Internal compensation 756 resistance

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

Reactants must diffuse through the network of pores of a catalyst particle to reach the internal area, and the products must diffuse back. The optimum porosity of a catalyst particle is deterrnined by tradeoffs making the pores smaller increases the surface area and thereby increases the activity of the catalyst, but this gain is offset by the increased resistance to transport in the smaller pores increasing the pore volume to create larger pores for faster transport is compensated by a loss of physical strength. A simple quantitative development (46—48) follows for a first-order, isothermal, irreversible catalytic reaction in a spherical, porous catalyst particle. 

In general, to compensate for larger IR drops, a three-electrode setup is used Most of the current I is passed between (i) the working electrode (WE) and (ii) an auxiliary electrode or counter electrode (CE), between which most of the IR drop will occur. The potential is monitored between WE and (iii) a reference electrode (RE), which draws very little current it is most often an NHE, or a standard calomel electrode (Hg Hg2Cl21KC1), or an Ag AgN03 electrode. A hopefully small fraction of the overall internal resistance, known as the "uncompensated" resistance Ru, will still be present between WE and RE the goal is to make Ru/R tolerably small. Figure 6.9 shows the symbols used for three-electrode electrochemical cells. 

Some deviation of AEp is typically observed experimentally due to solution resistance, especially at higher scan rates. Many potentiostats can compensate for this phenomenon with internal resistance compensation. When ipip 1 and/or when AEp > 59/n mV, the electrochemical process is quasireversible. If no return wave is observed in the cychc... 

Experimentally, the emf can be measured either by compensating the circuit voltage (classical technique which became rare nowadays) or by using a voltmeter of very high internal resistance. The accuracy of emf determination of about 1 xV can be achieved in precise measurements, whereas common devices provide the accuracy of about 1 mV. The potential unit named Volt, which is used in the modern literature (particularly, below), is the so-called absolute (abs) Volt it differs slightly from the international (int) Volt value. The ratio abs/int is 1.00033. To determine the sign of emf, a conventional rule is adopted, which states that the left electrode should be considered as the reference one. 

Hack, H. P., Moran, P. J., and Scully, J. R., Influence of Electrolyte Resistance on Electrochemical Measurements and Procedures to Minimize or Compensate for Resistance Errors, The Measurement and Correction of Electrolyte Resistance in Electrochemical Tests, ASTM STP 1056, L. L. Scribner and S. R. Taylor, Eds., ASTM International, West Conshohocken, PA, pp. 1-16. 

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