$pH$ of ions
Tutorial 3
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 $ solution of calcium nitrite?
Find 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!
Characterize these ions from the acid-base point of view !
$Ca^{2+}$ : no influence
$NO_2^-$ : weak base
What is the pH of this solution?
Taking $y=[OH^-]$:
$y^2\;+\;10^{-10.86}\;y\;-\;10^{-10.86}\;0.20\;=\;0$
(Attention to the molarity of nitrite!)
$y\;=\;0.0122 $
$pOH\;=\;-log\; y\;=\;5.76$
$pH\;=\;14-5.76=8.24$
