PH reading different from phenol red vs. strip

[poolaris]

When I measure the ph in my spa with a regular ph test kit using phenol red reagent it shows a dark purple color. A test strip however shows normal 7.4
(the more usual red/orange). The only changes I made was going from chlorine to bromine and there is some of both in there.

Does Phenol red react to bromine?

[aylad]

I don't know if phenol red reacts to bromine (I'm sure chemgeek or waste can give you that info), but I do know that if your chlorine level is high, then your pH will read falsely high with phenol red, and possibly falsely low on a test strip because it bleaches the strip. I would trust phenol red results over a test strip anyday.

Janet

[chem geek]

Yes, the phenol red pH test is affected by either chlorine or bromine being high, usually above 10 ppm FC for chlorine or 22 ppm Total Bromine in the case of the Taylor kits.

[waterbear]

IF you are using an inexpensive phenol red test then it can show interfernece from chlorine as low a 3 ppm and bromine as low as 6 ppm and turn purple.