why it is necessary to acidify the silver nitrite solution?

When silver nitrate solution is used to identified chloride, bromine or iodide ions, the silver nitrate solution should be acidified with dilute nitric acid.

This is because all of silver chloride, silver bromide and silver iodide precipitate are insoluble in dilute acids, but all other insoluble silver salts (such as silver carbonate and silver phosphate) are soluble in dilute acid.

Therefore, when acidified silver nitrate solution is added to an unknown solution and a precipitate is formed, one can deduce that the solution must contain chloride, bromide or iodide ions. This is because all other insoluble silver salts are dissolved in the acid.

However, if the silver nitrate solution added is not acidified, the precipitate formed may be an insoluble silver salt other than silver chloride, silver bromide and silver iodide.

Silver Nitrite