pH dropped MUCH lower than Pool Math predicted?

[mgtfp]

Thanks for chiming in, Matt.

So, the endpoint is always "pale blue" (maybe pH 7.5-7.6?), no matter where the pH was after step 5? If you overshot in step 5 and got it to "dark blue" (maybe pH >8?), you should rather bring it down again with R-0009 before adding the Mannitol (or restart from scratch) then just titrating to the same pH as after step 5?

 

[JoyfulNoise]

It doesn’t matter where the pH starts after adding the mannitol because it’s the reaction of mannitol (sugar alcohol with diol groups) with boron that release the hydrogen ions that drops the pH. Base is then added to neutralize all the hydrogen ions created. So the amount of base (OH-) added is directly proportional to the amount of hydrogen ion (H+) ion created which is proportional to the boron concentration. Dropping the pH with R-0009 initially removes carbonate alkalinity which would interfere slightly with the test but also helps to get the BTB indicator colors setup right.

I’ll see if I scare up the original analytical chemistry description … I think I have a pdf saved somewhere

 

[Stoopalini]

One thing I don't understand is .... the color indicator is a way to tell pH, so you're watching for a particular color to tell you the pH of the test. How is it any different to look at a pH meter instead? If "straw yellow" indicates pH of 6.0, why can't I just look at the meter to determine pH of 6.0 instead?

From the test I ran, the sharp rise of pH in the S curve was very easily spotted; even with the concentration being so high in the R-0010 drops. With each drop of R-0010 equating to 4 ppm borates, it seems even if a more accurate test could be had with a less concentrated reagent, the R-0010 still gets us close enough ... doesn't it?

Watching my pH meter climb slowly with each R-0010 drop, and then suddenly jump, told me I had hit the sharp rise in the S curve. Isn't this "good enough" for the purposes of measure borates in a pool?

What I wasn't sure of is if I should count the drop which cause the sharp increase in pH or not though. I suppose the answer to that would depend on how close I was to the mid point of the S curve on the previous drop. But again, with a 4ppm resolution, it really shouldn't matter when targeting 50-60ppm borates.

 

[mgtfp]

That's what I thought, Matt, you want to neutralize the H+ that got created by the Mannitol reaction. And with first adding R-0009 and then R-0010 you're basically replacing carbonate alkalinity with "OH-" alkalinity.

So, when pH is let's say 7.5 (which should be close to pale blue according to the scale on my BTB) after adding R+0010 before adding the Mannitol, then pH 7.5 would be your endpoint. If it was 8.0, you'd titrate up to 8.0. But with BTB you're pretty blind at 8.0. Or is there still another factor why you want to start and end at "pale blue", and not at "dark blue"? Or "grass green" which would be close to neutral?

 

[mgtfp]

I’ll see if I scare up the original analytical chemistry description … I think I have a pdf saved somewhere

That would be great

 

[JoyfulNoise]

Let’s just all agree that the test, as it is performed with the chemicals on hand, is going to be a bit course. This is not an analytical chemistry lab where I have access to auto-titrators, pH probes and any flavor of sodium hydroxide reagent I choose to mix up. We’re working with what we got rather than what is ideal.

So in a test scenario like this, a probe can certainly be very useful if it’s well calibrated but a color indicator is pretty much “good enough”. As long as you starting pH is low enough to get rid of the carbonate alkalinity and then you hit the high pH you’re looking for, it’s pretty much as exact as it’s ever going to get. I’m not against using a pH probe, I just find the color indicators easier to work with. I’m not going for super-precision, I’m going for the ease of use. If I have to start busting out pH probes and taking copious notes to make a single measurement then I’ve violated the the KISS principle.

 

[mgtfp]

Maybe just to clarify: The pH-meter is only being used because the OP is colour blind. Otherwise I agree 100%, the BTB indicator really is good enough for this test.

Edit: ...and much easier to use.

 

[JoyfulNoise]

Maybe just to clarify: The pH-meter is only being used because the OP is colour blind. Otherwise I agree 100%, the BTB indicator really is good enough for this test.

