[pr0poon]

Right, i am a rather base level chemistry student who is trying to impress.

I am struggling to understand when and why certain salts/ and or reactions occur in solution, when on other occasions they dont. I have two examples but for now i will focus on one...

Number 1.
I have a solution of Trisodium Citrate and Citric Acid. A buffer at pH 4.2.
When i add to this solution some Sodium Methyl Hydroxybenzoate (SLOWLY) (see link..https://en.wikipedia.org/wiki/Sodium_methylparaben) this goes into solution, no problem, due its high aqueous solubility and stays in solution...fine.

When i add the material FAST (which does briefly increase the pH slightly), it tends to go in and then crash immediately and stay out of solution. I performed an ID (by IR) on the precipitate to only find it is Methyl Hydroxybenzoate...I presume it is the non-sodium (which is known to be less soluble).

Why does this crash on the rate of addition?

When and what denotes if something forms a salt, or takes up a H and forms a compound in solution? Why dont they both behave in the same way?

I know this is maybe a simple answer but im confused... Thanks for any help.

[phth]

These types of salt forming neutralization reactions have very downhill enthalpy terms associated with them.  In other words, they heat up rapidly and are kinetic reactions.  Yes, you're right: the protonation of the p-hydroxymethylbenz is favorable, and it makes the molecule much less soluble. 

In addition, the release of heat causes the stuff to agregate and undergo further unwanted side reactions such as encapsulating the methyl benzoate salt inside a coat of the less soluble form.   Add things together slowly for salt/neutralization reactions.  If you do not, then side reactions will occur for the reasons stated above.  Another example would be adding a base such as triethylamine...etc.  One thing you should be doing for things that are not very soluble is grinding them in a moatar and pestle to increase the surface area and decrease the chances of poor soubility causing problems.  It is easier to fix the problem before it occurs.

[orgopete]

Given the conditions of the equilibrium, there should not have been a difference. The only thing that I can think might happen is that the concentration of methyl p-hydroxybenzoate became high locally and precipitated. If you added it more slowly (or rapidly with a higher rate of mixing), the concentration of methyl p-hydroxybenzoate would remain low or sufficiently under the limiting concentration of the solution.

You may see the effect I am referring to when you neutralize an acid with base and a phenolthalein indicator. As you add base, you may see a pink color develop around the added base which disappears on mixing. As you approach neutrality, the pink color will dissipate more slowly. I think adding the sodium salt to the buffer is acting similarly, except there isn't a color change to indicate this. However, rapid addition may give the equivalent of a high local concentration of methyl p-hydroxybenzoate which precipitates. I just guessing.