Combined Chlorine Testing Question

revitup

Gold Supporter
Bronze Supporter
Nov 30, 2019
350
Pawleys Island, SC
I'm testing for CC using the Taylor K-2006C kit. When testing for CC I add the 5 drops of 0003 and the test sample turns a very very light pink. One drop of the 0871 turns it clear. I record that result as .5ppm CC. I sometimes will leave that test sample as is while I conduct other tests. It doesn't take long and that CC test sample turns pink again, a deeper pink than it turned when I originally added the 0003 for my CC test. What does that mean, if anything? Is my original .5ppm CC result still valid?
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,588
NW Ohio
Record the result when it changes to clear and you're done. That test will always return to pink when left sitting. And I mean always: one time someone who asked this question had spent over an hour adding more drops every few minutes to see if it would stop. It didn't.
 

JamesW

TFP Expert
Mar 2, 2011
21,079
Reagent R-0003 contains iodide, which reacts with combined chlorine and any other weak oxidants to form iodine, which then combines with the dpd reagents to create the red color.

Atmospheric oxygen can interfere with the test by oxidizing the iodide.

4I- + O2 + 4H+ --> 2I2 + 2H2O
 

imagineero

Member
Nov 24, 2019
18
sydney australia
On a somewhat related note, I’m using the clear choice labs kit which supposedly uses the same reagents as the taylors. When adding titrating drops to get to clear testing FC, the instructions say something like “if not totally sure, add another drop and if there’s no further colour change, don’t count the last drop.”

All good there

So we move on test CC and add 5 drops chlorine detection reagent to see if we get any pinkishness and if so, once again add titrating drops to figure the CC.

If I added an extra drop of titrating agent during testing for FC which turned out to be unnecessary and made no further change to the colour, has that drop been taken away from my CC measurement? Or does chlorine detection agent “reset” the whole thing?

For bonus points, would it make any difference to CC measurement if I added an extra 10 unnecessary titrating drops before adding detection reagent?