Honeywell Understanding Platinization and Its Importance for Conductivity Cells

Platinization and Platinum Black

5. Platinization and Platinum Black

Only the electrodes having constants from 5 to 50 must be replatinized if the velvety-black deposit has been rubbed off the electrodes in service or in cleaning, or if platinized electrodes are recommended and this black deposit is not present when the cell is received. Always replatinize if a brush was used in cleaning the electrodes. The indication of a need for replatinization of the electrodes is a long term drift of the measuring instrument caused by an apparent increase in cell constant as the platinum black coating is depleted from the electrode surfaces. The electrodes of the high constant cells are not visible since they are located near the middle of the flow channels. Therefore the need for platinization is only indicated by the effect on the measuring instrument. Do not platinize cells intended for measuring high purity water.

Before platinizing, clean the cell with detergent and brush as described in Section 4.1.

Support the cell in a cylindrical vessel with the end of the cell raised from the bottom. It is not necessary to remove the cell from the fittings for platinizing. However, the guard tube must be removed from the low constant cells. Pour in platinizing solution (p/n 31103011) to a level above the cross-channel.

To platinize the 5, 10, 20, 25 and 50 constant cells, immerse an auxiliary platinum electrode* in the solution to a point about midway between the cross-channel or tube hole and the open end of the cell. Both electrodes of the cell are platinized simultaneously by connecting the negative terminal of the battery (see Table 4.2 for voltage) to both leadwires of the cell.

ATTENTION

*This third electrode should be chemically pure platinum. Its shape is unimportant. It may be one of the electrodes in another conductivity cell or a platinum strip, sheet, rod, wire, etc.

Connect the positive terminal of the battery to the auxiliary platinum electrode. Note the time in seconds listed in Table 4.2. During the platinizing operation, move the cell up and down gently to keep the solution stirred. Then disconnect the battery and remove the cell. Rinse the cell thoroughly in tap water and then rinse in distilled water.

Pour the platinizing solution back into its container as it may be used a number of times.

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