end point through a dual platinum electrode as suggested in the apparatus manual. This detection apparatus was not further modified. The titration cell was constructed from a 125-ml three-neck, round-bottom flask with one 24/40 and two 19/38 T joints and stoppers. The electrode was placed in one of the outer joints and a drying tube in the other. The joint fitted with the drying tube served as a sample addition port also. The center joint was used t o hold the buret tip, which was sealed into a rubber stopper. The titration buret was an autozeroing type with attached closed reservoir and three-way Teflon (Du Pont) stopcock (available from Arthur H. Thomas Company). The electrode was constructed either
from a standard taper joint with two 16-gauge wires sealed into it about 1 cm apart, or the Beckman No. 39032 platinum pair electrode. The fluoroalcohols were dispensed from an automatic buret of 75-100 ml volume with an attached reservoir which could be protected from the atmosphere. Reagents. The Karl Fischer reagent was the single stabilized formulation, available along with the diluent, from Fisher Chemical Company. The K F R was prepared for use by adding diluent until the titer was about 1 mg HzO/ml KFR, and then transferring t o the titration buret. Hexafluoroisopropanol (HFIP), obtained from E. I. Du Pont Company, and trifluoroethanol (TFE), obtained from Halocarbon Products Corporation, were both dried before use by addition of a 250-ml beakerful of type 4A, 1/‘&. molecular sieve pellets, which had been freshly activated at 150 “C for 16 hours, to three liters of the alcohol. A period of 48 hours was allowed for the drying procedure t o be completed, and then the dry alcohol was transferred t o the dispensing buret for use. Procedure. A 75-ml volume of dried alcohol was titrated with the dilute K F R to obtain a blank value. Known amounts of water were carefully weighed into several dried flasks containing 75 ml of alcohol and then titrated. A second blank was run after the water calibration samples and the factor calculated for the K F R as mg H20/ml KFR. An alternate, and equivalent, procedure for calibrating the reagent was t o add weighed amounts of water t o a pretitrated cell and then titrate back t o the same end point. The samples were prepared by accurately weighing approximately 4 grams of ground nylon into a previously dried and stoppered 125-ml, three-neck flask. With the absolute minimum of exposure to the atmosphere, 75 ml of the alcohol was introduced into the flask along with a dry stirring bar. Two blanks were prepared along with the samples. The nylon samples were either allowed to dissolve at room temperature or were refluxed t o effect rapid solution. The room temperature dissolution took 2-3 hours with stirring, while refluxing the solvent [under dry Mg(C104)2]usually dissolved the samples in less than 1 hour. One of the blanks was titrated first, followed by the dissolved nylon samples, and finally the second blank. Each flask was titrated with the dilute K F R to a potential response which remained constant at about 700 mV for 30 seconds. Results were calculated as per cent water in the nylon sample, and were corrected for the water introduced with the alcohol. The above procedures were adhered t o with all solvents tried. DISCUSSION
(1) Kaname Muroi, Birnseki Kcigoku, 11, 351 (1962). (2) N. K. J. Syrnons and E. C. McKannan, ANAL. CHEM., 31,
Conventional “dead-stop” end points are measurements of sudden surges of current signalled by a pair of identical platinum electrodes polarized with a constant voltage (5). The problem with this type of detection system is that a change in
(3) G. Glockner and W. Meyer, Fase/~forschTextiltech, 10 (2), 83 (1959). (4) K. Yashiro, Jap. A ~ ~ l y 3, s f 413 , (1954).
(5) J. Mitchell and D. M. Smith, “Aquarnetry,” Interscience, New York, 1948, pp 71-102.
1990 (1959).
