782
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
CONCLUSIONS
on aging of the gas phase in Tenax
By use of the technique described herein, it is possible to trap and recover the more highly labile components in the gas phase of cigarette smoke. All of the isoprene and acrolein and 809" of the acetaldehyde was recovered. Pattern recognition of the chromatograms indicated that other major components in t h e gas phase were recovered also. T h e adsorption characteristics of many of these on Tenax have already been reported (2-8). Improvements need to be made in the storage of aged samples. Given these, this method should be applicable to remote sampling of cigarette smoke and other gases for subsequent analyses a t another site.
LITERATURE CITED A. D. Horton and M. R. Guerin, Tobacco, 176, 45 (1974) (Tob. Sci. No. 19). W . Bertsch, R . C. Chang, and A . Ziatkis, J , Chromafogr. Sci.. 12,175 119741
E -D. Peilizzari, J. E . Bunch, R. E. Berkiey, and J. McRae, Anal. Left.. 9 (1). 45 11976). A.'Zlatkis,' H. A: Lichtenstein, and A. Tishbee, Chromafographia, 6 (2),
67 (1973). J . S. Parsons and S. Mitzner. Eviron. Sci. Techno/.,9 (12), 1053 (1975). R van Wijk, J . Chromafogr. Sci., 8, 418 (1970). L . D. Butler and M. F. Burke, J , Chromafogr. Sci.. 14, 117 (1976). J. Janak, J . Ruzickova, and J, Novak, J . Chromafogr., 99, 689 (1974). "The Chemistry of Acrylonitrile", 2nd ed.. American Cyanamid Co., 1959.
ACKNOWLEDGMENT
RELEI\ED for review November 4, 1975. Accepted February
T h e assistance of W. H. Baldwin of ORNL Chemistry Division and C.-H. H o of the ORNL Bio/Organic Analysis Section in synthesizing the 3,3'-(trimethy1enedioxy)dipropionitrile is greatly appreciated. C. K. Bayne, Computer Sciences Division, performed statistical analyses of the data
7,1978. Research sponsored by the National Cancer Institute, The Council for Tobacco Research--USA, and the Department of Energy under contract with Union Carbide Corporation. S.G.Z. from Centre College, D a n d l e , Ky. 40422, was an O R A L summer research participant.
Effects of Surface Heterogeneity on the Sensitivity of Sulfide Ion-Selective Electrodes Janis Gulens" and Brian I k e d a ' General Chemistry Branch, Atomic Energy of Canada Limited, Chalk River, Ontario. Canada KOJ IJO
non-Nernstian response of the electrode. Polishing the electrode removed this film: restored the crystal to a shiny state. and the electrode response was again rapid and mol L level). LVhen the Nernstinn (at least a t the 3 X electrode which had "failed" a t the heavy water plant was polished and tested, negative deviations from Nernstian hehavior were observed ("super-Nernstian" response) at concentrations less than 3 X 10 mol L, I . This electrode would rapidly grow films, and one of these films was examined under a scanning electron microscope (SEILI). This communication presents the study of the calibration behavior of this electrode and the results of the SEM examination of the film on its surface. The super-Nernstian response observed at low sulfide concentrations is proposed to be due to the measurement of mixed potentials that arise as a result of various surface reactions. I t is further proposed that these surface reactions primarily involve metallic silver which gradually accumulates at the electrode surface, and that the primary source of the metallic silver is the internal silver metal contact to the silver sulfide membrane.
The limit of Nernstian response of Ag,S ion-selective electrodes changes from -lo-' mol L-' to mol L-' total dissolved sulfide with increased use of these electrodes. Films or deposits appear on the surfaces of such electrodes and two of these deposits have been examined by a scanning electron microscope. The non-Nernstian response at low concentrations is attributed to the measurement of mixed potentials which, it is proposed, arise primarily as the result of the gradual accumulation of metallic silver at the membrane/solution interface. The silver metal solid contact to the inner membrane surface is proposed to be the primary source of metallic silver. Films or deposits of Ag,S or Ag,O appear on the electrode surface as a consequence of the presence of metallic silver.
