conductance titration (1 Viewer)

[stargaze]

For conductance titration, the conductivity is high initially, then (at equivalence point) goes to its lowest conductance, then back up...

Now I know initially its because there are a high concentration of ions (because nothing has reacted yet)

Then at equivalence point, I'm not totally confident on why the conductivity falls to its lowest point. Looking at conquering chem, its kinda vague..
For example (p159 conquering chem, q26)...
When u titrate H2SO4 with Ba(OH)2... Is the conductivity falling at equivalence point because:
- As reaction occurs, BaSO4 forms which is extremely non-conductive [i.e. ions are becoming compounds --> lower ion concentration means less conductivity] --> not 100% on this...

Then, after equivalence point is reached, as more acid is acid, this just adds more ions into the solution, thus causign conductivity to rise

Can somebody just clarify .. point out stuff pls haha? thanks a lot

 

[Zatts]

Well the shape of the conductivity graph is like a concave up parabola, right? Well the downward slope is indicating that the ions are combining to form BaSO4, which is an insoluble salt. This causes the concentration of ions in your solution to decrease, which in turn causes the conductivity to decrease. Then, the upward slope indicates that all the Ba2+ ions in the flask have reacted with the SO42- ions. Also, all the hydroxide and hydrogen ions have reacted. Extra acid is being added, so the concentration of H+ ions and SO42- is increasing, and the conductivity increases.

This was using an example from the Surfing Chemistry book on The Acidic Environment (pg 35, Q 10). Hope it is of help.

 

[lum]

rather than parabola, it's more like an absolute function of a straight line graph shifted up, ie. a v shape

 

[Jumbo Cactuar]

rather than parabola, it's more like an absolute function of a straight line graph shifted up, ie. a v shape

It would be like that if you added only ions and not water with it.

stargaze is right in that the conductivity it proportional to the sum of the absolute charges. And since the conductivity is taken over a specific volume it is proportional to the concentration of the ions as well.

To summarise;

under the same atmospheric conditions;
Deionized water: G = 10^-7 Siemens
0.001 M KCl : G = 130 Siemens
0.002 M KCl : G = 260 Siemens
0.001 M Ba(OH)2 : G = 260 Siemens
0.001 M Ba(OH)2 + 0.002 M KCl : G = 520 Siemens

You get the idea! These values are pulled out of nowhere so don't take it too literally. Also in large concentrations these rules go very awry.

So if you have tightly bound precipitates (ie. BaSO4) you won't get any contribution. Some interesting results occur when metal cations complex.

ie Fe3+ ions are added to thiocynate ions through conductance titration.

Fe3+ + 6SCN- ---> (Fe(SCN)6)3-

initial conductivity G = 400 S (12 charge units)
endpoint conductivity G = 400 S (12 charge units)

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400 S ________________/