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: CONCLUSION : Careful observation of serum potassium concentration and the ECG should always be done during Bpd administration , particularly in elderly patients .
### Output Text:
CONCLUSION : Careful observation of serum <span class="chemical">potassium</span> concentration and the ECG should always be done during <span class="chemical">Bpd</span> administration , particularly in elderly patients . 