How can I find pKa of analyte ?

[Mr.Buffer]

Dear members ;

As you know , pKa of analyte is so important information to decide some parameters(e.g. mobil phase pH ) in hplc method development. I heard some web sites which you can write molecul name or input molecule structure to get molecul pKa/dipol moment information. But I have found it yet.
In short ;
How can I know/investigate/decide pKa of a analyte ?

Thank you in advance.

[tom jupille]

Knowing the *exact* pKa is not very useful, if for no other reason than it is usually determined in water, and so will be different from the pKa in a water-organic mobile phase mixture. I usually just catch a tame organic chemist and ask. For most purposes, an estimate based on functional groups is good enough; this table is taken from our "Advanced HPLC Method Development" course:

[carls]

There are several free web sites that calculate pKa but since this is a calculated value you have to scrutinize the results:

http://www.chemaxon.com/marvin/sketch/index.php

http://www.vcclab.org/lab/alogps/start.html


A source for experimental pKa's for well known compounds (you must provide the CAS#) is also available:

http://www.syrres.com/what-we-do/databa ... spx?id=386

[mbreslav]

I like retention order predictor (LC simulator by ACD Labs) that takes into account both LogP and pKa ==> LogD. I found a very close correlation with the experimental results, and also I liked pH optimization option that allows for finding an optimum pH for a particular separation.

[carls]

In my recollection ACD labs software is fairly expensive, correct?

Another free website showing many useful predicted physical properties, including logD:

http://www.chemicalize.org/

These parameters are calculated using Chemaxon's Marvin with calculator plugins (one of the previous links) but a bit faster than Chemaxon's website.