Anal. Chem. 1990, 62, 2155-2158
there were no obvious visible changes associated with the poor electrode performance. For the reasons noted above, this procedure is not a recommended method of construction. Electrodes with the most desirable performance were obtained from method C, i.e. by "air-drying" the membrane component mixture for 24 h before introducing the reference solution via an internal pipet. Figure 3 shows results from five representative electrodes with this type of fabrication technique. The average of the potential changes from pCa 3 to the other solutions is represented by (0);bars represent the range of potential variation among the electrodes. The rate of potential change from pCa 3 to 7 was 30 mVll0-fold change in Ca2+concentration. Even though the sensitivity rolls off below pCa 7 , a 20-mV change between pCa 8 and 9 remained. The calculated log kCa,K a t pCa 8 and 9 is -6.84 and -7.27, respectively. Repeated calibration of these electrodes showed that all of them could be reliably calibrated for 6 h after they were made and two of the five were still reliable 48 h later. After hanging dry for 24 h following calibration, the response characteristic of the three that changed is shown by (0). The rate of potential change/lO-fold change in Ca2+concentration was 25 mV (line) instead of 30 mV and the response began to "roll off" at pCa 6. In addition to the change in sensitivity, the selectivity was reduced; calculated values of log kCa,K at pCa 8 and 9 were -6.4 and -6.84, respectively.
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A comparison of data presented in Figure 3 with those in Figure 1 indicates that a "hardened" ETH-129 membrane retards deterioration upon exposure to aqueous solutions. However, even a "hardened" membrane eventually deteriorates even though not exposed to aqueous solution. Five electrodes that were allowed to air-dry for 3 weeks and then filled with reference solution showed no sensitivity to Ca*+. ACKNOWLEDGMENT We wish to thank T. Buhrer for providing us with the ETH-129 cocktail. Registry No. ETH 129, 74267-27-9; Ca, 7440-70-2.
LITERATURE CITED Brown, H. Mack; Owen, J. D. Ion-Sel. Electrode Rev. 1979, 1 . 145. Levy, S.; Tillotson, D. Can. J. Physlol. Pharmacol. 1987, 65,904. Ammann, D. Ion -Selective Microelectrodes : Springer-Veriag: Berlin, 1966. Brown, H. Mack; Pemberton, J. P.; Owen, J. D. Anal. Chim. Acta 1976, 85, 261 Ammann. D.; Guggi, M.; Pretsch, E.; Simon, W. Anal. Lett. 1975, 8 , 709. Ammann, D.; Buhrer, T.; Schefer. U.; Mulier, M.; Simon, W. Pfluegefs Arch. 1987, 409, 223. Buhrer, T.; Gehreg, P.; Simon, W. Anal. Sci. 1988, 4 , 547. Erne, D.; Ammann, D.; Simon, W. Chlmis 1979, 33, 88. Tsien, R.; Rink, T. Blochim. Blophys. Acta 1980, 623. Nikolskii, B. P. Zh. Fiziol. Khim. 1937, 10, 495.
RECEIVED for review March 26,1990. Accepted June 26,1990.
Voltammetric Determination of the Ion-Exchange Behavior of Poly(ester sulfonic acid) Anionomers in Acetonitrile Thomas Gennett*J and William C. Purdy Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 INTRODUCTION Over the past few years, electrodes modified with ionomeric polymers have been the focus of a significant amount of research (1-11). Recently, reports have described the ion-exchange properties of a series of Eastman Kodak AQ polymers in aqueous solutions (9-11). These poly(ester sulfonic acid) anionomers exhibited transport properties and ion-exchange selectivity comparable to those reported for Nafion (1,6,10, 11). In aqueous solutions the AQ membranes preferentially bind large hydrophobic cations and exclude negatively charged species from entering the membrane. The complete structure of the three different AQ polymers is not known; however, the proposed backbones of the AQ55, AQ38, and AQ29 polymers are illustrated in Figure 1 (12). The equivalent weights of the three polymers, as determined from the percent sulfonation, are 1500,2500, and 2500, respectively (12).
