L5 - Precipitation Titrations

Objectives

• conduct investigations and/or process data involving:
  • precipitation titrations

• This is very similar to standard titrations however the product is a Precipitate
• In most cases this is done on titration with silver ions - such as AgNO₃. This is because silver ions form many PPTs with different ions.
• You still use indicators and wait for the colour change at the end-point.

Examples of Precipitation Titration Calculations

• Let's try to determine the Br^- concentration in a 500ml water sample.
• We will use 0.1 molL^-1 AgNO₃ in the burette and add a few drops of an indicator.
• We will titrate this against 25ml aliquots of the water sample and the average titre (after we get two concordant results) was 18.73ml.

Workings

Step 1: Balance the equation for the formation of the PPT

Ag⁺_(aq) + Br^-_(aq) → AgBr_(s)

Step 2: Calculate moles of the AgNO₃ used using C x (V/1000)

(18.73/1000) x 0.1 = 0.001873 moles

Step 3: Calculate moles of the AgBr in the sample (hence the moles of the Bromide ion)

1:1 ratio so Br- = 0.001873 moles

Step 4: Calculate the concentration of the Br- in the sample using moles/(volume/1000)

0.001873/(25/1000) = 0.07492 molL^-1