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: From these data we conclude that the penile pain following intracorporeal injections is most likely due to the acidity of the medication , which can be overcome by elevating the pH to a neutral level .
### Output Text:
From these data we conclude that the penile pain following intracorporeal injections is most likely due to the acidity of the medication , which can be overcome by elevating the pH to a neutral level . 