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: The serum level of potassium was observed to be 8 . 4 mequiv L - 1 .
### Output Text:
The serum level of <span class="chemical">potassium</span> was observed to be 8 . 4 mequiv L - 1 . 