f - Acids, alkalis and titrations (copy)

2f (2.28-2.33)

Litmus paper

Litmus paper

Acidic - red

Alkaline - blue

Neutral - purple

Phenolphthalein

Acidic: colourless

Alkaline: bright pink

Methyl orange

Acidic: red

Alkaline: yellow

pH scale

0-3: strongly acidic
4-6: weakly acidic
7: neutral
8-10: weakly alkaline
11-14: strongly alkaline

Universal indicator

• Add a few drops to solution and match colour with chart

Acid

• Source of hydrogen ions (H⁺) in aqueous solution

• Have pH less than 7

Alkali

• Source of hydroxide ions (OH⁻) in aqueous solution

• Soluble base

• Have pH more than 7

• Can neutralise acids

Acid-alkali titration

• Allow you to find how much acid is needed to neutralise a quantity of alkali (or vice-versa)

1. Using pipette and pipette filler, add 25cm³ of alkali to conical flask, along with 2/3 drops of indicator

2. Fill burette with acid
   Do this below eye level so acid doesn’t spill in your eyes

3. Using burette, add acid to alkali a bit at a time - swirl conical flask regularly
   Go slowly nearer to the end-point (colour change)

4. Indicator changes colour when all alkali has been neutralised e.g. phenolphthalein pink in alkali, colourless in acid

5. Record volume of acid used to neutralise alkali

   Repeat until you achieve concordant results (3 results within 0.2cm³)