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: In the normal - sodium group the highest mean potassium deficit was 176 mmol on day 9 , after which some potassium was regained ; in the low - sodium group the highest deficit was 276 mmol on day 13 .
### Output Text:
In the normal - <span class="chemical">sodium</span> group the highest mean <span class="chemical">potassium</span> deficit was 176 mmol on day 9 , after which some <span class="chemical">potassium</span> was regained ; in the low - <span class="chemical">sodium</span> group the highest deficit was 276 mmol on day 13 . 