| MadSci Network: Chemistry |
Acidifying with sulfuric acid might not be a good idea, unless you are using a very dilute silver nitrate solution. Silver sulfate does not dissolve very well, and could easily itself form a white precipitate if you tried to acidify silver nitrate with sulfuric acid. Nitric acid is what you have to use. The test for chloride ion in solution is to add acidified silver nitrate. You observe a white precipitate that gradually turns to grey or lilac colour in the light over a few minutes. What is happening is that insoluble silver chloride is formed, which then slowly decomposes to silver metal and dissolved chlorine with the energy from sunlight. The problem is that there are many other silver salts that are insoluble, and that can form as white precipitates if you add silver nitrate to a solution. Adding nitric acid can stop the precipitation of those that are based on weak acids, e.g. silver sulfite, silver carbonate, silver citrate. The other problem is that if you add silver nitrate to a solution that is at neutral or slightly alkaline pH, you are likely to get a brown precipitate of silver oxide, that could mask the characteristic chloride reaction (i.e. make it hard to see accurately).
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