Calcium compounds, particularly calcium carbonate, react with citric acid to dissolve, producing carbon dioxide, water, and calcium citrate.
The Reaction Explained
Citric acid is a weak organic acid. When it reacts with calcium carbonate (CaCO3), a common form of calcium, the following occurs:
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Dissolution: The citric acid donates protons (H+) to the carbonate ions (CO3^2-) from the calcium carbonate.
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Carbon Dioxide Formation: This protonation of carbonate ions leads to the formation of carbonic acid (H2CO3), which is unstable and decomposes into carbon dioxide (CO2) gas and water (H2O). This is the bubbling you might observe.
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Calcium Citrate Formation: The calcium ions (Ca^2+) from the calcium carbonate then combine with the citrate ions from the citric acid to form calcium citrate. Calcium citrate is generally more soluble than calcium carbonate, thus the calcium carbonate "dissolves."
The overall (simplified) reaction can be represented as:
CaCO3 (s) + 2 C6H8O7 (aq) -> Ca(C6H5O7)2 (aq) + H2O (l) + CO2 (g)
Where:
- CaCO3 is Calcium Carbonate (s=solid)
- C6H8O7 is Citric Acid (aq=aqueous/dissolved in water)
- Ca(C6H5O7)2 is Calcium Citrate (aq=aqueous/dissolved in water)
- H2O is Water (l=liquid)
- CO2 is Carbon Dioxide (g=gas)
Factors Affecting the Reaction
- Concentration: Higher concentrations of citric acid will result in a faster reaction rate.
- Temperature: Increasing the temperature generally speeds up the reaction.
- Surface Area: Using finely powdered calcium carbonate will increase the surface area available for reaction, leading to a faster reaction rate compared to using larger chunks.
- Stirring/Mixing: Agitation helps to bring fresh reactants into contact, which can accelerate the dissolution process.
Examples and Applications
- Descaling: Citric acid is used as a descaler in household appliances (like kettles and coffee machines) to remove calcium carbonate deposits (limescale).
- Food Industry: Citric acid is used as a food additive to control pH and also can react with calcium salts present.
- Dietary Supplements: Calcium citrate is sometimes used as a calcium supplement as it is believed to be more easily absorbed than calcium carbonate, particularly for people with low stomach acid.
