GCSE OCR Chemistry C5.1b-c The Titration technique

C5.1b To describe the technique of titration using acid/alkali titrations.

C5.1c explain the relationship between the volume of a solution of known concentration of a substance and the volume or concentration of another substance that react completely together.

The titration technique

The titration technique is used to measure the unknown concentration of a solution given that you know the concentration of a solution that will react with the unknown.

For example, you can use a standard solution of sodium carbonate (Na₂CO_{3 (aq)}) (see my previous post here) to find the concentration of a solution of hydrochloric acid.

Finding the concentration of a solution of hydrochloric acid using sodium carbonate solution.

To carry out a titration you will need:

50ml burette

250ml conical flask

25ml pipette

white tile

small filter funnel

2×100ml beakers

pipette filler

acid/base indicator e.g. methyl orange or phenolphthalein

distilled water wash bottle

0.1M sodium carbonate solution

hydrochloric acid of unknown concentration

How to carry out the titration:

1.   Rinse the burette with the sodium carbonate solution and then fill it with the same solution and don’t forget to fill the jet below the tap!!

2.   Record the initial burette reading in the rangefinder column in your results table (see below). The diagram shows how to read the burette:

3.   Rinse the pipette with some of your hydrochloric acid solution then transfer a 25ml aliquot (!) of the acid to your clean conical flask.

4.   You can add a few drops of either indicator.  In the analysis below we’ll discuss the effect of each indicator.

5.   Run the sodium carbonate into the flask until near the end point swirling the flask. With phenolphthalein the end point will be pink, with methyl orange it will be orange or yellow.  You’ll overshoot this titration because it’s your rangefinder. Record the final burette reading.

6.   Repeat your titration method recording your results in the table and making sure you rinse the apparatus before using it again.

7.   You need two results within 0.10ml.

Here is a typical set of results in a useful table

Pipette solution	Hydrochloric acid				unknown
Burette solution	Sodium carbonate solution				0.1 mol.dm-3
Indicator	Phenolphthalein
		Rangefinder	1	2	3
Burette readings	Final	12.67	11.30	11.60
	Initial	0.00	0.10	0.30
Volume used (Titre) /ml		12.67	11.20	11.30
Mean titre /ml	11.20  +  11.30  =  22.50/2  =  11.25ml

Calculation

First, calculate the number of moles of sodium carbonate that were used in the titration using n=cV

so

Second, work out the number of moles this amount of sodium carbonate neutralised.  Use the equation for the reaction that happens when phenolphthalein is the indicator.

Na₂CO₃   +    HCl     ⟶     NaHCO₃    +   NaCl

As 1 mole carbonate reacts with 1 mole of acid then the number of moles of hydrochloric acid in the 25ml aliquot were 0.00001125moles.

Third, use n=cV again to calculate the hydrochloric acid concentration. 

  

so

Notes:

First, you should have realised by now that sodium carbonate can react in two different ways with hydrochloric acid:

Na₂CO₃   +    HCl     ⟶     NaHCO₃    +   NaCl

And

NaHCO₃   +    HCl     ⟶     NaCl    +   H₂O   +    CO₂

Phenolphthalein indicates the first pH change and methyl orange indicates the second pH change.

You’ll have noticed too that we prefer to titrate from colourless acid solution to alkaline pink.  We do this because it is easier to observe the colour change.

Errors can creep into the titration procedure if you do not rinse the burette with the alkali.  If your burette is wet then you will use a diluted solution of alkali and at least your first titration result will require more alkali than is actually needed. 

Again if you do not rinse the pipette with acid there will be no effect if the pipette is dry but if wet then the solution in the pipette will be slightly diluted and result in a lower value for the concentration.

And if the jet of the burette is not filled before the titration then the rangefinder volume will be greater than is actually needed.