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 sigmoid Emax model offered a good description of the relation between terbutaline concentrations and potassium effects .
### Output Text:
The sigmoid Emax model offered a good description of the relation between <span class="chemical">terbutaline</span> concentrations and <span class="chemical">potassium</span> effects . 