Identifying Coloured Cations

Three of the cations we work with have characteristic colours, although iron (II) can often be very pale.

Cu²⁺ = blue
Fe²⁺ = pale green
Fe³⁺ = orange

While this is very useful for deciding which ion might be present in an unknown solution, but we also need some chemical reaction to confirm which cation is present.

NOTE: There is an error in the video regarding the copper-ammonia complex ion. This is corrected in the text below.

One of the easiest tests to do is the addition of a few drops of NaOH:

Cu²⁺ + 2OH^- → Cu(OH)₂ (pale blue precipitate)
Fe²⁺ + 2OH^- → Fe(OH)₂ (forest green precipitate)
Fe³⁺ + 3OH^- → Fe(OH)₃ (orange/rust precipitate)

The only issue with these (on their own) is that the Fe²⁺ reaction often has an orange/rust coloured precipitate develop on top if it is left for a while. Therefore, we need some more definite experiments:

Iron (III)

If you need to confirm that the orange precipitate was definitely due to Fe³⁺, get a new sample and add a few drops of potassium thiocyanate (KSCN). This goes blood-red, due to a complex ion being formed:

Fe³⁺ + SCN^- ↔ [FeSCN]²⁺

Fe²⁺ will not have this effect; in fact, there will be no reaction at all.

Copper (II)

It seems a bit pointless to confirm the presence of Cu²⁺ as its blue colour is obvious, and the precipitate it forms with NaOH is unique. However, we are still expected to do the following experiment as well:

A new sample is collected, and excess (or concentrated) ammonia is added. It goes from a pale blue solution to a pale blue precipitate, then very quickly develops into a royal blue solution. This is another complex ion:

Cu²⁺ + 4NH₃ ↔ [Cu(NH₃)₄]²⁺
Please note that this is a correction of the equation shown in the video.

As Fe²⁺ is a very pale green colour, it can be confused for Cu²⁺ initially. If ammonia is added to Fe²⁺, it will make a forest green precipitate due to the hydroxide ions in the ammonia (ammonia is a base).