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: To investigate the initial potassium loss and development of hypokalaemia during the administration of an oral diuretic , metabolic balance studies were performed in ten patients with essential hypertension who had shown hypokalaemia under prior oral diuretic treatment .
### Output Text:
To investigate the initial <span class="chemical">potassium</span> loss and development of hypokalaemia during the administration of an oral diuretic , metabolic balance studies were performed in ten patients with essential hypertension who had shown hypokalaemia under prior oral diuretic treatment . 