In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: While severe hypokalemia may cause muscle weakness , severe hypomagnesemia is associated with muscle spasms and tetany which cannot be corrected by potassium and calcium supplementation alone ( 1 , 2 ) .
### Output Text:
While severe hypokalemia may cause muscle weakness , severe hypomagnesemia is associated with muscle spasms and tetany which cannot be corrected by <span class="chemical">potassium</span> and <span class="chemical">calcium</span> supplementation alone ( 1 , 2 ) . 