Testing with high TC

[chem geek]

The following is the effect of high FC levels on the various other tests:

[EDIT]
Adding DPD chlorine test per Evan's post below this one.

DPD Chlorine
The DPD chlorine test, where you measure the intensity of pink/red, will usually start to bleach out above 5 ppm FC and may become completely bleached out above 10 ppm FC making one think that they have no chlorine when in fact they have very high chlorine levels. An OTO chlorine test, where you measure the intensity of yellow, will not bleach out and will instead turn to orange, red or even brown at very high chlorine levels. A FAS-DPD chlorine test, where you count the drops going from pink to clear, will not bleach out, but at high chlorine levels adding the DPD powder may show a flash of pink and then become clear at which point you can just add more DPD powder until a pink/red color stays.
[END-EDIT]

pH
The pH test is mostly affected above 10 ppm FC though the actual level depends on the TA level where a higher TA causes less variation of the pH from the neutralizer (which tries to be pH neutral in its effects, but isn't perfect). Taylor wrote the following with regard to chlorine and the pH test:

FALSE READINGS: high levels of chlorine (usually >10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple+orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

TA
Taylor says that "high halogen level may change indicator reaction from green/red to blue/yellow; to prevent, add thiosulfate prior to testing." You already add thiosulfate in the TA test (it's reagent R-0007), but you can add more if the FC is higher. There's nothing wrong with the blue to yellow transition, however -- it is still valid.

CYA
Taylor doesn't talk about this, but it seems that chlorine bound to CYA does not form the melamine-cyanurate precipitate which means that the CYA test will read artificially too low when the FC is high. This should be able to be handled by adding around 2 drops of R-0007 thiosulfate to the water sample for every 10 ppm FC before you add the CYA reagent.

I don't believe that the CH test is affected by high FC.