A-level Applied Science/Finding out about substances/Qualitative

The basic question addressed by qualitative analysis is: Is there any X in the sample?

In contrast, quantitative analysis answers the question: How much X is there in the sample?

In practice, qualitative analysis records if the amount of X is above a certain threshold value.

Uses of qualitative analysis

Reagent

Effects

6M HCl

Increases [H+]

Increases [Cl-]
Decreases [OH-]
Dissolves insoluble carbonates, chromates, hydroxides, some sulfates
Destroys hydroxo and NH3 complexes
Precipitates insoluble chlorides

6M HNO3

Increases [H+]

Decreases [OH-]
Dissolves insoluble carbonates, chromates, and hydroxides
Dissolves insoluble sulfides by oxidizing sulfide ion
Destroys hydroxo and ammonia complexes
Good oxidizing agent when hot

6 M NaOH

Increases [OH-]

Decreases [H+]
Forms hydroxo complexes
Precipitates insoluble hydroxides

6M NH3

Increases [NH3]

Increases [OH-]
Decreases [H+]
Precipitates insoluble hydroxides
Forms NH3 complexes
Forms a basic buffer with NH4+

Limitations

Group I: Ag+, Hg22+, Pb2+

Precipitated in 1 M HCl

Group II: Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+ and Sb5+, Sn2+ and Sn4+

Precipitated in 0.1 M H2S solution at pH 0.5

Group III: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+

Precipitated in 0.1 M H2S solution at pH 9

Group IV: Ba2+, Ca2+, K+, Mg2+, Na+, NH4+

Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M (NH4)2CO3 solution at pH 10; the other ions are soluble

Many reagents are used in qualitative analysis, but only a few are involved in nearly every group procedure. The four most commonly used reagents are 6M HCl, 6M HNO3, 6M NaOH, 6M NH3. Understanding the uses of the reagents is helpful when planning an analysis.

Standard procedures

Analysis of mine water sample

A. Cations

1. To a 2 cm³ mine water sample: Add sodium hydroxide solution dropwise until it is in excess.
2. Filter the mixture from (1) and add sulphuric acid dropwise to the filtrate.
3. Add aqueous ammonia solution to the acidified filtrate from (2).
4. To a 2 cm³ mine water sample: Add aqueous ammonia solution dropwise until it is in excess.
5. Filter the mixture from (4) and add sulphuric acid dropwise to the filtrate.
6. To a drop of mine water sample: Add a drop of potassium thiocyanate solution.
7. To a drop of mine water sample: Add a drop of potassium hexacyanoferrate (III) solution.
8. Flame test the mine water sample.

B. Anions

1. Test the pH of the mine water sample. What anions would react to form gases at this pH?
2. To a drop of mine water sample: Add a drop of barium nitrate solution.
3. To a drop of mine water sample: Add a drop of silver nitrate solution.
4. To a 1 cm³ mine water sample: Add powdered Al and aqueous NaOH – if a reaction occurs test gas with damp red litmus paper.

Questions

1. Which ions have you identified as being present?
2. Which ions are definitely not present?
3. Are there any ions which you are unsure about?
4. Write a report of this practical work. Discuss the limitations of qualitative analysis. Give a full account of all your experiments and observations.