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: In all of them normal serum potassium levels reached within 2 to 4 days of stopping sulindac .
### Output Text:
In all of them normal serum <span class="chemical">potassium</span> levels reached within 2 to 4 days of stopping <span class="chemical">sulindac</span> . 