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: Patient tolerance study of topical chlorhexidine diphosphanilate : a new topical agent for burns .
### Output Text:
Patient tolerance study of topical <span class="chemical">chlorhexidine diphosphanilate</span> : a new topical agent for burns . 