[dun13203171]

Guys I'm struggling to understand why quinine sulphate is dissolved in sulphuric acid rather than HCL for example for fluorescence analysis.

I understand that quinine sulphate is a salt? A dehydrated solution of quinine and sulphuric acid?

I've read that optimal fluorescence occurs in acidified solution, ie with sulphuric acid but I can't find a reason why.

What structure is produced when quinine sulphate is placed in sulphuric acid solution? I'm suspecting it acts as a base, accepting h+ at both nitrogens and contribute few protons themselves?

All help is greatly appreciated! Thanks

[Hunter2]

It is also a matter of solubility. Them sulphate dissolves in sulfuric acid easier as in water. In water also decomposition can occur. It looks like it needs the quarternum nitrogen's

[dun13203171]

Thanks for the reply

Why does sulphate ions dissolve better in sulphuric acid than water and what drives its composition in water. I have done the usual Google search but having no luck.

[Arkcon]

The answer to these sorts of questions can be hard to find, this may boil down to:

1) A reaction with the salt and the sulfate anion.  This is almost chemistry trivia.  Someone like Linus Pauling would put it in his textbook, but not everyone's head is filled with random trivia.

2) Side reaction:  maybe HCl has a bad effect.  Maybe nitric destroys quinine.  How many acids do you know?  Will you try them all, and write a reference for us?

3)  They tried one thing, it worked, and won't try anything else.  This is called validation.  Quinine sulfate, in sulfuric acid behaves in a certain way.  They've tried deliberately fiddling with it slightly, time and time again, and found the variation minimal.  Switching to a different acid would require the same rigor.  They can't just swap it out once and say, "Meh ... looks about the same."

[Corribus]

Typically, chloride ions (and other halogen ions) quench fluorescence, including that of quinine, due to the large spin-orbit coupling constants of heavy atoms. This is most likely the primary reason why sulfuric acid, as opposed to HCl, is used.

[dun13203171]

Thanks everyone for the replys, very insightfull.

[dun13203171]

Another quick question,

there are two pka values im looking at for a compound. There are two predominating emission peaks via fluoresence, one represents a monocationic species the other, a dicationic species.

Am I right in saying the higher pka value will relate to that nitrogen that remains protonated in both forms, ie it is the stronger base, thus holding onto the proton more strongly and thus larger pka vakue.

cheers in advance