Accurate pH test during shock levels with R-007?

[JesseWV]

Can R-007 be used to neutralize high FC before performing the pH test to get an accurate reading or will this further interfere with the chemistry?

 

[Richard320]

I'm pretty sure it messes things up.

How about you be our guinea pig? Test your tap water pH, then add some R-007 and try it again.

I expect a full report by morning.

 

[JesseWV]

Normal chlorine level, accurate pH reading.


Shock level chlorine, pH reading is inaccurately high.


Chlorine neutralized, pH still reading high.


Obviously there is still something that the R-007 is not neutralizing which is still affecting the pH test.

At least now I know, and knowing is half the battle! :knowing:

 

[Richard320]

Excellent! Bookmarked for future reference.

 

[UnderWaterVanya]

My understanding is that the phenol red from Taylor is already buffered and that neutralizers have an impact themselves on pH.

Sent via Tapatalk...

 

[chem geek]

Taylor uses a proprietary blend of chlorine neutralizers that try to net out to not change the pH when chlorine is present. The sodium thiosulfate in the R-0007, however, tends to be of high pH and will not react with chlorine in a pH neutral way so will affect the pH test which is why Taylor recommends not to use it. With CYA in the water, the pH test may still be OK for a short time even at FC higher than 10 ppm, but for simplicity and to be conservative we generally say not to consider the pH test to be valid when the FC is higher than 10 ppm.

 

[JesseWV]

So what amount of sodium thiosulfate should to be added to what volume of water to neutralize a reasonable amount of chlorine without being considered "heavily used" and thus also causing an erroneous high pH reading?

Also, I have to wonder what the break point is for getting a correct reading even at high FC levels? This makes me want to do some experimenting.

No, I would leave the pH alone. You don't need it higher to kill algae. And odds are that his pH reading may be correct. When the pH reads falsely high, it's false reading is quite high. It won't falsely read a lower reading. If you get a reading of 7.5 or below, odds are it's a correct reading. It's when it looks like 7.8 or 8.0 or higher that it may not be that high. Remember what Taylor says about this process:

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.

The part that Taylor is missing is that the "quickly and completely" occurs when there is no CYA in the water. With CYA in the water it appears that this conversion process takes longer, perhaps 30 seconds or a minute, depending on the FC/CYA ratio. We have just been conservative and simplistic about high chlorine interfering with pH just in case the CYA is low or people don't take a reading quickly. Also, adding a hypochlorite source of chlorine in large quantities WILL raise the pH significantly, especially when CYA is present and borates are not used and if the TA isn't high.

 

[JasonLion]

You can't win with sodium thiosulfate and the PH test. Adding enough to have any meaningful effect on the FC level will throw off the PH reading. If you absolutely must have PH readings when FC levels are high, you will need a completely different approach. However, there really is hardly any situation that comes up when you are following our recommendations where you need to measure PH at high FC levels.

The PH test gets less and less precise as the FC level goes from 10 to about 18. Somewhere between an FC of 18 and 21 the results switch to being completely unusable as the indicator dye gets chemically converted to a different compound by the chlorine.

 

[JesseWV]

So ideally pH levels should be adjusted before shocking. Obviously if you've already added shock level chlorine it's too late. Should pH really be ignored until after the shock process is over?

This bothers me because one of the criteria for completing the shock process is "the water is clear." I can see this being a problem for someone with a high CSI. With improper pH the water may never become clear due to calcium precipitates. I suppose the filter will eventually get it but that could take much longer than necessary if the pH was in the proper range in the first place, correct?

You can't win with sodium thiosulfate and the PH test. Adding enough to have any meaningful effect on the FC level will throw off the PH reading. If you absolutely must have PH readings when FC levels are high, you will need a completely different approach. However, there really is hardly any situation that comes up when you are following our recommendations where you need to measure PH at high FC levels.

The PH test gets less and less precise as the FC level goes from 10 to about 18. Somewhere between an FC of 18 and 21 the results switch to being completely unusable as the indicator dye gets chemically converted to a different compound by the chlorine.

 

[Richard320]

So ideally pH levels should be adjusted before shocking. Obviously if you've already added shock level chlorine it's too late. Should pH really be ignored until after the shock process is over?

This bothers me because one of the criteria for completing the shock process is "the water is clear." I can see this being a problem for someone with a high CSI. With improper pH the water may never become clear due to calcium precipitates. I suppose the filter will eventually get it but that could take much longer than necessary if the pH was in the proper range in the first place, correct?

You can't win with sodium thiosulfate and the PH test. Adding enough to have any meaningful effect on the FC level will throw off the PH reading. If you absolutely must have PH readings when FC levels are high, you will need a completely different approach. However, there really is hardly any situation that comes up when you are following our recommendations where you need to measure PH at high FC levels.

The PH test gets less and less precise as the FC level goes from 10 to about 18. Somewhere between an FC of 18 and 21 the results switch to being completely unusable as the indicator dye gets chemically converted to a different compound by the chlorine.

If the pH is correct, or slightly at the low end, there should be no problem. While bleach is alkaline, after it breaks down the net pH change is very small.

Now you also see why we get so insistent upon a full set of readings. If someone intends to start the season with astronomical CH and/or CYA and a green pool, we encourage a partial drain before pouring expensive chemicals in.

 

[Zadd]

In the CPO class they taught that the higher pH level the less effective the chlorine will be at activating and killing organics. Was this just bunk or is there validity in those lessons. If so I would think that it requires pH in the lower side before undertaking the shock process.

 

[JasonLion]

Chlorine is significantly less effective when the PH is high and CYA is zero. As soon as you add CYA that effect is almost completely eliminated. We deal with nearly all outdoor pools, which invariably are using CYA, so the PH effect on chlorine can normally be ignored as the CYA cancels it out.

 

[chem geek]

There's a double-edged sword effect when CYA is present. CYA buffers hypochlorous acid so resists changes to its concentration when the pH changes, but the other way around is also true which is that when a hypochlorite source of chlorine is added, the pH rises more than it otherwise would if there were no CYA. This thread discusses whether one should lower the pH before shocking.

If there is no CYA in the water, but one has a TA of 80 ppm, then if one adds 10 ppm FC using chlorinating liquid or bleach then the pH would rise from 7.5 to 7.76 so not a huge amount. Though the active chlorine (hypochlorous acid) level would drop by about 50% going from 7.5 to 8.0, it's absolute level is already very, very high at 3.4 ppm. When CYA is present, the active chlorine level drops by around 15% for that same pH change so not very much.

Now let's shock the pool with an FC that is 40% of the CYA level, assuming we are starting with no FC and that the TA is 80 ppm:

12 ppm FC added with 30 ppm CYA: pH goes from 7.5 to 8.26; active chlorine is 0.22 ppm (at pH 7.5 it would be 0.29 ppm)
32 ppm FC added with 80 ppm CYA: pH goes from 7.5 to 8.95; active chlorine is 0.18 ppm (at pH 7.5 it would be 0.31 ppm)

So the rise in pH does lower the active chlorine level by about 1/3rd from its intended target though shocking just accelerates things and there's not a single "magic" number for it. The bigger concern is how high the pH rises, especially if there are metals in the water since that can lead to metal staining. It could also cause calcium carbonate precipitation (scaling) though in practice may just make the water more dull/cloudy which wouldn't be noticeable with algae turning from green to gray though it may lead to a slower clearing of cloudy water since some of that cloudiness is associated with calcium carbonate and not dead algae.