| Popis: |
Electrodes used continuously in the field for up to 5 yr were removed, examined visually, and tested in both a quinhydrone solution and a 1:5 soil-water slurry. Comparison with pre-installation quinhydrone readings showed that all but 3 of 102 electrodes tested were still within ±10 mV of the standard value. Water observed inside more than half the electrodes had no apparent adverse impact on electrode performance. The post-removal quinhydrone test correctly identified one electrode that developed faulty readings in the first vear of field use. Test values for two other potentially defective electrodes fell within acceptable standards after rinsing them in an HCl-HNO 3 solution and retesting. Platinum poisoning with coatings could not be detected visually, and we conclude that poisoning, though possibly present at minimal levels, did not significantly impair electrode performance after 5 yr. Post-removal testing in a constantly stirred slurry gave readings that were a little more variable than quinhydrone readings but were much less variable than actual field readings. Anomalous slurry readings within replicated electrode sets suggest that, despite good quinhydrone readings, some electrodes may not have responded properly under actual field use. Field data from one such set confirmed the presence of a defective electrode. We conclude that the slurry test provides valuable information that complements the quinhydrone test and could be used as a pre-installation test to help identify potentially faulty electrodes. |
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