Electrical Properties of Microelectrodes

Generally, micropipette electrodes should be stored in distilled water or alcohol. If stored in the filling electrolyte, bacterial growth may occur, and strong electrolytes tend to erode fine tips. Storage in the dark at low temperature retards bacterial growth. Micropipettes stored in and filled with distilled water or alcohol can be filled with electrolyte for use by the diffusion technique mentioned above. Electrolyte-filled electrodes stored in air are frequently damaged by crystallization of salts on their tips, with resultant tip fracture. 

When a metal is placed in contact with an electrolyte, a potential difference is observed at the liquid-metal interface, as noted in Chapter 2. This is similar to the work-function potential difference which occurs when two dissimilar metals are brought into contact, or the potential difference associated with a semiconductor p n junction. The value of potential difference associated with a metal electrode-electrolyte interface is a function of the metal and contacting electrolyte. Theoretical treatment of this situation is complex and one should refer to a text on electrochemistry such as those by Macinnes (1961) or Newman (1973). Certain types of electrodes are extremely sensitive to various trace impurities in the contacting electrolyte and may react quite differently in seemingly similar circumstances. 

During the passage of electric current in an electrolyte, the various ions in solution carry different proportions of the current. The numerical value of each fraction is called the transference number of the associated ion. Techniques for determining these numbers were worked out by J. W. Hittorf (1853, 1856, 1858, 1859). In a simple electrode-electrolyte system of two silver electrodes in contact with silver nitrate solution, the following reactions occur with passage of current Per faraday passed, the electrode reaction at the anode is 

Between the electrodes, current is carried partly by the silver ions and partly by the negatively charged nitrate ions. This leads to a situation which is analogous to space charge formation in a thermionic vacuum tube. Since the Ag ions are only partly responsible for the current in the electrolyte, they are not transported away from the anode immediately upon formation. There is, then, an accumulation of silver ions about the anode and a depletion of silver ions at the cathode. These phenomena can be observed in a Hittorf transference apparatus. Similar phenomena occur in mixed systems, for example, platinum electrodes in contact with sodium chloride solution. 

Whether metal or fluid-filled microelectrodes are used, the interface effect is still felt. In the case of metallic electrodes, the interface occurs at the active surface of the electrode. With fluid-filled micropipettes, the metal-electrolyte interface exists in the pipette stem where electrical connection is made to the recording or stimulating electronics. There is the additional complication of a two-electrolyte interface at the electrode tip, since the electrolyte filling the lumen is usually different from the external electrolyte. 

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