'
Sulfide ion-selective electrodes. based on a sik er sulfide membrane, can be calibrated in alkaline solutions to give Nernstian response to total sulfide concentrations as low as mol L-' but only if excess reducing agent (ascorbic acid or hydrazine) is present to remove dissolved oxygen (1,2). A continuous H2S-in-water monitor, based on the sulfide ionselective electrode, was developed and used to measure the dissolved H2Sconcentration in the liquid effluent from a heacy water plant (3). During operation of the monitor a t the heavy water plant, a sulfide ion-selective electrode failed to give mol L-' level and was replaced. Nernstian response a t the Previously, some sulfide ion-selective electrodes had acquired a dull film or tarnish on their surface with use (2),and t h e appearance of this film was accompanied by slow and
EXPERIMENTAL All sulfide electrodes used were manufactured by Orion Research Inc., Model 94-16A, and have an internal solid metallic silver contact ( 4 ) . Their potentials were measured by an Orion hIodel 801 voltmeter relative t o a saturated calomel reference electrode, the latter being separated from the sample solution by a 1 mol L KC1 bridge solution. An Orion Model 605 Electrode Switch was used to simultaneously measure the potentials of several electrodes. All measurements were performed at room temperature, 300 f 3 K. Reagent grade chemicals and distilled-deionized water were used throughout. Sulfide solutions were prepared from crystals of Na2S.9H20 in 1 mol L,-' NaOH-0.1 mol L-' ascorbic acid solution ( 5 ) . Calibration curves were obtained by the standard
'Present address, Department of Chemistry,rniversitv of Guelph. Guelph, Ontario K l G 2W1. 0003-2700/78/0350-0782501 0010
C
1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, M A Y 1978
OJJ
I
-
\
A-A'
I3
I
- &p I-: _ -1- 6 , :c L1 0 L i CL il [ j 1:Ui
-. Figure 1. Simultaneous calibration of failed electrode C and a new electrode H in 0.3 mol L-' NaOH-0.1 mol L - ' ascorbic acid solution. (0)electrode C; ( A ) electrode H addition procedure, and the concentration of free sulfide in solution was assumed to be equal t o the total amount of sulfide added. Various electrode polishing procedures were used during the course of these experiments. Polishing strips provided by Orion Research (94-82-01) were used initially. Later, more thorough polishing to remove visible pits and scratches from the surface was done using AB Metadi diamond polishing compound (0.25-pm diameter) on AB Microcloth (Buehler Ltd.) with a final polish using Fisher Gammal (0.05-pm alumina) on AB Microcloth. The SEM results were obtained using a Cambridge Scientific Instruments S-410.
RESULTS Calibration. The "failed" electrode from the heavy water plant, electrode "C",was polished with Gammal and calibrated simultaneously with a new unused sulfide ion-selective electrode, electrode "H", Figure 1. Electrode H gave Nernstian response to sulfide concentrations as low as lo-' mol L-' while C began to deviate from Nernstian response at -5 X mol L-'. Since both electrodes were immersed in the same solution, the response of electrode H was clearly superior to that of electrode C. After further use, the performance of both electrodes deteriorated: electrode H deviated mol L-' while C began to from Nernstian response a t deviate from Nernstian response at 10-j mol L-l. The extent of deviation from Nernstian response was much larger for C than for H. Similar calibration curves were obtained with other sulfide electrodes (used previously to varying degrees in the laboratory): Nernstian response was obtained a t concentrations greater than 10-6mol L-',super-Nernstian response at lower concentrations. T h e concentration a t which the potential began to deviate from Nernstian values varied from one electrode to another but was generally in the range of 10-'to mol L-l. Electrodes which deviated most from Nernstian response were characterized by their ability to rapidly grow films on their surface, even over the interval of 30-60 min required to obtain a calibration curve. On some electrodes, these films would appear only as a light grey tarnish while on others, a heavy course granular deposit would be observed. Other electrodes, such as H, remained shiny and bright but still gave super-Nernstian response. Pits, clearly visible t o the eye, appeared on all the electrodes with time. Another characteristic of electrodes which gave extremely poor Nernstian response was their long response time in this super-Nernstian region, e.g., steady potentials were reached in 10-15 min for concentrations less than 10" mol L ~ ' .Electrodes, such as H, which consistently gave the lowest limit of Nernstian response,
- -
-
\
d
i:Er
5
/
783
~~
-.
