Re: New Pool Owner/Chemistry Worry?
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As far as a more accurate way of testing CYA, the $15 Aquachem test kit from walmart will test more accurately then strips...as will the Taylor K-2006 and K-2005 kits. You seem to have the basic Taylor kit that only tests FC, TC, Acid/Base demand (a useless test....it makes assumptions about the ALK and if not in normal range the demand test results are not accurate), and possibly ALK. You really need a more complete kit. The Taylor K-2006 with the FAS-DPD chlorine test is good...A better 'bang for your buck is the PoolSolutions ps234s available on the sister website to this forum.
Evan (waterbear),
The acid and base demand tests actually do not make any assumptions about the alkalinity which is why such tests are in fact reasonable. The tests simply add real acid or base to your existing pH measurement sample and you note the color change. The tables simply convert the number of drops into the amount of acid or base needed in scaled-up pool volumes.
If your alkalinity is very high, then adding the drops will only make the color change a little bit because the pH won't move much. If the alkalinity is very low, then the drops will make the color move a lot. The tables have nothing to do with alkalinity and are simply volume conversion tables from your drops into the amount needed for pool volumes (plus a conversion to the type of acid or base you use if that is different than what is in the drops).
I have a spreadsheet that does the calculations for me so I don't use nor need the acid/base demand test for figuring out how much acid/base to add. However, I do use the drops to make the color move when I'm in that hard to discern range of pH between 7.4 and 7.6. By adding a drop one way and then a drop or two the other way, I can verify that I am indeed straddled between two of the colors.
Richard
Re: New Pool Owner/Chemistry Worry?
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Originally Posted by chem geek
Evan (waterbear),
The acid and base demand tests actually do not make any assumptions about the alkalinity which is why such tests are in fact reasonable. The tests simply add real acid or base to your existing pH measurement sample and you note the color change. The tables simply convert the number of drops into the amount of acid or base needed in scaled-up pool volumes.
In actual practice I have seen these tests overdose and underdose pools. If the TA is not a factor coming into play then it is the tables themselves introducing error when trying to change a minute quatity such as a drop into pints and pounds of chemical. These demand tests, in my experience, are not very worthwhile. I do use them when testing water and usually recommend putting in half of the amount suggested and retesting. In the majority of cases (at least with lowering pH) this is sufficient if the TA is not way out of line. If TA is low I even lessen the amount and if it is high I recommed retesting and reajusting pH until the goal is reached!
If your alkalinity is very high, then adding the drops will only make the color change a little bit because the pH won't move much. If the alkalinity is very low, then the drops will make the color move a lot.
My point exactly. Lower TA values will make pH shift quickly and high TA values will keep pH from changing and then it changes very fast with just a small addional amount pH adjusting chemical added. I am not sure but I believe the tables are based on a 'normal' alkalinity range and since an imprecise drop in a very small sample is being extrapolated into a sample of thousands of gallons of water any error introduced is magnified greatly!
The tables have nothing to do with alkalinity and are simply volume conversion tables from your drops into the amount needed for pool volumes (plus a conversion to the type of acid or base you use if that is different than what is in the drops).
I have a spreadsheet that does the calculations for me so I don't use nor need the acid/base demand test for figuring out how much acid/base to add. However, I do use the drops to make the color move when I'm in that hard to discern range of pH between 7.4 and 7.6.
The colors produced by phenol red are very distinct and a comparator is not needed at all if your are familiar with them. In fact, it is very easy to differentiate between a pH of 7.4 (orange color) and a pH of 7.6 (pinkish red color) without a comparator!
The phenol red color range is as follows:
purple 8.2 or above
purplish red 7.8- 8.0
red (actually pinkish) 7.6
orange 7.4-7.5 (this is the color to shoot for when adjusting pH)
yellow orange 7.0-7.2
yellow 6.8 and below
If you use your demand reagents to take the indicator through all of its colors (IMHO, the best use for them) you wil soon learn to recognize them without the comparator scale, especially the orange color of pK[a] at pH of about 7.4 for phenol red (sorry, there are no subscripts so I substituted brackets.) (Actually, I believe the actual pK[a] for phenol red is 7.3, certainly close enough for balancing a pool!) You can even double check with a pH meter but you will find if you do the colors of the indicator and pH are extremely close, if not spot on, to what I say above!
In actual practice my customers have done much better at maintaining a proper pH in their pools when I tell them to just make the color orange with their inexpensive 2 way (oto and phenol red) testers! We use a LaMotte Waterlink Express colorimeter at work and when I test their water with it I find that they are in the range of 7.4-7.5.
By adding a drop one way and then a drop or two the other way, I can verify that I am indeed straddled between two of the colors.
Richard
Understand that I come from the point of view of actually adjusting pH in pools, my own and my customers who tend not to be as versed in chemistry as I am.
Re: New Pool Owner/Chemistry Worry?
Evan (waterbear),
While I agree that the test is imprecise because +/- even half a drop can be a somewhat large dosage difference, I say again that the tables have nothing to do with alkalinity and are not based on any "normal" alkalinity range. If there was very little alkalinity in the system and therefore adding a drop of acid made a large color change then in fact this is what would also happen in your pool when adding a relatively small amount of acid -- it would lower the pH quite a bit. So the drop test reflects (in theory) what would happen in your pool.
