R-0007 FC drop in a sample size


Well-known member
Mar 17, 2015
I did some testing and come up with the following calculations that I thought others may be interested in:

R-0007 is thiosulfate, used to lower FC, and is used with the TA test and drop based borate test.

Each drop of R-0007 in a 20ml sample will lower the FC by 11.0.
Each drop of R-0007 in a 25ml sample will lower the FC by 8.8.
Each drop of R-0007 in a 40ml sample will lower the FC by 5.5.
Each drop of R-0007 in a 50ml sample will lower the FC by 4.4.


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Thanks for that!!

This is just a side point but it's always worth repeating - thiosulfate reduction of chlorine to chloride is NOT pH neutral. There are actually several different thiosulfate/chlorine reactions that can occur and it depends on the pH of the solution. The dominant reaction that happens at pool water pH will raise pH. This is why one can not use R-0007 to neutralize chlorine in order to make the interference go away with high chlorine levels and R-0004 pH test reagent. The R-0004 drops already contain a blend of chlorine reducing agents that are designed to be pH neutral but it only works up to 10ppm FC or so.

I will go ahead a link this info over to the borate drop test thread. Since I use a 50mL water sample for that test, I typically add 2 drops which seems to be more than enough for neutralizing chlorine.
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