How to calculate how much 32% HCL acid is needed to set the pool (55000L vol) at pH7.2?

[l2oBiN]

I am looking for a formula to calculate how much acid (in mL) do I need to add to my pool in order to set the pH at 7.2 if my pool pH reading is 8.

 

Can anyone advise? (I am planning to enter this in excel as a calculator).

 

 

 

 

 

[John Cuthber]

It's impossible to tell.
The Pool's pH is 8 so it is slightly alkaline.
But we can't know if that is because it contains a small amount of a strong alkali, or a larger amount of a weak alkali.

The best way to get some sort of answer is to take a sample of the pool water- say 1 litre and add small  known volumes of the acid and measure the Ph change with each addition until you get to the pH you want.

(It may be easier to measure the volume of a diluted acid.)

Then you can scale up the amount of acid you need for the volume of the whole pool.

I'd start by only adding half that much to the pool- to see what happens. You can add the rest if it turns out not to be enough. But it's very hard to remove it if you overdo it.


Having said that, it's quite possible that adding the acid will have practically no effect on the pH.

The cement in many pools will dissolve slightly and make the water alkaline.
Adding acid will just make more cement dissolve.

 

[l2oBiN]

Interesting. How do the pool shops/pool test kits determine the volume of acid to add? 

[John Cuthber]

1 hour ago, l2oBiN said:

Interesting. How do the pool shops/pool test kits determine the volume of acid to add? 

Trial and error, or something like what I suggested, adding a known amount of acid to a known sample of the water.

[tinkerer]

On 11/11/2018 at 3:24 AM, John Cuthber said:

It's impossible to tell.
The Pool's pH is 8 so it is slightly alkaline.
But we can't know if that is because it contains a small amount of a strong alkali, or a larger amount of a weak alkali.

Having said that, it's quite possible that adding the acid will have practically no effect on the pH.The cement in many pools will dissolve slightly and make the water alkaline.
Adding acid will just make more cement dissolve.

 

I suspect this is not exactly so, regarding quantities of alkaline materials, whether much weak, or little strong: Ph is a measure of the TOTAL number of H+ ions in the solution, thus it would not matter WHERE those H+ ions had their origins, simply that there were LESS of them, given the 8.0Ph than there would be in "pure water", at Ph=7.0.

Furthermore, why would the added acid immediately begin to react with the cured cement contained in the structure? So long as the added H+s failed to completely neutralize the Ph, the cement would never know they were added.  NO?   imp 

On 11/11/2018 at 12:57 PM, John Cuthber said:

Trial and error, or something like what I suggested, adding a known amount of acid to a known sample of the water.

Quite so! Generally, the actual volume of water contained in a given swimming pool is very rarely known with any accuracy, unless filling the pool originally was metered; even then, if filling required several days (mine did), there would be a significant amount of water lost while filling to evaporation, as much as 1/2 inch per day, here in the hot and windy Desert. Calculating the pool's volume would be little more than an estimate, either, as the actual curvatures involved are poorly known.   imp

[John Cuthber]

12 hours ago, tinkerer said:

I suspect this is not exactly so,

I suspect that only one of us is a chemist.

12 hours ago, tinkerer said:

Furthermore, why would the added acid immediately begin to react with the cured cement contained in the structure? S

There are lots of materials that can be referred to as cement. One of the oldest and simplest is lime mortar which sets by reaction with CO2 in the air to form calcium carbonate, so I will pick that as an example (even though it's not a good choice for making pools)

Calcium carbonate is slightly soluble in water (about 15 mg/ litre).
If there's exposed mortar in the pool then the calcium carbonate will dissolve until the solution is saturated- about 15 grams will dissolve in each cubic metre. It will raise the pH slightly.

If I add enough HCl to react with that material then I will lower the pH.

But the water will no longer contain calcium carbonate, so it won't be a saturated solution any more.

More calcium carbonate will dissolve into the water until it's back to (nearly) 15 mg/l.
and the pH will rise again.

So I add more acid to get back to the value I want.

But that means that the water isn't saturated any more...

and so on.

[zapatos]

I have a swimming pool and have used test kits myself and also use a pool company. We have never been able to accurately address chlorine and acidity on the first try.

I get the impression that factors that can affect acidity include the interplay of water temperature changes, type of chlorine and its ingredients (often acid) added to the pool, status of the chlorine in the pool, how much the pool is used, and the type and status of the filter used. Meaning that acidity is a bit of a moving target.

[tinkerer]

6 hours ago, John Cuthber said:

"I suspect that only one of us is a chemist."

 

 

Sir, your remark, above, may be interpreted in many ways, few not demeaning in some way to both the forum, and the New Member. 

First, it has no relevance whatsoever to the OP. Thus, it can only be concluded that:

My post was so ignorant of the field of Chemistry, it did not deserve an answer, OR,

My post could not be successfully refuted, OR,

The intent was simply to belittle.

Please be aware, I came here hoping to learn, even if that means taking "licks" when I'm wrong. I have never claimed to be a Chemist, but have always thoroughly enjoyed the study of Chemistry. I must apologize if my inabilities so related bring about brusque dismissal.   imp