volumetric analysis: general theroy

Standard solution

A solution whose concentration is accurately known

How can a solution be standardised

Performing a titration

Primary solution

A substance that can be obtained in a stable, pure and solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration

Examples: anhydrous sodium carbonate (Na₂CO₃)

Hydrated ammonium iron (II) sulfate ((NH₄)2SO₄.FeSO₄.6H₂O)

Explain why deionised water is used

Tap water contains many different types of ions

These ions many interfere in the titration reaction and could affect the accuracy of the end point

Volumetric flask

Function: Used to make up solutions of known volume accurately

i) A certain mass of solid can be added and made up to a known volume

with deionised water

ii) A certain volume of liquid can be added and diluted up to a known volume

with deionised water

Conical flask

Function: The titration reaction takes place in the conical flask

Q: How is a conical flask prepared for use

• A clean dry conical flask should be used in a titration

Q: Why is a conical flask used for a titration rather than a beaker?

• The shape of a conical flask allows the two solutions to be swirled and mixed

without splashing so they can fully react

Pipette

Function: Transfers an exact volume of solution

i) To a volumetric flask to be diluted to a known volume

ii) To a conical flask to be titrated against

Preparing pipette for use

Preparing and filling a pipette for use

• Rinse the pipette with de-ionised water to clean it

• Rinse the pipette with the solution it will contain

• Place the pipette into the solution and using a pipette filler, draw the solution into the

pipette until it is above its graduation mark

• Allow some solution to run out into a waste beaker so the bottom of the meniscus rests on

the graduation mark looking at eye level

Delivering the solution in a pipette into a conical flask/beaker

• Place the pipette into the clean, dry flask/beaker and allow the liquid to freely drain out

• When almost empty tap the pipette against the side of the flask to ensure all of the solution

Describe the procedure and techniques used in volumetric analysis for making up a solution of

• Place the pipette into the clean, dry flask/beaker and allow the liquid to freely drain out

• When almost empty tap the pipette against the side of the flask to ensure all of the solution

is delivered

Describe the procedure and techniques used in volumetric analysis for making up a solution of

known volume from a given mass of a solid substance

• If the solid is in tablet form, crush into a powder using pestle and mortar

Example: For iron tablets

• Using a spatula add the solid into a weighing boat/clock glass on an

electronic balance until the desired mass is obtained

• Carefully empty the mass of solid into a beaker of deionised water - using a wash bottle

containing deionised water, rinse the weighing boat and sides of the beaker and add the

rinsings to the beaker

• Stir with a glass rod to ensure the solid dissolves completely

• Using a funnel add the solution from the beaker into a volumetric flask – using a wash

bottle containing deionised water, rinse the beaker and sides of the funnel and add the

rinsings to the volumetric flask

• Add deionised water to the volumetric flask until roughly 1 cm from its graduation mark

• At eye level, using a dropper, add deionised water dropwise until the bottom of the

meniscus rests on the graduation mark

• Stopper and invert the flask at least twenty times to ensure a homogenous solution

Describe the procedure and technique used in volumetric analysis for taking a given volume of

liquid and diluting it up to a known volume Example: For vinegar or bleach

• Pour the solution from its container into a beaker

• Use a suitably sized pipette previously rinsed with deionised water and rinsed with the

solution it will contain

• Place the pipette into the solution and using a pipette filler, draw the solution into the

pipette until it is above its graduation mark

• Allow some solution to run out into a waste beaker so the bottom of the meniscus rests on

the graduation mark looking at eye level

• Place the pipette into a new clean dry beaker and allow the liquid to freely drain out

• When almost empty tap the pipette against the side of the flask to ensure all of the solution

is delivered

• Using a funnel add the solution from the beaker into a volumetric flask – using a wash

bottle containing deionised water, rinse the beaker and sides of the funnel and add the

rinsings to the volumetric flask

• Add deionised water to the volumetric flask until roughly 1 cm from its graduation mark

• At eye level, using a dropper, add deionised water dropwise until the bottom of the

meniscus rests on the graduation mark

• Stopper and invert the flask at least twenty times to ensure a homogenous solution

Describe the procedure and techniques used in volumetric analysis for extracting an exact

volume of solution from a volumetric flask and delivering it into a conical flask

• Pour some of the solution to be delivered into a clean dry beaker

• Use a suitably sized pipette previously rinsed with deionised water and rinsed with the

solution it will contain

• Place the pipette into the solution and using a pipette filler, draw the solution into the

pipette until it is above its graduation mark

• Allow some solution to run out into a waste beaker so the bottom of the meniscus rests on

the graduation mark looking at eye level

• Place the pipette into a new clean dry conical flask and allow the liquid to freely drain out

• When almost empty tap the pipette against the side of the flask to ensure all of the solution

is delivered

Burette

Adds an exact volume of solution into the solution in the conical flask

during the titration

Preparing and filling burette

• Rinse the burette with de-ionised water to clean it

• Rinse the burette with the solution it will contain

• Place the solution in a beaker before the transfer

• Use a funnel

• Ensure the burette is clamped exactly vertically and ensure the 0 cm3

mark is at eye level

• Pour the solution from the beaker into the burette and fill the burette

with solution above the 0 cm3 mark

• Remove the funnel

• Open the tap and allow some solution to run out into a waste beaker so the bottom of the

meniscus rests on the 0 cm3 looking at eye level