Testing with high TC

[polyvue]

Thanks Anna! I guess unless someone like Chem Geek tells me otherwise, I'll have to assume that I can't do any other tests when shocking.

I don't know... I've tested pH and gotten reasonably accurate results at a shock level of ~12 PPM Free Chlorine. Lack of precision in color matching (interpretation), variations in (test tube) sample volume and titration can work together to exaggerate an error; they can also work to cancel each other out. Some tests, as AnnaK alludes to, above, commence with chlorine neutralizer (thiosulfate, in the tests I have) so you should be able to add an extra drop or two and correct for that. Don't try this with the pH test, however. In general, it's best to adjust pH before shocking to high levels (above 10 PPM).

I'm not aware of any interference from high chlorine levels with the Calcium Hardness or Cyanuric Acid test. Has anyone else encountered this?

[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.

[sturev]

Thanks Chem Geek!!! I love learning all this...

[polyvue]

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.

It would be interesting to test this with FC levels of 20, 30, 40 PPM... both with and without the thiosulfate.

[waterbear]

You all forgot one test that is affected by high chlorine levels...all DPD chlorine tests (comparator is shades of red/pink) (and to a lesser extent FAS-DPD tests).
They can "bleach out" and remain colorless at high chlorine levels (DPD over 10 ppm FC and FAS-DPD over 25 ppm---but are usable at higher FC levels with certain modifications.)
If your chlorine test is reading low and you suspect there is chlorine in the water (expecially if you saw a pink flash before the DPD test turned clear) do an OTO test (comparator is shades of yellow). It is a foolproof test that will ALWAYS tell you if you have chlorine (or bromine) present.
If it turns yellow to deep yellow it's most likely swimmable, if it's deep yellow to orange you are most likely shocking, if it's orange to brown stay out of the pool but throw in the whites because you have a pool of bleach!

[giroup01]

When testing high levels of chlorine with DPD, add the DPD1 and DPD2 in the empty test cell then add the sample water slowly - it should turn pink, then red then clear as more and more water is added. You can determine that way if you're in a 0 TC or high TC situation.

For CYA there was an article by Woto in the JSPSI describing the interferences of CYA with high chlorine, as I recall he does mention to add a few drops of thiosulfate to the sample water. I'll have to find the article.

And with the ProTeam borate titration, a high TC level bleaches out the indicator.

[waterbear]

And with the ProTeam borate titration, a high TC level bleaches out the indicator.

Which is why I usually use the LaMotte Borate strips! The color change in the Proteam indicator is just plain hard to read, bleachout or not! If memory serves correctly it is bromphenol blue. I do use the Proteam titration kit when I need to make adjustments to the borate levels (perhaps once a year) but the strips are good to tell me I am in the 'sweet spot' of 30-50 ppm!