Quote Originally Posted by chem geek View Post
I still have a concern that too vigorous a stirring might be outgassing chlorine. A simple way to verify that is to take two samples where for one you do vigorous stirring BEFORE adding any DPD powder of titrating drops while for the other you do not. Then measure the chlorine in each of these using a consistent technique -- say, moderate swirling. If you measure a difference, then vigorous swirling may in fact be changing the chlorine level so would not be accurate. If there is no difference, then it says that thorough mixing during the test is important so swirling should not be light.
I tried measuring 3 slightly different ways this morning, all with 10 ml water sample:

1) the way I usually do, with moderate swirling between drops -- 4.5 ppm FC

2) agitating the water sample, before adding powder, for 2 minutes (sloshing the water around as much as I could without actually losing any out of the open tube); then adding powder; then doing the habitual moderate swirling between drops -- 4.0 ppm FC

3) no pre-agitation of the water, but doing more aggressive swirling after each added drop -- 4.0 ppm FC

So, at these relatively modest chlorine levels, both of the approaches that involved more agitation of the water (either pre-test or during test) gave somewhat lower results. It wasn't a big difference, but maybe that difference would be relatively greater if the total chlorine levels were higher (as I think they were in the original post in this thread)?