RP1 - Performing a Titration & Volumetric Analysis

• Titration: A quantitative method used to determine the concentration of an unknown solution by reacting it with a standard solution of known concentration.

• Volumetric analysis: Use of volumetric techniques (e.g. A burette) to determine quantities in chemical reactions.

• End-point: An observable indication signalling the completion of a chemical reaction e.g. neutralisation.

Basic Principles

• Use a pipette to measure a fixed volume of solution with unknown concentration.

• Titrate against a standard solution in a burette.

• Use a suitable indicator (e.g., phenolphthalein, methyl orange).

• Perform repeat titres to ensure accuracy and concordant results.

Apparatus

• Volumetric pipette and filler – to measure a fixed volume of solution into a conical flask

• Burette – to gradually add measured volumes of a solution to the conical flask during the titration

• Burette holder and stand

• Conical flask – to hold a known volume of a solution transferred from the pipette

• White tile – placed under the conical flask to better visualise the endpoint colour change

Burette

1. Wash with distilled water to remove and soluble impurities

2. Rinse with the solution that will be placed in the burette to ensure any remaining water does not dilute the solution and affect its concentration

Conical flask

Rinse with distilled water ONLY

(If rinsed with the solution, this would add additional moles to the flask and therefore result in a larger volume of solution from the burette.)

Diagram of titration set-up

Illustration of titration setup showing burette clamped vertically above a conical flask on a white tile.

Selecting an Indicator  

A suitable results table is set out below

Notice that all volumes are recorded to two decimal places. When reading the volume from a burette with intervals of 0.1 cm3, volume where the bottom of the meniscus is on the mark should end with a ‘0;. Volumes where the bottom of the meniscus is between two marks should end in ‘5’.

Mean Titre

The ‘mean’ or ‘average’ titre is calculated from the concordant results only. These are titres within of eachother.

For example, above, the concordant results are those from trials 1 and 3.

The mean titre is calculated as

Percentage uncertainty = [(absolute uncertainty x number of readings)/measure value] x 100

Absolute uncertainty is inherent to the measuring equipment itself e.g. a burette’s absolute uncertainty ±0.05cm3.

Number of readings = 2 (initial volume and final volume)

Measured value = titre

Using the same apparatus, percentage uncertainty can only be reduced by increasing the measured value (the denominator).

To reduce the percentage uncertainty for a titre, the measured value would have to be greater. This means requiring a larger volume of solution to be added from the burette. This can be achieved by:

• Using a larger volume of the solution in the conical flask – this increases the number of moles in the conical flask therefore resulting a greater volume of solution from the burette to react with it

• Making up a standard solution of lower concentration for the burette – this means a greater volume of the solution will be needed from the burette to equal the number of moles of the solution in the conical flask

• Using a more concentrated solution in the conical flask* – this increases the number of moles in the conical flask therefore resulting a greater volume of solution from the burette to react with it. This may not be possible depending on what is being analysed*