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: Following major intracranial surgery in a 35 - year - old man , sodium pentothal was intravenously infused to minimize cerebral ischaemia .
### Output Text:
Following major intracranial surgery in a 35 - year - old man , <span class="chemical">sodium pentothal</span> was intravenously infused to minimize cerebral ischaemia . 