4.2 Acid-Base Titrations

A technique used to accurately measure the volume of one solution that reacts exactly with another solution

Finding the concentration of a solution, identification of unknown chemicals, and finding the purity of a substance

A volumetric flask Filled so that the bottom of the meniscus just touches the graduation line

100cm^3 - ±0.20cm^3 250cm^3 - ±0.30cm^3

• Accurately weigh the solid • Dissolve the solid in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark • Solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water • 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 • The volumetric flask is slowly inverted several times to mix the solution. Without doing this, results will not be consistent

Pipette and a burette

10cm^3 pipette - ±0.04cm^3 25cm^3 pipette - ±0.06cm^3 50cm^3 burette - ±0.10cm^3

To the nearest half division with the bottom of the meniscus on a mark or between two marks Measured to the nearest ±0.05cm^3 so that the reading always has two decimal places, the last being either 0 or 5

• Add a measured volume of one solution to a conical flask using a pipette • Add the other solution to a burette, and record the initial burette reading to the nearest 0.05cm^3 • Add a few drops of an indicator to the solution in the conical flask • Run the solution in the burette into the solution in the conical flask, SWIRL the conical flask to mix • Indicator eventually changes colour at the END POINT of the titration. Used to indicate the volume of one solution that exactly reacts with the volume of the second solution • Record the final burette reading, volume of solution added from burette is called the titre • Quick trial titration is carried out first to find approximate titre • Repeated accurately adding it dropwise nearing the end point • Carry out more until two accurate titres are concordant

The volume of solution added from the burette Calculated by subtracting the initial from the final burette reading

ONLY use the closest accurate titres • By repeating titres until two agree within 0.10cm^3, can reject inaccurate titres • If you were to include all titres in the mean, you have lost the accuracy of the titration technique

Both the concentration c1 and the reacting volume V1 of the solution Only the reacting volume V2 of the other solution

Step 1 - Work out the amount in mol of the solute in the solution for which you know BOTH the concentration c1 and volume V1 Step 2 - Use the equation to work out the amount in mol of the solute in the other solution Step 3 - Work out the unknown information about the solute in the other solution

• Prepare a solution of an unknown carbonate X2CO3 in a volumetric flask • Using a pipette, measure 25.00cm^3 of your prepared solution into a conical flask • Using a buratte, titrate the solution using 0.100moldm^-3 hydrochloric acid • Analyse your results to identify the carbonate