Anal. Chem. 1982, 54, 2625-2626
in excess of 25 "C. We found that there was a significant loss of TCDD when toluene solutions were evaporated to dryness at 50 "C (47 f 5%, n = 3; 66 f 2%, n = 3) and at 100 "C there was an almost complete loss of TCDD (2 f 1% , n = 3; 50 "C.
2625
(2) Burke, J. A.; Mllls, P. A.; Bostwick, D. C. J . Assoc. Off. Anal. Chem. 1988, 4 9 , 999-1003. (3) Bowers, W. D.; Parsons, M. L.; Clement, R. E.; Karasek, F. W. J . Chromatoor. 1981. 207. 203-21 1. (4) Hummel, E. A. J . Agrlc. Food Chem. 1977, 2 5 , 1049-1053. (5) Ryan, J. J.; Pilon, J. C. J . Chromatogr. 1080, 797, 171-180. (6) Harless, R. L.; Oswald, E. 0.; Wllkinson, M. K.; Dupuy, A. E.;McDaniel, D. D.; Tai, H. Anal. Chem. 1980, 52, 1239-1245. (7) Lamparskl, L. L.; Nestrick, T. J. Anal. Chem. 1980, 5 2 , 2045-2054. (8) VanNess, G. F.; Solch, J. G.; Taylor, M. L.; Tiernan, T. 0. Chemosphere 1980, 9 , 553-563. (9) Solomon, S.Anal. Chem. 1970, 5 7 , 1861-1863. (10) Beroza, M.; Bowman, M. C.; Bier], 6. A, Ana/, Chem, 1972, 44, 2411-2413. (11) Puchweln, G. Anal. Chem. 1981, 5 3 , 544-546. M (12) O'Keefe, P. W.; Smith, ~ ror, ~ R.; , e w , C.; Hiker, D.; Aldous, K.; Jelus-TyB.~J . Chromotogr. 1982, 242, 305-312. (13) Baughman, R.; Meselson, M. A&. Chem. Ser. 1973, No. 720, 92-104. (14) Smith, L. M. Anal. Chem. 1081, 5 3 , 2152-2154.
LITEXATURE C I T E D (1) Chlba, M.; Monley, H. V. J . Assoc. Off. Anal. Chem. 1088, 51, 55-62.
for review November IgB1*Resubmitted June 30, 1982. Accepted August 19, 1982.
Teflon Double-Junction Reference Electrode for Use in Organic Solvents d. F. Coetzee" and C. W. Gardner, dr. Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania
Especially during the past few years many indicator electrodes have been tested in nonaqueous media (1) and found to respond, under careftilly controlled conditions, in a manner useful for both fundamental and applied purposes. It is becoming increasingly clear, however, that in many potentiometric cells containing nonaqueous solvents the reference electrode, rather than the indicator electrode, may introduce the largest uncertainty in the measured cell potential. This is particularly true when an external reference electrode and salt bridge must be used because an internal reference electrode is not applicable, as is typically the case for aprotic solvents. [An exception is the poly(viny1ferrocene)-coated platinum electrode (2) which functions well in acetonitrile; it is less satisfactory in other solvents (both protic and aprotic) tested.] Common electirodes of the second kind, such as the Ag,AgCl and Hg,Hg2C12electrodes, are unsuitable as internal reference electrodes in aprotic solvents owing to stabilization of halo complexes relative to free halide ions, as shown in eq 1 and 2. Such electrodes can be used externally, of course,
+ Cl- e AgCI2Hg2C12J. t- C1- e HgO + HgC13AgC1.l
(1)
but they offer no advantage and have some disadvantages as compared to electrodes olf the first kind, especially the Ag+/Ag electrode which has been used extensively in many nonaqueous solvents (3). In this paper we describe an improved liquid-junction a8sembly for use with the Ag+/Ag or other external reference electrodes, especially the 13-,1-electrode that we are recommending elsewhere ( 4 ) . The junction is of the constrained diffusion boundary type ,and is formed inside a tightly rolled plug of Teflon tape. Liquid junctions formed in Teflon capillaries (5) and frits (6) have recently been introduced with aqueous reference electroldes, mainly for reasons of inertness and durability as compared to the numerous types of porous glass junctions described, We have an additional interest in a Teflon junction: its use allows construction of all-plasticware potentiometric cells to minimize adsorption of ions which complicates analysis of very dilute solutions in water and especially in those solvents in which certain ions have enhanced activities.
15260
Table I. Flow Rates and Impedances of Different Types of Constrained Liquid Junction with 0.1 M Sodium Perchlorate in Acetonitrile as Solvent ceramic frit flow rate, /IL h" impedance, (1kHz)
asbestos fiber
2.2
0.32
3.4 X l o 4
4.8
X
Teflon roll 2.6
lo6
3.3 X
lo4 -
EXPERIMENTAL SECTION Solvents. Acetonitrile, abbreviated AN (Matheson, Coleman and Bell), dimethyl sulfoxide, Me2S0 (J.T. Baker), and ethanol, EtOH (USI), were ACS reagent grade products and were used without further purification. Propylene carbonate, PC (Fisher reagent), was distilled in vacuo under a slow nitrogen bleed at I 5 "C. Reagents. Silver perchlorate (Alfa Products) was dried in vacuo at 80 "C for 24 h. Sodium perchlorate (G. F. Smith, anhydrous) was used without further purification. Tetraethylammonium perchlorate, abbreviated T W , was prepared (7)from tetraethylammonium bromide (Fisher) and perchloric acid (Baker, 70%) and was recrystallized twice from water and then dried in vacuo at 90 "C for 24 h. Apparatus and Experimental Procedures. Potentials were measured with a Keithley Model 616 digital electrometer connected to a Linear Instruments Model 255 or Sargent-Welsh Model SR-G strip-chart recorder. Liquid junctions were formed and tested in three types of constraining devices. (a) Asbestos fiber junctions were fabricated by heat-sealinga 0.02 mm diameter asbestos fiber into one end of an 8-mm i.d. glass tube; we had previously used such junctions extensively in many solvents. (b) Ceramicjunctions consisted of a 5.5 mm long X 0.6 mm diameter ceramic plug sealed into one end of a 6 mm i.d. glass tube (Fisher Scientific Co. item FSE-1203). (c) Teflon junctions were fabricated by rolling a 4 in. long X 1 in. wide strip of PTFE pipe thread tape (Gore-Tex, W. L. Grace and Associates) lengthwise around two thin wires. The wires were then retracted and the Teflon cylinder was rolled between gloved fingers to tighten it up slightly. The cylinder was inserted tightly into a '/a in. liquid chromatography Teflon fitting (General Valve Corp.) which had been bored out to '/e in. The protruding ends of the cylinder were cut off flush with the ends of the nut, and the assembly was threaded into a Teflon tube which then served as the salt bridge body. The
0003-2700/82/0354-2625$01.25/00 1982 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table 11. Performance of Ag+/AgDouble-Junction Reference Electrodes Containing Teflon Roll Outer Junctions in Various Solvents salt bridge fluidity of pure solvent, g. mL-'c P ' 125 'OU surface tensiol; of p;re solvent, dyn cm-l (20 "C)U "wettability" of Teflon junction electrode impedance.
W
EtOH
AN
PC
Me,SO
1M KC1 1.12
0.1 M NaC10, 0.73
0.1 M TEAP 2.27
0.1 M TEAP 0.47
0.1 M TEAP 0.55
72.58
22.32
19.10
40.50
43.54
poor
good
good
fair
fair
1.4 x 105
2.3 x 105
1.8 x
ca.
lo6
06
1.3 X
lo6