The use of ionomer-modified electrodes in nonaqueous electrochemistry has been limited because of several inherent problems with ionomeric membranes in nonaqueous solvents: swelling, solubility, lack of structural integrity, etc. However, we have found the AQ polymers to be stable in several nonaqueous electrochemical solutions. We report the ion-exchange and electrochemical behavior of platinum disk electrodes coated with an AQ55 polymer film.The investigations, conducted in acetonitrile, found a strong correlation of ionexchange properties and electrochemical response to the size Current address: Chemistry Department, Rochester Institute of Technology, Rochester, NY 14623. 0003-2700/90/0362-2155$02.50/0
of the electrolyte cation. Also, the nonaqueous ion-exchange selectivity was markedly different from that observed in aqueous systems.
EXPERIMENTAL SECTION An EG&G PAR Model 273 potentiostat/galvanostat and EG&G PAR Model 175 universal programmer in conjunction with a Hewlett-Packard 7015B x-y recorder were used for voltammetric analysis. An EG&G PAR Model 174A Polarographic Analyzer equipped with a Houston Instruments Model REO089 x-y recorder was used for differential pulse voltammetry. An IBM Instruments electrochemicalcell was used for all electrochemical experiments. The typical three-electrode cell consisted of a 1.6-mm platinum disk electrode, a saturated Ag/AgCl reference electrode, and a platinum wire auxiliary electrode. All electrolyte salts, NaClO, (Aldrich), KPF, (Aldrich), [(CHB),N]PF6(Aldrich), [(CH3CH2),N]C1O4(Eastman Kodak), and [ (C4H9),N]PF6(Aldrich Co.), were recrystallized twice and dried in vacuo before use. All electrochemical experiments were conducted in 0.01 M supporting electrolyte unless stated otherwise. The solvents, acetonitrile (Aldrich, HPLC grade) and methylene chloride (Aldrich,anhydrous spectro-grade),were used as received. Ru(bpy),C12 was purchased from Sigma and used as received. All electrochemical solutions were degassed for 20 min with nitrogen prior to data acquisition. Electrochemical experiments were performed under an inert gas atmosphere and at ambient temperature. The AQ55D polymer was purchased as a 28% dispersion from Eastman Kodak and diluted to a 1.0% aqueous dispersion with distilled water from a Millipore Milli-Q system. Prior to membrane coating, the platinum disk electrodes were polished with 1.0-, 0.3-, and 0.05-pm alumina on a Buehler Ecomet I1 polisher. After being polished, the electrodes were sonicated 0 1990 American Chemical Society
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-f-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
! ~ ~ - - C H ~ - - C H 2 - -CH2-CHI 0
C
X
SO, Na'
A055
I
0.1 yarnpi
SO3Na'
AQ29 AQ38
Flgure 1. Polymer backbone of the AQ55, AQ38, and A029 complexes. in water for 2 min to remove any residual alumina. The clean, dry electrode surface was coated with 10 p L of the 1.00% AQ55 dispersion; the resultant membrane thickness was approximately 1 pm. (We found the 1:20 (v/v) water/acetone dilution of the AQ55 28% dispersion described in the literature (IO, I I ) , to be a heterogeneous colloidal suspension. Membranes cast from the 1:20 (v/v) solution were ill-defined with inconsistent coverage of the electrode surface.) The coated electrodes were allowed to dry one by one of three methods: overnight at room temperature, 2 h at 60 "C, or 1 h at 80 "C. The film was removed at the end of the experiment with a wet polishing cloth. These procedures resulted in a membrane/electrode interface that exhibited reproducible physical and electrochemical behavior. R E S U L T S A N D DISCUSSION This investigation was concerned with the ion exchange and electrochemical response of the AQ55-coated electrodes to the R ~ ( b p y ) , ~ + /redox ,+ system. The AQ55 polymer undergoes minimal swelling, is insoluble, and maintains its structural integrity in acetonitrile. Therefore, acetonitrile was the solvent of choice for the ion-exchange and electrochemical investigations. Various supporting electrolytes, NaClO,, KPF6, [ (CH3),N1PF6, [ (CH3CH2),N1C104,and [ (CH4H9),N1PF6,were used to determine the effect that electrolyte cation size has on the ion-exchange properties of the membrane. Attempts to reproduce the reported behavior of AQ55modified electrodes in aqueous solutions proved unsuccessful. The room temperature cast AQ membranes (10, 11) were found to be highly soluble in aqueous solutions. Adjustments to the published electrode modification procedure with the AQ polymer were necessary. A solution processing technique similar to that reported by Martin et al. (13-15) for the coating of electrode