Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Amperometric endpoints  (Read 148 times)

MindNarrative and 3 Guests are viewing this topic.

MindNarrative

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Online Online
  • Posts: 3
Amperometric endpoints
« on: Yesterday at 04:39:26 PM »

As I understand it, the amperometric endpoint is the volume where the current vs. volume graph slope changes. But why would this be different from the volumetric endpoint? I have one chart each of the same titrant on two different concentrations of the same titrand, but one has the amperometric endpoint before the volumetric, and the other has it after. What gives?
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • **
  • Mole Snacks: +1415/-373
  • Offline Offline
  • Gender: Male
  • Posts: 21765
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Amperometric endpoints
« Reply #1 on: Yesterday at 09:22:25 PM »

What do you mean by "volumetric endpoint" - the one calculated from stoichiometry for known concentrations of titrant and titrand? Differences shouldn't be large, but they are not surprising.

Hint: take a look at the titration curve of acetic acid. At what pH would you expect the lowest conductivity of the solution? Is it the same pH titration ends at? Which comes first?
Logged
Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

MindNarrative

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Online Online
  • Posts: 3
Re: Amperometric endpoints
« Reply #2 on: Today at 07:04:41 AM »

By volumetric endpoint, yeah, it's the one calculated from stoichiometry. The lowest conductivity would be when the ion concentration is lowest, so....wait, would it be at 7.0? Now I'm all turned around.

These are my graphs, two concentrations of the same titrand:

http://i.imgur.com/0iDSfgc.png
http://i.imgur.com/VRHAhln.png

If I have it right, I'm supposed to extend the slopes on either side of the slope change, and the endpoint is where the lines intersect. The color changes happen at 25 mL and 10 mL, but the slope changes are on either end of the color change on both graphs.

Or is the amperometric endpoint where the conductivity = 0? My textbook doesn't have much on reading the graphs.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • **
  • Mole Snacks: +1415/-373
  • Offline Offline
  • Gender: Male
  • Posts: 21765
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Amperometric endpoints
« Reply #3 on: Today at 08:04:43 AM »

The lowest conductivity would be when the ion concentration is lowest, so....wait, would it be at 7.0?

Perhaps not exactly at 7.00, but conductivity is a sum of infiovidual ion conductivities. At pH 7.00 sum of concentrations of H+ and OH- is the lowest possible.

Quote
Or is the amperometric endpoint where the conductivity = 0? My textbook doesn't have much on reading the graphs.

No way to have conductivity of 0. Think about what is present in the solution.
Logged
Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

MindNarrative

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Online Online
  • Posts: 3
Re: Amperometric endpoints
« Reply #4 on: Today at 11:03:00 AM »

Not conductivity, I meant current. Again, turned around. The amperometric equivalence point is where the redox system is balanced, so net current = 0, so on the graph where y = 0. So that would mean the slope changes mark the region where the midlog of the titration begins and ends, like a typical titration curve, right?

The titrand is Fe2+ and the titrant is Ce 4+. As I understand it, before adding Ce, Fe2+ is being oxidized, which reduces the electrode, giving positive current. At the equivalence point, the oxidation of Fe 2+ and the reduction of Fe 3+ occur in equal proportions, so net current at the working electrode is 0. After the equivalence point, Fe is getting reduced, so the current is negative.

But I still don't understand why stoichiometric and amperometric equivalence points can be different, especially to the degree in my data. The current is 0 at 50% and 80% of the volume calculated by stoichiometry, which means there is something I'm missing, but I don't know what.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.072 seconds with 25 queries.