Anal. Chem. 1981, 53, 1857-1861
Flgure 3.
A
closest-packed model for UPD of Hg(I1) at a Au electrode.
for those values of potential. The average value of nsppis 1.60 f 0.02 equiv mol-l in 1.0 M for 0.75 I Edep I 0.45 V vs. SCE. This value is concluded to be in good agreement with the results of Sherwood and Bruckenstein (12). The values of nappfor 2.0 M "03 at Edep < 0.60 v are slightly larger than for 1.0 M H$0+ This resdted from dissolved oxides of nitrogen in the HN03. The values nappfor the HN03 solutions duplicated exactly the values for HzS04when N2 was dispersed through the supporting electrolyte and the stock solution of Hg(I1) in HN03 prior to the inject ions.
COMCLUSIONS The value of nRPP calculated from I-t curves for the UPD process in the coulometric Au electrode R i 1.60. Hence, the average valence of the Hg deposited by UPD is Calculated to be +0.40. We are hesitant to propose a mechanism for this deposition process. However, it may be significant that the average charge for mercury calculated for the closest-packed array in Figure 3 is 0.33 even though only integral valences are considered for individual atoms. According to this model, Hg is deposited into clusters with closest packing even when the total surface coverage of the Au electrode is far lese than the equivalent of one monolayer. Each Hg(1) is bound to six atoms of Hg(0) and each Hg(0) is bound to three ions of Hg(1).
185;7
It is not possible with coulometric electrodes to evaluate nappfor UPD of Hg(I1) in an amount approaching a full monolayer. This results because the decrease of available surface area for the electrode would cause the efficiency to decrease below a value of 100% for the UPD process and eq 1 would not be applicable. Identical values of area for the stripping peaks were obtained with the tubular Au electrode for deposition of a quantity of Hg(I1) much less than the equivalent of one monolayer for Edep in the UPD region as well as at values more negative than the Nernstian potential for the Hg(II)/bulk Hg(0) couple. Hence, it is concluded that the initial process leading to the eventual formation of bulk Hg(0) occurs witlh the same value of naplp as for Hg(I1) deposited in the underpotential region of the potential axis.
LITERATURE CITED (1) FuJlnaga,T.; Klhara, S. CRC Crit. Rev. Anal. Chem. 1977, 223. (2) Lorenz, W. J.; Hermann, H. D.; Wuthrlch, N.; Hilbert, F. J . Electrochem. SOC. 1974, 121, 1187. (3) Sherwocd, W. G.; Bruckenstein, S. J . Electrochem. SOC. 1978, 125, 1098. (4) Sherwocd, W. 0.Ph.D. Thesis, State Unlversity of New York, Buffalo, 1977; University Mlcroflims, Ann Arbor, MI. (5) Andrews, R. W.; Larscheile, J. H.; Johnson, D. C. Anal. Chem. 1976, 48, 212. (6) Sioda, R. E. Nectrochim. Acta 1970, 75, 783. (7) Kihara, S.; Yamamoto, T.; Motojima, K. Tahnfa 1972, 19, 329, 657. (8) Johnson, D. C.;Larochelie, J. H. Talanta 1973, 20, 959. (9) Takata, Y.; Muto, G. Anal. Chem. 1973, 45, 1864. (IO) Lankelma, J.; Poppe, H. J. Chromatogr. 1978, 125, 375. (11) Kissinger, P. T. Anal Chem. 1977, 49, 4. (12) Ruckl, R. J. Tahnfa U 8 0 , 27, 147. (13) Klhara, S. J. Electrmnal. Chem. 1973, 45, 31, 45. (14) Snider, B. 0.; Johnson, D. C. Anal. Chim. Acta 1979, 105, 25. (15) Lindstrom, T. R. Ph.13. Thesis, Iowa State Universlty, Ames, 1980; University Mlcrofllms, Ann Arbor, MI. (16) Schwartz, A. "Calculus and Anawical Oeometry", 2nd ed.;Holt, Rinehart and Winston: New York, 1967; pp 372-373.
RECEIVED for review May 1, 1981. Accepted July 24, 1981.
Selectivity Characteristics of Ammonia-Gas Sensors Based on a Polymer Membrane Electrode Yvonne M. Fraticelli an(d M. E. Meyerhoff * Department of Chemistty, University of Michigan, Ann Arbor, Mlchigan 48 109
Fundamental studies concernlng the response properties of newly devised polymer membrane electrode based amnnonla gas senslng systems toward various voiatlle amines are presented. Two prlnclpal Interference mechanisms are postulated; one Involves the lnlherent potentiometric response of the Internal nonactln-polymer membrane to protonated forms of the amlnes whlle the other involves the effect diffuslng amlnes have on the pH of Ihe Internal electrolyte buffer. The latter mechanlsm Is shown to predomlnate In the static ammonla gas sensor deslgn. In contlnuous flow arrangements, response and Interferences from volatlle amines are negligible due to the nonequlllbrlum nature of the detection process. For both gas-senslng modes, apparent selectlvlties observed are shown to be far superlor to those obtained wlth conventional pH electrode based ammonla sensors.
