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 concentration of serum potassium was high , and normal sinus rhythm was restored after correction of the serum potassium level .
### Output Text:
The concentration of serum <span class="chemical">potassium</span> was high , and normal sinus rhythm was restored after correction of the serum <span class="chemical">potassium</span> level . 