Apply equilibrium and concentration rules to chlorine disinfectants.
Method and worked examples
Chlorine reacts with water: Cl2+H2O ⇌ HOCl +H++Cl−. Hypochlorous acid also ionises: HOCl ⇌ H++OCl−. HOCl and hypochlorite are oxidising disinfectants, but HOCl is usually the more effective disinfectant at the same available-chlorine level. Increasing pH shifts the second equilibrium toward OCl−; decreasing pH favours HOCl.
Very acidic conditions can favour dissolved chlorine and hazardous chlorine release; acid must not be mixed with hypochlorite products. Disinfection is distinct from clarification and desalination: each addresses different contaminants. Concentration calculations require the stated volume, dilution and available-chlorine basis. The closed drills here concern chemical species and arithmetic, rather than directions for treating drinking water or mixing household chemicals.
Example 1
For HOCl ⇌ H++OCl−, predict the effect of adding OH−.
OH− consumes H+, lowering its concentration. The equilibrium shifts right to replace some H+, increasing the fraction present as OCl−.
Example 2
A hypothetical 2.0 mg L−1 disinfectant solution is diluted from 100mL to 400mL. Find final concentration.
Amount is conserved: c2=V2c1V1=2.0×400100=0.50 mg L−1, assuming no decomposition or reaction.
Watch out:Higher pH shifts HOCl toward OCl−, which is generally less effective at equal available-chlorine concentration.
Chlorine chemistry and pH-dependent disinfection | Year 12 Chemistry | Ascent Arcade