3.1 Ion selective Potentiometry

1. Dnyanasadhana College, Thane.Department of ChemistryM.Sc. Analytical Chemistry Sem-IIIon selective Potentiometry

2. 3.1 Ion selective Potentiometry • Solid state, precipitate, liquid –liquid, enzyme and gas sensing electrodes with applications, • Ion selective field effect transistors, biocatalytic membrane electrodes, • Enzyme Based Biosensors. [8L]

3. Ion selective Potentiometry Solid state, precipitate, liquid –liquid, enzyme and gas sensing electrodes with applications, ion selective field effect transistors, biocatalytic membrane electrodes, and enzyme based biosensors.

4. Classification of Ion selective electrodes Ion selective electrodes are classified into following types on the basis of nature of membrane.

7. Solid state, precipitate electrode • Construction: In fluoride ion selective electrode a crystal of lanthanum fluoride is sealed into the bottom of a plastic container to produce a fluoride ion electrode. The container is charged with a solution containing potassium chloride and potassium fluoride and carries a silver wire which is coated with silver chloride at its lower end: it thus constitutes a silver-silver chloride reference electrode. The lanthanum fluoride crystal is a conductor for fluoride ions which being small can move through the crystal from one lattice defect to another and equilibrium is established between the crystal face inside the electrode and the internal solution. Likewise, when the electrode is placed in a solution containing fluoride ions, equilibrium is established at the external surface of the crystal. In general, the fluoride ion activities at the two faces of the crystal are different and so a potential is established, and since the conditions at the internal face are constant, the resultant potential is proportional to the fluoride ion activity of the test solution.

8. 1.Solid state membrane electrode Fluoride ion selective electrode

9. Mechanism of fluoride ion selective electrode

10. Applications: Fluoride electrode has many applications such as fluoride determination in i) bone,ii) Air and stack gas samples,iii) Chromium plating baths, iv)Minerals, v) Water, vi) Toothpastes. Vii) Similar to fluoride ion selective electrode many other solid state membrane electrodes can be developed. In Table few are listing few such ion selective electrodes.

11. 2.Glass Electrode: Glass electrode is of glass membrane category. HCl 0.1M

13. Liquid Membrane Electrode: • Liquid Membrane Electrode:Ion exchange electrode are also called as liquid membrane electrode. • Ion exchange electrodes can be prepared using an organic liquid ion exchanger which is immiscible with water, or an ion-sensing material is dissolved in an organic solvent which is immiscible with water, and placed in a tube sealed at the lower end by a thin hydrophobic membrane such as 'Millipore' cellulose acetate filter: aqueous solutions will not penetrate this film.

14. Construction: The basis of the construction of such an electrode is indicated in Fig. The membrane (A) seals the bottom of the electrode vessel, which is divided by the central tube into an inner (B) and outer compartment (D). Compartment B contains an aqueous solution of known concentration of the chloride of the metal ion to be determined: this solution is also saturated with silver chloride and carries a silver electrode (C), which thus forms a reference electrode. The liquid ion exchange material is placed in reservoir D and the pores of the membrane become impregnated with the organic liquid which thus makes contact with the aqueous test solution in which the electrode is placed; this solution also carries a suitable reference electrode, e.g. a calomel electrode. • solid ion exchange membranes by dissolving the liquid ion exchange material together with poly (vinyl chloride) (PVC) in a suitable organic solvent such as tetrahydrofuran and then allowing the solvent to evaporate. A disc is cut from the flexible residue and cemented to a PVC tube to produce an electrode vessel, in which the PVC membrane replaces the cellulose acetate and reservoir material previously used, so that only a single compartment is needed.

16. 4. Gas Sensing Electrode (Gas permeable membrane electrode): • Based on the principle of ion selective electrodes many gas sensing electrodes have been developed in past few years. They are available primarily for the measurement of ammonia, carbon dioxide, and nitrogen oxide. This type of electrode has a gas permeable membrane and an internal buffer solution. The pH of the buffer solution changes as the gas reacts with it. The change is detected by a combination pH sensor within the housing. This type of electrode does not require an external reference electrode. In following Table some commercial gas sensing electrodes are summarized.

