Test for ions in qualitative analysis

Illustration by Richard Oo

1. Reagents are crucial in qualitative analysis

Qualitative analysis is difficult. You are hard-pressed to complete numerous tests within a short time, while many hypotheses run through your mind. It can be a mess! However, it can be enjoyable and rewarding if you are crystal clear about the logic of qualitative analysis.

To do this, we have to think from the perspective of reagents to understand:

1. their chemical properties
2. the range of expected observations
3. the conclusions or follow-up action to take

2. Add alkalis to test for cations (and nitrate anion)

Usually, we add sodium hydroxide or ammonia to test for cations. When a small amount of either alkali is added, they react in the same way to form precipitate. However, adding an excess causes different observations. For example, in the test for calcium or lead ions, the white precipitate redissolves in excess sodium hydroxide but not in excess ammonia.

Interestingly, sodium hydroxide is also used in the test for nitrate anion. It provides an alkaline medium for aluminium to reduce nitrate to ammonia, which is an alkaline gas that turns moist red litmus blue.

3. Add nitrate-containing reagent to test for most anions

We add dilute nitric acid to test for carbonate anion, as they react to produce effervescence of carbon dioxide gas. Furthermore, this removes any carbonate ion present in the sample, to prevent it from forming a precipitate with the other reagents that test for anions.

For halide anions, they are tested by silver nitrate. The silver cation forms precipitate of different colour with different halide.

Lastly, we use barium nitrate solution to test for sulfate anion.

4. Why do precipitates form?

When we test for cations, the reagents sodium hydroxide and ammonia are alkali. Therefore, their solutions will contain hydroxide anions. During precipitation, the hydroxide anions are strongly attracted to the unknown metal cations to form an insoluble metal hydroxide.

Likewise, when we test for anions with other reagents, precipitate is observed because two solutions react to give an insoluble salt.

5. Lab technique: how much of the reagent to add?

Quality, not quantity. For qualitative analysis, we only need to add a small amount of sample. 1 cm³ of sample is enough. That is about the width of your thumb.

When we add reagents like aqueous sodium hydroxide or aqueous ammonia to a sample, we must do so gradually. This is because the observations depend on the amount of reagent added.

1. First, add two to three drops gradually, to check if any precipitate forms
2. Then, add 2 cm³ at a time, such that the reagent is in excess to check if the precipitate dissolves

For the second part, do not spam the reagent. Sometimes, the precipitate appears to dissolve because there is simply too much solution. Don’t turn the lab into a trick eye museum.

6. Lab technique: how much is excess?

You can’t know what you can’t see. So watch the video on the addition of aqueous ammonia to an unknown sample.