surfaces with a Nafion membrane was developed for the AQ55 polymer. Systematically, a series of AQ55 membrane coated electrodes were prepared a t various temperatures, 50 "C, 80 "C, and 120 "C, for different time durations. The membranes cast at 120 "C shows signs of thermal degradation. The optimum conditions were to cast the membranes from a 1% aqueous solution of the AQ polymer for a period of 1 h a t a temperature above the Tgof the polymer. ( Tgis the glass transition temperature of the polymer film, for AQ55 T, = 55 "C.) The resultant membrane was insoluble in aqueous solutions without any apparent loss of the high selectivity of the membrane for large hydrophobic cations. Experimental results in aqueous solution for membranes cast with the above conditions correlated with the behavior reported in the literature (10, 11, 16). More importantly however, a dramatic 5- to 10-fold decrease in the electrochemical background currents for high-temperature-cast, as compared to room-temperature-cast, membranes, was observed. This decrease in background current increased the sensitivity of the entire electrochemical system to solution
1
I 1.4
I
Volts
I
1 .o Y8,
I
I
0.6
AIlAICl
Figure 2. Oxidation of 5 X lo-' M Ru(bpy),CI, in acetonitrile at a 1.6-mm platinum disk electrode coated with 10 pL of the 1 % AQ55 dispersion: (A) 0.01 M [(C4H9),]PF,;(B) 30 s after solution A made 0.01 M in NaCIO,; (C) 5 min after solution A made 0.01 M in NaCIO,. analytes. This effect could, in part, be caused by a lower water content in membranes cast a t the higher temperatures. Gravimetric analysis of membranes cast at high temperature as compared to those at room temperature resulted in an 8-12% decrease in the mass of the membrane cast at 80 "C. The variation in the physical properties of membranes caused by the high-temperature solution processing procedures are more than likely the result of a combination of changes in water content and/or the supermolecular structure of the nonionic and ionic domains within the ionomer. Investigations are presently underway to determine the actual morphological and physical changes caused by high-temperature casting. Figure 2A shows a cyclic voltammetric experiment for the oxidation of 5 x M R ~ ( b p y ) , ~ +in/ ~a+0.01 M [(C4H9),N]PF6 acetonitrile solution. The E" equals 1.24 V versus Ag/AgCl as compared to an E" equal to 1.33 V a t a bare platinum disk electrode (determined in a 0.5 mM solution). This potential shift is comparable to that observed for the aqueous electrochemical response of AQ55-modified electrodes (10, 11). The voltammetric response of the anodic peak current for the coated electrode was linear with respect to the square root of the scan rate; hence diffusion-controlled electron-transfer kinetics occur within the membrane. From differential pulse voltammetry a detection limit of M Ru(bpy),l+ in acetonitrile was observed, which is comparable to the detection limit reported for Nafion-coated electrodes in aqueous solutions (3). The ion-exchange distribution coefficient for R ~ ( b p y ) , ~ +into / ~ +the AQ55 membrane from a 0.01 M TBAHFP acetonitrile solution is 2.6 X lo4 as determined from differential pulse voltammetry concentration isotherms. This value compares favorably to reported aqueous values for Nafion (3),the AQ polymers (10, I I ) , and other ion-exchange films (17). A similar voltammetric response was also observed a t a 3.0 mm diameter glassy carbon electrode coated with AQ55. It is apparent from the voltammetric experiments that modification of an electrode surface with the AQ55 polymer results in a significant increase in amperometric sensitivity for the oxidation of Ru(bpy),2+. An interesting electrochemical response occurred when the size of the cationic species of the supporting electrolyte was varied. The voltammetric response of the AQ-coated and bare platinum disk electrodes in the presence of NaClO,, KPF6, [(CH3)4NlPF6, [(CH&H2)4NlC104, or [(C4Hd4N1PF6 was evaluated. For the alkali metal electrolytes NaClO, and KPF6, there was no enhancement of the amperometric response for
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
