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: Addition of excess calcium reversed the depression in both tissues , but calcium - independent catecholamine release by acetaldehyde was not blocked by NH4Ac .
### Output Text:
Addition of excess <span class="chemical">calcium</span> reversed the depression in both tissues , but <span class="chemical">calcium</span> - independent <span class="chemical">catecholamine</span> release by <span class="chemical">acetaldehyde</span> was not blocked by <span class="chemical">NH4Ac</span> . 