etermination of Water in Ion-Exchange Resins by the Karl Fischer and Drying Methods H. D. Sharma and N. Subramanian Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
HEUMANN AND ROCHON ( I ) have indicated that further studies are necessary to check the reliability of the Karl Fischer method for the determination of water contents in the conventional sulfonic acid type ion-exchange resins. They compared the values of water content in Dowex 50-W resin in the sodium form obtained by the Karl Fischer and drying methods and found that the former method had serious shortcomings with respect to displacement of water in the resin pores by the Karl Fischer reagent. The titrations required two hours in some cases and the resin retained 1 to 2 equivalents of H20/equivalent of resin. Earlier, Dickel and Hartmann (2) considered the Karl Fischer method superior to the drying method, which needed standardized conditions with respect to temperature, pressure, and the dehydrant. Pollio (3) showed that the titration method is applicable in the case of Amberlyst-15. During the course of studies on the nuclear magnetic resonance (NMR) spectra of Dowex 50-W ion-exchange resins, we were interested in a rapid method for the determination of water content in a large number of resin samples. This note presents comparative water analyses of Dowex AG 50-W resins by Karl Fischer titration and drying methods. EXPERIMENTAL
Dowex AG 50-W type resins with different crosslinkages were obtained from Bio-Rad Laboratories in the H+ form and were converted to the desired ionic forms by the usual column method. Samples with varying water contents were (1) W. R. Heumann and F. D. Rochon, ANAL.CHEM., 38, 638 (1966). (2) A. Dickel and J. W. Hartmann, Z . Physik. Chem. (Frankflirt), 23, l(1960). (3) F. X. Pollio, ANAL.CHEM., 35,2164 (1963).
taken for simultaneous determination of moisture content by the Karl Fischer and drying methods. In the titration method, samples weighing 50-100 mg were suspended in 20 ml of spectral grade methanol (Fisher). The Karl Fischer reagent obtained as a single stabilized solution from Fisher Scientific Co. was standardized using -50 mg of pure water. The titrations were carried out by employing the dead-stop technique, and the end point was determined by the use of a conductance meter supplied by Lab Industries, Berkeley, Calif. (No. 1002 Aquametry Meter electrode). The overall error in the values of the percentage of water (w/w) due to inaccuracies in titre values and in standardization of the reagent has been estimated to be not more than *2% in any case. For the drying method, -0.2 gram of resin samples were heated in an oven at 120 "Cfor 48 hours, conditions considered ideal for complete removal of water from the resin with very little decomposition of the resin beads (4).
RESULTS AND DISCUSSION The results of water content determinations by the two methods are given in Table I along with the time required for completion of Karl Fischer titrations. The agreement between the two sets of values is quite good within the limits of experimental error. For swollen resin (wet) samples, the titrations could be completed within 5 to 10 minutes. The titrations took a longer time when water content of resins (air dried) was low. (Perhaps aging of resin may have some effect on the exchange of solvent.) The time required by the Karl Fischer reagent to react completely with the water inside the resin beads increases as the cross-linkage of the resin increases, but in no (4) W. R. Heumann and F. S. Rochon, Can. J. Chem., 43, 3483 (1965).
Table I. Determination of Per Cent Water Content (w/w) in Dowex AG 50 W Resins by Karl Fischer and Drying Methods Water (wiw) Drying at 120 'C Time for for 48 hrs.a Ionic form Cross-linkage Karl Fischer, min K. F. H+ X8 8 15.6 f 0.1 15.21 f 0.11 6 51.2 f 0.1 51.44 f 0.04 Li+ X8b 14.32 f 0.02 15 14.8 f 0.1 X8 13.39 ri: 0.02 30 13.5 f 0.2 X16 45.74 f 0.02 6 44.9 i 0.2 Na+ 30 11.8 f 0.1 11.44 f 0.02 x2 X8 35 11.4 f 0.1 11.75 f 0.05 X16 35 10.7 f 0.2 10.86 f 0.02 40.58 f 0.02 Kf X8b 5 39.7 f 0 . 1 15 9.8 f 0.1 9.27 f 0.01 x2 20 9.6 f 0.1 9.58 f 0.10 X8 20 9.4 f 0.2 9.15 f 0.02 X16 lower when the resin was dried at 105 "C Thermogravimetric analyses of H+ form resin showed that the value of water content was -1.1 for 12 hours (52.4 as against 53.573. In the case of salt-form of resin, the values were generally 0.7% lower when the resins were dried at 105 "Cfor 12 hours. 6 Swollen resin (wet).
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Table 11. Water Determinations id Dowex AG 50-W - H + Form Resins by Karl Fischer, NMR, and Drying Methods Dowex AG 50-W Hf form
K. F.
x2 x4 X8 XI2 X16
74.5 65.3 51.8 45.6 38.4
a
Water (w/w) NMRa 74.0 64.6 54.0 44.2 36.0
Dryingo 78.3 66.5 53.0
45.6 37.0
From Ref. (5).
