The Antimony and Quinhydrone Electrodes

The quinhydrone electrode is of simpler construction than the antimony electrode. A piece of 24-gauge platinum wire about 1/2 in. long is introduced into the capillary, which is then sealed and beaded to the wire leaving a short tip exposed externally. The platinum wire should be cleaned in aqua regia briefly and washed in distilled water before using. After the bead has cooled, self-wetting solder is introduced into the lumen and the assembly is heated until the solder melts. After the solder has bonded to the platinum, but while it is still molten, a piece of clean copper wire is inserted into it to form the electrode connection when the solder cools. 

Although these electrodes are easily constructed, they have a number of limitations. Antimony is toxic and tends to contaminate solutions being measured, especially when used in strong acids. The electrode tip tends to break down and must be reformed and polished. 

The quinhydrone (platinum) electrode requires that a small amount of quinhydrone [C6H402-C6H4(0H)2] be added to the solution under test. 

This is necessary to insure hydronium ions in solution, since the electrode potential varies directly with hydronium ion concentration. In aqueous solution, quinhydrone exhibits the following redox behavior  

If the electrode is used against a hydrogen reference electrode, the cell so formed is described by 

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