Silver mirrors and Tollen's test скачать в хорошем качестве

Silver mirrors and Tollen's test

One of my favourite qualitative organic chemical reactions is the test for aldehyde functional group. Since I first did it when I was about 16 years old I was transfixed and mesmerized. I've been demonstrating it to others in all sorts of glass containers small and large since then. Justus von Liebig is regarded to be the first person to use the silvering process or electroless process in 1835. This process is still used today to manufacture mirrors and christmas baubles and other ornaments. The test itself is very straight forward. First, we make the Tollen's reagent or ammoniacal silver nitrate solution by adding conc. ammonia dropwise to a silver nitrate solution until a brown precipitate first forms and then re-dissolves to give a clear, colourless solution. The solution contains now the colourless complex ion [Ag(NH3)2]+ The pH is raised again by addition of potassium hydroxide solution. A dark brown almost black precipitate of silver(I) oxide forms straight away which re-dissolves with further addition of conc. ammonia to give a clear, colourless solution. Now that we have prepared the Tollen's reagent we need an aldehyde source to test, in this case a soluble reducing sugar (glucose). The glucose solution is poured in a 2 L round bottom flask together with the Tollen's reagent and swirled around to make sure the whole of the inner surface of the flask is wetted. The solution turns black at first but with further swirling a mirror surface starts to appear. The process is called electroless plating because no electricity is employed. The aldehyde group is a weak reducing agent which reduces the complex silver ion to silver metal and gets deposited on the inside wall of the flask. About a 2mm silver metal layer sticks to the glass. The aldehyde group in glucose oxidises to carboxylate ion, that is gluconate. In contrast, a ketone such as propanone (acetone) does not react. WARNING: After the demonstration, the silver residues and remaining solution MUST be disposed of down the sink with plenty of water immediately after silvering the flask. A highly unstable primary or contact explosive, fulminating silver, may form as small black crystals if the solution is left to stand for some time.