Hi TQ. They are quite old and probably have lost some CLO2 during that time period. If they have been in a dark place and cool or cold, they are still useful.
CLO2 gas tends to stay in a water solution (CDS) if kept below 50 degrees F (10C) and not exposed to light, especially sunlight. So, don't open them unless they are cold and be sure to recap the bottles as soon as you finish taking out a dose. If you have CLO2 test strips, you can test for CLO2 concentration, ppm.
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