$pH$ of ions
Tutorial 2
Influence of ions on pH
Cations
Most metal cations have no effect on pH.
Examples: $ Na^+, K^+, Ca^{2+}, Mg^{2+} ....... $
Exception: Some hydrated cations are weak acids (see → this table ).
Examples: $ Al^{3+}, Fe^{3+}, Zn^{2+} $
Anions:
Anions corresponding to strong acids do not influence pH.
Examples: $ Cl^- $, $ Br^- $, $ I^- $, $ ClO_4^- $, $ NO_3^- $
Some anions are both weak acids and weak bases. They therefore belong to two acid-base pairs with the constants $ pK_{a1} $ and $ pK_{a2} $ (see → this table ). They are called ampholythes.
Examples: $ HCO_3^- $, $ HSO_3^- $, $ H_2PO_4^- $, $ HPO_4^{2-} $, $ HS^- $, ...
The pH of their solutions is calculated as follows:
$pH$ $=$ $\frac{1}{2}pK_{a1}$ $+$ $\frac{1}{2}pK_{a2}$
The other anions are weak bases. (See → this table )
Examples: $ F^- $, $ CN^- $, $ PO_4^{3-} $, $ ClO_3^- $, $ ClO_2^- $, $ ClO^- $, ......
What is the pH of a $0.1 \; M $ iron(III) nitrate solution ?
Look for the ions present in solution!
For answers, use (possibly several times) the arrows ↑ Down! and ↓ Up!
Complete please this question before moving on to the next one!
In solution:
$Fe^{3+}$ and $NO_3^-$
Characterize these ions from the acid-base point of view !
$Fe^{3+}\cdot 6H_2O$ : weak acid
$NO_3^-$ : no influence
What is the pH of this solution?
Taking $x=[H_3O^+]$:
$x^2\;+\;10^{-2.83}\;x\;-\;10^{-2.83}\;0.10\;=\;0$
$x\;=\;0.0122 $
$pH\;=\;-log\; x\;=\;1.92$
