Fluoride selective electrode
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The fluoride electrode is one of the simplest kinds of soild state electrode.
the sensing element is a Crystal of LaF3, doped with EuF2 to create lattice vacancies. the crystal is an ionic conductor by virtue of the mobility of fluoride ions which jump between lattice vacancies. A cell constructed using such a crystal as a membrane separating two fluoride solutions acts as a concentration cell with transference where the fluoride transport number is one. As transference of charge through the ctystal is almost exclusively confinded to fluoride the electrode is highly specific to fluoride, with OH- the being the only significant interfering ion. Generally such ' alkaline error' can be avioded by Buffering the sample to a pH below 7
the cell diagram of a typical experimental arrangement is
Cu'/Ag,AgCl/KCl/solution/LaF3/KF,KCl/AgCl,Ag/Cu
where Cu'/Ag, AgCl/KCl is an external referance electrode and the KF,KCl/AgCl,Ag/Cu is an inter reference inside the fluoride selective electrode.
some commerically available reference electrodes have internal junction that minimises the liquid junction potienial between the sample solution and the electrolyte in the referance electrode (KCl). Because the internal electrolyte is at fixed composition and the electrode response is given by the Nernst equation
E=E0- RT/F ln a(F-)
where
E is the potenial
E0 is a constant
R is the ideal gas constant
T is the temperature in Kelvin
F is farady's number 96500
a(F-) is the activity of the flouride ion.
