What is the black stuff left after the silver deposits in Tollen's test?

Question

Initially, I tried doing it by the 'textbook' way, but as soon as I added NaOH to the silver nitrate,the brown precipitate just disappeared after a few seconds. Why is that?

Then, I started afresh and this time I just added ammonia solution to the silver nitrate. After the silver got precipitated, there was a black ppt left.What is this black stuff? It's a fine black powder.

And lastly, how do I get a good coating of silver?

Note: IF YOU DO NOT KNOW THE ANSWER TO EITHER OF MY FIRST TWO QUESTIONS, PLEASE DON'T ANSWER THE LAST ONE. I don't want useless links or cut and paste answers.

Answer 1

The brown silver oxide precipitate will disappear before you add the ammonia only if you have not added enough sodium hydroxide. You have to add enough sodium hydroxide to give a permanent precipitate before you add the ammonia. The black precipitate left is also silver - usually with a properly prepared reagent the black precipitate appears initially as silver starts to precipitate then it becomes more gray as the reaction proceeds.

To get a good mirror the textbook way is the best - adding the ammonia to the silver nitrate without the sodium hydroxide will not give a good reagent solution. Plus you need a clean glass surface free of grease or oil. Washing the glass with sodium hydroxide first can help.