Acid/Base Demand Test

[waterbear]

Evan (waterbear),

I figured out the source of our disagreement with regard to alkalinity and the table. We don't disagree after all! It's a matter of interpretation and I misinterpreted what you meant by the dependence of the table on alkalinity.

Once one has a specific concentration and type of chemical they are using for the acid and the base used in the drops, then the numbers in the tables are only a function of drop volume to 25 ml sample volume
Which Taylor kit do you have? The K-2005 and K-2006 use a 44 ml sample for the pH and demand testsand use reagents R-0004, R-0005, and R-0006.
and a conversion to account for the type of chemical used for the acid/base in the drops vs. the table. HOWEVER, this specific concentration of acid/base used in the drops is what determines the resolution of the table and the accuracy of the measurements and it is the determination of this concentration that has some "assumption" of alkalinity which determines the accuracy of the test (error from+/- 0.5 drop).

So it appears that the test makers assumed a rather high alkalinity since, as we both pointed out, lower alkalinity has one drop make a rather large color change and therefore each drop represents a rather large amount of acid/base to add and therefore makes the test very inaccurate. The test makers should have used a weaker (diluted) acid and base so that each drop represented a smaller amount of acid/base to add to a pool and so that the method would be more accurate even when the alkalinity is lower. This would mean using more drops in a higher alkalinity pool for typical changes, but so what. The alkalinity test itself typicaly uses about 10 drops anyway and the acid/base chemicals for the test are relatively cheap.
This is what I meant, exactly! I believe they assume an alkalinity of around 100-120 ppm from my own real world experience with the test. When the TA is lower than that there seems to be a greater error introduced.
Whew!

Richard

Think anyone else will understand all this (excluding Ben)?

[chem geek]

Which Taylor kit do you have? The K-2005 and K-2006 use a 44 ml sample for the pH and demand testsand use reagents R-0004, R-0005, and R-0006.

Think anyone else will understand all this (excluding Ben)?

I have the K-2005 which explains the difference. I'm glad I'm not using some sort of inferior Phenol Red. I'll plan to switch to the FAS-DPD test as well since the rough chlorine measurement is a bit frustrating.

Well, whether people understand it or not, the takeaway is the same -- the acid/base demand tests are pretty useless -- and the worst part of that is that they didn't have to be!

Richard

[medvampire]

Originally Posted by waterbear
Think anyone else will understand all this (excluding Ben)?

Yes and I really hate to say I following your discussion. I am color challenged so like chem geek I use the acid base drops to help me zero in on the pH. I do not use the acid base demand test to make actual changes to the pH in the pool knowing that ALK will affect the needed amount of acid. I am with water bear about pH meters. I still occasionally have to use one at work but try to use color indicators as much as possible. I understand the theory but know real world results are what count. My chemistry knowledge is mainly biochemistry so I do understand real world buffers. You should try adding protein buffers into the equation.
Steve