Acid-base neutralisation – a microscale titration
A microscale titration apparatus is prepared from pipettes, a syringe and some rubber or plastic tubing. This is then used to carry out a titration by filling the ‘burette’ with hydrochloric acid and placing 1 cm3 of sodium hydroxide solution in a 10 cm3 beaker. The aim is to calculate the exact concentration of the sodium hydroxide solution.
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Lesson organisation
Microscale techniques are a fairly recent innovation in school chemistry, but most students take readily to them. Manipulative skills are important, and students need to be capable of careful manipulation to carry this out successfully. Students also need to be familiar with the mole concept, and capable of performing the calculations from the results of the experiment.
On such a small scale, safety issues are minimal. Similarly, the time taken to carry out a titration should be much reduced as the volumes being reacted are so small. It should be possible for a class to carry out the practical work and calculations in a one-hour session.
Apparatus and chemicals
Eye protection
Each working group will require:
Microscale titration apparatus (see note 1):
Graduated glass pipette (2 cm3)
Pipette (1 cm3) + pipette filler to fit (or a 1 cm3 plastic syringe)
Plastic syringe (10 cm3)
Fine-tip poly(ethene) dropping pipette (see note 2)
Small lengths of rubber, plastic or silicone tubing
Beakers (10 cm3), 2
Clamp stand with two bosses and clamps
Hydrochloric acid, 0.10 mol dm-3 (Low hazard at this concentration), about 10 cm3
Sodium hydroxide, approx. 0.1 mol dm-3 (Irritant at this concentration), about 10 cm3 (see note 3)
Phenolphthalein indicator solution (Highly flammable), a few drops
Technical notes
Hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47A and CLEAPSS Recipe Card 31
Sodium hydroxide (Irritant at concentration used) Refer to CLEAPSS Hazcard 91 and CLEAPSS Recipe Card 65
Phenolphthalein indicator solution (Highly flammable) Refer to CLEAPSS Hazcard 32 and CLEAPSS Recipe Card 33
1 The microscale titration apparatus replaces the normal burette. To make the microscale titration apparatus, cut the tip end off a fine-tip poly(ethene) dropping pipette and push the tip carefully onto the end of a 2 cm3 graduated glass pipette. Clamp a plastic syringe, 10 cm3 capacity, above the adapted pipette, as shown in the picture, and connect the two with rubber, plastic, or silicone tubing. Because the diameters of the syringe nozzle and of the top of the pipette may be quite different, two pieces of tubing, one to fit each end, will probably be needed; these can then be joined by an adaptor. A suitable adaptor can be made by cutting the lower end off a 1 cm3 plastic syringe, such that the syringe body diameter fits the wider tubing, and the syringe tip fits the narrower tubing. See diagram and photograph.
It is possible for students to build their own microscale titration apparatus from supplied components, but this is likely to take the students more time than the titration itself! For that reason, it is probably preferable to prepare a class set of these in advance.
2 A suitable poly(ethene) dropping pipette would be fine-tip standard, non-sterile, 3.3 cm3 capacity, such as those available from Sigma-Aldrich.
3 Students are to calculate the concentration of the sodium hydroxide solution so the bottle should not be labelled with the exact concentration.
Procedure
HEALTH & SAFETY: Wear eye protection
a Clamp the microscale titration apparatus securely in position as in photograph and push the syringe plunger completely down.
b Fill the apparatus with 0.10 mol dm-3 hydrochloric acid as follows. Put about 5 cm3 of the acid in a 10 cm3 beaker and place the tip of the apparatus well down into the solution. Raise the syringe plunger slowly and gently, making sure no air bubbles are drawn in. Fill the pipette exactly to the zero mark. Release the plunger; the level should remain steady.
c Use the 1 cm3 pipette and pipette filler to transfer exactly 1.0 cm3 of the sodium hydroxide solution into a clean 10 cm3 beaker.
d Add one drop (no more!) of phenolphthalein indicator solution to the sodium hydroxide solution.
e Adjust the position of the microscale titration apparatus so that the tip is just below the surface of the sodium hydroxide and indicator solution in the beaker
f Titrate the acid solution into the alkali by pressing down on the syringe plunger very gently, swirling to allow each tiny addition to mix and react before adding more.
g Continue until the colour of the indicator just turns from pink to permanently colourless.
h Record the volume of hydrochloric acid added at that point.
i Repeat the titration until you get reproducible measurements – that is, the volume required is the same in successive titrations.
j Calculate the concentration of the sodium hydroxide solution as follows.
The equation for the neutralisation reaction is:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
From the equation you can see that one mole of hydrochloric acid reacts with one mole of
sodium hydroxide.
1 What was the reliable value for the volume of hydrochloric acid solution needed? Let us call this value V cm3.
2 Calculate the number of moles of hydrochloric acid in this volume using the formula: V/1000 x C , where C is the concentration of the hydrochloric acid in mol dm-3.
3 How many moles of sodium hydroxide were therefore present in the original 1 cm3 of sodium hydroxide solution placed in the beaker?
4 Now calculate how many moles of sodium hydroxide would have been present in 1000 cm3. This is the concentration of the sodium hydroxide solution in mol dm-3.
Teaching notes
This microscale technique minimises apparatus and chemical requirements, and takes less time to perform than titration on the usual scale. Although the solutions used do present minor hazards, the use of such small quantities reduces risks from those hazards to very low levels. Students should nevertheless take all usual precautions in handling these solutions. The main risk is from misuse of the syringe or pipettes, especially if containing hazardous solutions.
The technique also makes the point that quantitative chemical experimentation does not always have to be performed on the traditional ‘bucket’ scale at school level.
Health and safety checked, February 2008
Web links
Other examples of school use of microscale titration can be found at:
http://dwb.unl.edu/Chemistry/MicroScale/MScale18.html
http://www.sciencebyjones.com/antacid%20lab%20new%202006.doc
For a discussion of the benefits of microscale work in schools, including titrations, visit:
http://www.deq.idaho.gov/waste/educ_tools/chemical_roundup_reduced_scale_chemistry_fs.pdf
The next link is to a commercial site which can supply ready-made microscale titration kits:
http://www.sargentwelch.com/product.asp_Q_pn_E_WL5261-55_EA_A_Somerset+Titration+Kit_E_
(Website accessed August 2007)
Updated 29 Oct 2008Average rating: 
4 out of 5
Your reviews
Very nice.
Submitted by: leroy on 11 February 2010
I find that the syringe plunger tends to stick when doing the titration by this method. An alternative is to use a bulb off a 3 ml disposable pipette, make a pinhole in it at the side and then attach it to a 2 ml graduated pipette. Placing your finger over the pinhole allows a solution to be drawn up the pipette; carefully allowing air through the pinhole gives a controlled release of solution.
Submitted by: Paul Darcy on 14 July 2010
Answer the questions given?
Submitted by: trisha on 7 May 2009