Water to Acid vs Acid to Water

[BigDave]

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Acid+water posts moved from:
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... add 1 gallon of water, then 1/4 gallon of acid, and then 1 - 2 MORE gallons of water ... Maybe. I'd prefer to always add Acid to Water and never Water to Acid. Why not just recommend adding 1/4 gallon of acid to 2 gallons of water then add that to the pool? It certainly avoids any risk of flashing acid.

[PoolDoc]

In my experience, muriatic acid solutions are not strongly exothermic when water is added. In any case, what I suggested was adding 1/4 gallon muriatic to 1 full gallon of water already in the bucket. So, he WOULD be adding acid to water and not the reverse. And, I'm pretty sure that adding more water to what would then be 6% acid is not going to be noticeably exothermic.

In this case, if he tried to add 1/4 gallon of acid to 3 gallons of water, the practical risk is of splash back when adding full strength acid to a nearly full bucket. I'd much rather have him splatter 6% (or less) acid, than 31% acid.

[BigDave]

Yeah, ... that's why I said "Maybe" but I can still hear my ninth grade Chemistry teacher in my mind. Sorry to belabor the point but why use the bucket at all?

[PoolDoc]

Pouring concentrated acid in a shallow pool with no circulation is not a good idea; there's too much chance of ending up with a 'hot spot' that damages the liner

[PoolDoc]

Thank you so much!!!!! I will gather everything soonest, and start clearing up the algae tomorrow evening. :)
Thank you again for all of your help!!!!!

@ Traxl: please let us know how it goes!

[JimK]

Yeah, ... that's why I said "Maybe" but I can still hear my ninth grade Chemistry teacher in my mind. Sorry to belabor the point but why use the bucket at all?

I understand where you're coming from. ;)

Some years ago I saw a demonstration if what happens when you add water to the acid...BOOM! :eek: That certainly scared me enough to never want to add water to acid!

That said, I trust Ben's advice and accept his word that once diluted with water as he mentioned in this thread there won't be a problem. :)

[JimK]

Ben is absolutely right. 18" of water is not deep enough to safely add the acid directly to the pool. Diluting it first in a better controlled environment makes sense. Sheepishly, I didn't post that here - telling myself that we were already pretty far off topic.
I will still stand by A->W (unless it's already quite dilute). FWIW I've seen a drop of water dance on a volume of HCL and I've seen it explode splashing HCL all around.

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion. :eek:

[PoolDoc]

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion. :eek:

Was that lab acid, or 31% commercial acid?

I'm going to have to try that.

[PoolDoc]

Hm-mh. Still going to try it, but this video shows exactly what I would have expected (somewhat obnoxious sound). with commercial muriatic acid and water:
https://www.youtube.com/watch?v=hMnwhifRPuw

Mind you, the situation would be very different with sulfuric acid, where the reaction would be strongly exothermic. I haven't worked with other acids, outside of a lab.

[BigDave]

Perhaps the demo was with H2SO4 - not what I recall but it's been more than 35 years. It did make an impression on me. To this day I trust A->W and will likely continue to recommend it until there's a good reason to do otherwise.
I'll have to try it this summer when it's time to adjust my pool's pH.

[JimK]

Was that lab acid, or 31% commercial acid?

I'm going to have to try that.

Don't know. I wouldn't recommend trying it. Look it up on YouTube instead. ;)

[CarlD]

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion. :eek:

Hence why you ALWAYS pour the acid into the water, not the other way around! (Ka-BOOOM!!!!)

[chem geek]

The heat (enthalpy) of dilution for some acids is given in this link (http://sites.chem.colostate.edu/diverdi/all_courses/CRC%20reference%20data/enthalpy%20of%20dilution%20of%20acids.pdf). 31.45% HCl is around 10 so the heat of dilution is roughly 10 kJ/mol. 38.5% Sulfuric acid, by comparison, has a heat of dilution of around 28 kJ/mol so substantially higher. This link (http://dpuadweb.depauw.edu/harvey_web/Chem260/Chem%20260%20pdfs/Worksheet%20Keys/Enthalpy%20of%20Diluting%20Strong%20Acids%20Worksh eet%20Key.pdf) calculates the temperature rise for a specific example of adding acid to water and explains why adding water to acid is not a good idea. It basically has the water heat up so much that it can boil and splatter, causing acid to be propelled with it risking getting splashed with concentrated acid.

[Anonymous [GDPR] European in the UK]

I like adding the acid slowly to the pool all the way round and letting it fume, I grab my air guitar and sing "Smoke on the water"