L--c.0 ~~
ulbJ'iS
Figure 2. Variation in potential of electrode C and H in deaerated 1 mol L-' NaOH solution after thoroughly polishing C and soaking it in sulfide solution. (0)electrode C; ( A ) electrode H
attained equilibrium potentials within 2 min at concentrations greater than 5 X lo-' mol L-'. Once the region of Nernstian response was attained, all electrodes responded rapidly. Effects of Polishing, Polishing the electrodes removed surface films and decreased the response time. However, the difference in the limit of Kernstian response for a given electrode in a polished vs. a coated state rarely exceeded a factor of two. In one instance, electrode C was polished for 20-30 min on a metallurgical polishing wheel (AB microcloth and 0.U5-Prn alumina powder) to remove numerous deep pits visible on its surface. Electrode H was polished lightly by hand and both electrodes were soaked briefly in a 0.05 mol L- sulfide solution and rinsed with distilled water. Large negative potentials were noted for both electrodes on immersing them in the same blank solution of 1 mol L-' NaOH-0.1 mol L-' ascorbic acid (-703 mV and -780 mV for H and C, respectively). T h e subsequent calibration showed a large positice deviation f i r C from Nernstian behavior (from lo-' to lo-' mol L while H gave a positive deviation from lo-' to mol L-'. On removing the electrodes from solution, electrode C already had a grey film while H was still shiny. Sulfide contamination of the blank solution was suspected as this was the first observation of a positive deviation from Nernstian response. T h e experiment was repeated: the electrodes were polished lightly, soaked in 0.05 mol L-'sulfide solution for 10 rnin and rinsed with distilled water. T h e potentials of both electrodes in a 1 mol L-' NaOH solution were again very large and negative (-715 mV and -805 mV for H and C, respectively). On adding some C d ( N 0 J 2 to this solution, the potential of electrode H decreased immediately to -440 mV while the potential of electrode C decreased only to -790 mV. T h e electrodes were removed from this solution, rinsed and immersed in a fresh blank solution (1 mol L-' NaOH, deaerated with N2) where the time dependence of their potential was recorded, Figure 2. The potential of electrode H was observed to increase or decrease, depending whether electrode C was or was not in solution. Electrode C had a grey film on its surface on removal from this solution, while H was shiny and bright. SEM Examinations. T h e grey film on electrode C on completion of the experiment described above was examined 3 weeks later with a SEM. Low magnification examination revealed that the surface contained numerous deposits and scratches. Examination under higher magnification showed that three different types of deposits were present. Type 1. Amorphous particles thinly dispersed over the sample surface, Figure 3. X-ray spectra showed high concentrations of K and S and low concentrations of N a and Ag in the deposits, Figure 4.
'
784
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
Figure 3. Type 1 deposit on Ag2S substrate, electrode C. 500X
Figure 6. Type 3 deposit on Ag,S substrate, electrode C, 500X
I I -
Ad.3
S C O L
L i,
A V j h l
iii?6"
r ? v
Figure 4. Plot of log intensity vs. energy of x-ray spectra for deposits on electrode C. Intensity of all curves is arbitrary. Curve a, Type 1 deposit; b, Type 2; c, Type 3
Figure 5. Type 2 deposit on Ag,S
substrate, electrode C, 500X
Type 2. A semi-amorphous dendritic deposit, largely concentrated in one region of the sample surface, Figure 5. X-ray spectra showed the presence of Na, Ag, and S in these deposits, Figure 4. Type 3. Plate-like deposits dispersed over the finely pitted surface, Figure 6. X-ray spectra showed that these deposits were rich in silver and very low in sulfur, Figure 4. The concentration of Type 3 deposits on the surface was much greater than Type 1 and Type 2 combined.
