Antacids contain active ingredients such as aluminium hydroxide Al(OH)3, magnesium hydroxide Mg(OH)2, calcium carbonate CaCO3, milk of magnesia and sodium hydrogen carbonate NaHCO3. Antacids have some side effects, for example magnesium compounds can cause diarrhoea. Sometimes, antacids are mixed with alginate, which produces a neutralising layer as it floats on top of the stomach contents. It prevents heartburn as the stomach acid is neutralised as it rises up to the oesophagus. Antacids are also mixed with anti-foaming agents to prevent flatulence.
An acid is a proton donor and a base is a proton acceptor. The substances used in the experiment have the following pH:
CaCO3 = pH8.8
NaHCO3 = pH8.5
MgCO3 = pH8.2
They are proton acceptors and have a fairly high pH each, which indicates strong bases. The strong acid dissociates completely while the bases do not.
The pH is measured by:
pH= log10 [H+]
which we can use to calculate the concentration of hydrogen ions. Hydrochloric acid is a strong acid of pH1 so most indicators can be used in this experiment. An indicator is usually a weak base or acid and is coloured, which will change when the pH of the solution it is added to changes.
HIn (aq) H+ (aq) + In- (aq)
This equation shows that, in an acid the equilibrium will shift towards the left so it will change colour to that of HIn.
Variables
Independent Variable
The independent variable will be the type of tablets, in this experiment, Rennie and Gaviscon tablets will be used
Dependant Variable
The volume of hydrochloric acid that is needed to neutralise the antacid will be the dependent variable in this experiment.
Prediction
The Rennie tablets contain the active ingredients calcium carbonate and magnesium carbonate. So when hydrochloric acid is added to the tablets, the following reaction occurs.
CaCO3 + 4 HCl + MgCO3 MgCl2 + CaCl2 + 2 H2O + 2 CO2
Molar Mass of MgCO3 = 84.32
Molar Mass of CaCO3 = 100.09
In 1g:
Amount of MgCO3 = 1.00
84.32 = 0.01186 mol
Amount of CaCO3 = 1.00
100.09 = 0.00999 mol
So, Amount of MgCO3 and CaCO3 = 0.01186 + 0.00999 = 0.02185 mol
So, Amount of hydrochloric acid neutralised by 1g of Rennie tablet
= 4 x 0.02185
= 0.08740 mol
The Gaviscon tablets contain the active ingredients sodium calcium carbonate and hydrogen carbonate. So when hydrochloric acid is added to the tablets, the following reaction occurs.
NaHCO3 + CaCO3 + 3 HCl NaCl + CaCl2 + 2 H2O + 2 CO2
Molar Mass of CaCO3 = 100.09
Molar Mass of NaHCO3 = 84.01
In 1g:
Amount of NaHCO3 = 1.00
84.01 = 0.01190 mol
Amount of CaCO3 = 1.00
100.09 = 0.00999 mol
So, Amount of NaHCO3 and CaCO3 = 0.01190 + 0.00999 = 0.02189 mol
So, Amount of hydrochloric acid neutralised by 1g of Gaviscon tablet
= 3 x 0.02189
= 0.06568 mol
From the amount of hydrochloric acid that is needed to neutralise 1g of Rennie and 1g of Gaviscon. It shows that Rennie is slightly more effective than the Gaviscon tablet.
Apparatus
- Pipette
- 150 ml beakers
- Funnel
- Distilled water
-
200cm3 of hydrochloric acid
- Stirring rod
- Electronic scales
- Pestle and mortar
- Packet of Gaviscon tablets
- Burette and clamp stand
- Packet of Rennie tablets
- Phenolpthalein indicator
Method
- Gather all the apparatus needed for the investigation
-
Using the funnel, carefully pour 100cm3 of hydrochloric acid into the burette
- Weigh out 1g of Gaviscon tablets and grind into powder using the pestle and mortar
-
Carefully transfer the powdered tablets into a beaker, ensuring all traces of powder has been removed and add 10cm3 of distilled water
- Add 7 drops of phenolphthalein indicator and place the beaker under the burette.
-
Remove the funnel and add 5cm3 of hydrochloric acid to the beaker and stir with a stirring rod
-
Continue adding hydrochloric acid in steps of 1cm3 or less until the solution turns colourless
- Record the volume of hydrochloric acid added. This is the volume that is needed to neutralise the Gaviscon tablet.
-
Refill the burette to 100cm3 and repeat this process with the Rennie tablet
- Repeat entire method at least three times
Safety Precautions
- As hydrochloric acid is a strong acid, extra precaution must be taken, lab coats must be worn
- Safety glasses must be worn during the entire experiment especially when adding acid to the burette