Edit: ...and much easier to use.

Ah. Gotcha

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), then add the mannitol and don’t worry about where the pH drops down to (could be 5 or 4 or whatever). Just make sure you titrate all the way up to your starting point and maybe go a drop over to be sure.

Also, make sure the pH probe is calibrated using a 2 point calibration with the 4.01 and 7 calibrating solutions and give the probe time to stabilize its readings. You’d be surprised with even good probes how long it takes to get a stable reading after each drop is added.

 

[mgtfp]

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.

I just started typing, asking if going down to 4.5 would make sense if using a pH-Meter anyway. Saw your post popping up, and started typing again...

 

[mgtfp]

You’d be surprised with even hood probes how long it takes to get a stable reading after each drop is added.

I agree. I once did the TA test with my pH-Meter, out of sheer curiosity, but I had the indicator in, too. It was important to wait long enough at the beginning until the temperature- and pH-reading stabilised. After that, my PH60 was quite responsive to the pH-changes after each drop, but still needed to wait some time before adding the next drop.

Not ideal with the small Speedstir and small test tube. Would be much easier with a proper beaker and a properly mounted pH-meter on a full sized magnetic stirrer and unlimited supplies of reagents in whatever concentration.

But still a fun science experiment with the kids on a Sunday afternoon during lockdown (one of the many we had here in Melbourne).

And it was nice to see the "S-curve"...

 

[Stoopalini]

Ok, so I think I understand now. The hesitation on using the meter isn't because it's not going to give accurate results, but is because it's easier (for a non-colorblind person) to just use the indicator solution. Got it.

Being colorblind, there is no way I'll ever be able to tell baby blue, pale blue, dark blue, etc .. apart. Remembering a shade of blue and trying to get back to it? Ya, never going to happen lol. I can't even do the calcium test because I can't see the blue from the purple. I can ask my wife to look at the colors (like I do with the calcium test), but I won't have BTB for another week or 2, and besides, I really want to be able to do as much as I can without having to get someone else to look at colors.

So it appears my procedure was correct, but the target points I chose should be adjusted. Instead of dropping the pH to 6.0 with R-0009 and coming up to 8.3 with R-0010 ... I should be dropping to 4.5 (or lower) and coming up to somewhere around 7.0/7.5 ... basically, wherever the last drop lands me (not being exact due to the strength of the R-0010 not giving me fine enough control). It's just important to know the level I hit because it becomes the target for the subsequent step.

Then add the Mannitol and the pH will drop to whatever (not important). Finally, titrate R-0010 and count how many drops it takes to come back up to the 7.0/7.5 target (or whatever it had landed on previously).

Thanks guys, I appreciate the input. I'll run it again tomorrow sometime, just to compare how changing the 6.0 + 8.3 targets to 4.5 + 7.5 effects the outcome of the test.

 

[Stoopalini]

Oh, and the PH60 meter employs a 3-point calibration at 4.0, 7.0 and 10.0. It's been a great tool for me over the past 2 years!

 

[Bperry]

Today I checked the chemistry ... pH reading 8.25 with TA between 70 and 80, so let's call it 75.

Pool Math suggested 49oz of 20* MA to reduce pH to 8.0 (assuming 50ppm borates), so I added 49oz.

Later in the day, I retested and the pH had dropped to 7.68 with TA reading a solid 70.

So either my borate strips are bad, or the MA I'm using is stronger than it's supposed to be.

The test strip bottle has an expiration date of Oct 2022 ... but of course, they could still be bad. I tested some tap water with them, and it was a solid 0ppm.

View attachment 445970

Here is a pic of the pool water test:

View attachment 445971

The family tells me this is a solid 50ppm reading, but I cannot tell myself.

I wish the BTB had a faster ship time, but it hasn't even shipped yet and has an estimated delivery window of Aug 16 - Sep 1.

