4.9        describe how to carry out acid-alkali titrations.

Titrations are used to work out exactly how much acid is need to neutralise a certain amount of alkali. If the concentration of one is known, the concentration of the other can be calculated (see Section 1.27).  Accurate glassware is used for titrations.  A burette measures volumes to the nearest 0.05cm3.  Pipettes come in various sizes and deliver a precise and accurate volume each time they are used.

Method

1)   Use a pipette to put 25 cm3 of the alkali in a conical flask.

2)   Fill a burette with the acid.

3)   Put a few drops of indicator in the conical flask.

4)   Note the initial volume of acid in the burette.

5)   Add the acid a little at a time to the conical flask whilst swirling it.

6)   Stop when the indicator just changes colour and note the final volume of acid in the burette.  For example, if using methyl orange, the indicator would start off yellow in the alkali in the flask.  You need to stop when it JUST turns orange.  If you go to red, you have added too much acid.

Notes on Method

1)   The acid and the alkali can be either way around.

2)   Putting a white tile under the conical flask can help spot colour changes.

3)   Before they are used, it is good practice to rinse the burette and the pipette firstly with water, and then with the solution they are going to be used to measure.

4)   Readings (for burette and pipette) are taken at eye level from the bottom of the meniscus.

Concordant Titres

1)   It is usual to repeat titrations to get several titres (titre = the volume added from the burette)

2)   The aim is to get two titres within 0.10 cm3 of each other.

3)   Such results are called concordant titres.

4)   When the average titre is calculated, only concordant titres are used. The others are ignored.

	Run 1	Run 2	Run 3	Run 4
Final Burette Reading / cm3	21.30	21.25	23.85	21.90
Initial Burette Reading / cm3	1.30	0.80	3.25	1.80
Titre / cm3	20.00	20.45	20.60	20.10

The concordant values are from Run 1 and Run 4. Therefore the average titre for this titration is:

(20.00 cm3 + 20.10 cm3) ÷ 2   =  20.05 cm3

(Run 1 can be done as a quick rough titration to check the indicator colour change and approximate  volumes. In such cases it should be labelled Rough Titration, and its value is not used in calculations). 

 

Take readings from the bottom of the meniscus. Note that burettes read in the opposite direction to measuring cylinders.

This reads 21.30 cm3 and not 22.70 cm3

Pipette

These measure fixed volumes with accuracy. They come in different sizes. (Burettes measure variable volumes)