Potentiometry (BF pp )

Transcription

1 (BF pp ) Measurement of a potential, E, that reflects the concentration of an analyte species in solution according to a Nernst-like equation. E = E 0 + (0.059/n)log[Ox]/[Red] Ion, internal solution E m = -(RT/z i F) log a iβ /a i α β Selective Membrane Ion, external solution α px = -log[x] If activity of species i is held constant in one phase, the potential difference between the two phases (often called the membrane potential, E m ) responds in a Nernst-like fashion to the ions activity in the other phase.

2 This is the essence of ion-selective electrodes. Measurements with these devices are essentially determinations of membrane potentials. Junction potentials develop at the membrane. The performance of any single system is determined to a large extent by the degree to which the species of interest can be made to dominate the charge transport in part of the membrane. A key is the membrane s selectivity for the ion or analyte of interest. The ion or analyte should have a tranference number of 1 and all other species should be 0. t i = z i u i C i /Σ z j u j C j

3 There are several types of ion-selective membrane electrodes: (minimal solubility, ion conductivity and selectivity for species of interest) Crystalline Membranes Single crystal (LaF 3 for F - ) Polycrystalline (Ag 2 S for S 2- and Ag + ) Noncrystalline Membranes Glass materials Liquid ion exchangers (e.g., crown ethers) Immobilized liquids in rigid polymers

4 Glass Membrane Electrode Ion selective properties have been know since the early 20 th century. Membrane regularly used for measurement of ph and alkali metal ion activity. Hg/Hg 2 Cl 2 /KCl (sat d)/test soln./glass membrane/hcl (0.1 M)/AgCl/Ag SCE Internal Ref. Electrode Glass Electrode E m = constant + (RT/z i F) ln a i soln The properties of the test solution influence the overall cell potential difference at two points. One is the liquid junction between the test solution and the SCE reference. The other is at the membrane.

5 Test Solution Dry Glass Silicate network Internal Filling Solution nm 50 µm nm The silicate membrane has an affinity for certain cations, which are adsorbed at anionic sites. Action creates charge separation internally that leads to a potential difference. Even though H + conduction through the membrane does not occur, what happens at the interfaces controls the membrane potential

6 Equil. Adsorption SiO 4- sites Equil. Adsorption Test Solution Dry Glass Silicate network Internal Filling Solution nm 50 µm nm H + soln + Gl- glass H+ Gl - H + Gl - H + soln + Gl- glass {external} {internal} Charge neutrality must be maintained! Complicated mechanism!

7 There are several factors that can affect a ph measurement: Alkaline error, ph > 12 (Na + ) Acidic error, ph < 0.5 Dehydration Variations in junction potential

8 Potential measured develops due to a difference in activity of an ion or charged molecule across a membrane which is selective for the species of interest. E = K /z log [x] An example of a permselective membrane made of plastic (polymer). Remember that the membrane must be chemically inert, insoluble and exhibit a selective interaction with the target analyte. An active substance is placed in the polymer membrane. The active substance is either a cation exchanger (Ca +2 ), an anion exchanger (Br - ) or a neutral macrocycle.

9 LDR = 4-6 orders of mag. LOD = 10-5 to 10-6 M (S/N =3) Sensitivity = 0.059/z (V) Precision = < 5% (n=10) Stability = good Detection limit largely governed by leaching of the ions from the internal to the external solution through the membrane.

10 Sensing electrodes are available for NH 3, CO 2, HCN, HF, H 2 S, SO 2, NO 2 Inside would exists a ph electrode for measuring the change in ph of the internal solution CO 2(aq,ext) CO 2 (g, membrane) CO 2 (aq,int) CO 2 (aq) + H 2 O HCO 3- (aq) + H + E ind = K /z log [CO 2 ] CO 2 (aq, ext) + H 2 O H + (aq,int) + HCO 3 - (aq,int)