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Author Topic: Chemical Identification of an unknown alkali  (Read 51074 times)

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Edward

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Chemical Identification of an unknown alkali
« on: December 18, 2007, 07:15:22 AM »

A yellow powder, when dissolved in water, the pH is 12.9.     Could anyone please enlighten me on how to analyze an unknown sample (aqueous or solid) for its chemical composition (cation and anion) with special reference to an alkali or acid?
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #1 on: December 18, 2007, 09:51:43 PM »

There is a procedure of inorganic salt analysis, should be described in any decent analytical chemistry book. Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on. Same holds for anions. High pH may suggest salt of weak acid.
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AWK

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Re: Chemical Identification of an unknown alkali
« Reply #2 on: December 18, 2007, 10:31:25 PM »

A yellow powder, when dissolved in water, the pH is 12.9.     Could anyone please enlighten me on how to analyze an unknown sample (aqueous or solid) for its chemical composition (cation and anion) with special reference to an alkali or acid?

Such high pH suggests oxides, hydroxides (or cyanides) of alkali metals.
Only some oxides, peroxides and superoxides of these metals can be yellow.
Search: www.webelements.com
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Rabn

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Re: Chemical Identification of an unknown alkali
« Reply #3 on: December 23, 2007, 12:55:48 PM »

There is the EDTA titration technique to help identify cation.
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #4 on: December 23, 2007, 01:06:59 PM »

There is the EDTA titration technique to help identify cation.

As far as I know EDTA titration is used for quantitative determination, I am not aware of qualitative applications. Can you elaborate?
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Rabn

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Re: Chemical Identification of an unknown alkali
« Reply #5 on: December 23, 2007, 03:10:24 PM »

Using a colorimetric indicator, you can use the pH at which the hydroxide precipitates to give you an idea of what the cation is. There are plenty of tables that you can use to narrow down possible cations in a range of pH. using EDTA would help narrow the cation down to about 3 from which you could determine use other methods to determine the exact cation.
   If you have access to an atomic absorption specrophotometer you would be able to determine what the cation is by the wavelength of light that is emitted in the flame.
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #6 on: December 23, 2007, 09:26:55 PM »

Using a colorimetric indicator, you can use the pH at which the hydroxide precipitates to give you an idea of what the cation is. There are plenty of tables that you can use to narrow down possible cations in a range of pH. using EDTA would help narrow the cation down to about 3 from which you could determine use other methods to determine the exact cation.

Somehow it doesn't translate to a convenient procedure to me. Too many caveats and assumptions to make. Besides, it won't help if the hydroxide is soluble. Systematic cation analysis (or some spectroscopic approach) is IMHO a much better approach.
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #7 on: December 30, 2007, 08:35:46 PM »

Actually, my problem is a systematic analysis of an acid or alkali.  There are so many acids and bases (organic and inorganic) in this world.  I found some logical scheme for cation and anion analyses (including Ag, Hg, Pb, Fe, Al, Ni, Mg, Zn, Ba, K, Na, Sr, Ca; OH, PO4, CO3, HCO3, SO4, Cl, I), but can I simply apply to acid and bases identification.  Is the logical cation analysis which involves different solubility of salts in acid, bases, applicable to an acid or alkali medium?  I have problem in identifying the hydrogen ion moiety in an acid (e.g, sulfuric acid, nitric acid, etc), in identifying acetic acid, carbonic acid, dihydrogen-phosphate ion, phosphoric acid.  Likewise, I don't know how to approach the identification of an oxide, acetate ion, monohydrogen-phosphate ion.  Adding to the complexity is other organic acids such as carboxylic acids and phenols.  I don't know how many acids and bases there are in this world.  Moreover, I simply don't have an systematic way to identify an unknown powder that is acidic or alkaline.
Could any experts please help me out?  Many thanks! 
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #8 on: December 31, 2007, 12:49:33 AM »

Systematic inorganic analysis will work no matter whether you start with salt, acid or base. If you have an acid, you will just find out that there no cations, if you start with base - you will find no anion.

However, full analysis is not possible without spectroscopic methods.

Number of organic compounds (including bases/acids) is unlimited.

Please give some more information about the substance and circumstances - methods and tools to use depend on whether you are Dow Chemical Company empolyee or High School student :)
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Alpha-Omega

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Re: Chemical Identification of an unknown alkali
« Reply #9 on: December 31, 2007, 12:17:49 PM »

XRF, AA or ICP are your best bets....were you looking ffor a spot test?  Spot tests will give you alot of 411 but not the exact id.....for that you must use elemental analysis:  XRF, XRD, ICP, AA.

