[Borek]

No such thing as unique pKa/pKb, when you take finite accuracy of the determination into account there is simply no place for them all in the range of values pKa and pKb can take. Titration will be of no use here, too many factors for reliable results.

Organic compounds will be either easily melted, or evaporated, or decomposed when heating, while inorganics will usually stay unchanged on heating (that's not 100% correct, they can change form loosing crystalline water, sometimes they also can decompose loosing ammonia or carbon dixode, but the final result will differ from messy carbon leftowers you may expect after heating many organic compounds).

[Alpha-Omega]

I hate to say this; but, if your lab-supervisor is interested in generating resls that are undisputable...he/she will have to invest in some instrumentation that will resolve this issue.  Especially if this is a forensic lab.

[Edward]

I am very puzzled now.  You both think that my approach of titration + systematic qualitative analysis is not applicable, right?  Case 1, if I only need a preliminary id (not 100% confirmed) do you have any suggestions?  Case 2, if I want to confirm the presumptive results, any suggestions?  Given that I only have an AA spectrophotometer, HPLC, GC/MS, LC/MS/MS.  Case 3, if my boss is willing to invest, what is the most cost-effective and easy to develp instrument to acquire?
Thanks!

[Alpha-Omega]

ICP is around 60K...will usually pay for itself in 1-2 years depending on sample load..EDXRF..nice portable models...can take it with you...a bit more pricey...and WDXRF..higher due to the optics....but ICP  good choice...why not use that AA...you have it...and I showed you the Application Notes on the Perkin Elmer site...

[Edward]

Is AA only for metal ions id?

[Edward]

The list of commonly encountered acids are: acetic acid, carbolic acid, formic acid, HCl, monochoroacetic acid, nitric acid, oxalic acid, phosphoric acid, sulfuric acid, oxalic acid, chromic acid, selenious acid, formic acid, hydrofluoric acid.  Alkali: ammonia, CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl. 

[Alpha-Omega]

Well, if you are looking for all those acids IC is capable of detecting them all.  So if you used IC with AA you would be in business....you can detect Na but not OH-, carbonate is a go..but not HCO3 (to unstable).

If you look into IC columns Check out the IonPac AS11-HC....great for organic acids in complex matrices...

[Edward]

For OH-, add NH4Cl to give ammonia gas.  For HCO3, add MgSO4 --> Mg(HCO3)2 which on heating gives white ppt MgCO3.   Is this ok?  Do you know if forensic labs have any standard procedures for acid/ base?  There must be some homicidal cases or attack by corrosive liquids.  What is the algorithm ?  Do they use IC, AA straight away?  I can't find any protocols after exhaustive search in internet.
Thanks!

[Alpha-Omega]

Yes...they are OK...but they are not 100%.  A spot test or qual flash collormetric test is just that....it is an initial ID...it is meant to give you direction to do deeper more comprehensive analysis.

I cannot begin to tell you how valuable those tests can be at times...I had one for stainless steel.  If immersed in HNO3 it will NOT oxidize....OK that is fine and did you test the HNO3?  And how do you validate all this...these are just flash teast...

Whenever a major piece of equipment went down  spot est saved the day momentarilty they allow or cataloguing of the samples....but the results of those tests are in conclusive.

So I have a piece of Aluminum and some sodium hydroxide....well I think I have Al because the label says so....so I toss it in the 50% NaOH...and what happens...I get this violent reaction indicating Al...but then I subjected that solution to FTIR as a confirmation.  Even that is not enough in forensics....every single material you use for an anaysis must be validated.

Titrate acids and bases against an NIST Std...and you are good to go....you  run the same exact test using primary stds then you run your samples....if they fall on your cal curve witin allowed RSD you are good to go...

Homicidal??  Not in customs we dealt with the US Tariff and classifying materials that enter the USA...No dead floating bodies for me...that would be one reason I stay away from FBI....that is one of the questions on theoir application...."So how would you feel if you had to fish a dead body out of a body of water."

Well, so if you have never done that before how would you respond?  LOL

You can contact any Forensic lab:  There is an entire socity dedicated to this here is the link:

www.ascld.org/accreditation.html

they use AA, IC, FTIR, SEM, Microscopy, HPLC, IC, GC, UV/Vis, XRF, XRD, Polarimetry, autotitrators, Karl Fisher, NMR, MS...and a few more I cannot think of right now...ASE-Accelerated Solvent Extractors...

There is one old timey wet test for NaOH people use begins with a B....and people in IC constantly try to compare their reults...and wonder why they are different...HELLO...2 different methods...two different protocols...cannot compare apples and oranges....

Check this;  http://www.clarksonlab.com/caps.pdf

That is a pdf  and I attached a copy...if you cannot download it download it from the link

Here are a ew more:

http://www.atsdr.cdc.gov/toxprofiles/tp7-c6.pdf
http://scholar.lib.vt.edu/theses/available/etd-07242002-111511/unrestricted/ThesisDraft.pdf
http://www.lib.utexas.edu/chem/info/analytical.html

Inorganic Qual Analyses: 
http://chemlab.truman.edu/CHEM121Labs/QualAnalysis.htm
 

[Edward]

Thanks for your reply.  I just sent an email to the association of forensic lab directors requesting for standard protocol. And now starting to read your references.  Thanks!  My boss just wants some simple preliminary tests, not confirmatory tests.  I am thinking in this direction.

[Edward]

I read some textbook.  I don't know if my understanding of the way to derive the pKa is right or not:--  Measure the pH of the acid in question.  If it is a strong acid (? don't know the definition of strong acid), use a weak base (? what weak base is suitable and ? conc. usually used) to titrate.  Plot pH vs volume of weak base used.    Plot the first derivative and second derivative to help find the equivalence point(s).  Note the volume of base used to reach equivalence point(s).  Divide this volume by 2.  Read the corresponding pH value at this volume from the titration curve.  This pH corresponds to pKa value of the unknown.  Match the pKa value with table and find out the unknown acid. 

For unknown weak acid (? definition of weak acid), use strong base e.g. NaOH to titrate.

Is this the correct way of finding pKa?  I don't have a complete list of pKa for inorganic acids.  Please comment.

Thanks!

[Borek]

When titrating weak acid with strong base pKa = pH at 50% neutralization. It applies to acids with pKa in the 3-11 range, outside errors are getting large. But as I have alredy told you it is of no use - there are too many acids that can fit in the - say - 3.5+/-0.1 range (and don't expect better accuracy). When you have only a few with well separated pKa values it can work, when you can have anything this approach is IMHO a waste of time.

Using weak base for strong acid titration is completely off. This way you can determine pKb of the base, but only if you will add 50% excess of titrant.

[Edward]

If I have a list of acids we are interested in : acetic acid, carbolic acid, formic acid, HCl, monochoroacetic acid, nitric acid, oxalic acid, phosphoric acid, sulfuric acid, oxalic acid, chromic acid, selenious acid, formic acid, hydrofluoric acid.  Alkali: ammonia, CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl.  I have found out pKa of some but not carbolic, HCl, nitric, selenious acids and not CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl.  Can I check the pKa the identity by titration?  I have no way to confirm. 
Is there any method you know of that can detect these acids and bases? 

[Alpha-Omega]

OK more 411:

[Alpha-Omega]

Go to www.dionex.com and download AN93.  I cannot upload it due to size restrictions.