New borate drop test at piscines-apollo vs. test strip

mgtfp

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Hi @JoyfulNoise
I'm still not 100% clear about the reason for this step:
Then I would make sure that you are initially adjusting the pH with R-0009 until you get to a pH below 4.5 as that is when carbonate alkalinity is fully (almost) converted to aqueous CO2. Then adjust back up to 7.0-7.5 (doesn’t matter much)

I understand that at pH 4.5 almost all of the carbonates will be in the form of dissolved CO2. But the CO2 remains in the water unless we give it time to outgas with a corresponding TA-neutral pH-rise. But when we immediately bring pH back up again to 7.0 to 7.5 with sodium hydroxide, the carbonates will reshuffle again, and we are back to most of the carbonates being HCO3- as before. So, I don't fully understand the reason for reducing pH all the way down to TA=0. I understand that we need to get quite far down so that we can adjust pH with more than just a fraction of a drop of R0010 to where we want it before adding Mannitol.

So, what really is difference between lets say (just example numbers, don't nail me down on them)
  • adding 10 drops of R0009 to get down to pH 6.0 and then adding 1 drop of R0010 to get pH to maybe 7.2
and
  • adding 14 drops of R0009 to get down to pH 4.5 and then adding 2 drops of R0010 to get pH to maybe 7.3?

I did see a small difference in the end results (which may or may not be significant) as reported above, but I still don't fully get the reason for going all the way down to pH 4.5.
 

JoyfulNoise

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Hi @JoyfulNoise
I'm still not 100% clear about the reason for this step:


I understand that at pH 4.5 almost all of the carbonates will be in the form of dissolved CO2. But the CO2 remains in the water unless we give it time to outgas with a corresponding TA-neutral pH-rise. But when we immediately bring pH back up again to 7.0 to 7.5 with sodium hydroxide, the carbonates will reshuffle again, and we are back to most of the carbonates being HCO3- as before. So, I don't fully understand the reason for reducing pH all the way down to TA=0. I understand that we need to get quite far down so that we can adjust pH with more than just a fraction of a drop of R0010 to where we want it before adding Mannitol.

So, what really is difference between lets say (just example numbers, don't nail me down on them)
  • adding 10 drops of R0009 to get down to pH 6.0 and then adding 1 drop of R0010 to get pH to maybe 7.2
and
  • adding 14 drops of R0009 to get down to pH 4.5 and then adding 2 drops of R0010 to get pH to maybe 7.3?

I did see a small difference in the end results (which may or may not be significant) as reported above, but I still don't fully get the reason for going all the way down to pH 4.5.
You could play around with it and see what happens ;)

If you look at the dissolved inorganic carbon speciation diagram (DIC concentration versus pH), you'll note that at a pH of 4.5 and below, 99% of the DIC is dissolved CO2. As you know, when adding adding acid to water with bicarbonate ions in it (which is what the pool water TA is mostly comprised of), you drive all of the bicarbonate anion into carbonic acid (H2CO3). But carbonic acid is not really a stable species at STP conditions and so the equilibrium shifts towards dissolved CO2 gas and water. If you are agitating the water say with a SpeedStir, then a lot of that CO2 will outgas. The pH will not rise on its own because there's no bicarbonate left in the water, it's all CO2. pH only increases when there is sufficient enough bicarbonate (HCO3-) anion to absorb a proton (H+) and outgas as CO2. But when there is no bicarbonate around, that process cannot happen. Now sure, if you drop the pH and form dissolved CO2 but then immediately raise the pH back up again using a base (NaOH in this instance), then you will shift some of the dissolved CO2 that wasn't able to outgas back into bicarbonate. This is why it's a good idea to use a SpeedStir and wait a bit after adding the R-0009. But, that is something you can test if you want to.

The reason why you want to remove the carbonate alkalinity as best as you can is because it will interfere with the measurement. If there is sufficient alkalinity in the test sample and you add the mannitol to the water to react with the borate, then some of the hydrogen ions generated in that reaction will be buffered by the carbonate alkalinity. That will suppress some of the pH fall and then you'll likely see a negative interference (less boron than anticipated). Pool water is a bit complicated because there is some cyanurate alkalinity that will mess with the pH drop, but that pKa is around 6.5 and so it shouldn't have a huge negative impact on the borate measurement.

As I said previously, this is not the most precise process and it could be greatly improved. But it's good enough to get a borate value that's close to what you would expect. In a perfect test, I would want to use an initial water conditioning agent that would both remove the carbonate alkalinity and lower the CYA concentration (perhaps by adding melamine to remove CYA), then you might have to filter the sample and could work with that to get a more exact borate test. However, that would now cross over from the world of pool side testing into the realm of white lab coats, safety goggles, and flaming Bunsen burners .... not very convenient for most people.
 
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mgtfp

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Thanks, Matt now it's coming together in my head.

That's the bit I hadn't considered:

The pH will not rise on its own because there's no bicarbonate left in the water, it's all CO2. pH only increases when there is sufficient enough bicarbonate (HCO3-) anion to absorb a proton (H+) and outgas as CO2

I thought that, as you also mentioned, with the speedstir CO2 should outgass pretty quickly (like in a glass of coke that's being stirred), and that should be noticeable with the indicator and the pH-meter. But I guess, the pH-rise from outgassing that happens already while pH is still higher gets immediately (over-)compensated by adding more R0009, and once pH is all the way down, CO2 outgasses more or less without pH-rise.


However, that would now cross over from the world of pool side testing into the realm of white lab coats, safety goggles, and flaming Bunsen burners .... not very convenient for most people.

