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: Initial potassium loss and hypokalaemia during chlorthalidone administration in patients with essential hypertension : the influence of dietary sodium restriction .
### Output Text:
Initial <span class="chemical">potassium</span> loss and hypokalaemia during <span class="chemical">chlorthalidone</span> administration in patients with essential hypertension : the influence of dietary <span class="chemical">sodium</span> restriction . 