>
.. . .
Figure 7. Simultaneous calibration of electrode A and H in 1 mol L-' NaOH-0.1 mol L-' ascorbic acid solution. Curve a, electrode A after polishing; b, electrode A, after 6 h of film growth; c, electrode H
T h e crystal was removed from the SEM and polished lightly. Re-examination in the SEM showed only Type 3 deposits present. Another electrode that had last been used a year previously in the laboratory was also examined under the SEM in an attempt to obtain further information on the silver-rich Type 3 deposits. This electrode, electrode A, now showed very little response to sulfide ions even after thorough polishing, Figure 7 , curve a! and had already acquired a light grey coating during calibration. The electrode was polished and stored in a mol L-' sulfide solution in a 1 mol L-'NaOH-0.1 mol L-' ascorbic acid solution. After 2 h in this storage solution, the electrode appeared cloudy; after 6 h, a grey-white film had formed on the surface and the electrode was calibrated without removing the film, Figure 7 , curve b. While the coated electrode gave higher potentials than the freshly polished electrode, its potentials were still greatly different from those of electrode H, Figure 7 , curve c. After a further 6 h in the storage solution, a coarse sandy grey deposit was noted on the electrode surface. The appearance of this coarse deposit did not seem to change over a further 12-h immersion in a fresh storage solution. SEM analysis revealed that the surface was covered with a crystalline-like deposit, Figure 8, which consisted entirely of Ag and S. On examining the sample a t higher magnification in the SEM, the surface of the crystals was observed to "melt" or decompose, Figure 9. One area was exposed to the electron beam for 6 h but very few new particles formed after the first few minutes exposure to the beam.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
Figure 8. SEM micrograph of general surface of electrode A, 2KX
SEM micrograph of electrode exposure at higher magnification, 5KX
Figure 9.
A
after several minutes
DISCUSSION Morf e t al. (6) have presented theoretical arguments claiming t h a t the limit of Nernstian response for the Ag,S electrode was governed by the activity of Ag defects in the membrane surface. This limit was claimed to be -3 X 10F mol L-’and was supported by experimental evidence (6). However, Crombie e t al. ( I ) have shown that the Ag2S electrode gives Nernstian response a t -2 X 10 mol L-’ total sulfide if dissolved oxygen is removed from the alkaline solutions by the addition of ascorbic acid. Our previous work (2) agreed with t h e results of Crombie et al. (I). T h e observations of Morf et al. may be accounted for by the loss of sulfide due to oxidation by dissolved oxygen, and by the leaching of excess soluble silver salts from the membrane (Ag$ precipitated from excess Ag+ contains excess soluble silver salts ( 7 ) ) . T h e limit of Nernstian response for the Ag2S electrode is thus not governed by the activity of silver defects in the membrane surface b u t is, in principle. governed by the solubility of silver sulfide. T h e results presented in this paper reveal that, m practice, other factors can alter the limit of Nernstian response. The limit of Nernstian response varies from one electrode t o another and also changes with the “age” of an electrode. Since the potentials of both electrodes H and C were measured simultaneously in the same solution (Figure l ) ,the difference in response cannot be attributed to the loss of sulfide via reactions with traces of metal impurities or with traces of dissolved oxygen or other oxidants in solution. Baumann (8) has also observed super-Nernstian response at sulfide con-
’
785
