Acid-base titrations

A traditional acid-base titration

Information & rationale

Before giving this experiment I usually get the students to do a simple acid base titration to introduce them to the correct technique of titration. The experiment is usually to determine the concentration of a solution of sodium hydroxide (about 0.1 mol dm-3) using 0.100 mol dm-3 hydrochloric acid. Students learn about the importance of rinsing out the equipment etc. beforehand and obtaining precise and accurate results. I also introduce the ideas of uncertainty and how they can use the uncertainty written on the apparatus to work out the total percentage uncertainty. I will then ask them what other assumptions they have made and hopefully one of them will come up with ‘trusting’ the technician who has made up the concentration of the sodium hydroxide to be exactly 0.100 mol dm-3. In theory a simple acid base titration is all that is necessary to cover the mandatory laboratory components  stated in Topic 1.3 Reacting masses and volumes. "Use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution." and also in Topics 8 & 18, where (in Topic 8.2) it states "Candidates should have experience of acid-base titrations with different indicators". However I think it is better to also do an experiment where they have to prepare the standard solution. Hence this one on oxalic acid which also is more challenging for them to do the calculations afterwards. To help them do this I provide questions to guide them through the process. It also neatly brings in environmental issues for them to consider.

Teacher’s notes for this experiment

Explain to your students that oxalic acid is poisonous.  If rhubarb (which contains oxalic acid) was discovered today would it be a banned foodstuff?

This experiment builds on the simple NaOH(aq)/HC(aq)l titration above by having the students make up their own standard solution using a volumetric flask. This, together with the balance, increases the uncertainties associated with the apparatus. They should calculate the overall uncertainty in their answer. This introduces an interesting point. They need to determine the molar mass of the oxalic acid by titration then compare it with the calculated molar mass (90.04 + 18x) g mol−1 to find the value of x (which is 2). If they calculate their total uncertainty based on their value of x it will probably be very large. What they should be doing is calculating the uncertainty associated with the value they determine for the molar mass of the oxalic acid.

I have left the details of how to calculate the final answer and also the discussion questions on the practical sheet to download and hand out to students. This is because it is usually done early in the course and is a good practical to train them about propagating uncertainties.

I have written this experiment so that it has an environmental aspect to address Green chemistry by asking them to audit it. Traditional titrations use large quantities of chemicals.  What is surprising to the students is that the most expensive chemical they use is not the oxalic acid or the sodium hydroxide but the distilled water. A class of 16 students will use about 6 dm3 of distilled water. After discussing this I give them the green acid-base practical and future experiments in the programme are planned with much smaller quantities of chemicals.

For those that like 'recipe' practicals here is Gordon Ramsey showing how to make making rhubarb crumble.

making rhubarb crumble   

  

Standard Level Higher Level Student worksheet

A TRADITIONAL ACID - ALKALI TITRATION

Neutralisation occurs between hydrogen ions and hydroxide ions in aqueous solution:

H3O+(aq) + OH-(aq) ⇌ 2H2O(l)

( or H+(aq) + OH-(aq) ⇌ H2O(l) )
 
A suitable indicator is used which changes colour at the 'end point' of the reaction when the pH of the solution corresponds to the presence of the appropriate salt and water only. In this experiment you will use the traditional method of titration to determine the number of moles of water of crystallisation in solid crystals of ethane-1,2-dioic acid (oxalic acid) which has the formula (COOH)2.xH2O. This traditional method makes use of volumetric flasks, pipettes and burettes and relies heavily on accurate readings of relatively large volumes.ENVIRONMENTAL CARE: Although oxalic acid is quite poisonous it is a natural product (it occurs in the leaves of rhubarb) so dilute solutions of it should not affect the environment unduly and can be disposed of down the sink. At the end of this experiment you will carry out an environmental audit.SAFETY: You must use a pipette filler and not your mouth whenever you use a pipette. Remember oxalic acid is poisonous.PROCEDURE: Weigh out accurately about 1.5 g of crystals of oxalic acid. Dissolve it in distilled water and make up the total volume to 250 cm3 in a volumetric flask. Pipette 25.0 cm3 of 0.100 mol dm-3 sodium hydroxide solution into a conical flask, add two drops of phenolphthalein and run oxalic acid solution from a burette into the flask, swirling continuously, until the indicator just loses its colour. Repeat the procedure to get two accurate results (within 0.10 cm3 ).

CALCULATION

1. Write an equation for the reaction. (Remember oxalic acid is a diprotic acid, i.e. one mole of acid reacts with two moles of sodium hydroxide)

2. What amount (in mol) of NaOH is present in 25.0 cm3 of 0.100 mol dm-3 sodium hydroxide solution?

3. What amount (in mol) of oxalic acid was present in the average volume required to react exactly with the sodium hydroxide solution?

4. What amount (in mol) of oxalic acid was present in your 250 cm3 volumetric flask?

5. What is the mass of one mole of oxalic acid?

6. How many molecules of water of crystallisation are present in one mole?

DISCUSSION

1. Estimate the degree of uncertainty in your readings using the balance, the volumetric flask, the pipette and the burette. How accurately can you quote your answer?

2. Compare your answer with the correct answer and work out the percentage error.

3. Suggest any other reasons for possible error.

4. Audit your experiment both financially and environmentally. Work out the total amounts of all reactants (ignore the few drops of phenolphthalein) and the total cost. What is thrown away during the experiment? Scale up your values for the whole class.

This worksheet can also be downloaded from:

 
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