POTENTIALS AND THERMODYNAMICS OF CELLS (2)

ION-SELECTIVE ELECTRODES (ISE) Advantages - Exhibit wide response - Exhibit wide linear range - Low cost - Color or turbidity of analyte does not affect results - Come in different shapes and sizes

Ion-selective electrodes-example Electrodes that respond selectively to specific analytes in solution or in the gas phase. Ion exchange between heparin and Cl− associated with tetraalkylammonium ions in the membrane of the ion-selective electrode. Rotating ion-selective heparin electrode

OTHER GLASS ELECTRODES Glass Electrodes For Other Cations K+ -, NH4+-, Na+-selective electrodes - Mechanism is complex - Employs aluminosilicate glasses (Na2O, Al2O3, SiO2) - Minimizes interference from H+ when solution pH > 5 pH Non-glass Electrodes - Quinhydrone electrode (quinone – hydroquinone couple) - Antimony electrode

LIQUID MEMBRANE ELECTRODES - Employs water-immiscible substances impregnated in a polymeric membrane (PVC) - For direct measurement of polyvalent cations and some anions - The inner solution is a saturated solution of the target ion - Hydrophilic complexing agents (e.g. EDTA) are added to inner solutions to improve detection limits - Inner wire is Ag/AgCl

LIQUID MEMBRANE ELECTRODES Ion-Exchange Electrodes - The basis is the ability of phosphate ions to form stable complexes with calcium ions - Selective towards calcium - Employs cation-exchanger that has high affinity for calcium ions (diester of phosphoric acid) - Inner solution is a saturated solution of calcium chloride - Cell potential is given by

LIQUID MEMBRANE ELECTRODES Other Ion-Exchange Electrodes - Have poor selectivity and are limited to pharmaceutical formulations Examples - IEE for polycationic species (polyarginine, protamine) - IEE for polyanionic species (DNA) - IEE for detection of commonly abused drugs (large organic species)

LIQUID MEMBRANE ELECTRODES Neutral Carrier Electrodes - Employs neutral carriers such as crown ethers and cyclic polyesters - Carriers envelope target ions in their pockets Used for clinical analysis - detection of blood electrolytes - detection alkali and alkaline earth metal cations

LIQUID MEMBRANE ELECTRODES Neutral Carrier Electrodes Examples of Carriers - Monensin for sodium - Macrocyclic thioethers for Hg and Ag - Valinomycin for potassium ions - Calixarene derivatives for lead - 14-crown-4-ether for lithium

LIQUID MEMBRANE ELECTRODES Anion-Selective Electrodes - For sensing organic and inorganic anions Examples of Anions - Phosphate - Salicylate - Thiocyanate - Carbonate

SOLID-STATE ELECTRODES - Solid membranes that are selective primarily to anions Solid-state membrane may be - single crystals - polycrystalline pellets or - mixed crystals

SOLID-STATE ELECTRODES Examples - Most common is fluoride-ion-selective electrode (limited pH range of 0-8.5) (OH- is the only interfering ion due to similar size and charge) - Iodide electrode (high selectivity over Br- and Cl-) Chloride electrode (suffers interference from Br- and I-) Thiocynate (SCN-) and cyanide (CN-) electrodes

OTHER ELECTRODES - Coated-wire electrodes (CWE) - Solid-state electrodes without inner solutions - Made up of metallic wire or disk conductor (Cu, Ag, Pt) - Mechanism is not well understood due to lack of internal reference - Usually not reproducible For detection of amino acids, cocaine, methadone, sodium

APPLICATIONS OF ISEs - Used as detectors for automated flow analyzers (flow injection systems) - High-speed determination of blood electrolytes in hospitals (H+, K+, Cl-, Ca2+, Na+) - For measuring soil samples (NO3-, Cl-, Li+, Ca2+, Mg2+) - Coupling ion chromatography with potentiometric detection - Column detectors for capillary-zone electrophoresis

Electrochemical cells that use a solid electrolyte composed of zirconium dioxide containing Y2O3 (yttria-stabilized zirconia) are available to measure the oxygen content of gases at high temperature. In fact, sensors of this type are widely used to monitor the exhaust gas from the internal combustion engines of motor vehicles, so that the airto- fuel mixture can be controlled to minimize the emission of pollutants such as CO and NOX. This solid electrolyte shows good conductivity only at high temperatures (500-1,000°C),

Care and Maintenance of Electrodes for pH and Voltametric Measurements Summary The glass pH electrode How it works Calibration Errors Maintenance Reference electrodes Functioning Types of reference electrodes Problems

pH measurements with a Glass electrode The glass electrode used to measure pH is the most common ion-selective electrode. A typical pH combination electrode, incorporating both glass and reference electrodes in one body. Glass combination electrode with a silver-silver chloride reference electrode. The glass electrode is immersed in a solution of unknown pH so that the porous plug on the lower right is below the surface of the liquid. The two silver electrodes measure the voltage across the glass membrane.

