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: Patients who developed hyperkalemia were older and more likely to have diabetes , had higher baseline serum potassium levels and lower baseline potassium supplement doses , and were more likely to be treated with beta - blockers than controls ( n = 134 ) .
### Output Text:
Patients who developed hyperkalemia were older and more likely to have diabetes , had higher baseline serum <span class="chemical">potassium</span> levels and lower baseline <span class="chemical">potassium</span> supplement doses , and were more likely to be treated with beta - blockers than controls ( n = 134 ) . 