Chapter 12 and 15 Flashcards
Titration curves for weak acids and bases
Four distinctly different types of calculations are needed to derive a titration
curve for a weak acid or a weak base
1. At the beginning, the solution contains only the solute acid or base, and the
pH is calculated from the concentration of that solute and its dissociation
constant.
2. After various increments of titrant have been added, the solution consists of
a series of buffers. The pH of each buffer can be calculated from the
analytical concentrations of the conjugate base or acid and the residual
concentrations of the weak acid or base (Henderson-Hasselbalch equation).
3. At the equivalence point, the solution contains only the conjugate of the
weak acid or base being titrated and the pH is calculated from the
concentration of this product.
4. Beyond the equivalence point, the excess of strong acid or base titrant
represses the acidic or basic character of the reaction product to such an
extent that the pH is governed largely by the concentration of the excess
titrant
A typical pH curve for the titration of a weak
base with a strong acid.
The stoichiometric
point (S) occurs on the acidic side of pH 7
because the salt formed by the
neutralization reaction has an acidic cation
Lewis Acid
electron pair acceptor= metal ions
lewis base
electron pair donor=ligand
donates both electrons of the electron pair bond
Chelate effect
the ability of multidentate ligands to form more stable metal
complexes than those formed by similar monodentate ligands
Examples of Chelating agents; crown ether and cryptands
Equilibrium calculations involving EDTA
Equilibrium calculations involving EDTA
Ag+ + Y^4—-> AgY3-
Al3+ + Y^4- —->AlY -
M^n+ + Y^4- —>MY^(n-4)+
KMY = [MY^(n-4)+ ][M^n+][Y^4-]
a4 = [Y^4- ] / cT
Fraction of EDTA in Y^4- is given as
cT = [Y^4- ] + [HY^3- ] + [H2Y 2- ] + [H3Y- ] + [H4Y 2- ] Mn+ + Y4- MY^(n-4)+ KMY = [MY (n-4)+ ]/[Mn+]a4c T
where
K’MY = a4K MY = [MY4-^n-4)+ ][Mn+]c T
Therefore,
or
EDTA Titration Techniques
Direct titration
Back titration
displacement
Direct titration EDTA
When the metal ion indicator responds well
– Indicator, spectrophotometric and potentiometric
methods can be used to obtain the endpoint
back titration
when no suitable indicator is available
displacement
when no indicator for an analyte is available
MgY2-+M2+—>MY2-+Mg2+
