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Author Topic: Chemical Identification of an unknown alkali  (Read 49804 times)

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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #75 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 #76 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 #77 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 #78 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 #79 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 #80 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 #81 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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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #82 on: January 14, 2008, 09:21:25 PM »

No reply yet.  But I wonder what the people in the old days use when they did not have the sophisticated instruments such as IC, etc.  Did they not do any prelim. id?
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AWK

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Re: Chemical Identification of an unknown alkali
« Reply #83 on: January 14, 2008, 10:00:27 PM »

No reply yet.  But I wonder what the people in the old days use when they did not have the sophisticated instruments such as IC, etc.  Did they not do any prelim. id?
these preliminary tests are called qualitative analysis and eventually chemical-technical analysis if samples are not pure. Did you read any texbook on this subjest.
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #84 on: January 14, 2008, 10:33:05 PM »

I did read some textbooks.  Qualitative analysis is for id of cations and anions.  Seems not comprehensive enough for acids and some bases.  But I haven't seen a term chemical-technical analysis. Could you suggest any references pls?
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #85 on: January 15, 2008, 12:48:20 AM »

I found a  really old book, dated 1898, An introduction to Chemical Technical analysis.
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AWK

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Re: Chemical Identification of an unknown alkali
« Reply #86 on: January 15, 2008, 01:36:09 AM »

I did read some textbooks.  Qualitative analysis is for id of cations and anions.  Seems not comprehensive enough for acids and some bases.  But I haven't seen a term chemical-technical analysis. Could you suggest any references pls?
Acids contains anions, bases contains cations you can find in your samples. Qualitative analysis works this way over 150 years and is sufficient to identify also acids and bases. Simply if you cannot find any cation, and solution is sufficiently acidic, then you have the acid containing this anion. Of course this is valid for individual substances.
In the case of mixtures, inorganic qualitative analysis detects only cations and anions (also a few organic anions).
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #87 on: January 15, 2008, 04:27:20 AM »

Do you mean if I find Cl- ion by qualitative analysis and the pH is low (? how low), then I can say it is HCl?  In the same sense, if I find Na in a pH 13 liq that gives NH3 gas when NH4Cl is added, I can say this is NaOH?
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #88 on: January 15, 2008, 04:31:42 AM »

I am intending to use acid base titration for weak acid to decide its pKa and check from a table for the corresponding acid's ID.  This doesn't work for strong acid as Borek stated.  If I can find the cation, how can I say the anion is H+?  Also, I want to use qualitative analysis to test for the cation and anion in a base.  what do you think of this approach?
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #89 on: January 15, 2008, 04:34:06 AM »

sorry, should be "If I can find the anion, how can I say the cation is H+? 
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