2048
Anal. Chem. 1981. 5 3 , 2048-2053
ening. The broadened solution spectra of the naphthalene compounds thus retain the feature that the symmetry is not uniform through the band and the point of highest symmetry is skewed in the direction of the more distant R branch. The major significance of the experimentally observed inhomogeneity is that the rotational branches are showp to contribute to the solution phase spectra. This is in contrast with two-photon theory of liquids (2). Such a result is not surprising in view of the fact that it is well accepted that rotations contribute to Raman spectrometric line shapes in liquids (12,131. The theory of ref 2, which considers molecules to tumble rather than to rotate, remains valuable for relating two-photon polarization dependences to molecular structure and is a reasonably good approximation for making symmetry assignments. However, in examining quantitative liquid-phase spectra in detail, one must use the vapor-phase theory such as that of McClain and Harris (4) or Boesl et al. (11)in order to interpret the data.
symmetry of a band in the absence of molecular rotations. In the presence of rotations one must average over the rotations in order to attach this physical significance to the value of 6F/6@ In solution, this averaging is performed to some extent by line broadening. Since the spectra show rotational effects, the reported 6F/6G values should not be exactly equal to the actual values; however, the reported values are nonetheless useful for molecular identification.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
CONCLUSIONS In use of excited-state symmetry parameters for molecular identification, noncoincidence of the 6F/6G maximum relative to that of the raw data illustrates that polarization data should be obtained over the entire spectral band of interest. The physical significance of 6 ~ 1 is6 that ~ it is a measure of the total
(9) (10) (11) (12) (13)
Honig, B.; Jortner, J.; Szoke, A. J. Chem. Phys. 1967, 46, 2714. Monson, P. R.; McClain, W. M. J . Chem. Phys. 1971, 53, 29. McClaln, W. M. J. Chem. Phys. 1971, 55, 2789. McClain. W. M.; Harris, R. A. In "Excited States"; Llm, E. C., Ed.; Academic Press: New York, 1977; Vol. 3, pp 1-56. Wlrth, M. J.; Koskelo, A. C.; Sanders, M. J. Appl. Specfrosc. 1981, 35, 14. Wlrth, M. J.; Sanders, M. J.; Koskelo, A. C. Appl. Phys. Lett. 1981, 38, 295. Johnson, M. C.; Lytle, F. E. J. Appl. Phys. 1980, 51, 2445. Hochstrasser, R. M.; Sung, H. N.; Wessel, J. E. Chem. Fhys. Len. 1974, 24, 168. Hochstrasser, R. M.; Sung, H. N. J. Chem. Phys. 1977, 66, 3276. Goodman, L.; Rave, R. P. J . Chem. Phys. 1981, 74, 3658. Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. 1976, 15, 167. Gordon, R. G. J. Chem. Phys. 1965, 42, 3658. Nafie, L. A.; Peticolas, W. L. J . Chem. Phys. 1972, 57, 3145.
RECEIVED for review April 8, 1981. Accepted July 17, 1981.
Determination of the Surface Composition of Lead Electrode Membranes by Electron Spectroscopy for Chemical Analysis Sunetra N. Kar Chaudharl, F. C. Chang, and
K. L.
Department of Chemistry, University of Missouri-Kansas
Cheng"
City, Kansas City, Missouri 64 1 10
V. Y. Young Department of Chemlstty, Texas A& M University, College Station, Texas 77843
The Interfaces between an Ion selective electrode (ISE) membrane and the Internal reference solutlon and external analyte solution are important In the overall response of solld-state Ion-selectlve electrodes. Slnce the surface composltlon of a solid may differ from the bulk composltlon, we have used electron spectroscopy for chemlcal analysls (ESCA) In order to characterlre the surfaces of lead Ion 88lectlve electrode membranes. Both untreated and solutlonexposed membranes have been studled. From the latter, evldence for the hydratlon of the outermost monolayersof the membrane has been obtalned. Thls lmplles that Ion exchange processes are viable and may be an Important factor In the overall response of solld-state membrane electrodes. Flnally, the effectiveness of the use of EDTA and HCIO4as cleaning agents for the membrane has been correlated wlth changes In the membrane surface composition.
