Wierd PH readings...

[poolmom06]

I just test my water, my ph at first was 6.8, but I am fighting some algae and have added bleach to elevate my cl. So I assumed this was a false low reading, so I added a couple drops of Thiosulfate before my PH indicator and it read at 7.5ish, ok, so I went out and tested again the same way, but with 4 drops of Thiosulfate, the reading was 8.2, so now I have gotten readings from 6.8 to 8.2, I tried several more times with just two drops of Thiosulfate and got 7.6-7.8 consistently. Is this the actual reading or should I add something , if so, what?

[chem geek]

I just test my water, my ph at first was 6.8, but I am fighting some algae and have added bleach to elevate my cl. So I assumed this was a false low reading, so I added a couple drops of Thiosulfate before my PH indicator and it read at 7.5ish, ok, so I went out and tested again the same way, but with 4 drops of Thiosulfate, the reading was 8.2, so now I have gotten readings from 6.8 to 8.2, I tried several more times with just two drops of Thiosulfate and got 7.6-7.8 consistently. Is this the actual reading or should I add something , if so, what?
It is best not to add thiosulfate when doing the pH test. For the alkalinity test, it's fine (because it's a titration not significantly affected by the thiosulfate pH change), but not for the pH test. The reason is that the breakdown of chlorine by thiosulfate greatly changes the pH (usually increasing it). There are many different reactions that can occur with thiosulfate and chlorine -- all of them consume the chlorine, but they result in different products and different pH changes (and directions!). In my experience, the net effect is generally a rather strong rise in pH.

The Taylor kits recommend not using thiosulfate with the pH test, but most OTO kits do. I probably misread pH for 2 years before I figured this out and got the better kit. Anyway, this is all well and good but doesn't solve your problem. I assume you are adding the thiosulfate because you believe the high chlorine level is interfering with the pH test? That may be true and I recall something in the Taylor instructions talking about this, but I'm not at home right now so can't look that up.

Anyone out there got their Taylor instruction booklet? Please look up high chlorine interference with the pH test.

My recollection is that one of two colors gets bleached out so if you had a base demand test then you could add some drops to see if that's the right direction (and roughly how far away you started from). See, I knew there'd be a time when the acid/base demand tests would find some use!:D

Richard

[poolmom06]

I tested it again and it said 6.8 so I added 2 pounds of Borax to pull it up... how long until I can test it again? I was gonna turn the pump off tonight to let the dead algae settle for vaccuming but I may wait till later...

[Watermom]

I usually wait 3 or 4 hours after adding Borax before retesting.

[bradjo]

Chlorine over 10ppm converts phenol red to chlorphenol red (dichlorophenolsulfonephthalein), Chlorphenol red measures pH from about 5.2 to 6.6 if memory serves turning a dk purple at 6.6 unlike phenol red which is dk purple in the 8's. So you can make a serious error here.

Thiosulfate is base and can give a false high pH reading here's where a 44 ml sample for pH is a good thing.
hth,
Jo