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: In addition , a distal tubular acidification defect may result , thus impairing the normal compensatory drop in urine pH .
### Output Text:
In addition , a distal tubular acidification defect may result , thus impairing the normal compensatory drop in urine pH . 