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In: Chemistry

Describe the response mechanism of the fluoride-ion-selective electrode. Explain why the OH- is the major interfering...

Describe the response mechanism of the fluoride-ion-selective electrode. Explain why the OH- is the major interfering ion in F- ISE measrements.

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Expert Solution

Ion selective electrodes are differentiated by membranes like glass membrane electrode, liquid membrane electrodes, crystalline membrane electrodes etc. F-ISE is one type of crystalline membrane electrode. It has an internal reference electrode, an internal flouride standard and LaF3 crystal. Single crystal LaF3 is doped with europium(atomic no-63) to increase conductivity. At membrane-solution interface LaF3 is converted to (LaF2)+ .So one negative charge creates at the surface(due to removal of one Flouride ion). So the potential difference occurs at the both side and it measured(related to fluoride ion concentration).

It can experience interference from hydroxide ion(above pH 8) and poly valent cations. The hydroxides form complexes with lanthanide crystals. So it will interference with the fluoride determination across the membrane. Addition of pH 5 buffer eliminates the interference from hydroxide ions.


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