Qualitative Analysis and pH Measurement

This method is subjective because it relies on human eyesight to interpret colours, providing a low resolution (usually to the nearest 1.0 unit).

Using a pH Probe: For superior precision, chemists use a pH probe. This digital sensor is objective, displaying a numerical value with a high resolution (often to two decimal places).

Calibrating and Using a pH Probe

To ensure digital readings are accurate, a probe requires calibration using a buffer solution (a liquid with a perfectly stable, precisely known pH).

Procedure for Calibration and Measurement:

1. Rinse the probe with distilled water to remove any contaminants.
2. Place the probe into a pH 7.0 buffer solution.
3. Adjust the meter until the digital display reads exactly 7.0.
4. Rinse the probe again, then place it into a second buffer (usually pH 4.0 for acids or pH 10.0 for alkalis).
5. Rinse the probe a final time with distilled water.
6. Submerge the probe's bulb into the unknown sample solution.
7. Stir the solution gently and wait for the reading on the digital display to stabilise.
8. Record the numerical pH value.

Identifying Cations: Flame Tests

A flame test is a method of qualitative analysis used to identify positively charged metal ions (cations) by the specific colour of light they emit when heated.

To perform a flame test:

1. Dip an unreactive nichrome or platinum wire loop into concentrated hydrochloric acid ($HCl$).
2. Hold the wire in a roaring blue Bunsen burner flame until no colour is visible (this cleans the wire).
3. Dip the clean loop into the unknown solid sample.
4. Place the loop at the edge of the roaring blue flame and observe the colour change.

• Lithium ($Li^+$): Crimson red
• Sodium ($Na^+$): Yellow
• Potassium ($K^+$): Lilac
• Orange-red

A major limitation of flame tests is that mixtures of ions can mask colours. For example, the intense yellow flame of sodium easily hides the delicate lilac flame of potassium.

Identifying Cations: Precipitation Reactions

Adding sodium hydroxide ($NaOH$) to metal solutions causes many metals to form a precipitate (an insoluble solid). $NaOH$ should be added dropwise initially, and then in excess.

Comparing White Precipitates:

• Both Calcium ($Ca^{2+}$) and Zinc ($Zn^{2+}$) form white precipitates initially.
• Zinc hydroxide is amphoteric, meaning it reacts as both an acid and a base. When excess $NaOH$ is added, the zinc precipitate redissolves to form a colourless solution.

Comparing Coloured Precipitates:

• Copper(II) ($Cu^{2+}$): Forms a blue precipitate.
• Iron(II) ($Fe^{2+}$): Forms a green precipitate.
• Iron(III) ($Fe^{3+}$): Forms a brown precipitate.

Testing for Ammonium:

• To test for ammonium ions ($NH_4^+$), add $NaOH$ and gently warm the mixture.
• This produces ammonia gas ($NH_3$), which has a pungent smell and turns damp red litmus paper blue.

Identifying Anions: The Testing Sequence

You must always test for anions (negatively charged ions) in this exact sequence to avoid false positives:

1. Carbonates ($CO_3^{2-}$): Add dilute acid (e.g., $HCl$). If present, you will observe effervescence (fizzing). Bubble the gas through limewater; it will turn cloudy/milky.
2. Sulfates ($SO_4^{2-}$): Add dilute to remove carbonates, then add barium chloride (). A white precipitate confirms sulfate ions.

Confirmatory detail: Silver chloride dissolves in dilute ammonia, silver bromide dissolves in concentrated ammonia, but silver iodide is completely insoluble.

Worked Example: Analysing an Unknown Sample

A student is given an unknown solid, Compound X. When heated in a roaring blue flame, a crimson red colour is observed. The student dissolves Compound X in water and adds dilute nitric acid followed by silver nitrate solution; a cream precipitate forms. Identify Compound X and the ionic equation for the precipitate.

Step 1: Analyse the flame test. Crimson red indicates Lithium ($Li^+$).

Step 2: Analyse the precipitation test. A cream precipitate with silver nitrate indicates Bromide ($Br^-$).

Step 3: Identify the compound. $Li^+$ and $Br^-$ form Lithium bromide ($LiBr$).

Step 4: Ionic equation: $Ag^+(aq) + Br^-(aq) \rightarrow AgBr(s)$.