Potentiometirc gas sengors, based on internal glass, pH sensitive membranes, have found wide use in numerous 0003-2700/81/0353-1857$01.25/0
analytical situations due to their high selectivity for the analyte gas over common ionic species (1-5). The ammonia-selective sensor, in particular, has proven to be quite valuable in clinical, beverage, and natural water analyses, and to some extent in industrial, agricultural, and waste water methods (1,5-9). The main limitation of this sensor in the latter areas arises from the large response these probes exhibit toward certain volatile amines which may be present in such samples (10, 11). In practice this necessitates the measurement of total ammonia nitrogen (e.g., Kjeldahl method) rather than simply free inorganic ammonia. We recently introduced a new type of potentiometric ammonia-selectivesensor (12,131and further developed an automated system based on the same electrochemical detection concept (14). We now report the selectivity characteristics of these new sensors with respect to various volatile amines. These new ammonia-sensing systems (both static and automated) are based on the electrochemical detection of ammonium ions formed, from diffusing ammonia gas, within a thin film or flowing stream of internal electrolyte buffer. The 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table I. Volatile Amines Studied compd compd no. cyclohexylamine 1 ethylamine 2 morpholine 3 hydrazine 4 triethylamine 5 methylamine 6 phenylhydrazine 7 8 decylamine 9
10
N,N-dimethylcyclohexylamine
monoethanolamine
10.66 10.81 8.33 8.23 11.01 10.66 8.73 10.64 10.73 9.50
lower pH of the internal buffer relative to the sample solution (separated by a gas-permeable membrane) servgs to trap and convert ammonia gas to ammonium ions which are then sensed by a nonactin-poly(viny1 chloride) (PVC) membrane electrode (12-1 4).
In this study we examine the response properties of the static and automated ammonia-sensing arrangements to a variety of volatile amines known to interfere with the conventional pH-based gas sensors (5,10,11,15). Table I lists the compounds investigated along with their associated equilibrium constants. By studying the two basic mechanisms by which such compounds can interfere, one can choose conditions to minimize potentiometric response toward these substances. It will be shown that, unlike conventional ammonia-gas sensors, these new polymer membrane electrode based systems display remarkably good selectivity for ammonia over the volatile bases. EXPERIMENTAL SECTION Apparatus. Potentiometricand pH measurementswere made with either a Corning Model 12, Fisher Accumet Model 620, or an Altex SelecbIon 2000 pH/mV meter. An Orion 95-10 ammonia selective sensor body was used for thin film pH measurements while an HNU Model 10-10-00ammonia electrode was used to test the conventional ammonia gas sensor's response to the volatile amines. A static polymer membrane electrode-based ammonia gas sensor (tip diameter, 5 mm) was constructed as previously described (16). An automated sensing arrangement consisting of a gas dialysis chamber, a flowing internal electrolyte technique, and a tubular nonactin-PVC membrane electrode was also assembled as described earlier (14),except that a Rainin-Rabbit peristaltic pump was used instead of a Technicon Model AA-I. Reagents. All chemicals used were reagent grade. Standard solutions and buffers were prepared with distilleddeionizedwater. Ethylamine (70% in water), methylamine (40% in water), cyclohexylamine, and morpholine were obtained from Tridom Chemical Co. (Hauppauge,NY).Phenylhydrazine was a product of Matheson, Coleman and Bell (Norwood, OH), and dimethylcyclohexylamine was obtained from Eastman ChemicalProducts (Kingsport, TN). Monoethanolamine was a product of Fisher Scientific Co. (Fairlawn, NJ). For the automated ammonia-sensing system, tris(hydroxymethy1)aminomethanehydrogen chloride (Tris-HCl),pH 7.5,O.Ol mol/L was used as the internal electrolyte buffer and 0.0015 mol/L NaOH was used as the diluent reagent. For static ammonia sensor measurementsand subsequent thin film pH studies, the following buffers were prepared and evaluated as internal solutions: Tris-HC1, pH 7.5, 0.01 and 0.1 mol/L; Tris-HC1, pH 8.3, 0.01, 0.05, 0.1, and 0.2 mol/L; Tris-HC1, pH 9.0,0.05 mol/L. The external sample solution was always 1.5 X mol/L NaOH. Evaluating the Response of the Nonactin-PVC Membrane Electrode to Protonated Forms of the Amines. To determine the selectivity characteristics of the internal nonactin-PVC ammonium responsive membrane electrode to cationic forms of the volatile amines, we used a tubular nonactin-PVC membrane electrode (14,17).Standard solutions of NH4C1or the amines, prepared in Tris-HC1, pH 7.5, 0.1 mol/L, were continuously
