Calculating pH from number of drops of acid demand reagent?

Jump to latest Follow Reply
R

rick32086

Member
Jan 31, 2019
19
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
 
rick32086 said:
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.
Click to expand...
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.
 
rick32086 said:
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???)
Click to expand...
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

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.
www.troublefreepool.com
 
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.
 
JoyfulNoise said:
all the tables assume a certain TA range for their calculated amounts of acid.
Click to expand...
The tables for acid demand don't need to know the TA because they use actual acid in the pool water.

JoyfulNoise said:
The acid demand drops are very course in their concentration so they are not that good for any kind of measurement.
Click to expand...
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.
 
If the TA was 30, then the starting pH could be estimated at 9.1.

In my opinion, estimating the pH up to 8.5 using this method will be reasonably accurate and then it will drop off quite a bit.

1663903214307.png
 
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.
 
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.
 

Enjoying this content?

Support TFP with a donation.

Give Support
JoyfulNoise said:
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.
Click to expand...

I think this is the post you are referring to:

Question about Taylor K-2006 kit and pH testing

I mentioned in another thread that I use a Hanna HI-98128 pH meter, but always use the Taylor K-2006 kit to double check it. I know I'm doing it twice, but I have a reason. One, is I like the second decimal place readout of the Hanna. It doesn't make any difference except I can kind of watch...
www.troublefreepool.com www.troublefreepool.com

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.
 
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.
 
  • Haha
  • Like
Reactions: gingrbredman and jark87
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.
 
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.
 
SoDel said:
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.
Click to expand...

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.
 
  • Like
Reactions: SoDel

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply