[Edward]

He probably won't believe there is no simple preliminary, not asking for a confirmative, test for acids.  I told him once I could not find any standard method from forensic labs and he thought that there must be some.  And it is my job to find the solution, not to create problem.  I am really worried that I will be out of my job.

[Alpha-Omega]

OK...as I see this...if you use a pH meter you can thell him the material is ACIDIC or BASIC....you cannot tell him what it is....

If you use colormetric IDs you can determine this or that is there....but how the this or that is put together is a different ball game....

IC by itself is not a 100% either it will tell you what anions and or cations are in there....just look at those chromatograms I sent you...what do you see? 

Most analyses for a material ....LET ME BE VERY CAREFULE HERE...require more than one test to give ALL the information one requires....then you put all those pieces together and explain why something is what it is....

There are cases where one analysis will yield enough information to give you a 100% accurate positive picture/ID of what you are looking for.

XRD would be an example of this....it depends on what you are looking for and are required to report.

[Edward]

I see lactate instead of lactic acid in the chromatograph, that's the anion, not the exact identity of the acid. 

[Borek]

Imagine mixture containing 0.25M H⁺, 0.25 M Na⁺, 0.25 M NO₃^- and 0.25 M Cl^-. Can you tell what acid and what salt does it contain?

[Edward]

Really appreciate your explanation!

[Edward]

"Imagine mixture containing 0.25M H+, 0.25 M Na+, 0.25 M NO3- and 0.25 M Cl-. Can you tell what acid and what salt does it contain?"
Can I say I can't tell the answer with any method I use?

[Borek]

Yep. All you can say is what ions (or molecules) at what concentrations are present.

[Alpha-Omega]

With IC you will see the lactate anion.  That is correct.  The software will monitor the pH of your system during the analysis.  You can also do a pressure trace...good for diagnostic purposes...But NO it does not tell you it is lactic acid.

In IC there are so many things going on in that suppressor...difficult to say exactly what you have going on in there....you can infer...you cannot ... and these are UNKNOWNS you will be analyzing....this is just a piece of an unknown...not the whole picture...

[Edward]

I think I am going to present to my boss what methods are there and the limitations.  My understanding now from our discussion is no technology can tell the exact id of an unknown acid or base.

[Alpha-Omega]

I know I sent this earlier...OK WEIRD....VERY WEIRD

That is correct-to see lactate...not lactic acid....and you will not see lactic acid with IC....you can analyze a lactic acid std...but you will see the lactate anion...the only way you know you have lactic acid is because you are using a certified std..

The softaware for most IC systems will allow you to monitor pH while you are acquiring.Still, that does NOT tell you you have lactic acid...and there is so much activity going on inside a suppressor...hard to say what the pH is at any instant in time in there...

There is a USP method for sodium lactate assays by HPLC-the sodium salt of lactic acid....because I analyzed it myself for 6 months.....that will give you a lactic acid determination...

Here is a list of hits:  http://search.yahoo.com/search;_ylt=A0geu7DV7I5HFGYAcxtXNyoA?p=HPLC+method+lactic+acid&fr=&ei=UTF-8

Specifically check this list:  http://www.ttb.gov/ssd/methods_list_composite1-06.pdf

[Alpha-Omega]

I just took a Method Development Course...came across some information that might solve many of your problems.  The Acclaim OA HPLC Column will do all organic acids from C1 - C7...and alot more...

Check the following links:



http://www1.dionex.com/en-us/webdocs/41786_31996-01_AcclaimOA_V21.pdf

http://www1.dionex.com/en-us/webdocs/25925_Acclaim_OA_V22.pdf

http://www1.dionex.com/en-us/lp41188.html

[Edward]

Really very grateful for your message.  It's so kind of you to share with me the valuable information and always put my problem in your mind.  Thank you so much!
 I don't understand why this time the HPLC can identify exactly the organic acid itself, not its salt e.g. lactic acid instead of lactate. 
After all the discussion before, I abandoned the pKa titration approach. And I have drafted a method outline using chemical spot tests.

[Alpha-Omega]

OK...this is how you do it:

1) You have to research the column.....I sent you the links so you can see what is does...

2)  You ahve toi purchase the column and the standards for the acids you want to analyze....

3)  You run the standards...you have HPLC with UV (I remember you said that)

4)  Create a standard curve

5)  Run your unknowns against the standard curve..

6)  That and the retention times...will give you information regarding what you have...

This manual even tells you how to VALIDATE: Look at the example applications chromatograms...

http://www1.dionex.com/en-us/webdocs/41786_31996-01_AcclaimOA_V21.pdf

[Edward]

Seems to be a good method for organic acids.  We've got an Agilent 1100 and 1200 HPLC.  Don't know if the Dionex column is suitable for use in Agilent equipment.

[Alpha-Omega]

Yes they are used in Agilent HPLC Systems all the time...definitly compatible....