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Author Topic: Chemical Identification of an unknown alkali  (Read 46790 times)

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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #45 on: January 08, 2008, 02:36:55 AM »

OK more 411:
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #46 on: January 08, 2008, 02:39:54 AM »

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

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Re: Chemical Identification of an unknown alkali
« Reply #47 on: January 08, 2008, 07:03:34 PM »

Many thanks!  Frankly speaking, I am desparate to find some simple methods to detect unknown acids and bases.  The information you gave here is very useful for anions and organic acids determination.  Do you know if the columns for anions and organic acids can be bought separately from ion chromatography analyzer system.  Can it be used in our HPLC system (which is an Agilent 1100 model) which uses a UV detector? ? an ECD is necessary for this purpose.  I am still stuck with acid and base determination. Borek said that titration applies to pH 3-11 range, so in my list at least monochloroacetic, nitric and sulphuric acid are out.  If the unknown's pKa does not match those of other acids in my list, I can rule those out.  My titration method is for ruling out of acids that I am interested in. 
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #48 on: January 08, 2008, 07:26:16 PM »

HCl is out also.  My problem is how to find the identity of monochloroacetic, nitric, sulfuric acids.
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #49 on: January 08, 2008, 09:06:32 PM »

Borek said that titration applies to pH 3-11 range

pKa not pH, and I have not told "titration doesn't apply" but "error in the pKa determination using titrimetry gets high outside pKa 3-11 range".
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #50 on: January 09, 2008, 03:31:14 AM »

<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.>
Thanks Borek.  What does the above mean?  Do you mean determining pKa of strong acid is impossible?  Even after adding 50% more titrant: weak base?  Can I dilute the acid in question, say ten thousand  fold to get a a lower pH before titration?
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #51 on: January 09, 2008, 04:24:16 AM »

Quote
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.
Thanks Borek.  What does the above mean?  Do you mean determining pKa of strong acid is impossible?

Possible, but not by titration. Strong acids have pKa well below 3 (limit I have signalled earlier).

Quote
Even after adding 50% more titrant: weak base?

You are so confused I have no idea what to start with...

pH = -log([H+)

Strong acid - full dissociation, so if you have solution of strong monoprotic acid concentration of H+ is identical with acid concentration, and pH = -log(acid concentration).

During titration you are neutralizing H+ and what you are left with is a mixture of salt and fully dissociated strong acid. pH = -log(concentration of strong acid left). No place (or need) for pKa. After all acid has been titrated, pH depends only on the added excess of base.

Weak acid - partial dissociation, described by the acid dissociation constant Ka. pKa = -log(Ka). When you add strong (fully dissociated) base you neutralize acid - and you are left with salt and not yet neutralized acid in the solution. It happens that if the acid is weak enough (pKa > 3) pH can be calculated using so called Henderson-Hessalbalch equation:

pH = pKa + log(concentration of neutralized acid/concentration of not neutralized acid)

When exactly half of the acid is neutralized pH = pKa. It works reasonably correctly only for acids with pKa within the 3-11 range.

After full neutralization, pH of the solution once again depends only on the concentration of excess base.

This picture is slightly simplified, but it shpould be enough to understand what's going on.

For bases replace Ka with Kb and pH with pOH (pOH+pH=14), otherwise they behave identical.

Quote
Can I dilute the acid in question, say ten thousand  fold to get a a lower pH before titration?

If you dilute acid pH will go up, not down.

BTW: I was wrong, it should be 100% excess of titrant, not 50%. My mistake. But it still doesn't apply to your problem. You can't easily determine pKa of strong acid with titration, no matter if you use strong or weak base.
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #52 on: January 09, 2008, 05:44:43 AM »

IC requires a pump, a column (your separator) , a suppressor (to suppress the ions in your eluent, and a detector...a conductivity detector.

UV can be impemented with post column derivitization systems. Hex chrome analysis is an example..

You Agilent HPLC is for HPLC.  There are columns you can use...but these columns are made of PEEK and may not be compatible with a SS system. But that is a DIFFERNET ANIMAL.

If you are really interested in IC and what you can do; again, I suggest, www.dionex.com and you will have to read some of the literature on the method and its capabilities...

I can send you a power point my friend AVM just developed for teaching IC Applications on the ICS-3000 RFIC system.  Cannot upload here too big...If there is a place on this site I can send it to I would be more than glad to post it for you...
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #53 on: January 09, 2008, 12:27:26 PM »

pls send me the powerpoint.    Many thanks! Borek said that there are ways to id strong acids.  What are they?
« Last Edit: January 17, 2008, 05:23:26 AM by Edward »
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #54 on: January 09, 2008, 12:41:29 PM »

Done Deal....
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #55 on: January 09, 2008, 02:23:56 PM »

Powerpoint received.  Really appreciate your great *delete me*  Do you know of any ways to id strong acids?
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #56 on: January 09, 2008, 02:47:07 PM »

What *delete me*? 

Here is a GC Method using that ECD/also requires Liquid Solid Extraction- you say you have:

DETERMINATION OF CHLORINATED ACIDS IN WATER USING LIQUID-SOLID
EXTRACTION AND GAS CHROMATOGRAPHY WITH AN ELECTRON
CAPTURE DETECTOR

http://www.accustandard.com/asi/pdfs/epa_methods/515_2.pdf

Your best bet is to go to the EPA site and do a search of all their approved methods for determining you species of interest...you can gdo a serch under GC methods on their site...tyer is a way of limiting your criteria...

You can determine if a method is availble with what equipment you have.
EPA SITE INDEX:  http://www.epa.gov/epahome/index/

Here is a PDF with all their approved test methods INDEXED:  http://www.ch2m.com/webuploads/newsgenerator/Ext_Environment/Env_ACCS_AboutUs/EPAMETH.pdf

there is this site also/National Environmental Methods Index:  http://www.nemi.gov/
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #57 on: January 09, 2008, 07:29:36 PM »

Don't know why hel..p can't be displayed here.  Thanks for your assistance!
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #58 on: January 09, 2008, 07:37:43 PM »

Oh had no clue...I am too new...but thanks....
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #59 on: January 10, 2008, 04:37:38 AM »

hi, Alpha-Omega.  I read through the whole EPA list and found some useful target acids for my purpose:  chlorinated acids, monochloroacetic acids, phenol.  And I sent an enquiry email to EPA, asking for further information about these acids and others, as well as bases.  Then I found that you have already given me the chlorinated acids manual.  Marvellous!  Thanks for your very useful information.  I would like to find similar manuals of standard methods in forensic practice.  To my disappointment, I couldn't find any after a day's search thro' google.  I am very impressed by your knowledge in this area.

« Last Edit: January 16, 2012, 08:46:05 AM by Arkcon »
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