Li+ > Na+ > K+ > Rb+ > Cs+ as expected (7). Qualitative confirmation for this trend is obtained by the integration of in-water peaks in the NMR spectra of the resins in the above ionic forms. Quantitative results could not be obtained since in-water peak overlaps out-water peak (6, 8). In the case of Ca2+and Laa+forms of Dowex AG 50-W (X8) stable end points could be reached in 10-15 minutes provided the resins were taken in “wet” form. For air-dried resins, it took hours before the titrations could be completed. For all ionic forms, the Karl Fischer method is rapid and accurate if the resin is taken in the completely swollen (wet) form, Recently Blasius and Schmitt (9) have determined up to 0.5 (wiw) of water accurately by contacting the resin for 24 hours in excess Karl Fischer reagent and back-titrating the unreacted reagent. In conclusion, direct titration with Karl Fischer reagent can be relied upon to give rapid and accurate estimation of water content for the sulfonated cation exchange resins provided that water content in the resin is not very low ( 210 w/w) and errors up to =t2xin the estimation are tolerable. Where more accurate results are desired, the procedure of Blasius and Schmitt can be adopted using a more elaborate Karl Fischer apparatus.
case did the time required to attain a stable end point exceed 35 minutes. De Villiers and Parrish (5) estimated the water contents of Dowex 50-W resins in the H+ form (X2 to X16) by drying and also by the NMR method. Their values were confirmed in our laboratory by NMR chemical shift measurements and are compared with those obtained by the Karl Fischer method in Table 11. The moisture contents of Dowex AG 50-W (Xl-16) resins in the H+, Li+, Na+, K+, Rb+, and Cs+ forms were determined by us using the titration method. The results show a progressive decrease in the water content in the order H+ >
RECEIVED for review June 16, 1969. Accepted September 25, 1969. The authors gratefully acknowledge the financial support for this work by the National Research Council of Canada.
(5) . _ J. P. de Villiers and J. R. Parrish, J. Polymer Sei., Part A, 2, 1331 (1964). (6) . , H. D. Sharma and N. Subramanian, DeDartment of Chemistry. University of Waterloo, Waterloo, Ontarib, Canada, unpublished data, 1969.
(7) F. Helfferich, “Ion Exchange,” McGraw-Hill, New York, p 103, 1962. (8) J. E. Gordon, J . Phys. Chem., 66, 1150 (1962); Chem. Znd. (London), 1962,267. (9) E. Blasius and R. Schmitt, 2.Anal. Chem., 241, 4 (1968).
(a,
x
Gamma-Ray Titrations Using Potentiometric End Point Detection R. F. Sympson and W. D. Hunter Clippinger Laboratories, Ohio University, Athens, Ohio 45701
CHEMICAL DOSIMETERS in which radiation dosage is measured by the quantity of product formed by a radiation-induced chemical reaction have been in use for many years ( I , 2). The reverse process in which the concentration of a reagent is determined by measuring the radiation dosage required to cause it to react completely has found limited use in analytical chemistry. An aqueous system exposed to ionizing radiation develops a potential that has been called the “equivalent redox potential” or “equivalent reduction potential” and is characteristic of the radiation used and the solvent (pN, oxygen content, etc.) (3,4). The value of this potential was found to be 0.85 to 0.90 volt more positive than the normal hydrogen electrode. Cartledge (5)interpreted this potential to be a mixed potential dependent upon the steady-state concentrations and electrochemical characteristics of all radiolytic products present in (1) H.Fricke and S . Morse, Am. J. Roentgenol., 18,430 (1927). ( 2 ) J. W. T. Spinks and R. J. Woods, “Introduction to Radiation
Chemistry,” Wiley, New York, 1964, Chap. 4. (3) F. S. Dainton and E. Collinson, Ann. Rev. Phys. Chem., 2, 99 (1951); Discussions Faraday SOC.,12,251 (1952). (4) H. S. Henderson, E. G. Lovering, R. L. Waines, and E. J. Casey, Can. J . Chem., 37, 164 (1959). ( 5 ) G. H. Cartledge, Nature, 186, 370 (1960). 2064
*
the solution. The steady-state concentrations depend upon the rates of formation of the particles and their rates of decay by various paths. If a redox couple is added to the solution, reaction between the added couple and radiolytic products occurs until the activities of the components of the oxidationreduction couple are such that they no longer disturb the steady-state activities of the radiolytic products. If the E a of the added oxidation-reduction couple is more than a few tenths volt from the equivalent redox potential, one component of the couple is converted quantitatively to the other form. Any couple having a standard potential more than 0.2 volt positive to the equivalent redox potential is quantitatively converted to the reduced form, and any couple with a standard potential more than 0.2 volt negative to the equivalent redox potential is quantitatively converted to the oxidized form. It would seem that if a solution containing an oxidationreduction couple were irradiated by gamma-rays or x-rays of constant intensity, the time required to establish the equivalent redox potential would be proportional to the initial concentration of one of the components of the couple. This is not ,generally true, however, because in many radiationinduced reactions the yield of product varies with concentration of the reactant. If the yield is not constant over a con-
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