Anal. Chem. 1983, 55, 988-989
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BALANCE B E A M
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Flgure 1. Schematic view of the calibration technique applled to a Du Pont TGA balance.
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100 150 2 0 0 2 5 0 T E M P E R A T U R E ("C) Flgure 3. Effect on heatlng rate on calibration with tin during decom0
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positlon of calcium oxalate. Note that the variation in decomposition rate with heating rate does not affect the calibration.
TEMPERATURE ("C)
Flgure 2. Signal obtained on melting pure reference samples In an empty boat, using tin and lead. Note that an impure lead wire was used which resulted in a lower melting point.
and be of a material having low surface tension in the liquid phase. On heating, the hook melts and the platinum weight drops to the bottom of the boat. This causes an action and reaction which is immediately sensed by the balance creating an instantaneous mark on the chart. Since the platinum weight remains in the boat, no weight change takes place and a trace similar to that shown in Figure 2 is obtained. The hook on melting will either remain attached to the platinum wire by surface tension or fall to the bottom of the boat and will not affect the total weight of the boat. If there is any chance that the hook material might react with the sample, a partition of thin platinum foil may be placed in the boat to prevent contact. If necessary the hook can be made of materials other than metal and a variety of methods of suspending a platinum weight on a pure reference material can be easily conceived. In this technique action and reaction occur rapidly and it has been noted that, using microprocessor-controlledDSC equipment in which data is collected intermittently, the signal may go unrecorded. This can be overcome by using more rapid data collection. In addition, it is possible that the derivative mode can be used at high sensitivity to detect the transition for cases in which a large sample weight requires a low sensitivity weight range setting.
RESULTS AND DISCUSSION This technique has been tried and shown to work with hooks of tin (mp 232 "C), lead (mp 328 "C), zinc (mp 419 "C), and silver (mp 961 "C). Multiple samples have been used during a single run to give two reference points bracketing
the temperature range of interest during a TGA run. The reproducibility of melting point as a function of heating rate during actual decomposition of calcium oxalate is shown in Figure 3. The temperature displacement to slightly higher temperature in the 2 "C/min run is due to the fact that the sample was rapidly heated to 200 "C at 20 "C/min and then switched to 2 "C/min resulting in a transient in the heat flow pattern. The other three runs at 5,10, and 20 "C/min were reproducible to within fl "C. The fact that there is no base line change using this technique allows it to be used while actual samples are being run with little or no loss in the accuracy of the data obtained. More extensive studies of the accuracy and reproducibility of this technique have been made covering the temperature range up to 1200 "C. These will be reported in detail elsewhere (3). The simplicity and reproducibility of this method should make it a useful calibration technique which will complement the use of magnetic standards. It should be applicable to almost every design of the thermogravimetric boat.
LITERATURE CITED (1) Norem, S.D.; O'Nelll, M. J.; Gray, A. P. Thermochim.Acta 1970, 7 , 29-38. _. (2) Garn, P. D.; Menls, 0.; Weideman, H. G. J . Therm. Anal. 1981, 2 0 , 185-204. (3) McGhie, A. R.; Chiu, J.; Fair, P. G.; Blaine, R. L. Thermochim. Acta, in press.
RECEIVED for review December 20,1982. Accepted January 24, 1983. This work was supported by the National Science Foundation, MRL Program, under Grant No. DMR-7923647.
Preparation of Tetrabutylammonium Hydroxide for Atmospheric Sampling of Acidic Halogen Gases 6. W. Gandrud" and A. L. Lazrus National Center for Atmospheric Research, P. 0. Box 3000, Boulder, Colorado 80307
Recently tetrabutylammonium hydroxide (TBAH) has been applied to filter material to adsorb acidic vapors drawn through a filter (1-6). The primary emphasis in using TBAH has been placed on the capture of acidic halogen vapors in
the stratosphere and troposphere. The collection of acidic halogen vapors, such as HC1 and HF, is of interest because these gases are the most stable decomposition products of chlorofluorocarbons in the stratosphere. This ultraviolet-
0003-2700/83/0355-0988$01.50/0 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
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NUMBER OF 3 5 ML FRACTiONS COLLECTED AFTER ADDITION OF 12 5% TBAH SOLUTION
Flgure 1. Eluent pH as a functlon of collected fraction.
radiation-induced decomposition is a pertinent research problem due to its effect on the ozone layer which protects the earth's surface from biologically damaging UV radiation. TBAH impregnated filters have been used to study the acidic halogen vapors emitted from Guatemalan volcanoes (5) and recently in the debris from the Mt. Helens eruption (7). The procedure for impregnating a fiiter with TBAH involves either soaking the filter in a solution of TBAH in methanol or spraying the TBAH methanol solution onto the filter. The methanol is allowed to evaporate leaving the TBAH and any contamination on the filter. We have examined the TBAH solutions (25% (w/v) in methanol) available from several manufacturers and suppliers and have found that they all contain substantial amounts of the halogen contaminants, C1-, Br-, or I- (or combinations of these) ranging from 100 to 1700 pg/mlL. The halogen contaminant is due to the synthesis of the TBAH from a reaction of the tetrabutylammonium halogens and Ag20 (8). This proves to be a serious problem since in samples the collected halogen would represent only a small fraction of the total amount of halolgen ion on the filter. T o overcome the problem of halogen contamination in the TBAH solution, we examined several procedures employing both anion and cation exchange resins. The hydroxide form anion exchange resins coeluted higher levels of halogen than our analysis could allow and attempts to purify them proved fruitless most likely due t o the fact that the halogen anion would be more strongly retained by the resin than the hydroxide ion. The purification scheme that has evolved is a time-consuming one, but it will substantially lower the C1-, Br-, and I- halogen levels.
