I use generic ph strips for a daily check of the hot tub but lately the colors don’t seem to match up. I bought a different brand at the hardware store and the same thing happened. I then tested them in various acetic/basic solutions and none of the colors matched. What I’m saying is that none of the colors on any strips matched any color on the comparison chart. They used to. Do I need glasses?
the PH indicators are dyes, and dyes can fade, expecially with regard to sunlight. However, this seems to be a colour matching problem. It may be that the printed colours do not match the test strips, eg a problem in printing. It may be the colours of the lights you are using (daylight versus incandescent). It may be you are tring too hard to match colours and so you should just accept a slight discrepency in the shade of green