While activated sodium chlorite works great as a water purifier, it is a little more complicated than just how many drops are necessary.
You have to work backwards. After the proper contact time for the concentration you use, you want to have a little less than 0.8 PPM left over.
The concentration you need depends on the quality of the water you are trying to purify. Also, water temperature and PH effect the treatment times.
When I am working with lake or stream water, I start out with 4 drops MMS, activated, in a liter of water. I measure the free ClO2 10 minutes after applying the solution and am looking for around 3 PPM. If the water has passed through a 1 micron filter, it should be ready to go in about 15 minutes, so I wait 30 minutes just to be sure. If the water is unfiltered, the cryptosporidum take about 2 hours to kill off, so I wait 4 hours, just to be sure.
Bruce has indicated that with a 5% solution and using a 50% citric acid solution, 7 drops, activated, in 1 liter will give you a 10 PPM solution to start with. As the ClO2 purifies the water, the concentration drops because it is used up.
In the end, if you can smell ClO2, the concentration is probably above 1 PPM, and you need to add a little ascorbic acid to use up the residual ClO2.
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