GCSE OCR Gateway C4:2 b Sodium Hydroxide Tests for Cations

GCSE OCR Gateway C4:2 b Sodium Hydroxide Tests for Cations

Learning Objectives:

C4.2b To be able to describe tests to identify the aqueous cations: calcium, copper, iron (II), iron (III)
and zinc using sodium hydroxide solution.

One of the most powerful things in chemistry is our ability to identify unknown chemicals that are causing problems of pollution or contamination or sickness.

Of course, some chemists have to put up their hands and confess to polluting the planet.  But sadly chemists have not been quick to do this.  So pollutants have led to human misery and to the association of chemistry with names such as Minamata, Bhopal or Thalidomide.

Minamata is the name of a Bay on the coast of Japan. Back in the 1900’s it was a beautiful place where Japanese fishermen plied their trade as they had done for centuries catching fish, mainly tuna.

Then came the industry.  A chemical factory was built outside Minamata town and started production of fertiliser in 1908.  As it expanded both before and after WW2 its range of products increased.

The Minamata factory started ethanal (CH₃CHO) production in 1932, producing 210 tons that year. By 1960, production reached a peak of 45,245 tons.

Ethanal production used mercury sulphate as a catalyst. From August 1951, they changed the co-catalyst from manganese dioxide (MnO₂) to ferric sulfide (Fe₂S₃).

What they did not realise was that a side reaction then started. The process produced a small amount of the highly toxic organic mercury compound, methyl mercury (CH₃HgCl). The company released this compound into Minamata Bay from the change of the co-catalyst in 1951 until 1968, when they stopped this production method.

The identification of mercury in the water of the bay around the town was easy using typical chemical tests like those required for your school or college chemistry course.

Linking mercury to the disease symptoms emerging in the local population was more difficult.

Photos below show the effects of the mercury poisoning:

You can read more about the investigation here.

How to use Sodium Hydroxide to detect Metal Cations.

The procedure to detect metal cations is not complicated.

You place a few drops of the suspected metal cation solution on a spotting tile or in a test-tube and add a couple of drops of 1M sodium hydroxide solution. 

A cloudiness in the solution shows that a precipitate has formed.  The colour of the precipitate is characteristic of the metal cation. 

Metal Cation Mn+	Calcium Ca2+	Copper Cu2+	Iron (II) Fe2+	Zinc Zn2+	Iron (III) Fe3+
Colour of the precipitate	white	Pale blue	green	white	brown

Adding excess sodium hydroxide will tell the calcium and zinc apart because the zinc precipitate will dissolve in enough excess sodium hydroxide.

What’s happening in these precipitation reactions?

Each precipitate is a metal hydroxide (M(OH)_n) where n is the charge on the ion.

So for example:  iron (III) sulphate (Fe₂(SO₄)₃).

Iron(III)sulphate  +  sodium hydroxide  ⟶ Sodium sulphate  +  iron(III)hydroxide

Fe₂(SO₄)₃(aq)  +   6NaOH(aq)   ⟶  3Na₂SO₄(aq)   +  2Fe(OH)₃(s)

                                                                               brown ppt      

But there are in the reaction ions that do not change in the reaction these are called Spectator Ions

The spectator ions are highlighted in Red

The remaining ions form the ionic equation:

2Fe³⁺(aq)   +    6OH^—(aq)    ⟶    2Fe(OH)₃(s)

                                                  brown ppt      

Can you build the equations for the reactions between the other metal ions and sodium hydroxide where the charge on the metal ion is 2+?

The zinc precipitate dissolves because it is amphoteric. 

Amphoteric means that the precipitate reacts with both acids and bases.

Here are the equations:

Zinc sulphate  +  sodium hydroxide ⟶  sodium sulphate +  zinc hydroxide.

ZnSO₄(aq)   +    2NaOH(aq)    ⟶    Na₂SO₄(aq)    +   Zn(OH)₂(s)

                                                                              white ppt

Zn²⁺(aq)   +    2OH^—(aq)    ⟶    Zn(OH)₂(s)

Then with excess sodium hydroxide

Zinc hydroxide +  sodium hydroxide ⟶   sodium zincate

Zn(OH)₂ (s)    +   2NaOH (aq) ⟶       Na₂Zn(OH)_{4  (aq)} 

You might find that this post of mine here helps you build the precipitation equations.

In my next post I’m going to describe anion tests.