The composition of potash alum is
or
$
\mathrm{K}_2 \mathrm{SO}_4 \cdot \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3 \cdot 24 \mathrm{H}_2 \mathrm{O}
$
$
\mathrm{K}_2 \mathrm{Al}_2 \mathrm{~S}_4 \mathrm{H}_{48} \mathrm{O}_{40}
$
It is a double salt of potassium sulphate and aluminium sulphate