the oxidation of R ~ ( b p y ) , ~at+ an AQ-modified electrode. In fact there was no amperometric response detectable a t a lo4 M concentration of analyte ions. At this concentration a bare platinum disk electrode gives a significant amperometric response. Therefore, the smaller cations appear to exclude the large hydrophobic Ru(bpy)32+ions from the polymer membrane. In contrast, a preconcentration of the Ru(bpy),2+ ion into the AQ55 membrane occurred for all three tetraalkylammonium salts. This was evident from the electrochemical analysis which showed lowered detection limits for cyclic voltammetry to 10-7-10-8 M. This represents a significant preconcentration, i.e. loading, of the dication into the membrane. Furthermore, there was a correlation between the size of the alkyl group and detection limit for the analyte. The tetramethylammonium salt had a slightly better detection limit than either the ethyl- or butylammonium salts. Figure 2B shows the shift a t an AQ55-coated platinum electrode of Eo for the R ~ ( b p y ) , ~ +redox / ~ + couple caused by the addition of NaC104 (0.01 M) to a 0.01 M [(C4H9)4N]PF6 acetonitrile solution. An immediate positive shift of the oxidation potential by 150 mV was observed. As seen in Figure 2C, within a period of 5 min the electrochemical sensitivity was decreased to the point where the oxidation of Ru(bpy)?+ was no longer detectable with cyclic voltammetry. This behavior occurred whether the electrode was immersed directly into the analyte solution or underwent ex situ preconcentration of the cation before immersion into an electrolyte solution. A similar response was observed with addition of either KPF, or NaC104 to any 0.01 M tetraalkylammonium acetonitrile solution. This ion-exchange behavior is the direct opposite of that observed for R ~ ( b p y ) , ~in+aqueous solutions (10,11, 16). The identical experiment at a bare platinum disk electrode showed no change in the electrochemical behavior of the Ru(bpy),2+oxidation reaction. These data strongly suggest that the AQ55 membrane is more ion selective for the smaller Na+ and K+ cations than the large hydrophobic R ~ ( b p y ) , ~ + dication in acetonitrile. A thermodynamic analysis of this ion-exchange behavior of the AQ55 polymer begins with the Gibbs-Donnan equation (18,19). The extent of the reaction, A+ *B+ + *A+ B+, is given by
+
RT In Kb = -RT In [(6*,6,)/(6,6*B)]
+
-P
AV
where K$ is the ion selectivity coefficient, 6’s are the activity coefficients of the respective ions, the asterisk refers to ions in the ion-exchange membrane, P is the membrane swelling pressure, and A V is the partial molar volume difference of the membrane ion-counterion salts in the membrane phase. For infinite dilute solutions, if both A+ and B+are simple inorganic ions of approximately the same size, any selectivity results purely from activity coefficient effects. However, if the difference in size between A+ and B+is large, then the P AV term dominates the energetic contributions to the ion-exchange activity. The incorporation of the smaller ion into the membrane phase would be energetically more favorable because of increases in ionic volume which would result from incorporation of the larger ion. When A+ and B+ have different hydrophobic properties, another term must be added to the Gibbs-Donnan equation (17)
RT 1n Kb = -RT In [(6*A6,)/(6A6*B)] - P A V - AGH where AGH describes the contribution of the nonpolar or hydrophobic nature of an ion to the free energy change associated with the incorporation of this ion into the membrane phase. In other words, the hydrophobic-hydrophobic interactions between an ion and the hydrophobic region of a membrane become the driving force for the thermodynamics
2157
of the ion-exchange process. The contribution of this term will also be directly dependent on the hydrophobic/hydrophilic nature of the solvent. Therefore, any significant solvation effects would be included in the AGH term. If immersed in an aqueous solution, the AQ55 membrane undergoes significant swelling. When swelling of an ion-exchange membrane occurs the membrane swelling pressure, P , and changes in the ionic volumes, AV, are greatly reduced. As a result, the P A V term of the Gibbs-Donnan equation does not make a significant contribution to the total free energy of the ion-exchange reaction. The ion-exchange selectivity thus becomes dependent upon the solvent/membrane composition, the resultant chemical/ionic interactions of the ions with the membrane, and the activity coefficient effects. Hence, the aqueous ion-exchange selectivity of a large hydrophobic cation over a small hydrophilic cation in the swollen, amphiphilic AQ55 membrane would be largely influenced by the AGH term of eq 2. This explains the selectivity of the AQ55 membrane for large hydrophobic ions in aqueous solutions. Since the AQ55 membrane is primarily composed of long hydrophobic alkyl chains, the membrane would selectively bind the hydrophobic inorganic ions from the aqueous solution. In acetonitrile, the AQ55 polymer exhibits minimal swelling, and the organic character of the nonaqueous solvent reduces the