I read through the Borate drop based test thread, and it looks like maybe I can use the pH meter to do some testing until I get the BTB. My understanding is ... I would follow the procedure to lower the pH to 6.0 (to convert any boron to broic acid I believe?), then raise the pH back up to some target (from the thread, 8.3 seems like a decent target), then add the mannitol, followed by R-0010 drops to bring the pH back to the 8.3 target, counting the number of drops to determine borate count. Not sure if this is accurate though, as a lot of chemistry talk in that thread goes over my head.

If the borates are indeed less than 50ppm, then it means I've lost some water somehow. I'll try and do a bucket test over the next 24 hours, just to check.

The borate strips I had went bad about a month after being opened, so don’t trust those. Just an FYI in case it helps.

 

[Stoopalini]

Update for today ... bucket test shows no signs of a leak, so there's good news there.

After church, I ran the drop test again, but using the 4.5 and 7.0/7.5 targets ... here are the results:

1st test:

1. Collected a 50ml water sample
2. Added 2 drops of R-0007 to neutralize the chlorine
3. Used R-0009 to reduce the pH with a target of 4.5
   1. After 13 drops, it ended up at 4.40
4. Added 3 drops of R-0010, which raised the pH up to 7.93
5. Added two level 1/8 tsp of Mannitol .. pH dropped to 4.76
6. Added drops of R-0010 to raise the pH back up with a target of 7.93
   1. At 9 drops, it was 7.52
   2. The 10th drop sent the pH up to 9.64

So 9 drops x 4 = 36ppm borates


2nd test:

1. Collected another 50ml water sample
2. Added 2 drops of R-0007 to neutralize the chlorine
3. Used R-0009 to reduce the pH with a target of 4.5
   1. After 13 drops, it ended up at 4.0
4. Added 2 drops of R-0010, which raised the pH up to 7.07
5. Added two level 1/8 tsp of Mannitol .. pH dropped to 4.57
6. Added drops of R-0010 to raise the pH back up with a target of 7.07
   1. At 9 drops, it was 6.90
   2. At 10 drops, it was 7.48
   3. The 11th drop sent it up to 9.01

So 9 drops again?

Based on how much my pH moves with the addition of MA, it seems the previous targets gave a more accurate reading ... as 28ppm borates would make more sense than 36ppm borates.

Thoughts?

 

[mgtfp]

I think I would just call that 30ppm. There are probably many tolerances coming together. BOR-test, TA-test, pH-test, pool volume, PoolMath approximations,...

Your TA test yesterday was 70ppm, that means 14 drops of R0009 should get a 50ml sample to about pH 4.5. With 13 drops you were already down to 4.0 (very steep curve there, one drop can have a big impact), so your TA is probably closer to 65 or even slightly lower (with a 50ml sample, you are actually doing a double precision TA-test as step one of the BOR-test).

Yesterday's tests were not quite down to TA 0, now it's probably a bit below zero TA at the start point.

I think, play a bit with your testing over time, and see how it matches PoolMath acid addition calculations. If in doubt, round your BOR test results down for the sake of acid calculations.

I also did some testing yesterday, after our discussion here gave me an idea. I posted my results in the deep end Borates thread. Read from here downwards:

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

What zollan said! I'd always been hoping for a drop test to validate my borates levels, and this test seems to work great (it correlates very well with predicted borate levels from PoolMath). Plus the instructions were very straightforward. Well done, you guys! (y) (y)

www.troublefreepool.com

 

[Stoopalini]

So I finally got the BTB and ran some tests yesterday.

I collected a sample of water, and checked the pH + TA. pH was 7.9, and TA was between 60 - 70. I logged 65 for it.

I then adjusted the pH with a target of 7.6, assuming 25ppm borates, and the app called for 21oz of 31.45% MA. I added it.
After the pool had mixed, I checked the pH and it was 7.68. I know my pool volume is correct, so 7.7 would make sense if borates were 32ppm and not 25ppm or 50ppm. I did this to compare the results against the drop based borate test.

