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Author Topic: Chemical Identification of an unknown alkali  (Read 57447 times)

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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #60 on: January 10, 2008, 01:22:09 PM »

http://www.fjc.gov/public/pdf.nsf/lookup/sciman00.pdf/$file/sciman00.pdf

here is the PDF Reference Manual on Scientific Evidence...
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #61 on: January 10, 2008, 01:56:47 PM »

Seems there are no standard methods for acid base identification in the website?!
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #62 on: January 10, 2008, 02:06:54 PM »

I would not think there would be...they would use methods from ASTM or EPA or USP/EP/JP/ACS...and they have to be documented to work...then they apply their chain of custody to those methods....

The species you are looking at...they would apply a quantitative intrumentation method to...like IC...something that has documentation behind it to show it is a validated method.
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #63 on: January 10, 2008, 04:08:41 PM »

You showed me ASTM, EPA.  what is USP/EP/JP/ACS?
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #64 on: January 10, 2008, 04:49:51 PM »

USP = United States Pharmacopia
EP = European Pharmacopia
JP = Japanese Pharmacopia
ACS = American Chemical Society
BP = British Pharmacopia

These books contain methods which are GLOBALLY accepted in the Pharmaceutical industry and the methods are also used in other industries.

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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #66 on: January 11, 2008, 07:02:37 PM »

Found some standard methods for flame test, titrimetric analysis in JP.  None in others.  Thanks for sharing these useful websites to me.
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #67 on: January 11, 2008, 10:48:52 PM »

Borek, could you please tell me the source of reference for the statement: titrimetric analysis for pKa is suitable for pH3-11? 
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #68 on: January 12, 2008, 01:08:04 AM »

No reference, but you may calculate it by yourself (although you will be probably forced to use some software, like my BATE, as these calculations can be challenging). The whole idea of pKa determination by titration is based on the Henderson-Hasselbalch equation and fact that pH of weak acid titrated 50% equals its pH. But this approach is based on simplyfying assumption that concentrations of HA (acid) and A- (conjuagted base from neutralized acid) are defined by the neutralization stoichiometry.  In the case of stronger acids HA will tend to dissociate "on its own", in the case of weaker acids A- will hydrolize "on its own" and the difference between pH at 50% and pKa will be larger. Perhaps 3-11 is slightly conservative, perhaps you can extend it to 2.5-11.5 - it all depends on the error you are ready to accept.

pKapH at 50%
0.001.50
1.001.67
2.002.18
3.003.02
4.004.00
5.005.00
6.006.00
7.007.00
8.008.00
9.009.00
10.0010.00
11.0010.97
12.0011.82
13.0012.32
14.0012.48

You may try to use some correction tables - like pH of 2.18 at 50% means pKa of 2.00 - but still, the stronger the acid the larger the error, as the dependence becomes more and more flat (ie differences in acid strength give smaller differences in pH).
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #69 on: January 13, 2008, 10:29:51 PM »

Do you know how much an autotitrator for acid base titration is?  What brand do you recommend?  Thanks!
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #70 on: January 13, 2008, 10:59:53 PM »

Only need to do pH titration automatically and for recording volume of base added and pH for equivalence point and pKa determination.  Preferably can plot pH titration curve and/or first or second derivative inflection point determination. Metrohm has some sophisticated equipment, but no price listed.
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #71 on: January 14, 2008, 03:35:24 AM »

Methrom and all other Instrumentation companies WILL NOT list prices.  You have to call them and give them your 411 and they will put  your regional sales rep in touch with you. 

They do not list prices....That rep will make sure that you purchase the most appropriate system for your purposes.  Explain the application and assay....they will hook you up.

I had a temp assignment where they were trying to use an autotitrator for an assay...there were two chemists....I could clearly see the instrument was inappropriate for the assay....now way was it going to detect those levels...in fact after a few runs...I could see their procedures for the 2 asays were reversed...No documentation trail....so the process engineer and I ran them together to prove the point....PROBLEM SOLVED!!! But the down time was significant...

You should talk to a sales rep....

These people sell used surplus autotitrators-they take bids-just like on ebay:  http://www.labx.com/v2/newad.cfm?catID=46


Be very careful buying from a reseller.  Most times you will do better buying from the provider/manufacturer-all around.

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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #72 on: January 14, 2008, 03:59:26 AM »

Thanks! you have got so many valuable experiences and useful tips.  I left a message on the Methrom website asking for an autotitrator suitable for my purpose and waiting for their reply. 
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #73 on: January 14, 2008, 04:24:13 AM »

Keep us posted, I wonder what they will tell you once they will learn you plan to use pKa for acid identification.
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #74 on: January 14, 2008, 09:04:32 AM »

They will tell him you cannot ID an acid by pKa....that his application will require further analysis and then they will try and sell him an IC system....

This link takes you to a number of downloadable applications for potentiometric analysis:

http://www.brinkmann.com/literature/default_B.asp?GRP=apps
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