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Sample H20a H?Ob Nylon 6*,* Nylon 6 ~ , ~
Table I. Typical Titration Data Using Fluoro-Alcohol Solvents Approx sample Approx ml weight, gram titration No. obsd Results Range 0.0155 14.7 6 1 .056cmg/ml 0.004 0.0115 12.1 4 1 ,056mg/ml 0.002 4.0 4.0 4.0 120
23.5 10.2 8.1 3.32
6 5 4 4
2a
0.535% 0.150% 0.20597, 0.00278%
0.00800 0.00714 0.00720 0 .oooO346
Nylon 66azd HFIP HFIP used as solvent. TFE used as solvent. c The concentration of the KFR used for determination of water in the nylon samples was slighhy different than that given here. The nylon 6 samples were typical Gulf-produced types, such as 401,615, and 406. The 66 sample was a Zytel polyamide.
dielectric of the solution causes a change in the current characteristics at the end point (6). When working on low levels of water in a polymer of low dielectric constant compared to the sample diluent (e.g., methanol), the “dead-stop” end-point current height is altered to such a degree that the responses for the blank and sample are quite dissimilar. The result is that the difference in dielectric between solvent and polymer solution is made up by adding excess K F R and overtitrating the water present. The greater the proportion of low dielectric material introduced to the cell, the more the effect is apparent. Bipotentiometric measurement of K F R end points has become common (7) and offers a reasonable solution t o the problem of dielectric changes resulting from addition of polymers t o polar solvents. The effect of changing system polarity was observed as a general narrowing of the potential break, with the upper potential (or end point) being unchanged. This indicated that the bipotentiometric end point detection system was a better choice for sensitivity and accuracy in this application of the K F R water determination. which is acidic enough to HFIP has a K, of 4.2 X dissolve nylon and give stable solutions over a period of time without deterioration of the polymer. TFE is less acidic than HFIP (K, = 4.3 X IO-l3) but still dissolves nylon rather easily. Both alcohols as obtained contain a greater amount of water than desirable (100-200 ppm). By introducing dry molecular sieves to the system, the water content was decreased to about 50 ppm, which is an acceptable level. The dry alcohol was stored in the dispensing buret where the low water content was maintained. The TFE generally contained larger amounts of water than HFIP but could still be dried to a usable level. The typical nylon sample titrated here contained a fairly small amount of water; therefore the strength of the standard KFR solution was decreased from about 6 mg H 2 0per ml to around 1 mg H 2 0 per ml. This was accomplished by using the commonly available K F R diluent. The dilution step gave a usable increase in volume measurement accuracy and produced no problems with reagent stability. Smaller amounts of K F R could also be added near the end point, and no changes were noted in the mechanics of the titration itself. (6) Robert C. Smith and Gene E. Kellum, ANAL.CHEM.,38, 67 (1966). (7) J. T. Stock, “Amperometric Titrations,” Interscience, New York, 1965, pp 166-174.
Nylon samples having a wide variety of water content were successfully analyzed. Most of the samples titrated were of the nylon 6 type; however, the water content of nylon 66 and 610 was also determined easily. The greatest amount of work was done using HFIP, but TFE was examined in great enough detail to assure its successful application in the procedure. Table I shows a compilation of data covering typical precision of reagent calibration in both-solvents and titration of several nylon samples in HFIP. The data indicate that the titration procedure is precise. The reagent calibration data show that the range between successive determinations is 0.002 to 0.004 mg H 2 0 per ml K F R which amounts to about a relative 0.2875 of the mean value. The precision of titration and K F R concentration values were identical using either HFIP or TFE. Titration of the water present in HFIP gave an excellent indication of precision with a relative 2u value of 1.275. The results for repeated titration of nylon 6 and 66 gave 2u values ranging from 0.00800 to 0.00714z water. The data showed the expected trends, the calibration of the reagent was the most precise and samples gave 9 5 x confidence limit values of about double that for the determination of water in the solvent. The solvent ordinarily contributed less than 2 5 x of the total water in the sample, and at the 0.5 level, less than 15 75 of the total water came from the blank. Water contents as great as 15 have been successfully analyzed, and the lower limit was easily extended downward to less than 0.1 water using the described procedure. The reaction of water in the fluorinated alcohols was observed to proceed at a more rapid pace than in the methanol system. The last traces of water in the methanol react rather slowly but with the fluorinated alcohols the titration could be brought to the end point quickly. When working at low levels of water, the rapid reaction is a great aid to accurate analysis. Nylon samples were dried under vacuum and titrated for residual water, then spiked with known amounts of water and retitrated. Results showed that, as expected, virtually 10075 of added water was recovered. We have utilized this procedure for routine analysis of water in nylon for about two years and have found it to be entirely satisfactory.
z
x
R E C E I V Ereview D ~ ~ ~May 11,1970. Accepted July 13,1970.
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