Why is the potassium cyanide acidified slightly?
The presence of an acid in solution helps to strengthen the polarity of the carbon-oxygen double bond. The electrons in that bond are strongly attracted towards the hydrogen ions in the solution. Why isn’t a lot more acid added to give a really low pH? You might think that the more acid you added, the more you could increase the polarity of the carbon-oxygen double bond. Unfortunately, there is a competing effect. The more acid you add, the more the cyanide ions get converted into hydrogen cyanide. Since cyanide ions are what actually attacks the slightly positive carbon, removing them isn’t helpful! A pH of 4 – 5 is found experimentally to give you the best rate of reaction. It increases the polarity of the double bond by a useful amount, but without removing too many of the cyanide ions as HCN.
