High FC - pH measurement options?

[UnderWaterVanya]

While I realize that most of the time our stock answer is that pH cannot be measured accurately during any condition where FC > 10; a few members have high CYA and restricted water options. Also some of us would like to know what pH is doing during shocking.

From what I understand, the phenol red tests for pH do not work reliably at FC > 10. So is there any way to reduce FC without destroying the accuracy of the tests available?

Idea: Add distilled water 1:1, 2:1, 3:1 to reduce FC < 10.
Concerns: Unclear what impact adding pH 7.0 to an existing pH solution is.

Idea: Use electronic pH monitor (or other non-Phenol red)
Concerns: Unknown accuracy overall, unknown if FC level impacts these.

Idea: Use Sodium thiosulfate (chlorine reducing agent) on pool water.
Concerns: Thought to impact phenol red test to an unknown degree.

Idea: as above - add Polyquat 60 to the pool bucket to increase chlorine demand and speed the process.
Concerns: Unknown impact on pH.

Idea: Set a bucket of pool water in sunlight to increase loss to sun - do not add bleach to this when increasing pool back to shock level. Measure pH once the FC has dropped below 10.
Concerns: May only be helpful if the pool is being covered during sunlight. Unclear if this will help in many cases - FC levels could be high enough that it will not drop quickly enough.


Anyone with critique and input please chime in.

 

[onBalance]

Using a pH meter is accurate, despite FC level.

 

[chem geek]

Dilution with distilled water (that has nearly 0 ppm TA) would work because it won't change the pH in a measurable way because of the pH buffering in the sample being diluted. The electronic pH meter would work, but can be expensive. The other methods reducing chlorine levels won't work because they can affect the pH themselves due to the pH of their buffered solutions and the dechlorination will usually lower the pH (remember that chlorine usage/consumption is an acidic process though it depends on the specific dechlorinating chemical that is used). The Taylor phenol red solution has a special combination of chlorine reducing agents that try to be as pH neutral as possible when converting chlorine to chloride.

However, in a pool with CYA, the oxidation of phenol red to chlorphenol red appears to be somewhat slow taking a minute or two. So if you measure the pH in a reasonable time, it is likely to be accurate. JamesW did some experiments that showed this. The fact is that shocking the pool to higher FC levels raises the pH. This is known and predictable. So when people see high pH when shocking, this isn't because the test is invalid but more likely to be a true reflection of the high pH. This is why I recommend lowering the pH first before shocking, most especially when shocking to high levels such as when the CYA is high or when going to yellow/mustard algae shock levels.

 

[jblizzle]

So when people see high pH when shocking, this isn't because the test is invalid but more likely to be a true reflection of the high pH. This is why I recommend lowering the pH first before shocking, most especially when shocking to high levels such as when the CYA is high or when going to yellow/mustard algae shock levels.

So is the forum's recommendation to not act on high pH measurements at high FC, really more due to the fact that the pH may truly be high due to the large addition of bleach, but will drop again when that FC is consumed? Or is the bleach really not that pH neutral?

I see where UWV is coming from when people are trying to maintain a 100+ CYA to just get through the end of the season. Am I understanding that if they are maintaining FC slightly above 10ppm, that the pH test is immediately valid, but may drift up in a minute or so? If they are not in "shock mode" and adding mass quantities of bleach, that the pH test should still be trusted and acted upon?

I had asked about this as well previously, if the ratio of the FC to CYA had any affect on the pH test. I thought I came away based on JasonLion's explanation that the pH quickly reads higher as the FC gets above 10ppm (regardless of the CYA) ... thus keeping the recommendation to only test pH below 10ppm.

 

[UnderWaterVanya]

Dilution with distilled water (that has nearly 0 ppm TA) would work because it won't change the pH in a measurable way because of the pH buffering in the sample being diluted. The electronic pH meter would work, but can be expensive. The other methods reducing chlorine levels won't work because they can affect the pH themselves due to the pH of their buffered solutions and the dechlorination will usually lower the pH (remember that chlorine usage/consumption is an acidic process though it depends on the specific dechlorinating chemical that is used). The Taylor phenol red solution has a special combination of chlorine reducing agents that try to be as pH neutral as possible when converting chlorine to chloride.

However, in a pool with CYA, the oxidation of phenol red to chlorphenol red appears to be somewhat slow taking a minute or two. So if you measure the pH in a reasonable time, it is likely to be accurate. JamesW did some experiments that showed this. The fact is that shocking the pool to higher FC levels raises the pH. This is known and predictable. So when people see high pH when shocking, this isn't because the test is invalid but more likely to be a true reflection of the high pH. This is why I recommend lowering the pH first before shocking, most especially when shocking to high levels such as when the CYA is high or when going to yellow/mustard algae shock levels.

