Reading PH

[Dianne]

I use Taylor reagents for my pool water testing. My PH reads 7.2 and total alkalinity is 180 ppm. I noticed that if I put a drop of Thiosulfate into the color comparator for testing PH (using Phenol Red), I get a different Ph of around 7.4 .
Is it good practice to add a drop of thiosulfate to eliminate the chlorine prior to testing PH?
I am curious if the chlorine(especially after shocking using Dichloro)could possibly "bleach out" the phenol red reagent color and give a false low result.
Also, what is the best light source to use when checking for PH using a color comparator?