TFP Expert Discussion - pH and Alkalinity

GSD_fanatic

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Thanks Matt...yes mine has the 4.0, along with 7.0 and 10.0

Glad you clarified using the 4.0, otherwise I may have been huh? lol The main reason for using the PH60 in this case is just having another way to verify. I trust the K2006 now that I've used it awhile, and can even see the transitions of color easier. But that is mostly because of repetition. At first I was having trouble recognizing the color change on some tests. I am color blind to a degree, mostly with the yellow and greens, but some shades of red as well. Like the PH test...at first I couldn't tell any difference comparing colors. But now I hold it up to a light/bright background and can easily see the color comparison.
 
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markayash

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Great video, my son had to do an experiment and measure the PH of several bottled water. I was shocked to see Dasani had very low PH compared to the rest. I am sure that's why he had to test the one the teacher told them to use.
Funny part was I couldn't find my test kit, ask wife if she moved it and was starting to think somebody climbed the fence and stole it until she said " ask Jayden"
" Oh yea its upstairs in my room"
 

Brentr

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The lecture will last a little over 90mins with no bathroom breaks. At the end there will be a 20 question review test. You’ll have 15mins to complete the exam … no calculators allowed, slide rules are optional. All answers must be in ink and you must “show your work”. Please record all answers with 3 decimal point precision.
Really 😀
 
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Leebo

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This was my favorite discussion yet! Very interesting information regarding the chemistry involved. One thing I was expecting to come up is why the accuracy of PH readings are affected by high chlorine levels (>10) from a chemistry perspective.
That’s a good question! I’ll make sure @mknauss catches this and we’ll see about working it in sometime in the future!
 

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mknauss

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One thing I was expecting to come up is why the accuracy of PH readings are affected by high chlorine levels (>10) from a chemistry perspective.
Sorry -- you should have been there to post the question on Chat!!
@JoyfulNoise -- can you give us a brief explanation?
 

JoyfulNoise

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Sorry of this picture is hard to see, but it shows the basics. Please note, that pH is only sensitive to free chlorine when using the phenol red indicator, pH probes are not affected by free chlorine levels.

Basically, the interference in the pH test that comes from presence of chlorine is due to several structural changes that converts phenol red into chlorophenol red. Essentially the chlorine atoms substitute for two hydrogen atoms on the phenol rings. Also, the sulfonate group (SO3-) gets converted to a thiol ring (-S-O-C-). The chemical reaction that converts phenol red into chlorophenol red is fairly slow at room temperature and can take several minutes to occur. As well, the Taylor pH reagent contains a blend of reducing agents that will convert oxidizing chlorine into chloride before it can react with the indicator. However, this must be carefully done so that the chlorine neutralizing reactions are pH neutral. Also, due to limitations in the chemistry, the chlorine reducing agents are only good up to about 10ppm FC.

Slide1.png
As you can see, both compounds are pH indicators but their colors and ranges are different. The phenol red that pool owners rely on, turns from yellow to magenta in a pH range of 6.8 to 8.2. That is very useful for standard pool pH testing. Chlorophenol red however is an acidic pH indicator with it's color range going from 4.8 to 6.7. Above a pH of 6.7, the color of chlorophenol red is locked into purple. So, when free chlorine is present above 10ppm FC, some of the phenol red gets converted slowly to chlorophenol red which then causes you to have a mixture of pH indicators. The chlorophenol red adds a purple color to the water while the phenol red is typically some shade of red/pink. This leads to a false red/purple color mixture that makes the solution appear too be a much higher pH than it really is.

In a range of FC from 10ppm to 20ppm, you can sometimes still do the standard phenol red test IF you read the results very fast since it takes time for the chlorophenol red to form. However, with an FC above 20ppm, the standard R-0004 reagent really doesn't work well. In that case, you can dilute the water sample 1:1 with DISTILLED water to try to cut the FC in half. Pool water has such high alkalinity that the pH of the sample will not change that much with dilution using DISTILLED water. Or, if you can, wait for the FC to return to normal and then test pH.
 

AUSpool

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Sorry of this picture is hard to see, but it shows the basics. Please note, that pH is only sensitive to free chlorine when using the phenol red indicator, pH probes are not affected by free chlorine levels.

Basically, the interference in the pH test that comes from presence of chlorine is due to several structural changes that converts phenol red into chlorophenol red. Essentially the chlorine atoms substitute for two hydrogen atoms on the phenol rings. Also, the sulfonate group (SO3-) gets converted to a thiol ring (-S-O-C-). The chemical reaction that converts phenol red into chlorophenol red is fairly slow at room temperature and can take several minutes to occur. As well, the Taylor pH reagent contains a blend of reducing agents that will convert oxidizing chlorine into chloride before it can react with the indicator. However, this must be carefully done so that the chlorine neutralizing reactions are pH neutral. Also, due to limitations in the chemistry, the chlorine reducing agents are only good up to about 10ppm FC.

View attachment 358560
As you can see, both compounds are pH indicators but their colors and ranges are different. The phenol red that pool owners rely on, turns from yellow to magenta in a pH range of 6.8 to 8.2. That is very useful for standard pool pH testing. Chlorophenol red however is an acidic pH indicator with it's color range going from 4.8 to 6.7. Above a pH of 6.7, the color of chlorophenol red is locked into purple. So, when free chlorine is present above 10ppm FC, some of the phenol red gets converted slowly to chlorophenol red which then causes you to have a mixture of pH indicators. The chlorophenol red adds a purple color to the water while the phenol red is typically some shade of red/pink. This leads to a false red/purple color mixture that makes the solution appear too be a much higher pH than it really is.

In a range of FC from 10ppm to 20ppm, you can sometimes still do the standard phenol red test IF you read the results very fast since it takes time for the chlorophenol red to form. However, with an FC above 20ppm, the standard R-0004 reagent really doesn't work well. In that case, you can dilute the water sample 1:1 with DISTILLED water to try to cut the FC in half. Pool water has such high alkalinity that the pH of the sample will not change that much with dilution using DISTILLED water. Or, if you can, wait for the FC to return to normal and then test pH.


o_Oo_O
So what your saying in simple terms is that FC above 10ppm will interfere with the phenol red pH test results. :)

But jokes aside, why calibrate at a pH of 4? Why not 7 and 10 which is either side of the ideal pool range?

Ooh, good work and thank you, to both yourself and Marty.
 
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tclayton10

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Just watched the replay, and was blown away at how Matt was able to show in layman's terms how all of these various chemicals interact with each other. I especially liked his comment towards the end where he advised to get the pH in order and let TA find its happy place. I'm definitely taking that to heart!

Tracy
 

JoyfulNoise

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o_Oo_O
So what your saying in simple terms is that FC above 10ppm will interfere with the phenol red pH test results. :)

But jokes aside, why calibrate at a pH of 4? Why not 7 and 10 which is either side of the ideal pool range?

Ooh, good work and thank you, to both yourself and Marty.

You only need to calibrate a pH probe at 4.0 if you plan to use it to measure TA. The titration test for TA starts with water at normal pool pH (7 range) and then adds a strong acid dropwise until the pH reaches 4.5 . If you plan to do the test without using the green/red indicator, then you need to make sure your pH probe is linear at the point you want to stop the test at. Since pH probes are not linear over large ranges, making sure it is properly calibrated at a pH of 4.0 is very important.
 

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