centrations less than -3 X lo4 mol L-’ in alkaline solutions containing ascorbic acid, and has also noted that the response time of the electrode varied with its history of use and the solution concentration. “Nernstian response in a solid-state membrane device requires that all the solid phases of the membrane be in equilibrium with the sample solution” (9). The S E M analysis of electrode C reveals that its surface is porous and nonuniform in composition, and thus it is unlikely that a state of equilibrium is attained in solution. T h e super-Nernstian response observed a t low sulfide concentrations is attributed t o the measurement of mixed potentials which arise as t h e result of the following. (i) Porosity of Electrode Surface. SEM micrographs of electrode C show that its surface appears to be highly pitted and porous. Depending on the depth and nature of these pores, different areas of the electrode surface are exposed to solutions of different sulfide concentrations, resulting in a mixed potential. T h e behavior of electrode C in Figure 2 supports the interpretation of the S E M micrograph that the surface is porous, as C appears to be the source of sulfide for t h e solution. As this was the only electrode exhibiting such effects and previous work (2) had shown that electrode response was rapid and accurate to step-function changes in concentration (both increasing and decreasing) over the range to lo4 mol L-’, it is possible that the sulfide was carried over from one solution to the next in a crack in the epoxy seal around the membrane, and not in the porous surface. Nonetheless, the apparent porosity of the electrode surface is of concern as a possible source of problems. Sorrentino et al. (10) have associated the deterioration in t h e electrochemical response of an AgI/Ag,S membrane electrode, after long exposure to solutions of high iodide concentration, with deterioration and roughness of t h e electrode surface. The response of the electrode to iodide was slow and unsteady with sub-Nernstian slopes. SEM examination revealed that the surface had become rough and porous, and it was postulated that the AgI had been leached from the membrane surface by the formation of soluble iodide complexes (10). T h e resultant membrane surface thus consisted of a rough and porous Ag,S matrix that was responsible for the slow, sub-Nernstian response. However, we are unable to explain, a t this time, the apparent porosity of electrode C unless it is inherent to the manufacturing process of Ag,S membranes. (ii) Kinetics of Surface Reactions. T h e concentration of silver ions at the electrode surface is exceedingly small and is governed primarily by the solubility of silver sulfide (pK,, -50). In alkaline solutions, silver oxide and hydroxide complexes are formed K lizAg20t ‘i2HZOt Ag’ + OHAg’ + 2 0 H - + Ag(OH),^ K = 50
= 2 X
lo-’
(1) (2)
In the presence of added sulfide ions, any Ag(OH),- formed a t the electrode surface should be converted entirely to Ag,S 2Ag(OH)p
+
S2-+ Ag,S
+
40H-
K
=
1048’3
(3)
In solutions containing 1 mol L-’ NaOH and only 10-7-l@--6 mol L-’ S2-, the kinetics of this transformation may be such that equilibrium is attained only very slowly. Thus, levels of Ag’ in excess of that predicted by the solubility of Ag,S could exist a t the electrode surface for a sufficient time to give rise to a mixed potential. If the silver-rich Type 3 deposits are primarily Ag,O, they provide support for such reactions. Unfortunately, x-ray spectra from the S E M experiments cannot be used to detect oxygen to differentiate whether the Type 3 deposit is primarily Ag,O or silver metal. X-ray photoelectron spectroscopy studies have also shown t h a t it
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
786
is not possible to distinguish, on the basis of the Ag binding energies, Ag,O from Ag2S or silver metal ( 1 1 ) . X-ray diffraction studies should, in principle, be able to differentiate between these species and such experiments are now underway (12). (iii) Localized Corrosion Reactions. The silver-rich Type 3 deposits can also be areas of metallic silver. A corrosion cell can be established where the anodic reactions 2Ag
+ S2-+ Ag,S + 2e
(4)
2Ag
+
(5)
2 0 H - + Ag,O t H,O t 2e
occur on the membrane surface, while the corresponding cathodic reaction Ag'
+ e + Ag
(6)
