Calculating pH from number of drops of acid demand reagent?

rick32086

Member
Jan 31, 2019
18
Southern California
Howdy,

I use the Taylor K-2006 kit to test my water. I've always had trouble pinning down pH when it's in the purple range, meaning 7.6 and up pretty much look the same to me. But once I starting adding the R-0005 acid demand reagent, I can see when I hit 7.5.

I want to start using TFP's guidelines for keeping pH between 7.6 and 7.8... not inch up to 8.0 and beyond. So somehow I have to figure out how to calculate current pH using the number of drops of R-0005. I just can't figure it out.

So my question is: If the indicator test (R-0004) shows pH is "purple" and it takes just one drop to get it to 7.5, what was the beginning pH? And what if it takes 2 drops? (since I understand pH is logarithmic???)

Tx
 

reggiehammond

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I want to start using TFP's guidelines for keeping pH between 7.6 and 7.8... not inch up to 8.0 and beyond. So somehow I have to figure out how to calculate current pH using the number of drops of R-0005. I just can't figure it out.
I’m not sure you need to worry that much, the general wisdom is that anything in the 7’s is fine for swimming. It might only be important if you add a SWCG and need to manage CSI, Etc.
 

JamesW

TFP Expert
Mar 2, 2011
31,503
So my question is: If the indicator test (R-0004) shows pH is "purple" and it takes just one drop to get it to 7.5, what was the beginning pH? And what if it takes 2 drops? (since I understand pH is logarithmic???)
Look up the acid demand in the chart and then use pool math to see how much the pH would drop.

For example, if the Taylor book said that you need 15 oz acid, you know the starting pH was 8.1.

You can use either section, but the top section is more accurate.

Using the bottom section, you get a starting pH of 7.86.

1663895587420.png

1663895698175.png

 

JoyfulNoise

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That’s not really how it works. All the acid demand test is telling you is how much acid you need to get to a pH of 7.4. The problem is, that depends on your starting TA and all the tables assume a certain TA range for their calculated amounts of acid. So in a way, the acid demand test is really a measure of how strong all the alkaline buffering species are. The acid demand drops are very course in their concentration so they are not that good for any kind of measurement. Add to that the fact that PoolMath isn’t that good at calculating large changes in pH and your really not improving your measurement accuracy any.

Chem Geek discussed all the problems with using acid demand for calculating pH in an old thread. I don’t have the link but the conclusion is the acid demand drops really aren’t going to improve your testing any.

The real solution is to purchase a digital pH meter if you having trouble with the phenol red drops.
 

JamesW

TFP Expert
Mar 2, 2011
31,503
all the tables assume a certain TA range for their calculated amounts of acid.
The tables for acid demand don't need to know the TA because they use actual acid in the pool water.

The acid demand drops are very course in their concentration so they are not that good for any kind of measurement.
They can give you a specific amount of acid to add to reach a specific pH.

PoolMath can figure out where the pH would have started to get to a specific pH.

PoolMath accounts for TA and borates.

It's not perfect, but I would argue that it is also not that far off.

For example, if the acid demand chart said that it would take 15 oz acid to lower the pH to 7.5, the starting pH could be estimated to be at 8.1 if the TA was 100 or 8.6 if the TA was 60.

1663902374485.png


1663902360256.png

In any case, most people would not find the acid demand test to be very useful.

Most people keep the pH below 8.0 and they can read the test just fine.

A calibrated meter can be useful if one has a problem reading the colors.
 

JoyfulNoise

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I wouldn’t trust PoolMath to do the calculations. There are serious truncation errors in the code. The Java code will throw lots of NAN errors if you go too far outside of the normal pool water limits.

You can use Richard’s advanced spreadsheet to do the calculations.
 

JamesW

TFP Expert
Mar 2, 2011
31,503
Maybe get a pH meter and do the test method to see if they are close to the same number.

Overall, I don't think that the acid demand test is very useful for most people.

If someone wants to use it for whatever reason, I don't see any problems with using the acid demand test.

Maybe get someone to read the color for you or maybe get a color reading app for your phone.
 

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mgtfp

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Chem Geek discussed all the problems with using acid demand for calculating pH in an old thread. I don’t have the link but the conclusion is the acid demand drops really aren’t going to improve your testing any.

I think this is the post you are referring to:


I considered this approach once, as I also struggle to differentiate the darker orange/red shades, but decided to get a pH-meter instead. Don't regret it.
 

JoyfulNoise

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The Apera PH60 is the one I have and, when fully calibrated (3 point calibration) is dead-on accurate with the phenol red drops. Problem is, it does cost an arm and a leg … ok, maybe a finger and a toe … it ain’t cheap. But it’s also certified and warrantied. I use it to test my pool water and my RO tap water.
 

mgtfp

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I second the Apera PH60. In the long run you'll save some money because you can replace the sensor probe and don't have to buy a complete new meter when the probe fails.
 

SoDel

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May 27, 2022
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I’m also thinking about getting one. The only thing stopping me is wondering how often do you need to calibrate it and can it sit in the preservative unattended while the pool is closed (about 5 or 6 mos. here)? I have an inexpensive one I got from Amazon just to try but it seems to need very frequent calibration which I’m sometimes (most times :) ) too lazy to deal with, and knowing myself, when I put it away for the off-season it won’t be touched again until the pool opens.
 

JoyfulNoise

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I’m also thinking about getting one. The only thing stopping me is wondering how often do you need to calibrate it and can it sit in the preservative unattended while the pool is closed (about 5 or 6 mos. here)? I have an inexpensive one I got from Amazon just to try but it seems to need very frequent calibration which I’m sometimes (most times :) ) too lazy to deal with, and knowing myself, when I put it away for the off-season it won’t be touched again until the pool opens.

3-point calibration monthly if used a couple of time per week. I have the combo pH/EC probe as I use it to test my RO tap. If it is stored properly in its KCl solution, it holds it calibration very well. If you’re not going to use it over winter, then it can simply be stored with its cap “wet” (there are visible droplets of water in the cap meaning the humidity is keep high). Then for start up just soak it in KCl for 24hrs prior to recalibration and use.

My pH readings are dead on with the phenol red and the EC readings are very accurate with respect to my IntelliChlor output as well as my K-1766.
 
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JamesW

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Mar 2, 2011
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