Now, that said, you are absolutely right about the impracticality of the test due to the errors it can introduce. Not only is their the +/- 1 drop error, but the reagents themselves may be in error and oftentimes the adjustment is rather small so 1 or 2 drops is a very small amount of liquid that can get contaminated (by an unclean dropper tip) whereas most other tests uses 5 drops or more so any such contamination errors are a smaller fraction. At any rate, we agree on the test not being accurate -- we just disagree on the technicality of whether the test is "independent" of alkalinity.
As for the colors in the phenol red test, I don't think I made myself clear. I can obviously tell the difference between 7.4 and 7.6, but I have a hard time knowing if the color I am seeing is really a 7.5 or a 7.4, for example. That is when the extra drops of the acid and base demand test come in handy. Then again, I am using the Taylor complete test and not Ben's test and my understanding is that Ben's test uses a different phenol red. Do you know what the difference is? Is it more concentrated to produce a deeper color? If so, then that would probably help me.
It sounds like I'm too overly concerned with hitting the right pH. The main reason I try to get that parameter right is that a 0.1 shift affects water balance much more than the 10 ppm shifts in other parameters (CH, CYA, TA, TDS) so I really wanted to get that right. Of course, there's a rather large +/- 0.3 leeway allowed for water balance, but I figure with all the errors introduced elsewhere I wanted to at least get the pH parameter right. Anyway, like I say, it's probably overkill.
Thanks for your feedback. I'm getting educated.
Richard
Re: New Pool Owner/Chemistry Worry?
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 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.
Whew!
Richard
Re: New Pool Owner/Chemistry Worry?
Quote:
Originally Posted by chem geek
Evan (waterbear),
While I agree that the test is imprecise because +/- even half a drop can be a somewhat large dosage difference, I say again that the tables have nothing to do with alkalinity and are not based on any "normal" alkalinity range. If there was very little alkalinity in the system and therefore adding a drop of acid made a large color change then in fact this is what would also happen in your pool when adding a relatively small amount of acid -- it would lower the pH quite a bit. So the drop test reflects (in theory) what would happen in your pool.
In theory, but it does not hold out in actual practice much of the time.
Now, that said, you are absolutely right about the impracticality of the test due to the errors it can introduce. Not only is their the +/- 1 drop error, but the reagents themselves may be in error and oftentimes the adjustment is rather small so 1 or 2 drops is a very small amount of liquid that can get contaminated (by an unclean dropper tip) whereas most other tests uses 5 drops or more so any such contamination errors are a smaller fraction. At any rate, we agree on the test not being accurate -- we just disagree on the technicality of whether the test is "independent" of alkalinity.
As for the colors in the phenol red test, I don't think I made myself clear. I can obviously tell the difference between 7.4 and 7.6, but I have a hard time knowing if the color I am seeing is really a 7.5 or a 7.4, for example. That is when the extra drops of the acid and base demand test come in handy. Then again, I am using the Taylor complete test and not Ben's test and my understanding is that Ben's test uses a different phenol red. Do you know what the difference is? Is it more concentrated to produce a deeper color? If so, then that would probably help me.
The K-2005 (DPD for chlorine) and K-2006 (FAS-DPD for chlorine) use the phenol red r-0004 which is for use with a 44 ml sample so the acid/base demand tests can be done. Ben's kit (and the Taylor K-1000) use a small comparator tube that looks to be about 9 ml, although I have not measured it, and uses reagetn R-0014. I have both comparators and both reagents and often cross check and the colors produces are almost identical. I assume the only difference is in the concentration of the reagent so the same color intensity will be produced in different sized samples. Taylor also makes other phenol red reagents for use with their other comparators. I am not familier with them but the comparators use liquid standards (much like the LaMotte high end comparators do) and supposedly provide a much more accurate color match.
It sounds like I'm too overly concerned with hitting the right pH. The main reason I try to get that parameter right is that a 0.1 shift affects water balance much more than the 10 ppm shifts in other parameters (CH, CYA, TA, TDS) so I really wanted to get that right. Of course, there's a rather large +/- 0.3 leeway allowed for water balance, but I figure with all the errors introduced elsewhere I wanted to at least get the pH parameter right. Anyway, like I say, it's probably overkill.
If you are so concerend about precise pH measurements then perhaps a pH meter might be the way to go. Personally I find them a pain in the a** to use. When I was in High School I was in a special program where I went to school half a day and spent half a day doing research at the University of Miami School of Marine and Atmospheric Science in oceanographic physical chemistry (yeah, I was one of those 'brainy kids':rolleyes:) on the nucleation and precipitation of calcium carbonate from various simple seawater like solutions, real seawater samples and Copenhagen standard seawater to investigate how various ions would affect the precipitation time and the form that precipitated. A pH meter was an integral part of the equipment and it seemed I spent more time doing three point calibrations, cleaning the electrodes with conc. H2SO4 with KMnO4 dissoved in it, and replacing defective electrodes than running samples! (Of course I also had to prepare my own buffers and standards). I have never really cared for using pH meters since! I have played around with some of the inexpensive 'pocket meters' with my aquariums and, while they do produce accurate results, they still suffer from the need for constant calibration and making sure the electrode does not dry out that I find them more trouble than they are worth! Perhaps they would work for you.
Thanks for your feedback. I'm getting educated.
Richard
Hope this info (and some of my background) is helpful
Re: New Pool Owner/Chemistry Worry?
Quote:
Originally Posted by chem geek
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)?;)
Re: New Pool Owner/Chemistry Worry?
Quote:
Originally Posted by waterbear
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
Re: Acid/Base Demand Test
Quote:
Originally Posted by waterbear
Think anyone else will understand all this (excluding Ben)?;)
Yes and I really hate to say I following your discussion.:o 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.:eek:
Steve