pumped directly through the tubular electrode (with associated salt-bridge/referenceelectrode (14))and steady-state equilibrium potentials recorded. An ionic strength of 0.1 mol/L was used (diluting buffer) so that 0.01 mol/L concentrationsof the amines would not alter the pH of the buffer. Procedures Used To Study the Effect of Amines on the pH of Thin Films of Electrolyte Buffers. To evaluate the effect of diffusing amines and ammonia on the pH of the thin film of electrolytebuffer in the static ammonia sensor,we utilized an Orion ammonia-gassensor and replaced the manufacturer's internal solution with a variety of buffers. In addition, we substituted a polytetrafluoroethylene,0.2 pm pore size, gas-permeable membrane (12) for the one supplied by Orion. The pH of the buffers was initially determined with a separate pH electrode and meter and then placed into the outer body of the Orion probe. The internal pH sensing element was then partially inserted (enough so that the buffer solution covered the reference electrode) and the pH meter calibrated to the known pH value of that buffer. The inner pH electrode was then completelytightened into place forming a thin film of buffer between the gas-permeable membrane and the pH glass membrane. The electrode was inserted into a 1.5 X mol/L NaOH solution and standard additions of NH4Clor methylamine (Me-NH2)were made. Steady-state pH values within the thin film were recorded after each addition. Evaluating the Selectivity of Automated and Static Gas-Sensing Systems. By use of the automated system previously described (14), aqueous standards of NH&l and each volatile amine (10-6-10-3 mol/L) were prepared, placed in the sampling cups, and subsequently aspirated into the autoanalyzer using the following conditions: sample to wash ratio, 1:2, sampling rate, 30/h, dilution ratio, 1:3.9. The diluent used was 1.5 X 10" mol/L NaOH and the flowing internal buffer was Tris-HC1,pH 7.5,O.Ol mol/L. Peak heights obtained for each sample were plotted vs. log NH3 or amine concentration. The response properties of the static polymer membrane electrode-based ammonia sensor using various internal buffers were evaluated by observing the potentiometric response upon additions of NH4Clor amines into a well-stirred 1.5 X mol/L NaOH solution. Steady-stateequilibrium potential were recorded and plotted vs. log NH3 or amine concentration.
(a
RESULTS AND DISCUSSION To fully understand the basic mechanisms by which volatile amines could potentially interfere with these new gas sensors, it is important to document the theoretical nature of the response of our new ammonia gas sensor. For the conventional static gas-sensing arrangement, an ammonium-sensitive nonactin-PVC membrane electrode is used as an internal indicator electrode. A thin film of internal electrolyte buffer exists between the surface of the polymer electrode and a Teflon gas-permeable membrane. The polymer electrode responds to ammonium ions formed within the thin film according to the Nernst equation
2.3RT E = E ' + - F log ["+,I. where E is the cell potential and [ 3h refers the concentration of a particular species within the thin buffer film. (Concentration may be used in place of activity if we assume that the ionic strength remains constant.) When a steady-state potential is observed upon addition of ammonia to the sample solution, the equilibrium expression
must be satisified, where K is the pK, of the ammonia-ammonium equilibrium. Solving for [NH4+]i,, and substituting into eq 1 we have
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12,
OCTOBER 1981
1859
II
!
125-
w
ui a 75-
4
A
r'
-loglNH4+ or RNH3+l,mol/L. I
Potentlometric response of nonactin-PVC melmbrane electrode to ammonium and protonated amines. Numbers refer to amines listed in Table I. Figure 1.
If the pH of .the buf€er film remains constant, K[H+]in remains constant and th.us (4) The concentration of dissolved ammonia in the thin film is related to the partial pressure of ammonia present via Henry's law (5) where Kh is Henry's constant and P[NH3]inis the partial pressure of ammonia. Uinder measurement conditions with a sample solution containing dissolved ammonia, [NHJ-ple, equilibrium across the gas-permeable membrane is achieved when the partial pressure of ammonia is equal on both sides of the membrane and eq 2 is satisfied. Therefore .p["31in
= P["3lsample
(6;)
and PPWlsample
= Kh["31mple
(7)
Rearranging and substituting into eq 4 we have
Whereas, eq 8 predicts Nernstian response, in practice subNernstian slopes of between 45 and 55 mV/decade have been typical for such electrodes (12, 13). Assuming constant pH of the internal buffer film (ideal case), any positive interference observed with this new sensor must be a function of the inherent selectivity of the internal ammonium responsive membrane electrode. With regurd to specific volatile amines, eq 1 must be rewritten in the form of the Nicolsky equation
where R-NH3+ represents the protonated form of the organic amine and k is the selectivity constant with respect t u that cation (18). When volatile amines tire present in a sample, diffusion across the gas-permeable membrane into the internal buffer solution of lower pH results in the following reaction: R-NH2
+ HZO * R-NHs+ + OH-
4
I
3
-logINH30rAMINEI,mol/L
Figure 2. Typlcai calibration curves of automated ammonia gas sensing system for amrnonla and volatile amines.
In a controlled pH einviroment within the thin film, the riasponse of the ammonium-selective polymer membrane electrode to R-NH3+ will be determined by the selectivity constant, k. The equilibrium amount of R-NH3+ formed is dependent on the pH of the internal buffer and the pK, value of the particular amine. We examined the selectivity properties of the polymer membrane electrode io the amines listed in Table I at pH 7.5 where most are present in predominantly protonated form. Figure 1 shows the steady-state potentiometric response of a tubular nonactin-PVC membrane electrode to NH4C1and the cationic amines. I t can be seen that apparent selectivities of 0.01 or less are observed for all the amines tested, indicating the high degree of selectivity the nonactin membrane has for NH4+ over protonated amines. Phenylhydrazine gave the greatest response; however, this response was partially due to ammonia impurities in this reagent. Following distillation, the response was similar to that observed for the other amine13. We next examined the response of an automated ammonia-gas sensing arrangement,assembled with a flowing internal electrolyte technique and gas dialysis chamber, to the volatile amines. Figure 2 shows the typical calibration curves obtaineld for pure aqueous standards of NH&l and the amines. The samples were diluted with 1.5 X lo9 mol/L NaOH within the system yielding a t least 50% of the total concentrations as unprotonated volatile amines (based on pKa values shown in Table I). It can be seen that in the automated gas-sensing mode, response to the individual amines was equal to or less than that observed for the protonated amines that werle pumped directly through the polymer membrane electrode. This is because, unlike the static sensor's response which is described by the above equations, the automated system is based on a nonequilibrium process. That is, the sample spends a limited fixed time in the dialysis chamber and the recipient internal buffer stream (Tris-HC1,pH 7.5,O.Ol mol/L) is of equal volume as the sample stream (no thin film effect). Thus, the full buffer trap effect predicted by equilibrium theory cannot be obtained in the brief period of time the sample spends in the dialyzer (e.g., eq 2 and 6 are not satisfied). Consequently, better selectivities than expected are observed because the volatile amines diffuse through the gas-permeable membrane at a slower rate than ammonia. Experimentally, the automated arrangement yielded apparent selectivity constants of lz 5 0.OO:l. In the typical static gas-sensing arrangement, because measurements are based on equilibrium conditions (eq 2 andl 6 satisfied), a different mechanism of potential amine interferences became apparent. Using our previous internal buffer
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table 11. The Effect of Methylamine on Ammonia Determinations in Mixed Solutions automated probe static probe [NH, 1, mol/L [NH, I, mol/L re1 error, [Me-",], re1 error, [Me-", I, mol/L taken foundaSb % mol/L taken foundC % 1.00 x 10-5 LOO x 10-5 9.91 x 10-6 -0.9 1.50 X 1.50X lo-' 1.33 X lo-' -11.3 1.00 x 10-4 1.00x 10-5 1.04 x 10-5 +4.0 1.50 X lo-' 1.50 X 1.72 X lo-' t14.6 1.00 x 10-3 1.00 x 10-5 1.10 x 10+io.o 5.0 x 10-5 1.50 x 10-5 1.47x 10-5 -2.0 1.00 x 10-2 1.00 x 10-5 1.55 x 10-5 t55.0 1.50 x 10-4 1.50 x 1.39 x 10-5 -7.3 a Average of two determinations. Values obtained from least-squares fit of aqueous calibration curve data between 5X and 4 X mol/L NH,Cl. Average of three determinations.
r-7-
-50
9.5
'5 -130 7.5-
.210
, 6
5
4
-log I NH3 I,mol/L I
6
I
5 +glNH3
I
4 or MeNn21, mol/L
3
5
5
4
3
-loglMeNHpi, mol/L
I
3
Figure 3. Potentiometric response of statlc polymer membrane electrode-based ammonia sensor to ammonia (0)and methylamine (A)using Tris-HCI, pH 7.5, 0.01 mol/L as internal electrolyte buffer.
solution (12),Tris-HC1, pH 7.5,O.Ol mol/L, we determined the sensor response to both Me-NH2 and NH&l in a sample mol/L NaOH. Figure 3 shows the results solution of 1.5 X of that experiment. It can be seen that a negative response to Me-NH2 was obtained. Steady-state response times upon addition of M e N H 2were usually 8-12 min while for NH4C1,