17. Construction: • The essential features of a gas sensing electrode can be in the Fig. The membrane A is permeable to the dissolved gas in the test solution and may be a micro porous membrane manufactured from either polytetrafluoroethylene or from polypropylene, both of which materials are water repellent and are not penetrated by aqueous solutions, but they allow gas molecules to pass through: this kind of membrane is used with ammonia, carbon dioxide, and nitrogen dioxide. Alternatively, the membrane is a very thin homogeneous film, commonly of silicone rubber, through which the gas diffuses (sulphur dioxide, hydrogen sulphide). • Electrode C is now a glass pH electrode or other suitable ion-selective electrode and a silver-silver chloride reference electrode is also incorporated in B. • The internal solution in B contains sodium chloride and an electrolyte appropriate to the gas which is being determined: for NH3 gas NH4Cl, CO2 gas, NaHCO3; NO2 gas, NaNO2, SO2, K2S2O5 , H2S, a citrate buffer. Membrane A is small in area, and the volume of liquid in B is also small so that it rapidly equilibrates with the test solution

18. Electrode C is now a glass pH electrode or other suitable ion-selective electrode and a silver-silver chloride reference electrode is also incorporated in B. C Gas sensing electrode B • A • - • ---------------- The internal solution in B contains sodium chloride and an electrolyte appropriate to the gas which is being determined: for NH3 gas NH4Cl, A The membrane A is permeable to the dissolved gas in the test solution and may be a micro porous membrane manufactured from either polytetrafluoroethylene or from polypropylene,

20. Applications Ion Selective Electrodes • Ion selective electrodes along with pH-sensitive glass electrode are widely used in clinical, biological, water, air, oceanographic, and pharmaceutical research and routine analytical determinations. So far there are reliable commercially available electrodes for detecting • H+, F–, Cl–, Br–, I–, Cd2+, Cu2+, CN–, BF − Pb2+, NO−3 , ClO−4 , Ag+, S2–, Na+, K+, and SCN–, for NH3, H2S, SO2, CO2, nitrogen oxides gases, and for several different enzymes. Glass membrane electrode is the most commonly used ion selective electrode. pH measurements have many applications some of which are summarized below: • • pH metric or electrometric methods help in detecting the end point of acid-base titration more accurately and precisely as compared to indicator methods. • • pH metry is an important analytical tool in studying the acid-base equilibria which is controlled by the carbon dioxide-bicarbonate-carbonate equilibrium system in most natural waters.

21. • The estimation of alkalinity and acidity based on pH metry serves a useful information of buffering capacity of water. • • In water and wastewater treatment, it gives an estimate of available of alkalinity to react with the coagulant viz alum, Ferrous sulphate etc. or otherwise to be supplemented with lime. • • Industrial process control especially in batch or flow-through configurations; through online pH monitoring and chemical dosing system. • • In neutralization of wastewaters, to evaluate the doses of acid or alkali to be added. • • Development of biosensors based pH sensitive immobilized enzymes and other academic studies. • Further the flexibility in available configurations allows the ions mentioned above to be monitored in a single sample solution (batch mode) or continuously in a flow through apparatus (flow-injection analysis).

22. 5. Ion selective field effect transistors • A novel development of the use of ion-selective electrodes is the incorporation of a very thin ion-selective membrane (C) into a modified metal oxide semiconductor field effect transistor (A) which is encased in a non-conducting shield (B) .When the membrane is placed in contact with a test solution containing an appropriate ion, a potential is developed, and this • potential affects the current flowing through the transistor between terminals Tl and T2

23. By calibration against solutions containing known activities of the ion being determined, measurement of the current can be used to ascertain the activity of the ion in the test solution. • Such measurements can be carried out with very small volumes of liquid, and find application in biochemical analyses. However, the simpler ion-selective electrodes discussed above can be readily adapted for dealing with small volumes, and even for intracellular measurements. • In an alternative procedure designed to deal with minute volumes of liquid, Walter set up a 'layer cell' based upon the technique employed in 'instant colour' photographic films, Such a cell designed to determine potassium ions made use of two layer assemblies terminating in valinomycin electrodes, so that with a standard potassium chloride solution added to one assembly, and the solution under test to the other, with the two assemblies joined by a Salt bridge,a concentration cell was set up, measurement of the e.m.f. of which made possible the calculation of the potassium ion concentration in the test solution.

24. B A T1 T2 ---------------------------------------- C

25. 6. Biocatalytic membrane electrodes, enzyme based biosensors • Ex. Enzyme based biosensors: Potentiometric Biosensors /Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme Produces a product whose concentration is monitored by the ion-selective electrode.

26. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. • One example of an enzyme electrode is the urea electrode, which is based on the catalytic hydrolysis of urea by enzyme urease. CO(NH2)2(aq) + 2H2O(l) +2H+---------------------- 2NH4+ + CO2 • The progress of the electrode can be followed by using glass electrode which is sensitive to ammonium ions. The final concentration of ammonium ions determined can be related to urea present. The urease is incorporated into a polyacrylamide gel which is allowed to set on the bulb of the glass electrode and may be held in position by nylon gauze. Preferably, the urease can be chemically immobilised on to bovine serum albumin or even on to nylon. When the electrode is inserted into a solution containing urea, ammonium ions are produced, diffuse through the gel and cause a response by the ammonium ion probe: