Titration Experiment: Standardising Sodium Hydroxide Against Oxalic Acid

Three conical flasks showing the phenolphthalein end point in a titration: deep pink, pale pink, then colourless
The end point. Phenolphthalein runs from pink through pale pink to colourless as the acid neutralises the last of the alkali.

A titration is a way of finding the concentration of a solution you do not know, by measuring exactly how much of a solution you do know it takes to react with it completely. You run the known solution in from a burette, drop by drop, until an indicator changes colour. That colour change is the end point: the moment neither the acid nor the base is in excess. In this Grade 12 experiment you make up a standard oxalic acid solution, then use it to work out the concentration of a sodium hydroxide solution. It is a prescribed formal assessment for CAPS Physical Sciences, Term 2.

Where this fits in the curriculum

Subject Physical Sciences
Grade 12
Term 2
Topic Acids and bases, Chemical change
Status Prescribed experiment for formal assessment (SBA)
Time needed 90 minutes, one double period

This runs in two parts and both carry marks. Part A is preparing the standard solution. Part B is the titration itself. Part A has to happen first, because without a solution of known concentration there is nothing to measure the sodium hydroxide against.

The chemistry behind it

Phenolphthalein is pink in alkali and colourless in acid. The sodium hydroxide in the flask starts pink. You add oxalic acid until the pink disappears and stays gone.

2NaOH(aq) + (COOH)2(aq) → Na2(COO)2(aq) + 2H2O(ℓ)

Two moles of sodium hydroxide react with one mole of oxalic acid. That 2:1 ratio is where most learners lose marks, and a result that comes out exactly half or exactly double the expected value almost always traces back to it.

Oxalic acid is used because it is a primary standard. It can be weighed out pure and dry, so a solution made from it has a concentration you can rely on. Sodium hydroxide cannot be used that way, because it draws water and carbon dioxide out of the air. What you weigh is never quite what you think you weighed, which is exactly why its concentration has to be found by titration rather than calculated.

Before you weigh anything: which oxalic acid?

Oxalic acid comes in two forms and the mass you need is different for each. This is the single most common way this practical goes wrong, and nothing on the bench looks wrong while it happens.

Form Formula Molar mass Mass for 250 cm³ of 0,1 mol·dm⁻³
Dihydrate (COOH)2·2H2O 126 g·mol⁻¹ 3,15 g
Anhydrous (COOH)2 90 g·mol⁻¹ 2,25 g

Use the dihydrate. It is the form the formal assessment is written around, and it is what ships in our kit. Weighing the anhydrous mass of a dihydrate powder gives a solution roughly 30 % weaker than intended, and every calculation the class does afterwards will be wrong.

What you need

Labelled diagram of a titration setup showing the burette, tap, retort stand, conical flask, volumetric pipette and volumetric flask
The setup for Part B, with the volumetric flask and pipette used in Part A alongside.

Apparatus

Item Qty
Burette, 50 ml, with PTFE stopcock 1
Burette stand 1
Burette clamp 1
Retort clamp with bosshead 1
Volumetric flask, 250 ml, with stopper 1
Conical flask, 250 ml 3
Volumetric pipette, 20 ml, one mark 1
Pipette filler 1
Filter funnel, glass 1
Watch glass 1
Spatula, stainless steel 1
Wash bottle 1
Beaker, 250 ml 1
Dropper bottle 1

Chemicals

  • Oxalic acid dihydrate, 3,15 g per group
  • Sodium hydroxide pellets
  • Phenolphthalein indicator solution
  • Distilled water

You supply

A mass meter reading to 0,01 g. Most Grade 12 labs already have one.

Method

Part A: making the standard solution

  1. Weigh a clean dry watch glass and write the reading down.
  2. Add oxalic acid dihydrate until you have about 3,15 g on the watch glass. It does not have to land exactly on 3,15. Write down what you actually weighed, to two decimals, because that is the number you calculate with.
  3. Tip it all into the volumetric flask through the funnel. Rinse the watch glass and the funnel into the flask so nothing is left behind.
  4. Half fill with distilled water, stopper and swirl until it has all dissolved.
  5. Top up to the graduation mark. The bottom of the meniscus must sit on the line.
  6. Stopper and invert several times to mix properly.
  7. Work out the concentration: n = m ÷ 126, then c = n ÷ 0,250.

Part B: the titration

  1. Rinse the burette with a little of the oxalic acid solution, then fill it using the funnel.
  2. Run a few cm³ out through the tap to clear the air bubble from the tip. Top up and record the initial reading to two decimals.
  3. Rinse the pipette with a little sodium hydroxide, then pipette 20 cm³ into a conical flask rinsed with water only.
  4. Add three drops of phenolphthalein. The solution turns pink.
  5. Run acid in, swirling constantly. Close the tap the moment the pink disappears. Record the reading. This is your rough run.
  6. Refill. Fresh 20 cm³ of sodium hydroxide, fresh flask, three drops of indicator.
  7. Run acid in quickly to within 1 cm³ of the rough figure, then go drop by drop until the pink goes and stays gone.
  8. Repeat until three readings agree to within 0,1 cm³. Average those three and ignore the rough run.
  9. Rinse the burette with water straight away. Left standing, oxalic acid leaves a deposit in the tip that ruins it.

What you should see

Pink to colourless, on a single drop. The change should be sharp rather than gradual. Expect a titre somewhere between 8 and 20 cm³, depending on how the sodium hydroxide was made up. Three readings agreeing to within 0,1 cm³ is what the practical is marked on.

If it does not work

What you see What caused it
Readings will not converge Air bubble left in the burette tip, so the initial reading was meaningless
Titre much larger than expected Burette rinsed with water instead of acid, diluting it. Or the sodium hydroxide was made up stronger than intended
Colour goes on the first drop The pipette delivered less than 20 cm³, usually an air bubble. Or the alkali is very dilute
Pink returns when you swirl Not at the end point yet. Keep going, one drop at a time
Answer exactly double or exactly half The 2:1 mole ratio was missed. Almost always this
Cloudy white solid in the flask Tap water was used somewhere instead of distilled

Which way the colour goes, and why it trips people up

Some published methods for this practical have the direction backwards, telling learners to add acid "until the solution turns pink". That is the wrong way round here. You are running acid into a pink alkali, so it goes pink to colourless. The colourless to pink wording belongs to the reverse titration, where the alkali sits in the burette and the colour appears rather than disappears. Anyone following it as written will run well past the end point and get a titre that is far too large.

Settle which way round it is before the lesson starts. It is the single most common reason a class ends up with unusable results.

Safety

Sodium hydroxide is corrosive and burns skin and eyes. Oxalic acid is harmful if swallowed and irritates skin. Goggles and gloves for both parts, including making up the solutions. Add sodium hydroxide to water, never water to the pellets. Never pipette by mouth.

If either solution goes on skin or in an eye, rinse under a strongly running tap for at least 15 minutes and get help. Safety data sheets for all three chemicals ship with the kit.

Disposal

The titrated flasks are close to neutral and can go down the drain with plenty of running water. Dilute any leftover sodium hydroxide heavily first.

Free worksheet and marking memo

Both are free to download, no sign up.

Buy this experiment

We are putting together a complete Titration Experiment Kit for Grade 12, with the glassware, the chemicals, the safety data sheets and a printed teacher guide in one box, plus an annual refill of just the consumables. Both are coming shortly.

In the meantime, every item in the apparatus table above is in stock and sold separately.