Membranes Permeable to a Single Species

Consider first a system in which two solutions of an electrolyte of concentra- 

An important practical example of the second case is the lanthanum fluoride single-crystal electrode, permeable to fluoride ion but not to cations. Construct a cell 

In practice, a single crystal of europium-doped lanthanum fluoride (K,p = 2 X 10 ) responds in a matter of seconds according to (13-4) to fluoride ion activity in solutions containing fluoride from about 1 Af to 10 M. Europium doping increases fluoride mobility by introducing lattice disorder. Hydroxide ion, beginning at about 10 Af, is the principal interference, probably the result of competition due to the low solubility of lanthanum hydroxide. 

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