Acids, Alkalis and Titrations

Litmus

• comes from lichen → natural

• not suitable for titration as colour change is not sharp

• there’s a purple transition colour in neutral solutions → endpoint is difficult to determine

Synthetic Indicators

• organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkali

• ex: phenolphthalein and methyl orange

• used to show endpoint in titrations as they have very sharp change of colour when an acid has been neutralised by an alkali and vice versa

Two Colour Indicators

Two colours indicators are used to distinguish between acids and alkalis

pH Scale

• 0-14

• acids = <7, alkalis = >7

• 0-3 = strong acid

• extremely acidic = below 1

• 4-6 = weak acid

• 8-10 = weak alkali

• 11-14 = strong alkali

• 7 = neutral

Universal Indicator

• wide range indicator

• only approx. value for pH

• mixture of different plant indicators which operate across a broad range and useful for estimating the pH of an unknown solution

• few drops are added to the chart which indicates the pH match with specific colours

• charts vary by manufacturer so charts are provided for specific indicator formula

Acids

• when added to water, produce H⁺ ions

• these ions make a solution acidic

Alkali

• when added to water, produce OH^- ions

• these ions make an aqueous solution alkaline

pH Scale Measures

• numerical scale measuring how acidic or alkaline a substance is

• measure of the amount hydrogen ions

Neutralisation

• reaction occurs when an acid reacts with an alkali

• when these substances react, the H⁺ ions react with the OH^– ions to produce water

• not all acid reactions are neutralisations

• important for treatment of soils to raise the pH as some crops can’t tolerate pH levels below 7 → achieved by adding bases like limestone and quicklime

Titrations

• method of analysing conc. of solutions

• acid-base titrations determine how much alkali is needed to neutralise an acid

• can also be used to prepare salts

Practical (Acid-Base Titration): Method

• use pipette and pipette filler and place exactly 25cc of NaOH in the conical flask

• fill burette with HCl, place an empty beaker under the tap

• run small portion of acid through burette to remove any air bubbles

• record starting point on burette to nearest 0.05cm³

• place conical flask on white tile so burette tip is inside the flask

• add few drops of indicator to the solution in the conical flask

• perform rough titration by taking burette reading and running the solution in 1-3 cc portions, while swirling vigorously

• quickly close tap when end point is reached (sharp colour change) and record volume, placing eye level with meniscus

• repeat with fresh NaOH

• as rough end-point is approached, add solution from burette drop by drop until indicator changes colour

• record volume to nearest 0.05 cm³

• repeat until you have 2 concordant results (within 0.1cc of each other) to increase accuracy