Taylor K2006 vs. test strip results

[chem geek]

Yes, it's just chemical equilibrium such as taught in 1st year college chemistry or in high school. The difficulty is that there are 13 simultaneous chemical equilibria among the chlorine bound to CYA, the protonated CYA variants, and hypochlorous acid / hypochlorite ion though I use these to calculate the active chlorine (hypochlorous acid) concentration in this spreadsheet and I derive the FC/CYA ratio approximation in this post.

As for the rate at which chlorine releases from CYA when hypochlorous acid is used/consumed, it is very fast where half the FC gets released in 0.25 seconds and this happens regardless of the type of wet-based chemical test (i.e. test strips, OTO, DPD, FAS-DPD) so your assumption is incorrect. Test strips are just pre-packaged wet-chemistry methods -- it is still chlorine reacting with a dye that is adhered to fibers in the test pad, though a different indicator dye is typically used. The methods all work by redox where chlorine oxidizes the indicator dye to a different colored form. The intensity of color is proportional to the quantity of chlorine, so is clearly dependent on how long you keep the strip dipped into the water and on how big a drop you get when you pull the strip out and let it continue to react. The size of that drop will not only be related to how quickly you remove the strip from the water, but to the surface tension of the water itself.

It is only methods that measure instantaneous chemical quantities that correlate with the active chlorine level, such as ORP, amperometric techniques, and membrane chemical-specific sensors. Test strips are also affected by temperature (I forgot to mention that part) because you do the measurement after a certain amount of time but reaction rates are temperature dependent.

If there had been no release of bound chlorine, then when CYA is present you would not have measured any chlorine with the test strips because when the FC is around 10% of the CYA level, the active chlorine level is the same as when there is only 0.1 ppm FC with no CYA. So it wouldn't show up.

Now, that said, the chlorine test strips tend to be one of the better ones. The CYA test strips tend to be one of the worst. Usually you only see Total Hardness strips and not Calcium Hardness so they are measuring magnesium in addition to calcium. Total Alkalinity is also so-so -- usually better than CYA, but often worse than the chlorine tests. And of course, the resolution for these tests isn't very good since they are generally visual "compare to a standard" tests. The FAS-DPD chlorine test, on the other hand, is a "count the drops" test that has a resolution of 0.5 ppm when using a 10 ml sample and 0.2 ppm when using a 25 ml sample. The test accuracy is within one drop or 10%, whichever is greater, and that's a conservative accuracy range -- you can usually do better to within 5% if you are careful.