The potential difference between inner and outer silver-silver chloride electrodes depends on the chloride concentration in each electrode compartment and on the potential difference across the glass membrane. Because [Cl−] is fixed in each compartment and because [H+] is fixed on the inside of the glass membrane, the only variable is the pH of analyte solution outside the glass membrane. The voltage of the ideal pH electrode changes by 59.16 mV for every pH-unit change of analyte activity at 25°C.

Glass pH Electrode H+ ion selective electrode Nernst Equation Note that the electrode measures the activity of hydrogen ions and that pH is the negative log of the activity of hydrogen ions The Nernst equation predicts that for every decade change in H+ ion activity (1 unit change in pH) the potential will change by about 60 mV

Glass pH Electrodes + - a sensing part of electrode, a bulb made from a specific glass sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode internal solution, usually 0.1M HCl for pH electrodes internal electrode, usually silver chloride electrode or calomel electrode body of electrode, made from non-conductive glass or plastics. reference electrode, usually the same type as 4 junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber. Ag/AgCl reference electrode Glass pH electrode

The Gel Layer The glass has a lithium silicate skeleton that forms a thin hydrated layer on both sides of the membrane. Ions can penetrate this thin layer and alter the electrochemical potential. Without the hydrated layer no pH measurements would be possible The structure of the glass has been optimised so that virtually only H+ ions can enter the gel layer The glass electrode is the best ion selective electrode invented, it can measure down to 10-13 M H+ions, ug/L is ppb. Li ions not H ions are responsible for carrying the electrical charge through the glass electrode. We know this from the fact that the internal filling solution, usually 0.1M HCl does not interfere with the measurements even at pH12.

Calibrating the Glass pH Electrode In performing a pH measurement it is essential to: Choose at least two standard buffers so that your measurement fits within the pH range of the buffers. E.g.: sample pH 5, select pH 4 and pH 7 standard buffers sample pH 3, select pH 2 and pH 4 Always determine the response of your glass electrode by recording the voltage or mV reading More important to use two buffers that cover the range of measurements than to use three buffer that don’t

Useful Buffers oC 0.05 M K tetroxalate KH tartrate satd. 25oC 0.05 M KH Phthalate 0.025 M KH2PO4, Na2HPO4 0.01 M Borax Ca(OH)2 satd. 25oC 1.67 4.01 6.98 9.46 13.43 5 6.95 9.39 31.21 10 4.00 6.92 9.33 13.00 15 6.90 9.27 12.81 20 1.68 6.88 9.22 12.63 25 3.56 6.86 9.18 12.45 30 1.69 3.55 6.85 9.14 12.30 35 4.02 6.84 9.10 12.14 40 1.70 3.54 4.03 9.07 11.99 45 4.04 6.83 9.04 11.84 50 1.71 4.06 9.01 11.70 55 1.72 4.07 8.99 11.58

Useful Buffers oC 0.05 M K tetroxalate KH tartrate satd. 25oC 0.05 M KH Phthalate 0.025 M KH2PO4, Na2HPO4 0.01 M Borax Ca(OH)2 satd. 25oC 60 1.72 3.56 4.09 6.84 8.96 11.45 70 1.74 3.58 4.12 6.85 8.93 80 1.77 3.61 4.16 6.86 8.89 90 1.80 3.65 4.20 6.88 8.85 95 1.81 3.68 4.23 6.89 8.83

Response of the Glass Electrode Measure the voltage response of the glass electrode and not just the pH response when calibrating the electrode. A new electrode should give a response of about 60 mV/pH under ambient conditions. pH 7-4 the electrode potential should increase by +180 mV pH 7-10 the electrode potential should decrease by -180 mV As the electrode ages the slope decreases and the response time also becomes sluggish. The electrode should be discarded when the response is appreciably below 60 mV and the response time is slow. It is advisable to keep a usage book and record the mV and pH every time the electrode is calibrated. This will provide a history of performance and deterioration of the electrode

Errors in pH measurements Improper calibration Sodium (alkaline) error, occurs at high pH and is due to the electrode responding to Na+ ions. The pH reading is lower than the actual pH (actual pH is higher) Acid error occurs at very low pH Temperature effects

Temperature Effects oC pKa H2O Neutral pH NaOH 10-5 M 10 14.54 7.27 9.54 20 14.17 7.08 9.17 25 14.00 7.00 9.00 30 13.83 6.92 8.88 40 13.54 6.77 8.54 50 13.26 6.63 8.26 60 13.02 6.51 8.02 The normal temperature compensation on the meter only compensates for the mV output from the pH sensor (electrode) and does not correct for solution changes To compensate for changes in solution pH with temperature, it is best to perform a pH/temp. calibration on your process solution and enter this into the program on your pH meter. Most modern research grade pH meters have this capability.

