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Summary of Potentiometry: pH and Ion Selective Electrodes Potentiometric Sensors I = controlled at 0 Amps E eq is measured In general E eq = const – 0.0592.

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Presentation on theme: "Summary of Potentiometry: pH and Ion Selective Electrodes Potentiometric Sensors I = controlled at 0 Amps E eq is measured In general E eq = const – 0.0592."— Presentation transcript:

1 Summary of Potentiometry: pH and Ion Selective Electrodes Potentiometric Sensors I = controlled at 0 Amps E eq is measured In general E eq = const – log a X

2 Electrode Potentials (review) Electrochemical cell – two half cells Convention, write both as reductions 2 AgCl (s) + 2 e - = 2 Ag (s) + 2 Cl - (cathode) - [2 H e - = H 2 ( gas )] (Pt, NHE reference, anode) E cell = E cathode - E anode Cell reaction is the sum of the two above 2 AgCl (s) + H 2 = 2 Ag (s) + 2 Cl H + E cell = E Ag/AgCl – E H+/H2 by convention E H+/H2 = 0 E cell = E Ag/AgCl = 0.46 V  G = - n F E cell ;  G = positive, non-spontaneous, electrolytic cell

3 Measurements in Potentiometry; I = 0 Amps; equilibrium Cell: working electrode + reference electrode (E half cell = const) Working or indicator E cell = E WE – E ref - E junction

4 Variable resistor Simple potentiometric measuring circuit galvanometer Move slidewire (arrow) until G shows I = 0, then V = E cell = E eq In practice this is all automatic in modern potentiometers or pH meters V V voltmeter

5 Reference electrodes: critical to both potentiometry and voltammetry They keep a nearly constant half cell potential during experiment Normal Hydrogen, Pt| H 2 (1 atm), HCL (0.01 M), NHE THE STANDARD, E = 0 V, but not practical Standard Calomel Electrode Hg| Hg 2 Cl 2 (s), KCl (sat’d.) SCE Set up as self-contained Half cell Contact to test solution

6 Half cell potential of the SCE – serves as a reference against which other E’s are measured Hg 2 Cl 2 (s) + 2 e - = 2 Hg (l) + 2 Cl - Use Nernst equation: E = E o - [RT/nF] ln (a Cl 2 a Hg 2 /a calomel ) ; but a of pure solids =1 only a Cl remains in the log term, and E = E o’ - [0.0592/2] log [Cl - ] 2 or E = E o’ log [Cl - ] ; sat’d KCl is ~3.5 M at 25 o C So E SCE = V vs. NHE at 25 o C Alternative reference: Ag|AgCl (s), KCl (sat’d.) E Ag/AgCl = V vs. NHE at 25 o C

7 Half cell potential of the SCE – serves as a reference against which other E’s are measured Hg 2 Cl 2 (s) + 2 e - = 2 Hg (l) + 2 Cl - Use Nernst equation: E = E o - [RT/nF] ln (a Cl 2 a Hg 2 /a calomel ) ; but a of pure solids =1 only a Cl remains in the log term, and E = E o’ - [0.0592/2] log [Cl - ] 2 or E = E o’ log [Cl - ] ; sat’d KCl is ~3.5 M at 25 o C So E SCE = V vs. NHE Alternative reference: Ag|AgCl (s), KCl (sat’d.) E Ag/AgCl = V vs. NHE

8 Ion Selective Electrodes (ISE) - sensor surface usually a membrane That adsorbs the ions, E eq measured at I = 0 Amps In pH electrode, the membrane is a very thin glass layer Stand alone glass pH electrode must be used with reference Glass pH electrode combined with internal reference electrode 0.1 M HCl Glass membranes are made of SiO 2, Li 2 O (or Na 2 O) and BaO (or CaO)

9 ISE’s obey Nernst-like equations (25 o C) E = const + [0.0592/z] log a ion ISE measure activity, not conc. if the ion Is H +, E = const pH; pH = -log a H+ Fast response is important, < 1 s in buffer for most pH electrodes E, mV Log a ion Nernstian region, slope = /z ISE Most pH meters read pH directly, But must be calibrated daily

10 How a glass pH electrode responds to H + ions Ag/AgCl 0.1 N HCl a H+ = const E2E2 E1E1 Inner hydrated layer outer hydrated layer Dry glass 0.1  m50  m Test solution a H+, soln Li + in glass can exchange with H + (both small ions), giving rise to E 1 and E 2 H+ DOES NOT cross the membrane (must store in in water or buffer to maintain hydrated layer) E M = E 1 – E 2 + E int ref ; or E B = E 1 – E 2 (boundary E) E cell = const + E B a2a2 a1a1

11 E B is related to a 1 and a 2, but a 2 is constant (0.1 M HCl) These ion activities control the membrane potentials, E 1 and E 2 and so control E B, at 25 o C E B = E 1 – E 2 = log (a 1 /a 2 ) E cell = const log (a 1 ) E cell = const pH In practice, pH meter incorporates these equations And relates them to measurements with standard buffer, And the output is a direct measurement of pH: pH = pH std + F (E cell - E std )/2.303 RT, R = gas const, T = abs. temperature

12 Errors and Interferences in pH electrode measurements Alkaline error: In basic solutions or high salt conc. NaCl, KCl, Na + and K + interfere by adsorbing to the glass membrane then pH obs < true pH e.g. 0.1 M NaOH, pH 13, [Na + ] is 0.1 M, [H + ]= 1 x Large error due to high [Na + ], low [H + ] In general fpr ISEs, Nicolsky equation E cell = const log [a 1 + Σ K j a j ], j = 1….n interfering ions K j = selectivity coefficient Acid error pH < 1, origin unknown pH electrodes reliable between pH 1 and 13 only

13 ISE’s for ions other than H+ glass membranes, Na +, K +, NH different composition than pH solid state membranes, F -, S - liquid membranes, Ca + gas sensitive electrodes CO 2, H 2 S, NH 3 enzyme electrodes – biological molecules can all be used with pH meter in mV-mode

14 FLUORIDE ISE Fluoride ISE – Solid State Detection limit M

15 Impregnated with ion exchanger; Ca ++ ISE, Ca(dodecylphosphate) + Polymer like PVC; Phosphate groups bind Ca ++ Crystals or solid state have the analyte Ion present, e.g. LaF 3 Detection limit ~10 -8 to M

16 Urea Enzyme ISE (NH) 2 CO + 2 H 2 O + H + Urease enzyme 2 NH 4+ + HCO 3 - Detection limit ~10 -6 M


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