Notes Practical

Test for Ions in Qualitative Analysis

The definitive guide to qualitative analysis

1. Feeling the heat of 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.

At the O Level practical exam, you will be given a table that tells you how to test for each type of ion. While helpful, this is a slow way of thinking. To game it, you have to think from the perspective of reagents.

  1. Know the function of each reagent
  2. Know the expected observations
  3. Narrow your hypotheses based on the observation

2. Function of reagents in qualitative analysis

Aqueous Sodium Hydroxide, NaOH (aq)

ObservationsConclusion or Follow-up
No ppt. Upon adding aluminium foil and on warming, ammonia gas that turns moist red litmus paper blue is produced.Nitrate, NO3
No ppt. On warming, ammonia gas that turns moist red litmus paper blue is produced.Ammonium, NH4+
White ppt, insoluble in excessCalcium, Ca2+
White ppt, soluble in excess giving a colourless solutionInconclusive, can be Al3+, Pb2+ or Zn2+
Light blue ppt, insoluble in excessCopper(II), Cu2+
Green ppt, insoluble in excessIron(II), Fe2+
Red-brown ppt, insoluble in excessIron(III), Fe3+

We usually use aqueous sodium hydroxide to test for cations. However, it can also test for nitrate anion, if sodium hydroxide is added together with an aluminium foil and warmed.

The only inconclusive observation is when you see white precipitate soluble in excess aqueous sodium hydroxide. To distinguish between Al3+, Pb2+ and Zn2+, you will need to:

  • Add aqueous ammonia, which gives white precipitate soluble in excess only when zinc, Zn2+ is present
  • Add solution containing chloride ion, which gives white lead(II) chloride precipitate only when lead(II), Pb2+ is present

Aqueous Ammonia, NH3 (aq)

ObservationsConclusion or Follow-up
No pptInconclusive, can be Ca2+ or NH4+
White ppt, insoluble in excess Inconclusive, can be Al3+ or Pb2+
White ppt, soluble in excess giving a colourless solutionZinc, Zn2+
Light blue ppt, soluble in excess giving a dark blue solutionCopper(II), Cu2+
Green ppt, insoluble in excessIron(II), Fe2+
Red-brown ppt, insoluble in excessIron(III), Fe3+

We use aqueous ammonia to solely test for cations.

Compared to aqueous sodium hydroxide, we can add aqueous ammonia to conclusively test for zinc cation. Furthermore, the result for copper(II) ion is more pronounced, giving you a mysterious dark blue solution in excess aqueous ammonia.

Dilute Nitric Acid, HNO3 (aq)

ObservationsConclusion
Effervescence of colourless, odourless gas that gives white ppt with limewaterCarbonate, CO32-

We add dilute nitric acid to conclusively test for carbonate. This is because acid reacts with carbonate to give salt, water, and carbon dioxide. Crucially, the production of carbon dioxide can be observed as effervescence, which is just a cheem word for bubbling.

Furthermore, we also use dilute nitric acid to acidify the sample before testing for other anions, like chloride, iodide, and sulfate.

Dilute Nitric Acid + Aqueous Silver Nitrate, AgNO3 (aq)

ObservationsConclusion
White pptChloride, Cl
Yellow pptIodide, I

Before testing for halides, we have to first acidify the sample by adding a few drops of nitric acid. This is to remove any carbonate ion that may also be present in the solution. Otherwise, carbonate will also react with aqueous silver nitrate, yielding white precipitate of silver carbonate. This is a false positive.

Dilute Nitric Acid + Aqueous Barium Nitrate, Ba(NO3)2 (aq)

ObservationsConclusion
White pptSulfate, SO42-

Likewise, we have to first acidify the sample with a few drops of dilute nitric acid to remove any carbonate ion.


3. 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 ionic compound, like barium sulfate in the test for the sulfate anion.


4. Lab technique: how much to add for qualitative analysis?

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, 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.


5. Lab technique: how do I know if the precipitate is dissolving in excess?

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

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