tpspier, please look at the Taylor video at this link where you should click on the "Pool / Spa" under "Liquid Kits" on the left and then click on the "To Test (Free and Combined) Chlorine using FAS-DPD [updated 3/5/10]" under the "K-2006 Complete™ Kit with FAS-DPD:" under the video area. Look at this video and notice how they stir. Do you call that "a very light swirl between adding drops" or "swirling more aggressively (almost shaking) between drops" or something in between. One should definitely not be doing just a light swirl. It should be a fast back-and-forth to cause a thorough swirl and mixing. It shouldn't be so vigorous as to cause splashing or getting air bubbles into the sample.

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.