4.2 Acid/Base Titre

4.2 Acid and Base titrations

Ø  A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution.

Ø  They are used for:

1.     Finding the concentration of a solution

2.     Identification of unknown chemicals

3.     Finding the purity of a substance

Ø  Checking the purity of a substance is an aspect of quality control, such as for medicines, food and cosmetics as they are compounds manufactured for human use.

Ø  Pharmaceuticals must have a high level of purity because the drug must not harm the user

Ø  A standard solution is a solution of known concentration.

Ø  A volumetric flask is used to make up a standard solution very accurately the flasks are manufactured in various sixes and can measure volumes very precisely

Ø  Typical tolerances of the flasks we will use:

+/- 0.20cm3 for a 100cm3 flask

+/- 0.30 cm3 for a 250 cm3 flask

Ø  The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark. If too much water is added the solution will be too dilute and must be prepared again.

Ø  The flask must be inverted a couple of times and if this stage is omitted, titration results are unlikely to be consistent.

Apparatus:

Ø  A solution of an acid is titrated against a solution of a base using a pipette and a burette.

Ø  They have the tolerances of:
+/- 0.04 cm3 for a 10 cm3 pipette

+/- 0.06 cm3 for a 25cm3 pipette

+/- 0.10 cm3 for a 50 cm3 burette

Ø  The reading of a burette is recorded to the nearest half division with the bottom of the meniscus on a mark or between two marks.

Ø  Each burette reading is measured to the nearest +/- 0.05 cm3 so the reading always has two decimal places – so the last digit will always be a 0 or 5

Ø  The end point is sued to indicate the volume of one solution that exactly reacts with the volume of the second solution

Mean titre:

Ø  Important to use inly your closes accurate titres

Ø  Repeat titres until two agree within 0.10 cm3

Ø  If all the titres are used, then the accuracy of the titration technique is lost

Titration calculations:

Ø  From the results you will know:
both the concentration and the reacting volume of one for the solutions

Only the reacting volume of the other solution

Method:

Ø  Step 1: work out the amount in mol of the solute in the solution for which you know both the concentration and volume

Ø  Step 2: Use the equation to work out the amount in mol of the solute in the other solution

Ø  Step 3: Work out the unknown information about the solute in the other solution