hydrophobic-hydrophobic interactions between the polymer membrane and the R ~ ( b p y ) , ~ions + in solution. However, there are solvation effects for both the electrolyte and analyte which makes a contribution to the magnitude of the AGH term. For example, in acetonitrile solutions the alkali-metal electrolytes would have a different contribution associated with the “selective solvation” of the electrolyte cation by the hydrophilic region of the AQ membrane than the alkylammonium salts. Also, since the AQ membrane does not undergo any significant swelling, the contributions of ionic volume differential become important; Le. the P AV term is no longer negligible. We found that the Na+ and K+ saturated membranes no longer preconcentrated the R ~ ( b p y ) , ~from + the nonaqueous solution phase, if the solution phase contained the respective alkali-metal electrolytes. In fact the analyte ions were excluded from the membrane. This exclusion of analyte ions decreases the voltammetric sensitivity of the membrane-coated electrode to Ru(bpy),2+ in the presence of KPF6 or NaC104. Therefore, in acetonitrile, the P A V term, combined with energetic contributions of the selective solvation effects attributed to the alkali-metal ions “dissolved” in the AQ membrane (AGH term) and not the analyte hydrophobic or activity contributions, exhibit primary control over the ion-exchange properties of the membrane. The exchange of R ~ ( b p y ) , ~for + a large, hydrophobic electrolyte cation should minimize the contributions from the change in ionic volumes, AV, and solvation effects. The enhanced detection limits of cyclic voltammetry for Ru(bpy)gz+ in the presence of the tetraalkylammonium salts demonstrate the effect of minimizing the P A V and solvation contributions while isolating the effects associated with the activity coefficients.
CONCLUSIONS In nonaqueous solvents the possibility exists to directly control the selectivity of the AQ55 ion-exchange membrane by regulating the composition of electrolyte salts in the acetonitrile solution. Through experimental manipulations of solvent composition, control over the contribution of the activity coefficients, ionic volumes, and free energy change associated with chemical (hydrophobiehydrophilic) interactions is achieved. The experimental results from adding the NaC10, to the [(C4Hg),N]PF6acetonitrile solution are an important example of this behavior. When the smaller alkali cations are
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Anal. Chem. 1990, 62,2158-2160
added to the tetraalkylammonium ion solution, a decrease in the voltammetric response of the electrode to the analyte, Ru(bpy),2+, occurs. Eventually, complete expulsion of the analyte ion from the membrane results. This is evident by the complete loss of electrochemical response. With all the changes in double-layer composition as these ion-exchange reactions are occurring, it is not surprising that the E" should shift. A complete electrochemical analysis of the double-layer and ion-exchange processes is necessary to conclusively determine the actual cause of the potential shift. This is one of the first reported studies on the ion-exchange selectivity of ionomers in nonaqueous solvents. We have shown a unique and extremely important ion-exchange process, the direct control of the selectivity of an ion-exchange membrane by manipulation of solvent and electrolyte composition. Also, we have found the AQ polymers to have a distinct advantage over Nafion with regard to the conditions for solution processing. For the AQ polymers the technique is accomplished with relative ease from aqueous solutions at 80 "C instead of from the dimethyl sulfoxide solutions at 180 "C necessary for the preparation of Ndion membranes (7,13, 14). We are currently expanding our investigations on the AQ polymers to the evaluation of ion-exchange and selectivity coefficients with various nonaquoeus solvents and electrolytes. Also, initial analysis of ion-exchange behavior for other inorganic and organic redox systems has begun. The AQ polymers should prove to be a boon to electroanalysis with their "controlled" selectivity, low cost, and ease of preparation. The applications are widespread, starting with the possibility to control the response of ion-selective electrodes. Also, the remarkably low detection limits observed with these modified electrode systems indicate the possibility of extensive applications to the electrochemical detection for
flow-injection analysis and high-performance liquid chromatography.
ACKNOWLEDGMENT The authors wish to acknowledge discussions with Dr. Robert B. Moore 111.