That's where close the circle: With a Bunsen burner you could turn that flan into a creme brulee :unsure:
 
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JoyfulNoise

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I may play around with the borate test a bit more now that we have had a few discussions. I don't have an Apera pH meter yet (it's on my Amazon wish list) but I may get one soon so I can fiddle with the chemistry a bit. It would be nice to mix up my own batch of sodium hydroxide to see if I can get some better results. If only I had owned a pool while I was still in my "lab rat" days ... oh the fun I would have had doing some weekend wet-chemistry ... "Ummm, no boss, I have no idea why all of our reagents are so depleted and our chemical expense budget is so high ...🙄 🤷‍♂️ "
 
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mgtfp

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I think we should use some of the donations TFP is raising for scholarships to enable pool ownership for chemistry students. Sounds like money well spent.

"Ummm, no boss, I have no idea why all of our reagents are so depleted and our chemical expense budget is so high ...🙄 🤷‍♂️ "

Similar to the boss asking: "Why is there an ice cream van in front of the building, and why are you needing so much liquid nitrogen these days?"

Aah, the good old lab days...
 
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mgtfp

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Another thing I was wondering about, is how well defined the NaOH concentration in R-0010 actually is. Its intended use is not as a titrant, but as "Calcium Buffer" in the CH test, where the exact amount of NaOH is not too critical. Taylor might not set the same tolerances on molarity as for example for R-0009 that is intended to be used as a titrant, and is explicitly specified as 0.12N. Is the R-0010 concentration somewhere specified? MSDS is a bit useless there, more of a legal document.
 

tfpdubld

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Am I doing something wrong because this test was super easy!? I'm guessing the 0.5% BTB is impacting the process but for me the blue was super obvious each time and matched almost exactly... is there a reason to use the 0.04% BTB and does 0.5% potentially take out some of the shade guessing and make for a bit easier process?

Materials
K-2006
BTB 0.5%
Mannitol

Procedure
  1. Collect a 25/50mL sample of pool water
  2. Add 2 drops of R-0007
  3. Add 1 drop BTB 0.5%
  4. Add R-0009 until the water turns straw yellow
  5. Add R-0010 until the water turns blue
  6. Add 1/4 teaspoon Mannitol. If the water has boron in it the sample will turn yellow again.
  7. Add R-0010 until the water transitions to the blue from step 5. Record number of R-0010 used.
  8. Multiply the number of drops of R-0010 by volume multiplier to calculate ppm boron/borates
Volume Multiplier

For 50mL each R-0010 drop x 4
For 25mL each R-0010 drop x 7.9

Results
  1. 25 ml water ✅
  2. 2 drops r-007 ✅
  3. 1 drop BTB so blue! almost matches the speedstir taylor blue. ☑️
  4. needed 5 drops R-009 - 4 was close but not straw ✅
  5. needed to add 1 drop R-0010 and it popped back to blue from #3 ☑️
  6. back to straw yellow
  7. added 6 drops and on the 6th drop popped back to blue from #3 ☑️
  8. 6 * 7.9 = 47.4ppm B

I consistently get the same shade of blue and 6 drops of R-0010 with a 25mL sample. I added the exact amount of boric acid that pool math said to go from 0 -> 50 a month or so back so 44ppm @ 50mL & 47.4ppm @ 25mL seems spot on! I also tried with the 0.04% BTB and got the same results but it was more work thinking about the shades of blue.

THANK YOU for developing this test! Adjusting to 60ppm now that I'm not guessing with test strips...

Update: I like the 50mL version of the test better because of the increased accuracy. After adding boric acid based on the 25mL and retesting this morning it was clear if I'd used the 50mL results it would have given a better outcome. So I just added a bit more boric acid based on the 50mL test to get up to 60ppm.
 
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AUSpool

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Another thing I was wondering about, is how well defined the NaOH concentration in R-0010 actually is. Its intended use is not as a titrant, but as "Calcium Buffer" in the CH test, where the exact amount of NaOH is not too critical. Taylor might not set the same tolerances on molarity as for example for R-0009 that is intended to be used as a titrant, and is explicitly specified as 0.12N. Is the R-0010 concentration somewhere specified? MSDS is a bit useless there, more of a legal document.

I was thinking the R0006 base demand reagent might work but the SDS is just as bad, its probably just a solution of soda ash.

And here’s why the drop test is such a bonus to have, luckily I kind of knew where I should be but I’ll have to go back to the drop test after our next major rain event.

A3735339-E3A5-4707-BEF3-1460A82A8F9D.jpeg
 

mgtfp

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@JoyfulNoise, when I did my tests last week, I had the impression that after bringing pH up with R0010, pH would start drifting down again. So, I did one more test today with a focus on that effect.

I did the test again with R0004 to be a bit more sensitive in the higher pH range.

Started with R0007, R0004, then precalculated number of drops of R0009 to TA zero point. Then waited about a minute with the stirrer stirring to let the CO2 outgass. No Manitol at that point.

Then 1 drop of R0010:

Screenshot_20220820-125645-800.png

A minute later, Speedstir stirring:

Screenshot_20220820-125958-005.png

Another drop of R0010, and pH went up again:

Screenshot_20220820-130053-940.png

My interpretation is that, after driving out CO2 while at pH 4.2, the water will then be undersaturated with CO2 when pH is back up above 7. And while waiting and stirring, CO2 gets reabsorbed into the water, driving pH down.

Does that make sense? Just curious, not trying to squeeze the last bit of precision out of the test, all of my fiddling was within one drop more or less anyway, pretty good.
 

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JoyfulNoise

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Pure water, or water low in carbonate alkalinity, will absorb a small amount of CO2 from the air. That is one of the ways we get acid rain (amongst other atmospheric pollutants that can a lower pH). The amount is tiny though, at most it wouldn’t be higher than 10-15ppm.
 
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