pH test with high FC

[Jaimslaw]

Moderator note: Moved from this thread- Saltwater Pool is always reading High Chlorine

Its also true that with FC above 10, other tests can be effected. Most prominently, the pH.

Do you know if the PH readings will be false high or low? My FC now is a little too high at 10. The HDX (Home Depot) test kit has a one drop neutralizing agent and it shows my PH quite a bit higher than two other kits that do not have the added neutralizing drop (unless it's already part of those in PH test solutions).

Little off topic, I Know.

 

[pabeader]

At FC of 10 or more pH will tend to read high. That's what I've read on many posts here.

 

[tim5055]

The Taylor reagents do have chlorine neutralizer included so you do not have to add it.

 

[JoyfulNoise]

So what happens is this - the phenol red indicator dye reacts with chlorine forming a new dye called chlorphenol red. The pH range for that dye is much lower, down at 6.5 or so. When chlorphenol red is added to water above that pH, the dye turns purple.

So, when you get above 10ppm FC, the chlorine neutralizers in the Taylor drops can't handle all of the excess FC above that value and you start to form chlorphenol. HOWEVER, the reaction rate of chlorine with phenol red is a little slow and concentration dependent. So, when your at 10ppm FC, you can still get a decent pH read if you read the color quickly as it takes time for the chlorphenol red to form. The real limit for the Taylor drops is more like an FC above 20ppm because you really can't read the test fast enough before the purple color start to affect the test.

One other thing to mention is this - you should NEVER add any other chlorine neutralizers to the pH test (specifically the sodium thiosulfate from the R-0007 drops). The chlorine neutralizers formulated into the R-0004 (R-0014) pH drops are design to be pH neutral. The R-0007 drops will react with chlorine and cause the pH of the resultant solution to go up. This will skew the pH test higher as well. Other pH tests (like the PoolMaster series) requires adding thiosulfate drops to the water before testing to neutralize the FC. But, since those kits are really not designed for FC above 5ppm, both the pH and TC tests are fairly useless.

 

[JoyfulNoise]

And final thing to mention which is not something you should do in general but something you can do in a pinch if your FC is between 10ppm and 20ppm - you can dilute your pool water sample in a 1:1 ratio with DISTILLED water (not tap or bottled spring water) thus cutting your FC in half and still get a good pH read from the drops. This works because the pH of distilled water is about 7.0 and it has 0 TA. Your pool water has a TA much higher than that so when you add distilled water to the pool water, the TA of the pool water buffers the resulting mixture and keep the pH from changing. I wouldn't do a greater than 1:1 dilution and, if you're close to 10ppm FC, you can use 2 parts pool water to one part distilled and the results should be fairly accurate.

 

[Jaimslaw]

I might tend to believe a Chlorine reading of 10 is accounting for different PH readings in three different test kits. A calibrated ph meter comes in at 7.2. The Taylor is a little hard for me to get use to as the color is more of a darker peach...not a light or pale peach that would indicate low ph. But in fact, could the higher chlorine content be accounting for a peach color that is actually a reading of 7.1 or 7.2? Anyways, I have read some posts that the peach color indicates a ph of 7.5, and is the color just before is starts to move more toward the higher ph color of pink. Anyone else know if that's the case?

My other two tests kits show (I) high PH of 8.0 (Home dept, HDX kit) and (ii) the so called ideal of 7.6 (color match exact on that vial).

I'm going to wait to get my Chlorine levels back to normal and retest at that time. BTW, All other chems are in optimal ranges.

 

[JoyfulNoise]

For the pH probe - how and when was it calibrated? Typically the better pH probes use a 2-point calibration method with buffered pH Calibration solutions (4.5, 7 and 10). 3-point calibration probes are the best but they are typically quite expensive. Single-point probes or those that are "factory calibrated" are suspect or of limited-use.

I would trust the Taylor drops. Even at an FC of 10ppm, you are not high enough to cause problems with the Taylor drops. If you want, post a picture of the tester block and we can all give you our interpretation. As for pH in general, a +/-0.2 units reading is all you really need for pool water. Trying to achieve super-high precision on a pH test for pool water is really not that important.

 

[Jaimslaw]

For the pH probe - how and when was it calibrated? Typically the better pH probes use a 2-point calibration method with buffered pH Calibration solutions (4.5, 7 and 10). 3-point calibration probes are the best but they are typically quite expensive. Single-point probes or those that are "factory calibrated" are suspect or of limited-use.

I would trust the Taylor drops. Even at an FC of 10ppm, you are not high enough to cause problems with the Taylor drops. If you want, post a picture of the tester block and we can all give you our interpretation. As for pH in general, a +/-0.2 units reading is all you really need for pool water. Trying to achieve super-high precision on a pH test for pool water is really not that important.

My Ph meter is dual calibration using the the two buffering agents you refer to, mixed with deionized water. It was a mid range ($100) meter - but even the $300 plus meters got their fair share of one and two star reviews on Amazon, so I am not putting a lot of stock in mine - just thought I would give it a try. (I have a soft spot for gadgets anyways).

I suspect that my variations in PH is Chlorine level related as I do not recall these variations when levels were lower. Will try that dilution method as I already bought a gallon of distilled water to clean the meter after each test. And I will take some high rez photos of the test vials and post them for useful feedback.

I'm not overly anxious about this PH situation - more curious than anything else, being an element of my nature. I do know that with having my pool replaster about 40 days ago, I am now going to be really really diligent about pool chem levels to preserve that new finish - far more than I was before when I would only get around to testing on weekends and, with my SWG creep, having persistent high PH levels on most of the weekdays. I presume those high PH levels would account for the white flakes that used to blast out of the spa return and settle onthe floor of my spa in a white pile.

What I do want to do in this whole excercise is to get comfortable with what I am reading when I test the PH, even though this is one of the easier and less problematic of the chem tests. I know + or - .2 is not the end of the world, although +/- 2 in PH can move the CSI level more than I would have guess - that being from before playing around with the Pool Math calculator, which is just another great function of that useful tool.

In sum, I've shut off the SWG and will wait and re-test when my Chlorine levels return to normal (3-4 ppm).

Thanks.

 

[hagedl]

What is the highest pH you have seen in an otherwise chemically balanced pool with SWG? Is there a way to test for pH greater than 8? Thanks.