occurs either a t the solid silver contact a t the "back" of the membrane or a t an interstitial site. The observed dendritic growth of Ag2S, the Type 2 deposit, implies that the Ag,S deposit has grown preferentially in one direction a t a specific site on the surface (13),in keeping with Equation 4. (ic)rrzjluence of Redox Reactions. Koebel et al. ( 1 4 ) have studied the conductivity of Ag2S membranes by incorporating them into two different cells Cell 1 Cell I1
AgiAg*aglAg,SIAg'aq/Ag Ag/Ag,S/Ag+,,/Ag
(7) (8)
T h e ionic conductivity of the $-Ag2S membrane a t room temperature was the same in both cells, tii = 0.36 x R-' cm-', but the electronic conductivity increased greatly from 9-' cm-' in cell I to t i , = 1.163 a value of K , = 0.0055 X X (I-' cm-' in cell 11. T h e much higher electronic conductivity of cell I1 where metallic silver contacts the membrane was attributed to "the excess of silver which acts as an electron donor" (14). These workers (14) also studied the kinetics of the electron exchange reaction a t the Ag,S/solution interface in the following cells: Cell I11 Cell IV
Ag/Ag,S/Fe3'aq-Fe2+aq/Pt (9) Pt/Fe3+,q-FeZ+a,/Ag,S/Fe3'aq-Fe2+,q/Pt( 1 0 )
The polarization resistance was measured to be -2000 R cm2 for cell 111 and -0.5 x 106 R cm2 for cell IV (membrane thickness was 0.041 cm). The authors concluded that "if the Ag2S is contacted with Ag, the electronic conductivity is high but the results obtained with the system AgzS/Feaq3+- Feaq2+ suggest that even in this case the redox reaction has little or no effect on t h e measured potential because of the high resistance of the electron exchange reaction a t the interface" (14).
If t,he Type 3 deposit observed in the S E M analysis is primarily metallic silver, then the membrane/solution interface is considerably different from that studied by Koebel et al. (14). The Type 3 deposit may not be a "deposit" as such, as it appears from the scratches on it to be part of the electrode surface that has not been eroded or dissolved. Since the S E M examination showed a high surface concentration of such deposits, the interface in this case is primarily Ag-Ag,S/ solution. The potential of a metallic silver electrode responds to redox couples in solution (the heterogeneous rate constant for the F e ( o ~ a l a t e ) ~ - / F e ( o x a l a t couple e ) ~ ~ - in 0.5 mol L-' potassium oxalate solution is 9 x cm s-l a t Pt and 1 x cm SS' a t Ag (Is)),and thus it is postulated that t,he potential measured a t the Ag-Ag,S/solution interface can be affected by the presence of redox couples in solution. T h e influence of the above factors on the measured potential in a given solution will vary from one electrode to another depending on how the electrode was manufactured, used, etc. However. in our opinion, the appearance of metallic
silver at the membrane surface is the most important factor that gives rise to mixed potentials. This accumulation of metallic silver also explains the change in response of a given electrode from Nernstian to super-Nernstian, the difference in response between electrodes and the gradual appearance of surface films on some electrodes. T h e mixed potential observed is primarily determined by the equilibrium potential of the electrode reaction having the highest exchange current (16). Thus the contribution of the various reactions involving metallic silver to the measured potential is governed by the magnitude of their exchange currents relative to the exchange current for a given concentration of silver ions (or sulfide ions as postulated by Camrnan and Rechnitz ( 1 6 ) )a t the Ag,S/solution interface. T h e exchange currents for the corrosion and redox reactions involving metallic silver depend not only on the magnitude of the exchange current density, but also on the surface area of metallic silver at, the electrode/solution boundary. T h e latter increases gradually with time and use, with a resulting increase in the exchange currents for the corrosion and redox reactions up to the point that the measured potential becomes a mixed potential and super-Nernstian response is observed. Metallic silver gradually accumulates at the electrode surface as a consequence of the following. (i) The finite current drawn by the measuring instrument
