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: Plasma potassium concentrations were on average 1 mmol / L lower during the chlorthalidone phase compared to amiloride therapy .
### Output Text:
Plasma <span class="chemical">potassium</span> concentrations were on average 1 mmol / L lower during the <span class="chemical">chlorthalidone</span> phase compared to <span class="chemical">amiloride</span> therapy . 