Sciencemadness Discussion Board

ammonium alum and sodium carbonate

vampirexevipex - 23-5-2012 at 16:47

Hello i recently reacted some (NH4)Al(SO4)2 and sodium carbonate, i want to know what happened. Did the sodium carbonate reacted with the aluminum sulfate making alumimum carbonate and sodium sulfate, or did it react with the ammonia giving some other compouund? The solution also emitted some gases, i doubt they could be CO2 because its odor was rather pungent smelling.

Pyridinium - 23-5-2012 at 20:07

in an alkaline solution (as with Na2CO3) you can get some deprotonation of NH4+ which results in NH3. Ammonia is definitely pungent.

vampirexevipex - 24-5-2012 at 05:28

And what are the 2 different precipitates in the solution? One is floating while the other one is resting at the bottom of the solution. Both are white.

[Edited on 22-02-12 by vampirexevipex]

ScienceSquirrel - 24-5-2012 at 05:36

The white solids are probably aluminium hydroxide.
Some of it is floating because it contains small bubbles of occluded carbon dioxide.

weiming1998 - 24-5-2012 at 05:42

Salts disassociates in solution to ions, even double salts, so ammonium alum dissolved in water would be a mix of Al3+, NH4+ and SO4-. The Al3+ reacts with water in an equilibrium, ultimately forming small amounts of H+ ions. These react with the CO3(2-) to form H2CO3, which decomposes to form CO2, which drives the continued hydrolysis of Al3+ to H+ forward, leaving you with CO2 and a precipitate of Al(OH)3. In such a basic solution, the odour could be made by small amounts of dissolved ammonium carbonate forming small amounts of free ammonia, which is responsible for the smell. The other precipitate that settled down could potentially be Na2SO4 formed by the reaction of Na+ ions with SO4(2-) ions, which precipitates out because Na2SO4 only has a limited solubility in water.