Adjust TA when chlorine very, very high?

[chem geek]

Sandy,

Your TA will yo-yo along with pH, but even going from a pH of 8.0 to 7.0 by adding acid (or using Tri-Chlor) would only cause the TA to go from 80 to 67 if your CYA were 30. If your CYA were 100, then that TA would go from 80 to 61. So we can probably eliminate that TA/pH relationship as the main source of your drop in TA, especially if you measured the TA of 80 and 60 at the same pH level (did you?).

So that leaves carbon dioxide outgassing, but if that were occurring then you should be seeing a significant rise in pH that you have to counteract by adding lots of acid. Is this what you are seeing?

As for the TA test and interference from chlorine, you do add Sodium Thiosulfate drops as part of the TA test and this is intended to breakdown the chlorine because a high chlorine level will make the test go from blue to yellow (which is much harder to see) instead of green to red. To avoid this (if you see blue/yellow instead of green/red), just add more drops (say 4, instead of 2) of thiosulfate to your sample before titrating.

Last, but not least, what is your CYA level? The shock level you will need to kill the hard-to-kill algae (if that's what you have) is dependent on your CYA level.

P.S. When you add lots of chlorine, the pH will increase a lot as will your TA. As the chlorine gets used up (or if you add acid), then the pH and TA will get lowered pretty much back to where they were.

P.P.S. If you really do have this hard-to-kill presumed yellow/mustard algae, then it may take a chlorine level of at least what is found in the column "1.00" in this chart. If you have high CYA, this could mean exceptionally high levels of chlorine to kill this stuff.

Richard