Titrations are used to determine the concentration of an acid or alkali solution.


Method: Titration

Set up a clamp stand with a burette fixed parallel to it. Place a conical flask underneath the burette
Rinse the conical flask and the burette with distilled water
Fill the burette with the acid, ensure that the bottom of the meniscus sits on the maximum measuring line of the burette. The concentration of this acid must be known.
Fill the conical flask with a set volume of the acid solution using a pipette.
Add an appropriate pH indicator to the alkali
First, there should be a rough titration to form a ‘general’ idea to where the end point is (or the exact point of neutralisation). To perform this, add the acid in the burette to the alkali solution, swirling the mixture until the solution turns clear.
To do an accurate titration, take the initial reading for the amount of acid in the burette that remains. Allow the acid to run through the burette until 2cm3 of the end point to which now add the acid dropwise. Stop adding the acid when the solution turns changes colour.
Work out the total amount of acid used in the accurate titration to neutralise the alkali.
Total amount of acid used = Final reading – Initial Reading

Repeat the titration again multiple times, until there are at least 3 concordant results
Use these values to find the mean volume of acid used (remove any anomalous results if any)

Method: Standard Solution

Work out the amount in moles of the alkali
Moles = Concentration x Volume

Work out the amount in grams of the alkali
Mass = Moles x Mr

Using a weighing boat and digital balance, measure the required mass and pour this into a beaker
Add distilled water to the beaker until all of the solution has been dissolved
Using a funnel, pour the solution in the beaker into a 250cm3 volumetric flask
Rinse the beaker and funnel with distilled water and add this to the flask too (ensures all of the solute is in the flask)
Fill the rest of the flask upto the 250cm3 with distilled water (ensure the meniscus sits on the measurement line)
Place a bung on the flask and shake the solution
Calculate the concentration of the solution*

*e.g. 40cm3 of 0.250 mol dm-3 KOH was used to neutralise 22.0 cm3 of HNO3 solution. Calculate the concentration of the nitric acid in mol dm-3

KOH + HNO3 -> KNO3 + H2O

For the KOH, there is a known volume and concentration but an unknown amount of moles.
Moles = Concentration x volume

First convert the 40cm3 into dm-3 (which is the same as concetration) to do this multiply the volume in cm3 by x10-3
40cm3 x 10-3 = 0.04

Multiply the volume (in dm-3) by the concentration
0.04 dm-3 x 0.250 mol dm-3 = 0.01 moles

*notice how the dm-3 can cancel, therefore leaving just the moles

Now the number of moles for the KOH is known, as it is a 1:1 ratio (as there is one mole of KOH and HNO3) the moles can be copied to the HNO3. Therefore there is a known volume and a known amount of moles, but an unknown concentration
Concentration = moles / volume

Convert the volume of HNO3 from cm3 to dm-3 by multiply it by x10-3
22cm3 x 10-3 = 0.022 dm-3

0.01 moles / 0.022dm-3 = 0.455 moles

Indicators

Indicators must be used that changes colour quickly over a small pH range. These must be used to determine the exact point of neutralisation compared to other indicators such as Universal Indicator that changes gradually
Methyl orange – red in acid, yellow in alkali
Phenolphthalein – colourless in acid, pink in alkali