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Salt in salt out?
Chromatography Forum: LC Message Board: Salt in salt out?
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By indium on Saturday, July 24, 2004 - 02:05 pm:
If a HCl salt of an amine is subjected to RP-HPLC using water and acetonitrile do you get the HCl salt out at the other end?
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By Anon on Sunday, July 25, 2004 - 08:50 am:
This would be true if you don't have ionic retention occuring.
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By Chris Pohl on Sunday, July 25, 2004 - 03:02 pm:
This is dependent on the pKa of the amine. Only for salts of very strong bases such as guanidine would there be much chance of keeping the HCl with the amine. In most cases, physical separation of the amine from the amine salt and the HCl (the amine should always have the longest retention) will shift the equilibrium to favor the amine form and HCl should elute unretained.
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By David on Monday, July 26, 2004 - 04:22 am:
Chris- you are clearly very experienced in the area of analysis of ionizable compounds. Is it possible for you to explain your reasoning for the above in some more detail? It would be helpful for me if we called the base B and the amine salt BH+Cl- and then I could try to follow.
Many thanks
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By Chris Pohl on Monday, July 26, 2004 - 11:49 am:
David,
For the sake of discussion, let's take the case of a base B with a pKb of seven. Furthermore, let's assume that the pH of your unbuffered mobile phase is 7 (generally this isn't a valid assumption). If we inject the hydrochloride salt of the base (i.e. BH+Cl-) into the mobile phase under these conditions it will be present as an equimolar mixture of B, HCl and BH+Cl- since at pKb the base should be 50% ionized. Now, since the base has greater retention than the HCl or the BH+Cl-, the base will be left behind on the stationary phase. The HCl should have no retention whatsoever so it will race ahead of the BH+Cl-. Now, the remaining BH+Cl- will once again reach equilibrium with a new equimolar mixture of B, HCl and BH+Cl-. If you imagine this process happening repeatedly (although in reality this is not a discontinuous process), I think you can see how in the end you will end up with B and HCl unless you have a very strong base and even then you won't maintain the base 100% in the salt form unless it is a quaternary cation.
Chromatography Forum: LC Message Board: Salt in salt out?
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By indium on Saturday, July 24, 2004 - 02:05 pm:
If a HCl salt of an amine is subjected to RP-HPLC using water and acetonitrile do you get the HCl salt out at the other end?
-------------------------------------------------------------------------------------------------------
By Anon on Sunday, July 25, 2004 - 08:50 am:
This would be true if you don't have ionic retention occuring.
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Sunday, July 25, 2004 - 03:02 pm:
This is dependent on the pKa of the amine. Only for salts of very strong bases such as guanidine would there be much chance of keeping the HCl with the amine. In most cases, physical separation of the amine from the amine salt and the HCl (the amine should always have the longest retention) will shift the equilibrium to favor the amine form and HCl should elute unretained.
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By David on Monday, July 26, 2004 - 04:22 am:
Chris- you are clearly very experienced in the area of analysis of ionizable compounds. Is it possible for you to explain your reasoning for the above in some more detail? It would be helpful for me if we called the base B and the amine salt BH+Cl- and then I could try to follow.
Many thanks
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Monday, July 26, 2004 - 11:49 am:
David,
For the sake of discussion, let's take the case of a base B with a pKb of seven. Furthermore, let's assume that the pH of your unbuffered mobile phase is 7 (generally this isn't a valid assumption). If we inject the hydrochloride salt of the base (i.e. BH+Cl-) into the mobile phase under these conditions it will be present as an equimolar mixture of B, HCl and BH+Cl- since at pKb the base should be 50% ionized. Now, since the base has greater retention than the HCl or the BH+Cl-, the base will be left behind on the stationary phase. The HCl should have no retention whatsoever so it will race ahead of the BH+Cl-. Now, the remaining BH+Cl- will once again reach equilibrium with a new equimolar mixture of B, HCl and BH+Cl-. If you imagine this process happening repeatedly (although in reality this is not a discontinuous process), I think you can see how in the end you will end up with B and HCl unless you have a very strong base and even then you won't maintain the base 100% in the salt form unless it is a quaternary cation.