(4). (ii) The rapid diffusion of silver metal through the membrane from the back surface, due to its large diffusion coefficient ( 4 ) . Growth of single crystals of silver metal has been observed on the surface of sulfide ion-selective electrodes used in a continuous monitor at -.%O K (17). Consequently, silver metal may gradually accumulate a t the electrode surface even while the electrode is sitting on a shelf, unused. Heating of the membrane during electrode polishing, both in the manufacturing process and during use in the laboratory, may also contribute to an increased rate of diffusion of silver. (iii) Redox reactions between the solution and the membrane surface ( 4 ) . Ascorbic acid which is added to the solutions to reduce dissolved oxygen is a sufficiently strong reducing agent to reduce silver ions (18). The difference in the limit of Sernstian response for various electrodes is primarily due to the differing amounts of metallic silver on the electrode surface. This amount is governed by the history of the electrode-the length of time since its manufacture, its length of use, and the conditions of use (temperature, exposure to reducing agents, extent of polishing. etc.). Differences in porosity of the membrane may also contribute to the difference in response. T h e films on the electrode surfaces appear to be Ag20 or Ag2S,and could occur as a result of the corrosion of metallic silver, Equat,ions 4 and 5 . Super-Nernstian response is always obtained on electrodes that grow films rapidly (such as electrode C or A), but electrodes that remain shiny and bright can also give super-Kernstian response (electrode H). T h e different behavior of these electrodes is attributed to different amounts of metallic silver on the surfaces: electrode H presumably contains sufficient metallic silver to give rise to mixed potentials but not to grow films that are sufficiently dense to be visible. Muller et al. (19) have observed that an Orion 94-16 sulfide electrode tarnished on exposure to solutions with a chloride concentration greater than 3 X mol L presumably due to the corrosion of metallic silver on the membrane surface
',
Ag
-L
C1-
AgCl
+
e
(1.1)
T h e difference in the film thickness observed for various electrodes may account for the different, S E M results for electrodes C and A. T h e film on electrode C formed over a 2-h period while immersed in a 1 mol L ' NaOH solution, while
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
electrode A was immersed for 25 h in a mol L-l sulfide solution in 1 mol L-' N a O H 4 . 1 mol L-' ascorbic acid solution. Consequently, electrode A is covered with a thick film of Ag2S and t h e Type 2 and Type 3 deposits of electrode C are not observed. T h e apparent decomposition or "melting" of the deposit on the electrode A under high magnification is puzzling, but may be related to similar observations of Bresgen (20). Bresgen concluded t h a t the process occurring was the rapid growth (under t h e influence of the electron beam) of Ag2S crystals from a mechanically deformed Ag2S film, with the deformed spots serving as starting points for crystallization on t h e surface (20). T h e behavior and characteristics of sulfide ion-selective electrodes, both in solution and under a SEM, are consistent with the proposal t h a t metallic silver accumulates a t the membrane/solution interface and gives rise to mixed potentials. Evidence for the presence of metallic silver a t this interface is, at present, circumstantial but further experiments are continuing (12). If the presence of metallic silver a t the interface is verified, the problems of mixed potentials could be overcome by eliminating the source of the metallic silver, Le., by replacing the internal solid silver contact with an internal liquid contact (silver wire immersed in a solution of fixed silver ion concentration).
787
cussions on the interpretation of these results.
