Field Methods of Monitoring Aquatic Systems Unit 10 – Ion-Selective Electrodes Copyright © 2006 by DBS

ISE Theory Uses potentiometry to measure concentration –Membrane responds selectively to a given ion –mV reading between sensing and reference electrode (stable mV) Samples must be aqueous to avoid damage to membrane Pros: Much cheaper than IC or AA instruments Cons: Limited life-span

ISE Theory Variety of ISE’s Polymer membrane –Ca 2+, BF 4-, NO 3 -, ClO 4 -, K +, Mg 2+ Gas-sensing –NH 3, CO 2, HCN, HF, H 2 S, SO 2, NO 2 Crystalline membrane (solid state) –Br -, Cd 2+, Cl -, Cu 2+, F -, I -, Pb 2+, Ag/S 2-, SCN -

Theory Ion Selective Electrodes (including the most common pH electrode) work on the basic pricipal of the galvanic cell By measuring the electric potential generated across a membrane by "selected" ions, and comparing it to a reference electrode, a net charge is determined The strength of this charge is directly proportional to the concentration of the selected ion

Theory Solid-state electrodes follow the Nernst equation: E cell = E RT log [M] nF Where E = voltage (V), E 0 = constant, 59.2 = slope, n = charge, and [M] = concentration of ion x RT / F = Where R = J mol -1 K -1, T = temperature (K) and F = C eq -1 E log[M]

Question Write down the E cell expression for a fluoride ISE in mV E cell = E log [F - ] 1 V = 1000 mV, For E cell (mV) E cell = K log [F - ]

Calcium ISE ISE, reference electrode and voltmeter (mV) PVC membrane coated with organic molecule that selectiveley absorbs Ca 2+ Ca 2+ concentration gradient generates potential E cell = K log [Ca 2+ ] Where E = voltage (mV), K = constant, 29.6 = slope

Ionic Strength Adjuster Ca ISA = 4 M KCl It is likely that samples will be higher ionic strength than standards Variable ionic strength of solutions varies electrode response, also calibration curve is affected ISA adds ionic strength (does not add ion under study) Swamps ionic effects of host solutions and gives uniform ionic strength in all solutions Ca ISA does not buffer solution or complex interfering ions like F - ISAB

Procedure Requires ISA, electrode and standards 1.Set up electrode and allow to equilibriate in high concentration solution (30 mins) 2.Make up standards and analyze with ISA from low to high 0.1, 1, 10, 100, 1000 ppm Ca 2+ 3.Make a graph of E cell (mV) vs. log[Ca 2+ ], ensure slope is at or near Analyze unknowns 5.Calculate concentration of samples

Question Why is it important that a refrigerated sample or standard be removed from storage 30 minutes prior to measurement?

Fluoride ISE Occurs in minerals somay be naturally present Low incidence of cavities with high levels of F - –Decay of hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 –F - replaces OH - with F - to form more resistant fluorapatite Ca 10 (PO 4 ) 6 F 2 F - > 3 ppm causes decay and poisoning –In SW US F - > 4ppm requiring removal

Fluoride ISE LaF 3 crystal doped with EuF 2 F - passes through holes in crystal lattice produced by EuF 2 impurity F - concentration gradient across the crystal generates potential E cell = K log [F - ] Where E = voltage (mV), K = constant, = slope

Ionic Strength Adjuster ISA Buffer maintains pH 5.2 – 5.7 reducing interference from OH - and formation of dangerous HF ISA masks Al 3+ and M 2+ (complexes them with citrate) that would otherwise bind F - strongly F - measurement is best carried out under conditions of high ionic strength Minimizes change in electrode response as a function of the concentration of other ions in solution

Measurement Considerations 1.Read operating instructions 2.Agitation – supplies constant flow of ions to sensor 3.Response time – at least 15 mins for equilibrium 4.Calibration – at least 3 standards 5.Rinsing – do not use a cloth! Shake off excess water 6.Conditioning – ISE needs to remain most even when not in use

Measurement Considerations 1.Read operating instructions 2.Agitation – supplies constant flow of ions to sensor 3.Response time – at least 15 mins for equilibrium 4.Calibration – at least 3 standards 5.Rinsing – do not use a cloth! Shake off excess water 6.Conditioning – ISE needs to remain most even when not in use

General Considerations 1.Electrodes with a polymer membrane must not come in contact with organic solvents 2. Do not store in water for extended periods—dry before storing 3. Store Combined Ion Selective Electrodes in dilute ISA (ionic strength adjuster) solution—for long term storage, remove reference solution and store dry. 4. Clean crystal membranes with a mild abrasive, then rinse with water. Toothpaste is an excellent cleaning agent, for fluoride electrodes use a fluoride toothpaste

Latest Technology New NextSens ISE with USB (no meter required) Source: Ex-Stick

Journal Articles Frank and Ross (1966) Science, Vol. 154, pp Frant, MS. (1994) History of the Early Commercialization of Ion- Selective Electrodes. Analyst, Vol. 199, pp Meyerhoff, ME and Opdycke, W.N. (1986) Ion Selective Electrodes. Advances in Clinical Chemistry, Vol. 25, pp

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Text Books Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil. Wiley-VCH. Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker. Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods. CRC Press. Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis Publishers. Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books. Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative methods. Taylor & Francis. Reeve, R.N. (2002) Introduction to environmental analysis. Wiley. Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the Examination of Water and Wastewater, 20th Edition. Published by American Public Health Association, American Water Works Association and Water Environment Federation.