6 replies to this topic

[pilot]

As genaral information, 99% pure Acetic acid has a pH value around ~4 (3.92 by MSDS), but an Aqueous solution of Acetic acid (demineralized water + Acetic acid in a 10:1 ratio vol/vol) is giving a pH = 2.43 - 2.44 while the DM water pH was 5.62.

This lowering of the pH is an uncertainty for me. Since the pH value is proportional to the conc. of [H+] ion or [OH-] ion, (and here I am not adding any [H+] ion), how does the pH gets lowered?

It was totally amazing to me, I couldn't understand why its happing! Can anybody explain this to me?

[djack77494]

pilot,
Do not be amazed. Acetic acid behaves similarly to several other acidic and basic compounds which go through a minimum/maximum at less than the maximum concentration. Another good example is sulfuric acid, where the pH goes up and the corrosivity goes down as the concentration increases to the maximum attainable.

The reason is simple. It takes water plus the acid to generate H+ ions. The polar nature of water helps these acids dissociate, forming H+ (or H3O+) ions which is measured (in an odd sort of way) as pH. If there is not enough water, less acid dissociation will occur and fewer H+ ions are produced - hence the pH increases.
Doug

[pilot]

Dear Doug,
Thanks for ur reply,
Is it you are saying due to H3O+ ions the pH is increasing, then why it don't behave in case of HCl or other acid or in some other weak acids. I am still not getting this can you elaborate, please or can you give me some referance.

[djack77494]

pilot,
I would think any chemistry book would serve as a reference. pH is nothing but a measure of the H+ or H3O+ ion concentration. So, how do you get H+ to form? Start with pure water which has a pH of 7. As you add a strong acid (including HCl, by the way), the acid gives up H+ ions and their concentration increases. With many acids such as HCl, this basically keeps increasing up to the solubility limit of the acid. (Note however that there is a very steep rise in pH at low acid concetrations, and this levels off as concentration increases.) For some acids, however, especially those that exist as liquids, you can increase the concentration almost without limit (i.e. to near 100%). Well the pH doesn't increase without limit. At some point, there is not enough water in the mixture to "pull apart" the acid and liberate additional H+ ions. At that point, the pH does NOT continue to increase as you increase the acid concentration, and, in fact, it will start to decrease.

Make sense?
Doug

[pilot]

Doug,
I don't deny with Your statement but my question is: If Acetic acid pH itself is 4 then how solution pH decrease. If I am not wrong, Is that in Acetic acid the [H+] ion is balanced & not free but when it goes to water it gets ionized & relieves [H+] ion which gives pH.

pilot

[Mr. pH]

pH is nothing but a measure of the H+ or H3O+ ion concentration.

Nope, it is a measure of H+ activity, not concentration. Fro very diluted solutions thats almost the same, but the higher the concentration (or - to be more precise - the higher the ionic strength of the solution) the higher the difference. See ionic strength and activity definitions for introduction.

[djack77494]

Thank you, Mr. pH. You learn something new every day. I hadn't known that before.