During shocking and in pools with very high CYA levels where the normal FC level must be very high to prevent algae growth, it becomes difficult to get an accurate pH reading because the high FC level causes a false reading. This is described in the Taylor's Pool & Spa Water Chemistry booklet as follows:
I've been thinking about possible approaches to working around this problem and one approach would be to add a measured amount of ammonia sufficient to convert all of the FC into monochloramine. If monochloramine does not convert phenol red into chlorphenol red because monochloramine is a weaker oxidizer than hypochlorous acid, then this approach may work. The formation of monochloramine from ammonia will somewhat raise the pH, but by a known amount that can be compensated for in the calculations if one knows the starting water chemistry parameters.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.
To test this theory out, we first need to create a stock solution of ammonia that is at a concentration where adding drops of it to a sample of known size will give us the result we desire. The Church & Dwight product known as Parson's Ammonia All-Purpose Cleaner has an MSDS indicating that it is <3% ammonia. Hach has an Ammonia Standard Solution that is N 1000 mg/L (i.e. 1000 mg Nitrogen per liter).
What we want is a solution whose concentration will neutralize the chlorine level in the sample. For simplicity, since high chlorine levels are typically measured using 10 ml samples where each drop of FAS-DPD titrating reagent is 0.5 ppm FC, we can make an ammonia solution where one drop from a 0.75 oz. Taylor bottle "neutralizes" 0.5 ppm FC. Taylor's dropper tips are standardized to 24 drops/ml. So the next piece of information we need to know is the sample size and this varies depending on the pH kit. For the Taylor K-2xxx series, the pH sample size is 44 ml while for the Taylor K-1xxx series (and the TF-100) the pH sample size is 7.5 ml. It takes an equal molar amount of ammonia to combine with chlorine to produce monochloramine, but in relative units of ppm N for ammonia and ppm Cl2 for chlorine, it takes 0.1975 ppm N of ammonia for every 1 ppm of chlorine. So for 0.5 ppm chlorine, it takes 0.0988 ppm N of ammonia.
Since we want one drop from a Taylor bottle to "neutralize" 0.5 ppm chlorine and since there are 24 drops/ml, the dilution ratio for the 44 ml sample size is 44/(1/24) = 1056 while for the 7.5 ml sample size it is 7.5/(1/24) = 180. So we want the concentration of ammonia solution to be 0.0988*1056 = 104 ppm N for the 44 ml sample size and 0.0988*180 = 17.8 ppm N for the 7.5 ml sample size. This implies diluting the Hach standard solution (with distilled or at least filtered water) by 1000/104 = 9.6 to 1 for the 44 ml sample size and 1000/17.8 = 56.2 to 1 for the 7.5 ml sample size. For simplicity we can just make these 10:1 and 50:1 dilutions. We just need to ensure that we add enough ammonia to get most of the FC down -- adding a little less just means a low remaining FC level while adding a little more just means some excess ammonia, neither of which are a problem.
So creating a stock solution is probably most easily done using a spare Taylor bottle since it has a nice calibrated dropper tip. Creating 20 ml of solution using the graduated cylinders in the kits, for the 10:1 (really 9.6:1) dilution for the 44 ml sample size, one would add 50 drops of the Hach standard and then fill up to the 20 ml line using distilled (or at least filtered) water. For the 50:1 (really 56.2:1) dilution for the 7.5 ml sample size, one would add 8-9 drops (it's 8-1/2) and then fill up to 20 ml. The resulting 20 ml of solution can be stored in a 0.75 ounce (22 ml) Taylor bottle.
As for adjusting the pH reading, this mostly depends on the starting FC, TA and pH. (more on this later...will edit this post).