ph and cloudy water

[Wolffe]

How do you properly test ph in a high chlorine environment? I was fighting off some mustard algae yesterday and while there are no traces of the algae today the water is still a little cloudy (pump running 24/7).

Measuring the ph with a Taylor 2005 kit before treatment, I saw that my ph had suddenly jumped very high (dark purple). I added a bit of acid before starting to shock. Now, after shocking (bleach and more bleach), I see that the ph test starts off really dark purple (high ph) but within a few seconds fades into the mid-range of colors. I know that the ph test can't be trusted with high cl but I'm not sure how to read this... or whether I can read this at all.

Does high cl make the ph test appear lower or higher than it actually is? I'm guessing my ph is actually still high and that the cl is making it fade to the lighter color.

[chem geek]

According to the Taylor "Pool & Spa Water Chemistry" manual:

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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.

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I've noticed that roughly speaking each drop of thiosulfate seems to raise the pH in the test by about 0.05-0.1 so the effect is pretty strong (unfortunately); otherwise, one could just simply neutralize the chlorine.

Richard

[Wolffe]

Thanks! I think I'm going to leave the pH until my CL levels come down enough to get a good reading. Right now, as soon as I mix the test it's a dark color looking like the ph is off the chart... then after 3 or 4 seconds it fades and looks to be about 7.2.

I started fighting the algae yesterday morning (I found the bottom of the pool covered in a light yellowish brown "powder"). After vacuuming everything up, and brushing the sides and bottoms, the pool was looking really cloudy in the deep end. It looked a bit better this morning, and right now the water is sparkling again. I'm going to keep it up until tommorrow, then when the CL levels come down again I'll deal with the pH because I have no idea what level it's at right now.