Ok so then its just the phenol red test that is not accurate above 10FC?
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The pH can be inaccurately high when FC is above 10. Above 10ppm FC, you start see the effects of chlorphenol red formation but between 10ppm and 20ppm the reaction is fairly slow. You can still do the pH test if you add the drops and do the reading waiting 10 to 15 secs. Above 25ppm FC, the chlorphenol red formation is too fast.
However, there is an additional work-around that will still give accurate results on pH - take a pool water sample that has high FC (below 20ppm) and dilute it 1:1 with distilled water. Distilled water has very low to near zero alkalinity and a pH of about 7.0 or so. Because of that low alkalinity, there will be almost no shift in pH upon dilution but a reduction in half of the FC level. You then read your pH normally.
The Taylor R-0004 pH Indicator solution has a proprietary combination of chlorine neutralizers already in it that try and keep the pH the same when chlorine gets neutralized, but there's only enough to neutralize roughly 10 ppm FC or so of chlorine. They didn't want to add more since they didn't want to adversely affect results by this neutralizer solution (i.e. too much could affect the pH). Handling 0-10 ppm FC seemed reasonable.
The quote from Taylor's Pool & Spa Water Chemistry booklet on the issue of a high FC causing a false high pH reading is as follows:
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.
Testing pH When FC is > 10
One way to correct the chlorine interference by adding 1 drop of R-0007 to a 50 mL sample; then add that treated sample to the comparator. However, adding R-0007 to high FC water is known to cause changes in pH therefore rendering the test results useless.
In general you DO NOT want to use R-0007 drops with the Taylor phenol red. When sodium thiosulfate reacts with chlorine, there are several dechlorinating reactions that can occur and none of them are pH neutral. That means if you add the R-0007, you will shift the pH of the resulting solutions.
A better way to test pH in high FC water is to mix a 1:1 solution of pool water with DISTILLED water. Then test pH. Doing that will cut the FC down by half but have almost no effect on pH (due to the much higher alkalinity of pool water compared to distilled water).
Distilled water is typically at a pH of 6.9-7.0. It has zero carbonate alkalinity and no mineral hardness. Diluting a pool water sample with it doesn’t change anything since the alkalinity of the pool water will resist any changes to pH introduced by the distilled water. That’s why it works better than adding R-0007 because the thiosulfate in R-0007 chemically reduces the chlorine but that reaction causes a shift in pH.