LITERATURE CITED (1) Eisenberg, A., Bailey, F. E., Eds. Coulombic Interactlons h Macromolecular SySt.9mS; ACS Symp. Ser. No. 302; Amerlcan Chemlcal Society: Washington, DC, 1986. (2) Chandler, G. K., Pletcher. D.,Eds. Electrochemistry: A Review of Modern Literature; Royal Society of Chemistry: London, 1985; Chapter 3. (3) Whitely, L. D.; Martin, C. R. Anal. Chem.1987, 49, 1746. (4) Stilwell, D. E.; Park, S. J. Electrochem. Soc.1988, 135, 2254, (5) Schneider, J.; Murray, R. W. Anal. Chem. 1982, 54, 1508. (6) Moore, R. B.; Wilkerson, J. E.; Martin, C. R. Anal. Chem. 1984, 56, 2572. (7) Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 1898. (8) Kaaret, T. W.; Evans, D. H. Anal. Chem. 1988, 60, 657. (9) Connelly, R. W.; McConkey, R. C.: Noonan, J. M.; Pearson, G. H. J. Polym. Sci. 1982, 20, 259. (10) Wang, J.; Lu, 2. J. Electroanal. Chem. Interfacial Electrochem. 1989, 266, 287. (11) Wang, J.; Golden, T. Anal. Chem. 1989, 67, 1397. (12) Lawnlczak, J., Eastman Chemical Products, Inc., private communication, 1990. (13) Moore, R. B.; Martin, C. R. Anal. Chem. 1988, 56, 2569. (14) Moore, R. B.; Martin, C. R. Macromolecules 1988, 27, 1334. (15) Whitely, L. D.; Martin, C. R. J. Phys. Chem. 1989, 93, 4650. (16) Gennett, T. Unpublished reports, 1989. (17) Wang, J. I n ElectroanalytlcalChemlstry;Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16, pp 1-88. (18) Miller, I.F.; Bernsteln, F.; Gregor, H. P. J. Chem. Phys. 1965, 4 3 , 1783. (19) Marinsky, J. A., Ed. Ion Exchange: A Serbs of Advances, Vol. 2 ; Marcel Dekker: New York, 1969; Chapter 3.
RECEIVED for review February 20,1990. Accepted June 19, 1990. The authors are indebted to the Natural Sciences and Engineering Research Council of Canada for financial support.
Importance of Clean Metallic Zinc for Hydrogen Isotope Analysis A. Tanweer Isotope Hydrology Laboratory, International Atomic Energy Agency, A-1400 Vienna, Austria Most laboratories using the zinc method (1-3) for reduction of water to hydrogen for D/H analysis by mass spectrometry were obtaining poor reproducibility of analysis. After several discussions it was discovered that people were careful about the amount of zinc, quantity of water, and temperature; but cleaning of the zinc was not strictly followed. In our laboratory, BDH zinc size (0.5-2.0 mm) is used for the D / H analysis, and an ultraclean surface is obtained by using the following sequence: (1)sieving the zinc fraction; (2) water rinsing; (3) stirring in 1% nitric acid; (4) complete drying under vacuum; ( 5 ) drying plus heating at 300 "C under vacuum.
EXPERIMENTAL SECTION Apparatus. The principal item is a high-vacuum 90" pattern
stopcock (Model 8195-47, ACE Glass, Inc., Vineland, NJ) with an O-ring glass joint (NO67807 Pyrex) added to one end to allow attachment to the vacuum line and a glass tube 20 mm in diameter, 90 mm long, and sealed to make a glass container on the other end which can hold 75 g of zinc. Other items include a glass rod to stir, a glass funnel, and a beaker. Reagent. Zn metal shot AnalaR was supplied by BDH Chemicals, Ltd., Poole, UK, with a grain size between 0.5 and
2.0 mm. Other materials used were 1%nitric acid and demineralized water. Procedure. The zinc was put into a glass beaker and washed with demineralized water. Four to five rinses were required until the oxide dust was removed; the water color changed from milky to clear. Quantities of about 75 g of zinc were washed in 200 ml of 190nitric acid; the zinc was stirred with a glass rod. In about 1 min, the shining surface of zinc could be seen. The acid was decanted and the zinc given four to five rinses with demineralized water to remove the acid. The entire cleaning procedure should be carried out in a fumehood while wearing safety gloves and glasses. Wet zinc was placed in the glass container to dry under vacuum using only a rotary pump. One should try to decant as much water as possible before pumping to avoid water entering the rotary pump. After -2 h the zinc will be thoroughly dry. Only now should the system be pumped with the diffusion pump. The furnace was placed around the container and the temperature increased gradually up to a maximum of 300 "C. After the system was pumped for about 2 h at 300 "C, the zinc was completely degased. The heating was switched off and the glass container cooled down to room temperature; the container was then closed and the diffusion pump switched off. The batch of zinc was then
0003-2700/90/0362-2158$02.50/00 1990 American Chemical Society
