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: Milk - alkali syndrome was first described 70 years ago in the context of the treatment of peptic ulcer disease with large amounts of calcium and alkali .
### Output Text:
Milk - alkali syndrome was first described 70 years ago in the context of the treatment of peptic ulcer disease with large amounts of <span class="chemical">calcium</span> and <span class="chemical">alkali</span> . 