Maintenance
5.4 Platinizing the Cell Electrodes
Only the electrodes having constants 10 and 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 new 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 loss in sensitivity (slow response of measuring instrument), erratic behavior of measuring instrument, or difficulty in balancing. 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 high purity water measurements.
Before platinizing, clean the cell with detergent and brush as described in Section 5.2.
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 a platinizing solution to a level above the cross-channel.
To platinize the 10 or 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. (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.) Both electrodes of the cell are platinized simultaneously by connecting the negative terminal of the battery (see Table 5-1)to both leadwires of the cell. Connect the positive terminal of the battery to the auxiliary platinum electrode. Note the time lapse and continue the platinizing operation for the time in seconds listed in Table 5-1.Then disconnect the battery and remove the cell. Rinse the cell thoroughly in tap water and then rinse in distilled water. During the platinizing operation, move the cell up and down gently to keep the solution stirred.
CAUTION
The preceding procedure produces a barely visible coating of platinum black on the electrode surfaces. Do not attempt to darken electrodes by additional platinization since this will affect the cell performance adversely.
Pour the platinizing solution back into its container as it may be used a number of times.
If the cell is not to be installed immediately after platinizing, it should be kept submerged in distilled water until put into use, as platinum black is not stable when dry.
Table 5-1 Voltage and Time Limits for Platinizing Cells
DC | 10 | 50 |
Volts | | |
| | |
6.0 | 100 sec. | 300 sec. |
| | |
12.0 | ---- | 240 sec. |
| | |
16 | 4905 Series Conductivity Cells – Installation and Maintenance | 12/07 |
