The wastewater from many industrial processes can be highly acidic or alkaline.For example, wastewater from dyeing factories can have a pH of 10-12. In the past,such wastewater went straight into rivers or into the sea (Figure4.40) where it caused environmental damages such as the killing of marine life and the corrosion to metal pipes and ships.
The government now repuires companies to treat their wastewater so that the pH is between 6 and 9. The treated water must then be discharged in to a sewer or watercourse and not into the sea.
4.8.2 Hydrolysis
Salts of strong acids and strong bases form neutral solutions in water, e.g. sodium chloride.
However, salts of weak acids and strong bases form alkaline solutions in water ,e.g. sodium ethanoate.
Let us look at an example. sodium ethanoate is fully ionized in water :
The CH COOH ions react with the H ions from the water :
CH COOH is a weak acid and doses not completely dissociate in water. Thus, the above reaction causes most of the hydrogen ions to be removed from the solution. The concentration of hydrogen ions decreases and the pH becomes larger than 7. The solution is alkaline.
Salts of strong acids and weak bases form acidic solutions in water, e.g. ammonium sulfate.
Ammonium sulfate is fully ionized in water :
The NH ions react with OH ions from the water :
NH is a weak base and only partially ionizes in water. Thus, the above reaction causes most of the hydroxide ions to be removed from the solution.