EXPERIMENTAL SECTION The ion chromatographic analysis for C1- was done with a 0.0025 M Naz B40,.10H,0 eluent. Br- and I- analyses were performed with neutron activation techniques. The column u ~ e dto contain the cation resin was a 2 cm diameter X 25 cm length glass column with a stopcock to control the flow. A glass, wool plug was used to retain the resin in the column. Preparation of Resin. For the preparation of 30 mL of approximately 10% TBAH, 15 mL of Dowex 50-X4 (available from Dow Chemical Co. of Midland, MI), hydrogen form, l00/200 mesh resin is placed in a chromatographic column. The resin is rinsed thoroughly with boiling deionized water and then converted to the Na+ form by passing 200 mL of 1 N hot NaOH through it. The flow through the resin is usually self-controlling but it should be kept below 1.5 mL/min. The excess hot NaOH i s needed to remove interstitial C1- contamination within the resin itself. After the conversion to a Na+ form, the resin is rinsed with 1000 mL of hot deionized water to remove the excess base. The N
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resin is now converted back to the H+ form with 400 mL of hot 0.8 M H2S04 (made with Ultrex concentrated H2S04) and then rinsed again with 1500 mL of hot deionized water. Purification of TBAH. Since the TBAH is dissolved in methanol, the resin eluent needs to be switched from deionized water to methanol. This can be done by gradually increasing the percentage of methanol in the rinse water until the eluent is entirely methanol. The next step is the conversion of the cationic resin from H+ form to the TBA' (tetrabutylammonium ion) form. This is accomplished by adding a 12.5% (w/w) solution of TBAH in methanol to the resin. The progress of the conversion as shown in Figure 1 can be monitored by observing the apparent pH of the eluent exiting the column with indicating pH paper. The completion of the conversion of the resin to the TBA+ form is indicated by the change of eluent pH from 1 to 10. The unexpected low pH observed in Figure 1 before the break through of TBAH is due to anion contaminants in the TBAH which yield strong acids upon reaction with the resin rather than water. The amount of TBAH required to convert from the H+ form agrees well with the capacity of the resin, 1.4 mequiv/mL of resin. Once the resin is converted to the TBA+form it is rinsed with methanol to remove the excess base indicated by the pH paper, followed by a rinse with 10% methanol in deionized water. This is helpful in removing the C1- contamination that was in the 12.5% (w/v) TBAH solution. To obtain a final TBAH solution with the lowest level of halogen contaminant, it is recommended that rinsing continue until the halogen contaminant reaches its lowest level in the rinse solution as indicated by ion chromatography. After this, a final rinse of the resin with 100 mL of methanol is performed to convert the eluent to pure methanol. To produce TBAH from this resin, a base is added in which the cation of the base is more strongly retained than the TBA+ cation. For this a 0.56 M solution of Ba(OH)2.8Hz0in methanol was selected. The Ba(OH)zsolution is added to the column and the collection of TBAH is started after the pH of the eluent changes from 5 to 10. The collection of TBAH is stopped when the presence of Ba2* in the eluent is indicated by a flame test. The concentration of the collected TBAH is determined by a nonaqueous titration of the base with benzoic acid in pyridine and a thymol blue indicator. With this procedure the purified TBAH will be 10-12% (w/v) in methanol with C1-, Br-, and Icontaminant levels ranging from 1to 5, 0.01 to 0.05, and 0.005 to 0.01 pgmL, respectively.
ACKNOWLEDGMENT The authors thank Charles Dickert of Rohm and Haas. in Philadelphia and Charles Felt of Dow Chemical in Midland, MI, for their advice on the use of ion exchange resins. We also wish to thank William Sedlacek of Los Alamos National Laboratory for providing the neutron activation analysis. Registry No. HCI, 7647-01-0; HF, 7664-39-3; HI, 10034-85-2; TBAH, 2052-49-5;Dowex 50-X4, 12624-04-3. LITERATURE CITED (1) Lazrus, A. L.; Gandrud, B. W.; Woodard, R. N.;Sedlacek, W. A. GQOphys. Res. Lett. 1975, 2, 439-441. (2) Lazrus, A. L.; Gandrud, B. W.; Woodard, R. N.; Sedlacek, W. A. J. Geophys. Res. 1976, 81, 1067-1070. (3) Mroz, E. J.; L a m s , A. L.; Bonelii, J. E. Geophys. Res. Lett. 1977, 4 , 149-1 50. (4) Lazrus, A. L.; Gandrud, B. W.; Greenberg, J.; Bonelli, J.; Mroz, E.; Sedlacek, W. A. Geophys. Res. Lett. 1977, 587-589. (5) Lazrus, A. L., Cadle, R. D.;Gandrud, B. W.; Greenberg, J. P.; Huebert, B. J.; Rose, W. I. J. Geophys. Res. 1979, 84, 7869-7875. (6) Otterson, D. A. Ion Chromatogr. Anal. Envlron. Pollut. 1978, 87-98. (7) Gandrud, B. W.; Lazrus, A. L. Science 1981, 21 I, 826-827. ( 8 ) Vogel, A. I. "Elementary Practlcal Organic Chemistry"; Wiley: New York, 1966.
RECEIVED for review June 11, 1982. Resubmitted December 9, 1982. Accepted December 9, 1982.
