Wednesday 27th June – period 3

Titration Demonstration

In an acid/base titration the precise volume of acid required to neutralise a measured volume of alkali is determined by experiment

The acid is best put into a graduated burette as alkalis can form carbonates which block the tap

titre_burette

The alkali is measured with a graduated pipette using a pi-pump

titre_pipette

All equipment is rinsed with water

The burette is then rinsed with about 10 – 15ml of acid to get rid of water drops and this is discarded

It is then filled with the acid and the air trapped in the jet below the tap is run out

The beaker used to hold the alkali is rinsed with a little alkali and discarded

The pipette is filled with alkali and again this is discarded

The equipment is now ready to carry out the titration

It is best to put a white tile on the base of the clamp stand to aid seeing the colour change of the indicator. The burette should be set with the bottom of the jet just below the level of the top of the conical flask.

The conical flask is rinsed with water each time to clean it. It does not have to be dried inside but it is best to keep the work area clean and dry.

For this experiment a 15ml pipette is used

15 ml of alkali is pipetted into the conical flask

A pi-pump is used to fill the pipette

The thumbwheel allows alkali to be drawn into the pipette until the bottom of  the meniscus touches the graduation mark

With a pi-pump it is best to draw in alkali into the pipette until it is about 1cm above the graduation line, lift the pipette tip out the solution and then let the liquid level slowly down to the line

The white lower lever allows air into the pump and the alkali to run into the conical flask. A small drip of alkali remains at the tip of the pipette – do not blow this out

5 drops of indicator are added- here Bromothymol Blue. It is orange!! But changes to blue when added to the alkali.

titre_ind_blue

The first titre is a rough titre to give an idea of how much acid has to be added each time. The burette is read and the value recorded in a table. Here 0.6 ml.

    

Acid is run from the burette with constant swirling till the indicator just goes orange.

titre_ind_yellow

The burette is reread and the value recorded. Here 17.3 ml.

    

The difference between the two reading gives the volume of acid reacted.

So the rough titre is 16.7 ml.

This gives a rough value. About 0.5ml is subtracted from this rough titre value and then added that to the current burette reading. This gives an idea of where the volume of the titre should be and saves time.

So 16.2 ml will be run in – the burette now reads 17.3 ml so adding 16.2 ml gives a target to aim for = 33.5 ml

The conical flask is rinsed, 15ml of alkali pipetted into it and 5 drops of indicator added. So, here for the first accurate titre, acid is run from the burette till the meniscus is at 33.5 ml. The endpoint is then reached by adding a drop of acid at a time till the indicator just changes colour.

         

Three consecutive titres are required – ie three titres within 0.2ml of each other.

         

         

(The initial target is shown in red writing – ie 16.2 ml added to the first reading on the burette)

The average is calculated. Note – the rough titre is not included in the average.

 

average = (16.4 + 16.4 + 16.3) / 3 = 16.37.ml