Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: 1 ... 5 6 [7] 8   Go Down

Author Topic: Chemical Identification of an unknown alkali  (Read 50819 times)

0 Members and 1 Guest are viewing this topic.

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #90 on: January 15, 2008, 05:22:29 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.

I have also stated it will not work for weak acids, as the accuracy of the pKa determination is low and there are too many acids with very similar pKa values. Alpha-Omega told you the same thing.
« Last Edit: January 16, 2012, 08:46:42 AM by Arkcon »
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #91 on: January 15, 2008, 05:23:11 AM »

If I can find the anion, how can I say the cation is H+? 

If you have eliminated all other possible cations you are left with H+ only.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #92 on: January 15, 2008, 07:28:36 PM »

My boss doesn't want to invest on IC, etc.  So the only thing I can count on is simple titration for acids like acetic, carbolic, chromic, formic, monochloroacetic, oxalic, phosphoric acids.  For HCl, HF, nitric, sulfuric acid I may use qualitative analysis for anions, and exclusion of other cations for H+.
I'd love to use IC if I had this instrument.  Many constraints when cost  is concerned.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #93 on: January 15, 2008, 09:05:51 PM »

Instead of titration you may as well try card reading - much cheaper and similar accuracy.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #94 on: January 15, 2008, 09:45:23 PM »

What is this please: card reading? 
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #95 on: January 15, 2008, 10:09:19 PM »

What is this please: card reading?

Fortune telling with deck of cards, like palm reading.

We told you several times that titration will not work as ID method yet you still cling to it.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #96 on: January 15, 2008, 10:22:09 PM »

Should I rather tell my boss we can do nothing except buy an IC?
Logged

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #97 on: January 15, 2008, 11:03:25 PM »

He probably won't believe there is no simple preliminary, not asking for a confirmative, test for acids.  I told him once I could not find any standard method from forensic labs and he thought that there must be some.  And it is my job to find the solution, not to create problem.  I am really worried that I will be out of my job.
Logged

Alpha-Omega

  • Full Member
  • ****
  • Mole Snacks: +360/-231
  • Offline Offline
  • Gender: Female
  • Posts: 700
  • Physical Inorganic Chemist
Re: Chemical Identification of an unknown alkali
« Reply #98 on: January 16, 2008, 02:15:37 AM »

OK...as I see this...if you use a pH meter you can thell him the material is ACIDIC or BASIC....you cannot tell him what it is....

If you use colormetric IDs you can determine this or that is there....but how the this or that is put together is a different ball game....

IC by itself is not a 100% either it will tell you what anions and or cations are in there....just look at those chromatograms I sent you...what do you see? 

Most analyses for a material ....LET ME BE VERY CAREFULE HERE...require more than one test to give ALL the information one requires....then you put all those pieces together and explain why something is what it is....

There are cases where one analysis will yield enough information to give you a 100% accurate positive picture/ID of what you are looking for.

XRD would be an example of this....it depends on what you are looking for and are required to report.
Logged

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #99 on: January 16, 2008, 04:54:07 AM »

I see lactate instead of lactic acid in the chromatograph, that's the anion, not the exact identity of the acid. 
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #100 on: January 16, 2008, 05:05:56 AM »

Imagine mixture containing 0.25M H+, 0.25 M Na+, 0.25 M NO3- and 0.25 M Cl-. Can you tell what acid and what salt does it contain?
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #101 on: January 16, 2008, 05:07:38 AM »

Really appreciate your explanation!
Logged

Edward

  • Regular Member
  • ***
  • Mole Snacks: +0/-1
  • Offline Offline
  • Posts: 61
Re: Chemical Identification of an unknown alkali
« Reply #102 on: January 16, 2008, 05:11:02 AM »

"Imagine mixture containing 0.25M H+, 0.25 M Na+, 0.25 M NO3- and 0.25 M Cl-. Can you tell what acid and what salt does it contain?"
Can I say I can't tell the answer with any method I use?
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1372/-369
  • Offline Offline
  • Gender: Male
  • Posts: 21011
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Chemical Identification of an unknown alkali
« Reply #103 on: January 16, 2008, 05:25:00 AM »

Yep. All you can say is what ions (or molecules) at what concentrations are present.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Alpha-Omega

  • Full Member
  • ****
  • Mole Snacks: +360/-231
  • Offline Offline
  • Gender: Female
  • Posts: 700
  • Physical Inorganic Chemist
Re: Chemical Identification of an unknown alkali
« Reply #104 on: January 16, 2008, 12:16:46 PM »

With IC you will see the lactate anion.  That is correct.  The software will monitor the pH of your system during the analysis.  You can also do a pressure trace...good for diagnostic purposes...But NO it does not tell you it is lactic acid.

In IC there are so many things going on in that suppressor...difficult to say exactly what you have going on in there....you can infer...you cannot ... and these are UNKNOWNS you will be analyzing....this is just a piece of an unknown...not the whole picture...
Logged
Pages: 1 ... 5 6 [7] 8   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.063 seconds with 24 queries.