If your material is in a powder form XRF is the fastest and easiet route. If you use AA or ICP you will have to subject that sample to digestion....you will be changing the matrix....

If you use XRF....you can get very fast qual ID....then for quant you can do a matrix match....sample prep is so easy aqnd so reliable....

Best of Luck!!!!!
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #10 on: December 31, 2007, 06:26:00 PM »

Thank you so much, Borek and Alpha-Omega!  If systematic inorganic analysis can't identify the cation in acid and anion in base, for base, can I use simple, separate tests to identify CO3 (CO2 gas on HNO4 addition), HCO3 (Add MgSO4 and heat --> white MgCO3 ppt), OH (Add aq. NH4Cl to give NH3 gas)? For other bases and all other acids I have no ways to identify the anion and cation respectively.  I work in a Clinical Toxicology Reference Lab and sometimes we receive unknown powder and liquid that when ingested, cause mouth swelling and bleeding in patients. We therefore need to identify unknown substances received with certainty.  There are limitless possibilities of powder or liquid but most probably they are found in household products such as detergent, soap, drain cleaner, etc; which can be organic or inorganic.  I don't have much knowledge in analytical chemistry, could Alpha-Omega please tell me what XRF, XRD are?  We don't have an Inductively coupled plasma analyzer.  But we have an atomic absorption spectrophotometer. We use it for Pb, Cu quantitation in blood.  How can it be applied to acid and base id in macro quantity?  You mentioned XRF, is this piece of instrument expensive and difficult to use?  I can report results based on systematic analysis and give comments that we only cover certain common household products such as detergent, caustic drain cleaner, HCl, soap, washing powder.  Is it possible to id these products by simple tests?  Thank you very much for your *delete me*
« Last Edit: January 16, 2012, 08:47:25 AM by Arkcon »
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #11 on: January 01, 2008, 01:09:03 AM »

If systematic inorganic analysis can't identify the cation in acid and anion in base, for base, can I use simple, separate tests to identify CO3 (CO2 gas on HNO4 addition), HCO3 (Add MgSO4 and heat --> white MgCO3 ppt), OH (Add aq. NH4Cl to give NH3 gas)?

These will be easily identified with the systematic approach, besides, your approach will give you false positives - systematic analysis is designed in such a way, that you remove all possible interfering cations/anions first.

Quote
I work in a Clinical Toxicology Reference Lab and sometimes we receive unknown powder and liquid that when ingested, cause mouth swelling and bleeding in patients.

I wonder - perhaps forensic labs have an established procedure for determining such substances?

Perhaps you can start with the list of the most probable substances and then look for the way of determining them assuming (at first) that the substance is on the list?

It is a very delicate thing, as false ID can be harmfull.

Quote
could Alpha-Omega please tell me what XRF, XRD are?

Google is your friend:

http://www.learnxrf.com/
« Last Edit: January 16, 2012, 08:45:07 AM by Arkcon »
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #12 on: January 01, 2008, 03:45:05 AM »

A website (http://chemlab.truman.edu/CHEM121Labs/QualAnalysis.htm) describes a systematic analytical approach which does not follow the procedures: Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on.  Is that approach free from false positives?  Besides, the approach doesn't indicate how to identify H, HCO3, OH, etc.  That is why I think of using separate tests for HCO3, OH.
I don't have access to forensic standard procedures.  Do you have any references or sources that I can refer to?
Thanks a lot!
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Borek

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Re: Chemical Identification of an unknown alkali
« Reply #13 on: January 01, 2008, 05:20:12 AM »

systematic analytical approach which does not follow the procedures: Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on.  Is that approach free from false positives?  Besides, the approach doesn't indicate how to identify H, HCO3, OH, etc.  That is why I think of using separate tests for HCO3, OH.

If I read it correctly it is designed to work only for a given subset of cations/anions.

Quote
I don't have access to forensic standard procedures.  Do you have any references or sources that I can refer to?

Google may help, I suppose US based users will have more to add.
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Edward

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Re: Chemical Identification of an unknown alkali
« Reply #14 on: January 01, 2008, 03:27:04 PM »

Do you have references for a more comprehensive systematic analysis of cations and anions?  For acids identification, is it possible to use titration method to determine their identity?  From titration with NaOH, is it possible to obtain molar mass and pKa?  From these data, can I identify all existing acids?  Are there any good references on this?
Thanks!
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