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Titration technique and standard solutions (Core Practicals 3 and 4)

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Titration technique questions ask why each step is done. Core Practicals 3 and 4 are taught in the Unit 2 titration lesson; this lesson gives the full mark-scheme detail.

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Making a standard solution

1. Weigh the solid in a weighing bottle; tip it into a beaker; reweigh the bottle (mass by difference).

2. Dissolve the solid in a small volume of deionised water, stirring.

3. Pour the solution into a 250.0 cm³ volumetric flask through a funnel.

4. Rinse the beaker, stirring rod and funnel with deionised water and add the washings to the flask (so no solid is lost).

5. Make up to the mark, adding the last drops with a dropping pipette, until the bottom of the meniscus sits on the line at eye level.

6. Stopper and invert several times so the solution is uniform.

Steps 4 and 6 were the ones most often missed in October 2024.

The titration

Rinse the pipette and burette with the solutions they will hold; rinse the conical flask with deionised water only (water does not change the moles of solute delivered).
Pipette 25.0 cm³ into the flask, add a few drops of indicator, stand it on a white tile to see the colour change.
Add from the burette, swirling; add dropwise near the end point.
Do a rough titration, then accurate ones until the titres are concordant.
Primary standards must be pure, stable and not absorb water or CO2. Sodium hydroxide is not a primary standard: it absorbs water and carbon dioxide from the air, so its concentration must be found by titration.

Indicators

Phenolphthalein: colourless in acid, pink in alkali. Alkali added from the burette to acid: colourless to (pale) pink.
Methyl orange: red in acid, yellow in alkali, orange at the end point. Acid added to alkali: yellow to orange.
Universal indicator is unsuitable: it changes colour gradually, so there is no sharp end point.

Always give the colour change in the direction it happens.

Worked example

Describe how to make 250.0 cm³ of a standard solution from about 2.6 g of anhydrous sodium carbonate.

Weigh the solid in a weighing bottle, transfer it to a beaker and reweigh the bottle.
Dissolve in deionised water; transfer to a 250.0 cm³ volumetric flask.
Rinse the beaker and rod, adding the washings to the flask.
Make up to the mark with deionised water (bottom of meniscus on the line); stopper and invert to mix.

Answer: Weigh by difference, dissolve, transfer with washings, make up to the mark, invert to mix.

Key idea: Weigh by difference, dissolve, transfer with washings, make up to the mark, invert. Rinse pipette and burette with their solutions; flask with water. White tile, swirl, dropwise near the end. Phenolphthalein colourless to pink.

Check yourself

Why can the conical flask be wet with deionised water before the pipetted solution is added?
- Water makes the end point sharper
- Water is needed to activate the indicator
- Water cools the reaction
- Water does not change the number of moles of solute in the flask
Show answer
Water does not change the number of moles of solute in the flask. The moles delivered by the pipette are fixed; extra water only dilutes.
Why is universal indicator not used in titrations?
- It is too expensive
- Its colour changes gradually, so there is no sharp end point
- It only works in alkalis
- It reacts with the acid
Show answer
Its colour changes gradually, so there is no sharp end point. A titration needs a sudden colour change at one drop.
Sodium hydroxide is added from a burette to hydrochloric acid containing phenolphthalein. What is the colour change at the end point?
- Pink to colourless
- Red to yellow
- Yellow to orange
- Colourless to pink
Show answer
Colourless to pink. Phenolphthalein is colourless in acid and turns pink when alkali is in excess.
Why is sodium hydroxide not used as a primary standard?
- It absorbs water and carbon dioxide from the air, so its mass does not give an exact amount
- It is too soluble
- It is a weak base
- It reacts with glass
Show answer
It absorbs water and carbon dioxide from the air, so its mass does not give an exact amount. A weighed sample would contain water and carbonate, not just NaOH.
What is the white tile for?
- To protect the bench from acid
- To keep the flask warm
- To make the colour change easier to see
- To catch drips from the burette
Show answer
To make the colour change easier to see. The colour change shows up clearly against white.
Why is the volumetric flask inverted several times after making up to the mark?
- To make the solution the same concentration throughout
- To remove air bubbles from the meniscus
- To dissolve the remaining solid on the stopper
- To cool the solution
Show answer
To make the solution the same concentration throughout. Without mixing, the solution near the top would be more dilute.
Explain why the beaker used to dissolve the solid is rinsed and the washings added to the volumetric flask.
Show answer
So that all the solid (solute) is transferred to the flask and none is lost.. Any solute left in the beaker would make the concentration too low.

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Last updated 8 November 2026. Spotted a mistake? Tell us.