Salts and Titration

A salt is the ionic product of an acid-base reaction. Knowing which salts are soluble, which preparation method to apply, and how to perform a volumetric calculation are among the most tested skills in CSEC Chemistry.

Salts: Types and Basicity¶

A salt is the ionic compound formed when the replaceable hydrogen of an acid is fully or partially replaced by a metal ion or ammonium ion.

A normal salt is formed when all replaceable hydrogen atoms are replaced:

$\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$

An acid salt is formed when only some of the replaceable hydrogen is replaced — the salt still contains ionisable hydrogen:

$\text{H}_2\text{SO}_4 + \text{NaOH} \rightarrow \text{NaHSO}_4 + \text{H}_2\text{O}$

The basicity of an acid is the number of replaceable hydrogen atoms per molecule:

• HCl — monobasic (one replaceable H)
• H₂SO₄ — dibasic (two replaceable H)
• H₃PO₄ — tribasic (three replaceable H)

Solubility Rules¶

The preparation method depends on whether the target salt is soluble or not, so check solubility first.

Knowing which salts dissolve in water determines which preparation method to use:

Methods of Salt Preparation¶

There are three preparation methods. Which one applies depends on whether the salt is soluble or insoluble, and whether the base used to make it is soluble or not.

Soluble Salts — Using Excess Insoluble Base¶

Used when the base (metal oxide, hydroxide, or carbonate) is insoluble.

1. Add excess base to the acid until no more dissolves and the solid remains.
2. Filter off the excess base.
3. Evaporate the filtrate to concentrate it.
4. Allow to crystallise; filter and dry the crystals.

Excess base ensures all the acid reacts. It can be filtered off because it is insoluble and the salt is in solution.

Soluble Sodium, Potassium, or Ammonium Salts — By Titration¶

Used when the base is NaOH, KOH, or NH₃(aq) — all soluble, so they cannot be filtered off.

1. Use an indicator to find the exact volume of acid that neutralises a known volume of alkali.
2. Repeat the titration using the same volumes but without the indicator (the indicator would contaminate the product).
3. Evaporate and crystallise the pure salt.

Insoluble Salts — By Precipitation¶

1. Mix solutions of two soluble salts, each containing one of the required ions.
2. The insoluble salt precipitates immediately.
3. Filter, wash with distilled water, and dry.

$\text{Pb(NO}_3)_2\text{(aq)} + \text{Na}_2\text{SO}_4\text{(aq)} \rightarrow \text{PbSO}_4\text{(s)}\downarrow + 2\text{NaNO}_3\text{(aq)}$

Uses and Dangers of Salts¶

Salts are not only laboratory products. Many are everyday substances in food, medicine, and construction, though some carry health risks at high doses.

Dangers: sodium nitrate and nitrite, when consumed in excess, are potentially carcinogenic and have been implicated in brain damage in infants. Their use in food preservation is regulated.

Neutralisation and Titration¶

Making a salt by titration requires knowing exactly when neutralisation is complete, and calculating concentrations from the volumes used. That is what volumetric analysis covers.

Neutralisation is always exothermic — the formation of water from H⁺ and OH⁻ releases heat. This can be used in a thermometric titration: the temperature peaks at the equivalence point. Plotting temperature against volume gives two straight lines whose intersection marks the end point.

In standard titrations, an indicator signals the end point by colour change:

• Phenolphthalein: colourless (acid) to pink (alkali)
• Methyl orange: red (acid) to yellow (alkali)

Volumetric Analysis Calculations¶

$n = c \times V \quad (V \text{ in dm}^3, \; 1 \text{ dm}^3 = 1000 \text{ cm}^3)$