new test kit - checking combined chlorine?

I got my new K-2006 kit in and started the learning process.

I know that to check combined chlorine, you begin to drop one drop the R-0871 reagent until the mixture turns from pink to colorless. I noticed that if allowed to sit for a short time (3 -10 sec?) the mixture begins to turn pink again.

At what point do you say the mixture is "colorless?" Do you keep adding drops until the mixture stays "colorless" even after sitting for a time or do you stop counting the first time it become colorless?

stop when it turns colorless, add the R-0003 reagent, and if it turns pink again retitrate to get your combined chlorine. If you let it sit you will have inaccurate results.

The combined chlorine test builds on the free chlorine test and uses the same sample.

Add the DPD powder, enough to turn the sample pink.
Add R-0871 drop by drop, counting drops, until the sample is clear. The last drop that turned the sample clear is the last drop you count.
If you're using a 10 ml sample, the number of drops divided by 2 is your FC. So, 10 drops = 5 ppm FC.

To the same sample add 2 drops of R-0003. If your sample turns pink, you have CCs and need to continue. If it stays clear, you're fine. If you have CCs it'll turn pink immediately upon adding the R-0003.

To see how much CCs you have, add R-0871 one drop at a time, counting and dividing by 2. Three drops would equal 1.5 ppm CC, for instance.

The sample is "colorless" when the last drop turns it colorless. It may go back to pink when you let it sit but you don't care. In titration chemistry, the lat drop to produce a color change is the one we count. If another drop doesn't make a noticable change in color then the previous drop was the "last drop".

Thanks Anna!

Quote:

---

Originally Posted by AnnaK

To the same sample add 2 drops of R-0003. If your sample turns pink, you have CCs and need to continue. If it stays clear, you're fine. If you have CCs it'll turn pink immediately upon adding the R-0003.

---

That should read " . . . add 5 drops of R-0003." Five, not two.

My apologies for a very misleading typo.

THANK YOU Waterbear and Anna! It was driving me crazy that the water sample would turn clear and then after a minute turn pink again. I didn't know what to count! Again, thank you, thank you, thank you.