Always love your responses. Thank you. This helps me understand this more and while I don't face any long term use of FC > 10 I know there are people on here who are trying to make it through the season or waiting for money for a truckload of water etc. Having some way of maintaining pH accurately in those longer term FC > 10 scenario's seems useful. Distilled water method seems the best - although you seem to indicate that perhaps a quick read on the pH would work also.

 

[chem geek]

So is the forum's recommendation to not act on high pH measurements at high FC, really more due to the fact that the pH may truly be high due to the large addition of bleach, but will drop again when that FC is consumed? Or is the bleach really not that pH neutral?

I see where UWV is coming from when people are trying to maintain a 100+ CYA to just get through the end of the season. Am I understanding that if they are maintaining FC slightly above 10ppm, that the pH test is immediately valid, but may drift up in a minute or so? If they are not in "shock mode" and adding mass quantities of bleach, that the pH test should still be trusted and acted upon?

I had asked about this as well previously, if the ratio of the FC to CYA had any affect on the pH test. I thought I came away based on JasonLion's explanation that the pH quickly reads higher as the FC gets above 10ppm (regardless of the CYA) ... thus keeping the recommendation to only test pH below 10ppm.

Nobody ever said that bleach was pH neutral upon addition. It is only closer to pH neutral when accounting for the acidity of chlorine usage/consumption. That is, it is closer to pH neutral over time when measuring the same FC level at two points in time. Hypochlorite sources of chlorine significantly increase the pH depending on how much you add and your TA and borates levels. Ironically, the presence of CYA has the pH rise more when you add hypochlorite because CYA tries to buffer the active chlorine (hypochlorous acid) level with the result that the pH goes up higher due to the following equation (which is 50/50 balanced only near a pH of 6.1 so at pool pH it is far to the right):

OCl^- + H₂CY^- ---> HClCY^- + OH^-
Hypochlorite Ion + Cyanurate Ion ---> "Chlorine Bound to CYA Ion" + Hydroxyl Ion

Without any CYA in the water, there is less movement of pH that is due to the following which is 50/50 balanced near a pH of 7.5:

OCl^- + H⁺ ---> HOCl
Hypochorite Ion + Hydrogen Ion ---> Hypochlorous Acid

Specifically, at a TA of 80 ppm and with a CYA of 50 ppm, if you raise the FC by 10 ppm using 6% Clorox bleach which has the minimum amount of excess lye in it, the pH rises from 7.5 to 8.1. If there were no CYA in the water, it would rise from 7.5 to under 7.8. If there is CYA but also 50 ppm Borates, then the pH rises from 7.5 to 7.7.

I've always suggested that one lower the pH first before shocking -- usually to 7.2 though if shocking to high levels (> 20 ppm FC added) than even 7.1 or 7.0 would be OK when borates are not present (with borates, 7.2 is still OK to 30-40 ppm).

Based on JamesW's experiments, it would appear that the pH is valid even above an FC of 10 ppm so long as one reads it soon after adding reagent. Unfortunately, Taylor would not confirm this effect nor do any experiment to validate it. It seems that the oxidation of the phenol red reagent is from the active chlorine (HOCl) and/or hypochlorite (OCl^-) so with CYA significantly lowering those levels the reaction slows down, though still ultimately occurs which is why waiting (at high FC levels) will still show the chlorphenol red after a minute or two.

We didn't change recommendations here because we didn't get any confirmation from Taylor and we should have others try this out with their pool water as well just in case there's anything in the pool water that would change our conclusion (i.e. perhaps the presence of some metals might catalyze the chlorination of phenol red -- I doubt it, but one never knows). At high shock levels of 20 ppm or above, one should see this color change effect over time and that would help confirm what is going on, especially if at very high FC of 30 ppm or above one eventually gets the telltale chlorphenol red color (dark purple).

 

[JamesW]

As best as I can tell, pH is probably testable when the FC is above 10 ppm, but the FC/CYA ratio is low. However, I am still cautious about trusting the numbers. Also, when shocking, the FC/CYA ratio is probably going to be high enough to cause at least some interference.

Overall, it's best to lower CYA to at least less than 80 ppm whenever possible. When one is going to try to maintain a pool with a CYA higher than 80 ppm, it is important to avoid needing to shock by maintaining a sufficient FC level.

The pH should be measured before and after shocking, and probably not during shocking.

 

[chem geek]

An FC that is 7.5% of the CYA level has the same active chlorine level as an FC of 0.06 ppm with no CYA, but at a shock level FC that is 40% of the CYA level this has an active chlorine level the same as an FC of 0.6 ppm with no CYA so about 10 times higher. So if it took 1 minute for the pH test to get interference at the usual minimum FC level, then it would take only 6 seconds at shock level, so pretty fast. This is speculative since the latter experiment hasn't been done, but it's something to consider as James has pointed out.