Cation Experiments

Today, Dominique worked through identifying some unknowns. Here are her results, along with some notes from errors made, or questions asked during this activity.

This one was used to show the amount of detail required. The ions identified on the right-hand side are the possible ions after each test. Barium must be considered as a possible ion when a white precipitate is formed with NaOH due to the concentrations of solutions that we are using. Barium hydroxide is a sparingly-soluble compound, so may precipitate despite what our flow chart says.

The third step was done just to confirm that it wasn't Barium.

While Ammonium is a possible cation, it is not going to be included in the assessment. The Achievement Standard criteria state that candidates will not be expected to distinguish between Sodium and Ammonium ions.

Although it is blatantly obvious that this is a Copper (II) solution, we must confirm it. There is no point adding NaOH, as the same thing can be achieved by adding a few drops of Ammonia solution (Ammonia is a base, so also provides Hydroxide Ions).

Examiners love using Zinc, Aluminium and Lead as unknown cations - just look at all of the wonderful complex ions!! Note the charge of the complex ion. Zn = 2+ and each OH = -, therefore the net charge is 2- (+2 + -4).

The addition of NaOH makes it obvious that this is is Iron (III). However, if you shake a solution of Iron (II) Hydroxide, it will oxidise the Iron (II) into Iron (III). Therefore, testing a new sample with KSCN is a vital step to confirm it is Iron (III), not Iron (II).

Barium is actually one of the easiest ones to identify, so long as you remember that it might form a white precipitate with NaOH.

Be very careful not to shake the test tube upon the formation of the green precipitate; this will lead to the Iron (II) being oxidised to Iron (III), making your results confusing.

While the underlined equation is accepted, it is incorrect; there is no such thing as Silver Hydroxide. If you can remember the correct equation for the precipitation of Silver Oxide (Silver ions + Hydroxide ions), then this counts towards Excellence.
The Silver Diammine complex ion equation is also an Excellence opportunity, and one that doesn't follow the "if you're unsure, it is four" rhyme/rule.

Aluminium is another one with a lovely complex ion. It does obey the "if you're unsure, it is four" rhyme . Note that this gives the complex ion a -1 charge.