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: Basal frequency ( BF ) evaluated by surface electrogram was 223 + / - 4 beats / min . in control Krebs - Ringer containing 137 mM Na and 1 . 35 mM Ca ( N ) .
### Output Text:
Basal frequency ( BF ) evaluated by surface electrogram was 223 + / - 4 beats / min . in control Krebs - Ringer containing 137 mM <span class="chemical">Na</span> and 1 . 35 mM <span class="chemical">Ca</span> ( N ) . 