Electrodes and Potentiometry

Mast cell (noncirculating white blood cell) with dark granules containing regulatory molecules such as heparin. (Mi indicates details of a cell s surface called microvilli and microfolds.) 

Rotation of heparin electrode improves sensitivity by an order of magnitude. [From Q. Ye and M. Meyerhoff, 

Ion exchange between heparin and Cl associated with tetraalkylammonium ions in the membrane of the ion-selective electrode. Ion-selective electrodes described in this chapter reach an equilibrium electric potential 

In the simplest case, analyte is an electroactive species that is part of a galvanic cell. An electroactive species is one that can donate or accept electrons at an electrode. We turn the unknown solution into a half-cell by inserting an electrode, such as a Pt wire, that can transfer electrons to or from the analyte. Because this electrode responds to analyte, it is called the indicator electrode. We then connect this half-cell to a second half-cell by a salt bridge. The second half-cell has a fixed composition, so it has a constant potential. Because of its constant potential, the second half-cell is called a reference electrode. The cell voltage is the difference between the variable potential of the analyte half-cell and the constant potential of the reference electrode. 

Dan received intravenous heparin in December 2004, but never stopped typing [Photo from Sally Harris.] 

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