Care and Storage of the Glass Electrode The best solution to store the glass electrode in is pure water. Storage in KCl and water which contains other salts will mean that cations can enter the gel layer and decrease electrode response. Storing combination glass electrodes (where the glass and reference electrode are combined in one probe) in water can cause problems with AgCl precipitating and blocking the junction (porous membrane) of the reference electrode. The best method, is to store the glass bulb in water and to leave the junction of the reference electrode exposed to air. Metrohm have produced a proprietary storage solution which they claim works and the ICMT should consider giving this a try

Two Electrode System

The Reference Electrode Generally speaking, most problems in electrochemical measurements can be traced to a problem with the reference electrode. These problems can be attributed to: Blockage of the junction (the area where the electrode makes electrical contact with the solution) Contamination of the internal filling solution These problems can be largely overcome if you use reference electrodes in which the internal filling solution can be easily renewed and the junction can flushed (renewed) with fresh electrolyte. Such electrodes are available from Orion and Metrohm. In the case of pH measurements, it is best to use separate single glass and separate reference electrodes. Combination pH electrodes have there place but for calibration it is best to have a meter setup in the lab. that uses a two electrode system.

Types of Reference Electrodes Silver/silver chloride -5o-100oC continuous use -5o-130oC intermittent use. NB: the superior temperature performance of the Ag/AgCl electrode Calomel Hg/Hg2Cl2 -5o-80oC

Types of Reference Electrodes Hg/HgSO4 electrode Useful electrode to use where Cl- ion cannot be tolerated Hg/HgO electrode Suitable for work at high pH

Potential of Reference Electrodes vs the SHE @ 25oC 3M KCl Satd. KCl 0.5M H2SO4 K2SO4 (sat’d) 0.1 M NaOH Ag/AgCl 0.210 0.222 (1M KCl) 0.1988 Hg/Hg2Cl2 0.2412 Hg/HgSO4 0.682 0.640 Hg/HgO 0.165

Problems with the Reference Electrode Blockage of the junction causes an increase in the electrodes’ impedance, which inturn makes the measurement more susceptible to noise pick-up. High impedances should be avoided where possible in electrochemical measurements. Blockage and/or contamination of the junction can result in a variable junction potential, which in-turn causes a variability in electrode response. The junction potential results from a separation of charge due to the different mobility of anions and cations and can be as large as 20 mV Na+ K+ Cl- Cl- The concentration of the electrolyte used in the Luggin probe? - +

Ag/AgCl Reference Electrode Care and Maintenance Problems with this electrode can often be traced to the presence of soluble AgCl. AgCl is sparingly soluble in highly concentrated chloride solutions due to formation of the silver chloride complex If the reference electrode is stored in water and solutions of low chloride activity, AgCl solid will form and lead to the junction blocking. The AgCl can decompose to Ag2O which is a black/purple colored deposit. For this reason it is best to store the electrode in 3M KCl solution. The dilemma is that this solution is not suitable for storing the glass electrode.

Ag/AgCl Reference Electrode Care and Maintenance Blockage of the junction can also take place due to precipitation of KCl. This problem can occur at low temperature and if the electrode is stored in air. For this reason the use of saturated KCl as the internal filling solution should be avoided. Better to use 3M KCl.

Types of Reference Electrodes (Ag/AgCl) Electrodes with renewable liquid junctions Metrohm reference electrodes Orion double junction reference electrode Single junction reference electrode

Copper/Copper Sulphate Reference Electrode

pH Meters There are various types of pH meters, hand-held, portable and research grade pH meters. A pH meters is an electrometer, the feature which distinguishes a pH meter from an ordinary volt meter is the input impedance. Research grade pH meters have an input impedance of the order of 1013 ohms. The input impedance enables the meter to measure the potential of glass electrodes which have an impedance of 50-500 M (106) ohms A reference electrode has an impedance of the order of 1-3 kilo ohms (103) The high impedance means that only a small current is taken from the system during a measurement, thus preventing a change in the equilibrium voltage. This is particularly important when recording an equilibrium voltage with a low exchange current density (10-8 amps cm-2). E.g. for a measurement of 1 V, if the impedance = 1013  the current = 10-13 amps.

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