Chlorine testing question

Apr 3, 2008
East TN
I took the cover off my pool, and there it was, the Green Monster. I couldn't bare the sight of it, so I decided to start killing it right away. I am waiting on the Taylor FAS-DPD Kit to get here, so I can properly measure for the high level of chlorine. Until that time, I am working with a simple 4-way test kit. Never really knew there was a proper test kit that I needed until I found the information here. I have read that you can dilute your test water with distilled water to get an approximate high lvl of FC (if I understand it correctly). My question is this. My test kit say to put 5 drops into the water. Can I put just 1 drop in the water and then take that reading and multiply it by 5? I am not a chemist by any means, so I don't know the whole reasoning for putting 5 drops in it. If it does work that way I would like to know the reasoning behind it, just for my own desire of knowing why things work the way they do.


LifeTime Supporter
Platinum Supporter
TFP Expert
May 7, 2007
Silver Spring, MD
No. With the "comparing to a color standard" tests the number of drops affects how clearly you can see the color but not which color appears. You put five drops in so that there is enough indicator to be easily seen.

With the standard OTO chlorine test, add five drops and compare various shades of yellow to a color standard, you are measuring TC. You really want to be measuring FC. Since CC is often non-zero when fighting algae and TC = FC + CC, there is no easy way to guess at what proportion of the TC is FC and what is CC.

At the start you can get by, TC will go quite low quickly and a low test result tells you that you still need to add lots of chlorine. But when TC starts to stay medium to high you won't be able to tell how much is FC and how much is CC and so won't know how much chlorine to add.

One alternative approach is to watch the color of the water. When the green starts to fade and turn gray or white then you have turned the corner and can slow down on chlorine. But it does tend to get kind of hit and miss and can lead to significant errors.

Nothing is as good as being properly prepared, and that means having the right test kit. If the algae just showed up this morning then you add some chlorine and you have probably beaten it. But dealing with a total green swamp can require vast amounts of chlorine and knowing when to stop through accurate testing becomes much more important.