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Everyday science
The analysis of Milk of Magnesia
by acid–base titration
John F. McCullagh
Abstract The physical and chemical properties of Milk of Magnesia provide examples of chemical
concepts and opportunities for investigative practical work for secondary chemistry students of all ages.
Basing the lesson on the contents of that familiar blue bottle, which is probably lurking somewhere
in a cupboard in most households, also showcases the role of chemistry in the formulation and
analysis of everyday pharmaceutical products. At key stage 3 (age 14), the formation of an insoluble
suspension of magnesium hydroxide from the mixing of two clear solutions of soluble salts provides
a very visual representation of a chemical reaction and the concept of chemical change. At GCSE
level (age 15–16), direct titration of a sample of this product with dilute hydrochloric acid results in
titre values consistent with the amount of magnesium hydroxide stated on the label and produces
a most visual endpoint as the cloudy mixture simultaneously clarifies and changes colour. Using a
back titration provides opportunities for calculations at A-level (age 16–19) and extends students’
understanding of quantitative analytical methods.
It is always pleasing when an everyday household prod- uct can take centre stage during a chemistry lesson. Better still when the substance of interest has a strong connection to the local area or region. The analysis of Milk of Magnesia, originally developed by the Irish chemist Sir James Murray (Box 1), provides an excel- lent example of an everyday neutralisation reaction, and allows students the opportunity to consider the contents of a pharmaceutical product. Both direct and back titra- tions can be carried out to determine the exact amount of the active substance, magnesium hydroxide, present in the formulation. Getting close to the value stated on the bottle can enhance students’ confidence and interest in chemistry and provide a context for a discussion of errors and product sampling.
Solubility and chemical change at
key stage 3
The concepts ‘solubility’ and ‘chemical change’ feature strongly in the junior chemistry curriculum and can be addressed in this simple demonstration. The addition of a small amount of magnesium hydroxide to a beaker of water and stirring should demonstrate its low solubility in water, in comparison with other ionic substances such as sodium chloride with which students will be familiar. The cloudy suspension already resembles Milk of Magnesia and the class could discuss everyday exam- ples of suspensions and solutions such as milk and dilute cordial. A centrifuge could be used to separate the insol- uble solid to the bottom of the tube and leave the clear
Box 1 Milk of Magnesia and Sir James Murray Born in 1788 in County Derry/Londonderry, Sir James trained as a doctor and, after qualification, he began work as an apothecary and physician at Belfast Dispensary and Fever Hospital (now the site of the Belfast City Hospital). His career flourished under the patronage of the Marquis of Donegall, who owned Belfast Castle. During this time, he experimented with electrical apparatus and, in about 1809, he developed and marketed Murray’s Fluid Magnesia. It was sold as a palatable laxative and a remedy for acidities, indigestion, heartburn, and gout. His most famous discovery has reportedly been put to other more unorthodox uses including mouth ulcer treatment, skin toning face masks and even helping a young actor to whiten his hair for a play. Based on Garvin and O’Rawe (1993)
McCullagh The analysis of Milk of Magnesia by acid–base titration
liquid above it. The low solubility of magnesium hydroxide can be further demonstrated by adding a small beaker containing approximately 100 cm^3 of 0.1 mol dm−3^ sodium hydroxide to a larger beaker containing approximately 200 cm^3 of 0.1 mol dm− magnesium sulfate solution to form a cloudy white precipitate of magnesium hydroxide. This reaction
(Figure 1) provides an example of a chemical change as
it produces new substances, and a simple diagram can be used to represent the reaction and support the idea of the individual ions combining to form the new compounds magnesium hydroxide and sodium sulfate.
MgSO 4 + 2NaOH → Mg(OH) 2 + Na 2 SO 4 (1)
Neutralisation
A sample of Milk of Magnesia can be used as an effective
demonstration of neutralisation for the topic of acids and bases at key stage 3 (age 14). When dilute hydro- chloric acid is slowly added to a mixture of Milk of Magnesia and water, students can see that the suspen- sion becomes less dense as the magnesium hydroxide reacts with the HCl to form a salt that dissolves, until finally a clear solution is obtained. A sample of 5 cm^3 of Milk of Magnesia mixed in a beaker with approximately
20 cm^3 of water (to make it easier to see) will require
about 14 cm^3 of 1.0 mol dm−3^ HCl for complete reac- tion (Figure 2).
Mg(OH) 2 + 2HCl → MgCl 2 + 2H 2 O (2)
With reference to the word equation, students could be asked to decide which of the compounds that appear in the word equation are present in the beaker at each of the following stages:
l after a small portion of acid has been added but not enough to remove all the cloudiness; l after just enouch acid has been added to remove all of the cloudiness; l after extra (the word ‘excess’ can be introduced later) acid has been added beyond the point where all the cloudiness has been removed.
