You have certainly performed the electrolysis of sodium chloride solutions before. Why don’t you investigate the electrolysis behavior of aqueous ammonium chloride solutions? Once again you can learn a lot of chemistry.
Electrolysis of ammonium chloride solution
Fill an ammonium chloride solution (w = 10%) into a U-tube with a diaphragm. Dip two clean graphite electrodes in and apply a DC voltage of 10 V. Gases develop on both electrodes. Wait a few minutes and then carefully sniff the individual electrodes. What do you notice?
Also tests the gases using chemical reactions. To do this, hold a damp potassium iodide/starch paper over the electrodes. At the anode it turns purple-blue: it is chlorine. The paper does not react at the with the gas produced on the cathode. The gas that forms on the cathode can be tested using the hydrogen-pop test – it is hydrogen.
Also test the gases with a damp pH paper. It will quickly turn red in the chlorine atmosphere.
You can also hold the opening of a bottle of concentrated hydrochloric acid to it: a mist of ammonium chloride forms on the cathode.
Chlorine is produced at the anode:
2 Cl– ———> Cl2 + 2 e–
The chlorine reacts with iodide to iodine, which forms the well-known blue complex with starch.
Cl2 + 2 I– ———> 2 Cl– + I2
In addition, chlorine reacts with water to form acids, hence the reaction of the damp indicator paper.
Hydrogen is produced at the cathode:
2 H+ + 2 e– ———> H2
But where do the protons come from? You can also detect ammonia at the cathode. It is formed from the ammonium ions. These also supply the protons. Therefore, you have to formulate the cathode process a little more precisely:
2 NH4+ + 2 e– ———> 2 NH3 + H2
The ammonia reacts again with water to form ammonium and hydroxide ions.
Compare this mechanism with the electrolysis of sodium chloride solutions, in which hydroxide ions are also formed. These, however, come directly from the water.
You can see that the NH3/ NH4+ system acts as a catalyst. We can therefore formulate the reaction process as a catalytic cycle. However, the ammonia is increasingly driven off as the solution becomes more and more basic.
Electrolysis of ammonium chloride solutions + pH indicator
Proceed as in experiment 1. Add a solution of bromothymol blue (w = 0.1% in ethanol; F) to the ammonium chloride solution beforehand. The mixture should now be bright yellowish green in color. Immediately after the start of the electrolysis you will see blue streaks on the cathode surface. The dye brightens strongly at the anode. Here it is partly destroyed by the chlorine.
Try other indicators too!
There are also indicators that show the redox position of the electrolyte solutions in the electrode spaces. You can also use it to conjure up colorful solutions. Ferroin is such a redox indicator. You can buy its solution directly from Merck.
Electrolysis of ammonium chloride solutions in the presence of ferroin
Proceed as in experiment 2. Do not add too little ferroin solution (c = 0.025 mol/l in dilute sulfuric acid; Xi) to the ammonium chloride solution beforehand. The solution should be slightly colored. After about ten minutes of electrolysis, the anode compartment turns deep blue, while the cathode compartment retains its red color.
Incidentally, ferroin is the indicator that is also used in the Belousov-Zhabotinskii reaction (a well-known “oscillating reaction”). Chemically, it is a 1,10-phenanthroline iron complex. Its solutions are colored red with divalent iron, blue with trivalent iron. Chlorine is able to oxidize the iron, and therefore change the color of the complex.
The ferroin (red) molecule consists of three 1,10 phenanthroline molecules coordinated to an iron(II) atom. On the right we can see the oxidized, iron(III) version (blue).
You can of course also carry out these experiments with sodium chloride solutions. However, you will then not smell ammonia, and you will not be able to see the metal sodium, because it cannot be separated from aqueous solutions due to it’s reactiveness.
The search for the “NH4 metal” plays a major role in the history of chemistry. It actually forms during the electrolysis of ammonium chloride solutions with mercury electrodes.
NH4+ + e– ———> NH4
The NH4 is a radical molecule. However, it is only known as an amalgam and can neither be produced nor is it stable in aqueous solutions. It only exists as a solvent in liquid ammonia at low temperatures.