pH shift in after water softening

[csaba]

Dear Everyone,

I am wondering about the following phenomenon in water softening applications. Even the ion exchange resin manufacturer could not answer my question so your feedback would be greatly appreciated.

The problem:

If you soften the raw water by removing the Ca++ and the Mg++ ions the water's pH shifts to the acidic direction.

What is its reason and is there some calculable correlation between the hardness degree of water and the pH value?

The resin used is a strongly acidic cation resin (SAC).

Thank you very much in advance!

Csaba

(ProMinent Ltd., Hungary)

http://www.prominent.com

Experts in chem-feed and water treatment

 

[quark]

If you are HCl as the regenerant then Mg++ and Ca++ or replaced by H+ ions and this increases hydrogen ion concentration in the water (remember the definition of acidity)

I really doubt why the water treatment guy didn't answer the question.

 

[csaba]

Dear Quark,

The resin used is regenerated by sodium chloride (NaCl) that is why I have difficulties in finding out the reason and correlation to the shift in the pH. I am not a chemical guy so it is hard for me to conclude. But you are right that an ion exchange guy should know it.

Is there some kind of correlation like 1 gpg decrease in water hardness equals with ... shift in pH???

Thanks!

Csaba

http://www.prominent.com

Experts in chem-feed and water treatment

 

[quark]

Sorry for the quick comment. I don't see any reason if the inlet water is neutral and there is no dissolved CO2.

Give a try at forum164

 

[25362]

I think quark is absolutely right. Only if the strongly acidic resin converts part of the bicarbonate (temporary hardness) into solvated CO[sub]2[/sub] aka carbonic acid (H[sub]2[/sub]CO[sub]3[/sub]), the pH may in fact drop.

A pH of 6.5 would indicate the presence of an equal number of mols of the bicarbonate ion and carbonic acid. A pH of 6 would indicate a molar ratio of H[sub]2[/sub]CO[sub]3[/sub]/HCO[sub]3[/sub][sup]-[/sup] ~ 3.

 

[DocMax]

Csba

Your question is strange, because in the water softening the pH will not change.
In the water softening you replace the only Ca2+ and Mg2+ with Na+, so the pH not change.
The reactions involved in the process are:

2R-Na + Ca2+ = R2Ca + 2Na+
2R-Na + Mg2+ = R2Mg + 2Na+

Can you give me some more details about your process, what is the chemical analysis of the raw water?

The only way to change the pH in the water is the lowering of alkalinity, but in the softening that not happen.
Did you feed any chemical in the raw water? Please explain better.
Are you sure that you use a SAC resin in the sodium cycle?.
This phenomeenon happen in every cycle of the softener or only at the start up of the plant?
What is the brand of your SAC resin?

Hi
DocMax

 

[csaba]

Dear All,

Thanks a lot for your help!

Meanwhile I also carried out some research: there should be really some kind of specific application problem still unknown for us that reduces the pH and it is - as you say - not the water softening process.

http://www.prominent.com

Experts in chem-feed and water treatment