The response of ion selective electrode membranes to interfering cations can not always be predicted on the basis of bulk stoichiometry. For example, lead ion selective electrodes (made of 1:l molar PbS:Ag2S)respond to Cu(I1) and Zn(I1) but not to Ni(I1) and Co(I1) (1,Z). From a solubility stand0003-2700/81/0353-2048$01.25/0
point, the response to Zn(I1) is not expected. Besides response, stability of the potential reading is also important in determining the analytical usefulness of ion-selective electrodes. It has been found that one of the factors affecting the stability of Pb ISEs is the process of cleaning the electrode surface (3); EDTA, HC104, and physical rubbing with Ajax powder have been utilized (4). However, the changes in the surface composition induced by these various cleaning processes have not been investigated. We believe that both the surface composition and its time stability are important factors which determine in part the magnitude and stability of the electrode response. In the present study, we have utilized ESCA (electron spectroscopy for chemical analysis) to investigate the surface composition of untreated Pb ISE membranes and Pb ISE membranes treated with EDTA and HCIO1. The composition of P b ISE membranes has previously been studied by non-surface-sensitivetechniques. Heijne, Linden, and Den Boef (5)studied a 30:70 mol % PbSAga system both with electrochemical techniques and with X-ray diffraction. They report that pressure sintering at elevated temperatures (750MPa and 150 "C for 7 h) is necessary to given an electrode of satisfactory response. Unheated cold-pressedpellets show hardly any response when employed as electrodes. Their study leaves several unanswered questions. The 3070 powder shows 0 1981 American Chemical Soclety
ANALYTICAL CHEMISTRY, VQL. 53, NO. 13, NOVEMBER 1981
Table I. Binding Energy Values and Fwhm of Different h v e l s of Pb, Ag, C, 0, N, and S fwhm, eV binding energy, eV _. 1 2 3 1 2 3 Pb 4f1,, (S2-) 138.1 138.4 137.9 0.90 1.0 1.0 Pb 4f1,, (SO,"-) 139.3 139.4 139.0 1.1 1.0 1.0 Pb4f,,, (Sz-) 143.0 143.3 142.8 0.90 1.0 1.0 144.3 144.5 143.9 1.1 1.0 1.0 Pb 4f,,, (SO:-) 1.0 1.0 Ag 3d5,, 368.7 368.9 368.5 1.0 374.8 375.0 374.6 1.0 1.0 1.0 Ag 3d,,!& *c 1s 285.0 285.0 285.0 1.7 1.5 1.5 1.6 1.5 c lsb xx 286,9 286.4 xx c 1SC xx 288.8 287.9 xx 1.6 1.5 0 1sa 532.3 532.2 531.9 1.3 1.5 1.5 1.5 1.5 0 lsb xx 533.6 433.4 xx xx 1.8 N 1s xx 400.5 xx xx xx 0.80 xx xx S 2p,,, (B-) 161.3 xx xx 1.0 xx xx S 2p3,, (SO:-) 168.8 xx xx 0.8 xx xx S 2p1,, (El2-) 162.4 xx xx 1.0 xx xx S 2pl,, (SO:-) 170.0 xx Lead ISE membrane a Fresh lead ISE membrane. dipped into Pb(I1) solution. Lead ISE membrane dipped first into Pb(I1) solution and then into ammoniacal *C 1s contaminant carbon peak. Reference solution. for all other binding energies.
2049
Table 11. Peak Intensity Values (countsls) for Different Levels of Pb, Ag, C, 0, N, S, and Ag intensity 1 2 3 3030 2530 Pb 4f1,, (P-) 4240 790 1470 1140 Pb4fl,, (SO:-) 2260 1910 Pb 4f,,, (S*-) 3180 760 590 Pb4f,,, (SO:-) 1080 1970 950 4450 Ag 3d,,, Ag 3d3,, 1370 640 3070 c 1sa 270 260 370 c lsb xx 130 190 c IsC xx 22 75 0 1sa 1030 480 340 0 lsb xx 330 310 N 1s xx xx 53 s 2PW2 ( S Z 3 170 s 2Pm (so,"-) 66 s 2P1/2 (sz-) 120 s 2P112 (so:-) 40 Fresh lead ISE membrane. Lead ISE membrane Lead ISE membrane dipped tnto Pb(I1) solution. dipped into Pb(I1) solution and then into EDTA.
Id
an X-ray tliffractogram consistent with the presence of acanthite (Ag2S)and galena (PbS), as do the sintered pelleh (albeit with reduced acanthite intensities). Cold-pressed pellets subNequently heated to 300 "C show galena peaks and even weaker acanthite peaks. These pellets are observed to be split parallel to their planes. The X-ray diffractograms of unheated cold-pressed pellets show a peak at 28 38" whose intensity dominates the diffractograms. This feature is not explained. We have employed electrodes prepared from 1:l molar FbS/Ag,S powders cold pressed at 104 MPa. These show good response with cleaning of the electrode surface, as mentioned above. According to the literature, P b ISEs have been prepared by a wide variety of seemingly contradictory methods, and yet all have been reported to give satisfactory responses. We feel that small differences in the relative amounts of AgzS and PbS cannot account for these observations. The need for both bulk and surface characterization of these membranes is evident.
-
EXPERIMENTAL SECTION The powder precursor of the lead ion selective electrode membranes was prepared by coprecipitation of PbS and AgzS (1:l mole ratio) from a solution containing AgN03and Pb(NOa)zby addition of a solution of NazS. The precipitate was filtered, washed, and dried at 110 OC overnight. Cast membranes were prepared by pressure sintering at 15000 psi (104 MPa) in a KBr die. All membranes weighted 0.3 g and had a diameter of 13mm. A 0.1 M Pb2+solutionwas made from Pb(C10d.3Hz0and all other reagents used (EDTA, ammonia, and HC103 were prepared from reagent grade chemicals. ESCA spectra were recorded for the following: the powder dusted on double sticky Scotch tape, the pressure sintered pellet, the pellet exposed to 0.1 M Pbz+solution followed by treatment with EDTA, the pellet treated with EDTA, and the pellet treated with HClOk A Hewlett-Packard 5950A ESCA spectrometer was employed; this instrument utilizes a monochromatized A1 K a source. The vacuum conditions were torr during analysis (the as follows: analyzer chamber, 2 X torr); X-ray chamber,