This should support students’ understanding of the idea that neutralisation is only complete when the amount of acid added is equivalent to the amount of magnesium hydroxide present, and thus lay the foun- dations for carrying out titrations later in their studies. The concept of neutralisation can be made more visual by adding a few drops of universal indicator solu- tion to the suspension of Milk of Magnesia and repeating the gradual addition of 1.0 mol dm−3^ HCl. The change in colour from blue through green to red ‘indicates’ when all the magnesium hydroxide has reacted and the solution has become neutral. Addition of more acid makes the solution acidic. Litmus can also be used or, keeping with the theme of everyday science, red cabbage indicator. (This indicator can be produced by heating chopped red cabbage leaves in water for five minutes, filtering and using the solution for testing.) By firstly demonstrating this neutralisation without an indicator, students are required to focus more closely on the turbidity of the mixture, and this highlights the importance of close observation in chemistry. It may also help to avoid the possible misconception that neutralisation itself must involve an indicator.
Key stage 4
At key stage 4 (age 14–16) a sample of Milk of Magnesia may be directly titrated with 1.0 mol dm−3^ hydrochloric acid using methyl orange as an indicator (Box 2). This activity could be used as part of a ‘value for money’ inves- tigation, with comparison to other antacid products or, as in this case, to compare the mass of magnesium hydroxide present to the amount stated on the package. Students should enjoy this aesthetically pleas- ing titration (Figure 3). During the early stages, the mixture takes on a yellow appearance not dissimilar to very thin scrambled egg. As the addition of acid is continued, the mixture becomes much thinner but is still clearly a suspension. Just before the endpoint, the mixture is quite transparent before turning to an orange solution at the equivalence point. Titre values are usually within 0.1 cm^3 of each other but there may
1- 1-
2- 2-
1- 1-
Figure 1. Magnesium sulphate and Sodium hydroxide (react to form) Magnesium hydroxide and Sodium Sulphate
Mg 2+
S
Na +
O
Mg 2+
O
Na +
S Na+
O
O
Na +
O O
O O
H
H H
H
O
O O
O
Figure 1 Magnesium sulfate and sodium hydroxide react to form magnesium hydroxide and sodium sulfate
1-
Magnesium hydroxide and Hydrochloric acid (react to form) Magnesium chloride and Water
Mg 2+
O Cl-
O (^) Cl-
Mg 2+
Cl-
Cl-
O
O
H
H H
H
H
H + H
H +
Figure 2 Magnesium hydroxide and hydrochloric acid react to form magnesium chloride and water
solution with an alkali (red to yellow), and to see how
the colour relates to the pH of the solution. The calcula-
tion based on the data from the back titration provides
good practice in calculating the number of moles of each
species and converting moles into grams and milligrams.
The direct titration of Milk of Magnesia provides an
example of Le Chatelier’s Principle in action. Magnesium
hydroxide is sufficiently soluble to produce an alkaline
solution in water, though the bulk of the compound is
suspended (undissolved) in the conical flask (see equa-
tion 3). As the added acid neutralises the hydroxide ions
present in the solution, more solid magnesium hydrox- ide dissolves to replace them. These hydroxide ions are in turn neutralised by the addition of more hydrogen ions. This continues until all of the magnesium hydrox- ide is neutralised.
Mg(OH) 2 (s) ⇌ Mg2+(aq) + 2OH−(aq)
↓ 2HCl(aq)
MgCl 2 (aq) + 2H 2 O(l) (3)
Box 3 Back titration method Apparatus Burette, 250 cm^3 volumetric flask, conical flasks, pipette (5 cm^3 ), pipette (25 cm^3 ), wash bottle, beaker (250 cm^3 ), small funnel Reagents Milk of Magnesia, 1.0 mol dm−3^ HCl, 0.1 mol dm−3^ NaOH, methyl orange indicator solution Health and safety 1.0 mol dm−3^ hydrochloric acid is low hazard (CLEAPPS). Safety glasses and protective gloves should be used. Ensure that eyewash facilities are available. Methyl orange is low risk (CLEAPPS). 0.1 mol dm−3^ NaOH is low hazard (CLEAPPS). Method Sample analysis 1 Using a pipette (5 cm^3 ), transfer 5 cm^3 of Milk of Magnesia into a 250 cm^3 volumetric flask. 2 Using a wash bottle, wash any of the suspension adhering to the inside of the pipette into the volumetric flask and continue to do so until the effluent appears clear and free of suspension. 3 Using a pipette (25 cm^3 ), add 25 cm^3 of 1.0 mol dm− HCl to the volumetric flask. The suspension will react and dissolve with the acid to produce a clear solution. Make to volume with distilled water and invert several times to ensure thorough mixing. 4 Using a pipette (25 cm^3 ), transfer 25 cm^3 of the solution into each of two conical flasks. 5 Add two drops of methyl orange indicator solution to each conical flask. 6 Titrate one of the flasks at a time with 0.1 mol dm− NaOH solution, using the second flask as a colour reference, until the solution first becomes yellow. 7 Repeat the titration to obtain at least three titre values and calculate the mean titre value. Blank analysis 1 Using a pipette (25 cm^3 ), add 25 cm^3 of 1.0 mol dm− HCl to a volumetric flask. Make to volume with distilled water and invert several times to ensure thorough mixing.
