In a nerve impulse, the hydrophilic ions K and Na must cross the hydrophobic cell membrane - PowerPoint PPT Presentation

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In a nerve impulse, the hydrophilic ions K and Na must cross the hydrophobic cell membrane

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Many times one must work at a pH where the metal hydroxide forms a precipitate. ... complexing agent; it can be used to quantitatively precipitate metal ions. ... – PowerPoint PPT presentation

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Title: In a nerve impulse, the hydrophilic ions K and Na must cross the hydrophobic cell membrane


1
EDTA Complexation and Equilibria
Read Chapter 13. This powerpoint will not be on
exam II.
In a nerve impulse, the hydrophilic ions K and
Na must cross the hydrophobic cell membrane
Ionophores carry these ions across the membrane.
2
Nonactin is an example of an ionophore
3
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4
EDTA is similar to nonactin in so much as it
forms strong 11 complexes.
5
EDTA is just one of many good chelating agents.
Chelating agents have more than one ligand bond.
6
EDTA forms six ligand bonds to Mn2.
7
EDTA is a hexaprotic acid system designated H6Y2.
8
EDTA Equilibria is like that of Tiron
One difference is that there is no MY2n-7
formation. Do you think that there is much MHYn-3
formation?
9
Tiron Equilibria
We introduced a, b, and gs and then wrote down
mass balance equations for cL and cFe in terms of
them. The cFe equation is a function of Fe3
and then a, b, and gs, so we can solve for
Fe3 as functions of these variables.
10
EDTA Equilibria
  • Write down the mass balance for EDTA
  • Write down the mass balance for Metal
  • Solve for the as and bs. Here the bs are for
    the fractions of metal.
  • Write MB for EDTA using the bs.

11
EDTA Equilibria
Use MB for EDTA to get 1 equation and 1 unknown.
Here CEDTA is the total concentration and EDTA
is the total free EDTA, i.e., H6Y2, H5Y,Y2-.
12
One can also write this equation as
Like a buffer
We can solve the above for xEDTA. One can
also write the equation is the following
suggestive form (cf. 13.6).
MnCMb0
The Mn is
13
Calculation at Equivalence Point
Starting with the master equation
At the equivalence point one has CEDTA CM so
14
Creating Titration Curves
Now insert the appropriate dilution formulas
And solve for Vt.
And the result is
15
Effect of pH on titration curves.
pH9
pH7
-logEDTA
pH5
Vt

16
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17
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18
Auxiliary Complexing Agents
Many times one must work at a pH where the metal
hydroxide forms a precipitate. Here we need to
add something that binds to the metal strongly
enough to prevent it from precipitating but
weakly enough to give up the metal when the EDTA
is added. The book considers the example of
adding NH3 to Zn2.
19
EDTA Equilibria with NH3
20
Begin with the mass balance of the metal.
21
Now lets use gs for relative amounts of Mn and
its ammonia complexes.
This simplifies to
Here g0 takes the usual form
where
22
Now combine MB for metal with MB for EDTA.
We now have one equation and one unknown
where EDTA is the unknown. Recall
23
With this defn of b1 the formula for Vt is
almost the same as before.
And the result is
24
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25
EDTA
  • EDTA is a great complexing agent it can be used
    to quantitatively precipitate metal ions.
  • We have worked out the equations for the
    multicomponent equilibria and can understand how
    pH and auxilliary complexing agents affect the
    metal concentrations.
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