Question

Cobalt has spilled into a pond and our task is to precipitate it out.

Your task is to choose a precipitation agent. It has been found that the cobalt is in the +2 state. After consulting your handy textbook, you see that the possible precipitants are carbonate, sulfide and hydroxide ions. Remember that the pH of the pond is already set.  

Based on what you know about equilibria, write the pertinent chemical reactions that would occur for each of the possible precipitants (not the precipitation reaction). Does the pH of the solution matter?  

a) Co⁺² titrated with Na₂S

b) Co⁺² titrated with Na₂CO₃

c) Co⁺² titrated with NaOH

Answer 1

Precipitation of Co2+ with various reagents

a) Co2+ titrated with H2S

reaction 1,

Co2+ + H2S <==> CoS + 2H+

b) Co2+ titrated with Na2CO3

reaction 2,

Co2+ + CO3^2- <==> CoCO3

c) Co2+ titrated with NaOH

reaction 3,

Co2+ + 2OH- <==> Ca(OH)2

The reactions for each ttration is shown above.

The pH matters in these cases, as can be seen in reaction 1, H+ is formed as a product of the reaction, so a low pH solution would have high H+ concentration and this would reverse the precipitation reaction according to LeChatellier's principle. So more Co2+ would dissolve in solution. Higher pH would favor precipitation reaction.

In reaction 2, a low pH (high H+) would favor the reverse reaction, so more of precipitate dissolve in solution. A high pH thus favors precipitation.

Again in reaction 3, a hgh pH (high OH-) would favor the forward reaction and precipitation of Co(OH)2 would occur spontaeously.