LITERATURE CITED (1) D. J. Crombie. G. J. Moodv. and J. D. R. Thomas. Anal. Chim. Acta. 80, 1 (1975). (2) J. (1976) Gulens and B. Labbate, A . Energy Can. Lid, AECL Rep , AECL-5542 (3) >:Guiens, K. Jessome, and C. K. MacNeill, Anal. Chim. Acta, 96, 23 (1978). (4) M. Koebei, Anal. Chem., 46, 1559 (1974). (5) Orion Research Incorporated Instruction Manual, Sulfide Electrode 94-16 (1974). (6) W. E. Morf, G. Kahr, and W. Simon, Anal. Chem., 46, 1538 (1974). (7) R. P. Buck, Anal. Chem, 48, 26R (1976). (8) E. W. Baumann, Anal. Chem., 46, 1345 (1974). (9) J. W. Ross in "Ion-Selective Electrodes", R. A. Durst, Ed., Nafl. Bur. Stand. ( U . S . ) , Spec. Pub/., 314, 82 (1969). (IO) M. H. Sorrentino and G. A. Rechnitz, Anal. Chem., 46, 943 (1974). (1 1) T. Dickinson, A. F. Povey, and P. M. S. Sherwood, J . Solid State Chem., 13, 237 (1975). (12) J. Gulens, D. Shoesmith, and P. Taylor, unpublished results. (13) J. A. Harrison and H. R. Thirsk in "Electroanalytical Chemistry", Vol. 5, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1971, p 89. (14) M. Koebei, N. Ibl, and A. M. Frei, Electrochim. Acta, 19, 287 (1974). (15) B. E. Conway, "Electrochemical Data", Greenwood Press, Westport, Conn., 1969, p 356. (16) K. Camman and G. A. Rechnitz, Anal. Chem., 48, 856 (1976). (17) J. W. Ross, Orion Research Inc., personal communication, 1977. (18) R. Beicher and C. L. Wilson in "New Methods of Analytical Chemistry", 2nd ed., Reinhold, New York, N.Y., 1964, p 97. (19) D. C. Mulier, P. W. West, and R. H. Muller, Anal. Chem., 41, 2038 (1969). (20) H. Bresgen. Werkst. Korros.. 27, 313 (1976).
ACKNOWLEDGMENT
RECEIVED for review November 14,1977. Accepted February
We acknowledge with pleasure the assistance of J. Baird for the S E M analyses. J.G. thanks J. Ross of Orion Research for his important contributions through stimulating dis-
10, 1978. This paper was presented in part a t t h e 2nd Joint Conference CIC/ACS, Montreal, Quebec, May 29-June 2, 1977.
Quantitative Determination of Cis:Trans Isomeric Ratios in Substituted Thiazolidines by Carbon- 13 Magnetic Resonance Spectrometry Joseph J. Pesek Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60 1 15
Quantitative determinations of the cis and trans isomers of alkyl substituted thiazolidine-4-carboxylic acids by carbon-13 magnetic resonance spectrometry are described. An average of all carbon peak intensities which show separate resonances for the cis and trans isomers (exclusive of carbonyl resonances) gives good agreement with proton spectra when a long repetition rate is used. Integrated peak areas give accurate results where the peaks of the two isomers are well-separated. Measurements of relaxation times indicate that differences exist for the two isomers and this appears to be the main source of error in quantitative determinations. Equalization of the relaxation times for the cis and trans isomers and suppression of the Nuclear Overhauser Enhancement (NOE) by addition of a paramagnetic relaxation agent give results for the compounds studied which are within 0.1% of those obtained from the continuous wave proton spectrum with a standard deviation between 0.3 and 0.7 %.
Carbon-13 magnetic resonance has proved to be a valuable tool for stereochemical structure elucidation. There have been extensive studies involving carbon-13 shieldings which are y 0003-2700/78/0350-0787$01 OO/O
to either a carbon or a heteroatom (1-23). It has been shown that a carbon-13 in the gauche conformation is shifted upfield from one in the anti conformation because of steric perturbations t h a t polarize the C-H bonds in the y position. Recently it has been reported that 13C resonances are shifted downfield when the nucleus is in the 6 position involving crowded syn-axial interactions (14-16). Because of these results, structural assignments can now be made with relative confidence for other systems involving conformers. Proton magnetic resonance has been used extensively in stereochemical structure elucidation ( I 7). Chemical shifts have been identified for various conformers in many different chemical systems. In addition to this qualitative information, proton magnetic resonance has been extremely useful in quantitative determinations in which more than one conformer is present. Similar determinations would be extremely useful if they could be done by 13C N M R because they could be applied to complex molecules which would be difficult or impossible t o analyze by proton NMR. T h e advantages in I3C NMR lie in the greater chemical shift range and the relative simplicity of the proton decoupled spectrum, Le., a single peak for each type of carbon atom. Although chemical shieldings for carbon-13 nuclei now seem to be well-established, there have been few investigations into C 1978 American Chemical Society