2 Using a pipette (25 cm^3 ), transfer 25 cm^3 of this solution into each of two conical flasks. 3 Add two drops of methyl orange indicator solution to each conical flask. 4 Titrate one of the flasks at a time with 0.1 mol dm− NaOH solution, using the second flask as a colour reference, until the solution first becomes yellow. 5 Repeat the titration to obtain at least three titre values and calculate the mean titre value.
Results and calculation Blank analysis Mean titre = 24.5 cm^3 Number of moles of 0.1 mol dm−3^ NaOH required = 24.5/1000 × 0.1 = 24.5 × 10 − Therefore 24.5 × 10 −4^ moles of HCl are present in the conical flask Therefore 24.5 × 10 −3^ moles of HCl are present in the blank sample Sample analysis Mean titre = 9.75 cm^3 Number of moles of 0.1 mol dm−3^ NaOH required = 9.75/1000 × 0.1 = 9.75 × 10− Therefore 9.75 × 10−4^ moles of HCl are present in the conical flask Therefore 9.75 × 10−3^ moles of HCl are present in the sample of Milk of Magnesia Therefore the number of moles of HCl neutralised by reaction with Mg(OH) 2 = 24.5 × 10 −3^ − 9.75 × 10 −3^ = 14.75 × 10 − Number of moles of Mg(OH) 2 present in sample = 7.375 × 10 − Mass of Mg(OH) 2 present in sample (5cm^3 ) = 7.375 × 10 −3^ × 58.33 = 0.430g (430 mg) The amount stated on the label = 415 mg % error = (430− 415)/415 × 100 = 3.6% The error in the result will arise from the combined error in the analysis of the blank and the analysis of the sample. The labelled value itself will also be prone to error and the actual value may vary from batch to batch.
McCullagh The analysis of Milk of Magnesia by acid–base titration
The equilibrium between solid and dissolved magne- sium hydroxide is brought to the right-hand side by the reaction of hydroxide ions with acid. The use of Milk of Magnesia to relieve indigestion and constipation provides opportunities to discuss the human digestive system, particularly the presence of hydrochlo- ric acid in the stomach and the role of osmosis in the large intestine. Milk of Magnesia is an example of a type of ‘hyperosmotic laxative’ and works by drawing water from nearby tissue by osmosis into the large intestine.
This softens and moistens the stool and therefore helps
increase bowel activity. You should expect to have a bowel movement within six hours of taking Milk of Magnesia. This study of a household pharmaceutical product not only showcases the chemistry behind its medicinal
properties and method of analysis, but also allows for a consideration of how its formulation must ensure that it looks, smells and sounds as attractive as possible. The class discussion could consider why peppermint oil is added, why the product is called ‘Milk of Magnesia’ and why it is packaged in an opaque rather than a transpar- ent bottle. As well as extending students’ knowledge and understanding of chemistry, this activity may help develop their appreciation of the role of marketing in the formulation and sale of pharmaceutical products.
Acknowledgements The author would like to thank Greg McCready for taking the photographs of the titrations and Ilva Prindule for technical assistance.
References Council for the Curriculum, Examinations and Assessment (CCEA) (2016) GCE Specification in Chemistry. Belfast: CCEA.
Garvin, W. and O’Rawe, D. (1993) Northern Ireland Scientists and Inventors. Belfast: Northern Ireland Education Support Unit.
John McCullagh is a Senior Lecturer in Science Education at Stranmillis University College Belfast
and a Fellow of the Royal Society of Chemistry. Email: j.mccullagh@stran.ac.uk
The analysis of Milk of Magnesia by acid–base titration McCullagh
@SphereScience email@spherescience.co.uk www.spherescience.co.uk
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