I then took another water sample and ran the drop based borate test:

I ran the test once with my 13 y/o son watching the colors, and we hit 8 drops (8x4 = 32ppm borates). I did have to play with the 0009 and 0010 reagents at the beginning because one drop of 0010 would give us green, and the 2nd drop would give us the deep blue. So the concentration of the 0010 made it difficult to get a pale or baby blue color. I had to use 0009 to drop the pH even lower after hitting straw yellow, in order to get the 0010 2nd drop to not overshoot the pale blue.

I ran the test a second time with my wife watching the colors, and she wasn't sure on the blue matching on the final step. It was somewhere around 8 or 9 drops as well though, but was difficult to really get an answer LOL.

During both tests, I dropped the pH meter in at each step to check my above prediction on using pH to do this test, and everything tracked. Comparing the pH of step 3 in my list above to the pH of step 6 with a pH meter, seems to be an accurate way to do this test. It also seems using a drop target of 6.0 with a raise target of 8.3 when using the meter and no BTB gave me closer results to the BTB test than using the 4.5 and 7.0/7.5 targets did. I realize 4.5 is probably a better target due to the conversion of carbonate alkalinity, but with my pool and testing, the 6.0 and 8.3 gave me the 32ppm result where the 4.5 and 7.0/7.5 targets gave me 36ppm borates. It's just one drop difference though...

After all this testing, I assumed 32ppm borates and calculated 15 lbs of boric acid (from duda diesel) to raise borates to 55ppm. So I added it, brushed the pool, ran the pump on high with the waterfall going, and also ran the robot.

About an hour later, I ran the borates test again. The BTB (with wife watching colors) resulted in 13 drops (13 x 4 = 52), but the pH meter showed at 13 drops it was still 0.3 pH points shy of the target pH. One more drop of 0010 sent it well over the target, so it looks like I hit the 55ppm borate target I was aiming for.

I also tested the pH + TA after adding the boric acid. pH dropped from 7.68 to 7.44 and TA didn't change at all. If I remember correctly, this is normal for the boric acid to lower pH a bit after adding.

In the future, I will probably do this test myself with the pH meter using 4.5 as a low end target to deal with carbonate alkalinity, and 8.3 as the upper target because it seems to produce a result more aligned to what my son and wife were seeing with the colors.

It seems ideally a less potent 0010 reagent would be really useful here, but in the end, this certainly worked out well given what I have on hand.

On another note, my wife just started her 1st year as a science teacher for a charter school (6th - 9th grades), so the BTB will go to good use since she said all the BTB she has in the school's chemistry inventory expired a while ago

 

[mgtfp]

Sounds like you've nailed it now, and all seems to be consistent.

Don't forget that it's not just the Borates test that's got tolerances. Even if you are lucky enough to know your pool volume e.g. from checking the water meter at filling, there are still other tolerances:

PoolMath for example doesn't actually solve the equilibrium equations, it just uses fitted curves or look-up tables, which will have its limitations over the whole parameter range.

Then I have my doubts that 31.45% MA is exactly 31.45% for the grade of MA that's being sold for building and pool purposes, rather than lab purposes.

How long to wait until checking pH after adding MA? Not long enough and it's not mixed in properly. Too long, and you might already have some outgassing, esp. when going into the lower 7s at higher TA.

And probably many more.

 

[JoyfulNoise]

You should never trust pH calculations that produce results with a magnitude of more than 0.2 units in either direction. Few people outside the world of analytical chemistry understand this fact. Analytical pH calculations in pool water would require solving a dozen different simultaneous equations in as many unknowns. There are some simplifying assumptions that can get that number down to 4 or 5 equations but, even then, you’re talking about a water sample that has enormous ionic strength (dissolved ionic compounds) which varies all of the assumed equilibrium “constants”. There really is no simple closed-formed solution to pH calculations.

Put simply, outside of using an expensive and sophisticated mathematical program designed to deal with analytical chemistry (and I can only think of a few), pH calculations are always “ball park” estimates.