Sulphur (element), brimstone
磺 centers on the element sulphur and its associated substance brimstone. It is a specific chemical/mineral term rather than a word with broad abstract extensions.
磺 combines 石 (stone/mineral) with 黄 (yellow), likely referring to the yellowish color of native sulphur. The exact historical development is uncertain, but the component choice fits the mineral meaning.
The left side 石 means stone or mineral, and the right side 黄 means yellow. Picture a yellow mineral stone: sulphur, also known as brimstone.
For コウ, imagine a yellow mineral stone that makes you cough when you smell it: cough -> コウ, and the yellow stone is sulphur.