[madscientist]

Hi all,

may sound stupid but im having a bit of trouble understanding acidity constants. I understand that they are a measure of how much a certain compound dissociates in water, small Ka ---> large pKa ----> less acidic (relative to) large Ka ---> small pka ---> more acidic. that makes sense.

What i dont understand is how can water have a pKa value of ~15.7? does water dissociate in water forming some OH and H3O ions? just doesnt gel with me that water can dissociate in water...

I.e   

Na(s) + H2O(l) -----> big boom

Na(s) + Na(s) ------> no reaction, no boom


in other words, i would have thought that water would be inert to water.

Please help an obviously mentally challenged person understand this 

cheers,

madscientist

[Fry]

What i dont understand is how can water have a pKa value of ~15.7? does water dissociate in water forming some OH and H3O ions? just doesnt gel with me that water can dissociate in water...

Water does have a dissociation constant, yes. Its very small though..

[english]

All weak bases and acids react with themselves, to a very small extent.

Ammonia, for example, auto-ionizes, just as water does. 

It must be noted that the amount of ionization is so small that the respective constants for these reactions are very small, with orders of magnitude normally < 10^-10...

[Borek]

2H₂O <-> H₃O⁺ + OH^-

water ionization constant

Ammonia:
2NH₃ <-> NH₂^- + NH₄⁺, pK = 32

Acetic acid:
2CH₃COOH <-> CH₃COO^- + CH₃COOH₂⁺, pK = 15.2

Sulfuric acid:
2H₂SO₄ <-> HSO₄^- + H₃SO₄⁺, pK = 2.9

and so on - like methanol, ethanol, HF, hydrazine, acetamide, acetonitrile etc.

[organoman]

OK lets make it more simple for u. As water is both acidic and basic. So lets asume that an acidic water molecule reacts with a basic one and the product is H3O+ and OH-.
Though this is not something reported anywhere but looks logical atleast for your question.
And dissociation is not the question of reacting it means getting dissolved. And of course water gets dissoved in water with some very low dissociation constant.

[Amy]

Dear All,

I read about Fluorescence. I am working with Fluorescein and I need to prepare Fluorescence solution in different concentration for optical measurement. From my reading, fluorescein has a pKa of 6.4 and multiple ionization equilibria. This leads to pH dependent absorption and emission over the range of 5 to 9.

1. What do you mean by the fluorescein has a pKa of 6.4? What do you mean by multiple equilibria. Which pka should I use to prepare fluorescein solution in buffer. I want to prepare the solution at pH 5 until 8.

2. If fluorescein changes its pH quite fast  either decreasing or increasing, is it true that I choose the pKa slightly lower or higher relating to its decrease or increase?

Thank you.

[Borek]

You force pH with buffer, fluoresceine pKa doesn't matter as long it is not your buffer acid/base conjugated pair.