Medical brain notes 14

  INTRODUCTION Generally speaking the actual concentration of a broad spectrum of solutes may be measured conveniently by forming an appropriate electrochemical cell. Thus, most electrochemical cells invariably comprise of  two  electrodes, namely : ( a )  an Indicator Electrode —the voltage of which solely depends on the thermodynamic activity ( i.e. , concentration) of one specific component in the solution ; and ( b )  a Reference   Electrode —the voltage of which must be absolutely independent of the nature and composition of the solutions   wherein it is immersed. Placing together of these two electrodes in a solution obviously gives rise to an electrochemical cell ; and consequently the voltage thus generated across the electrodes may be determined by connecting it either to a potentiometer or a millivoltmeter that has a sensitivity to measure ± 0.2 mV, besides possessing a high impedence-input of minimum 10 12  ohms ( Ω ).   Under thes...

  THEORY In a situation, where a metal M is placed in a solution containing its own ions M n + , an electrode potential is established across the two electrodes, whose actual value is provided by the Nernst equation as shown below : E = E –  + (RT/ n F)  1 n a  M n +                    ..( b )   From Eq. ( b ) the relationship to a cationic electrode,  i.e. , sensitive only to a cation concentration, may be expressed as : E = E –  Y n + , Y + (RT/ n F)  1 n a Y n + .........................(c) to an anionic electrode : E = E –  X n – , X – (RT/ n F)  1 n a  X n – .........................(d) or to a redox electrode :        ..........................(e) where,   E– = Standard electrode potential (SEP)    (or reduction potential of the half-cell involved),       a = Therm...