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: Diclofenac sodium ( Voltarol , Geigy Pharmaceuticals ) is a non - steroidal anti - inflammatory derivative of phenylacetic acid .
### Output Text:
<span class="chemical">Diclofenac sodium</span> ( <span class="chemical">Voltarol</span> , Geigy Pharmaceuticals ) is a non - steroidal anti - inflammatory derivative of <span class="chemical">phenylacetic acid</span> . 