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: Three hours later the patient felt better , the frequency of PVC reduced to 4 - 5 x / minute and on the third day ECG was normal , potassium level was 3 . 34 meq / L .
### Output Text:
Three hours later the patient felt better , the frequency of PVC reduced to 4 - 5 x / minute and on the third day ECG was normal , <span class="chemical">potassium</span> level was 3 . 34 meq / L . 