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: The present study was designed to investigate the effect of 5 non - steroidal anti - inflammatory drugs , sodium salicylate , phenylbutazone , indomethacin , ibuprofen and mefenamic acid , on seizures produced by pilocarpine .
### Output Text:
The present study was designed to investigate the effect of 5 non - steroidal anti - inflammatory drugs , <span class="chemical">sodium salicylate</span> , <span class="chemical">phenylbutazone</span> , <span class="chemical">indomethacin</span> , <span class="chemical">ibuprofen</span> and <span class="chemical">mefenamic acid</span> , on seizures produced by <span class="chemical">pilocarpine</span> . 