H3PO4 (Phosphoric Acid) is a weak acid. 1 It only partially dissociates into its ions when dissolved in water, resulting in a lower concentration of hydrogen ions (H+) compared to strong acids. This means that phosphoric acid has a limited ability to release H+ ions in solution and exhibits weaker acidic behavior.
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Key Takeaways: Is H3PO4 a Strong or Weak Acid?
- H3PO4 (phosphoric acid) is a weak acid, partially dissociating into ions when dissolved in water, resulting in a lower concentration of hydrogen ions (H+) compared to strong acids.
- The weak acidic behavior of H3PO4 is due to its partial ionization, with the equilibrium position favoring reactants (H3PO4) rather than products (H+ and (H2PO4)-).
- In contrast to strong acids, which completely dissociate in solution, H3PO4 only releases a few H+ ions, resulting in a higher pH value (less acidic).
Why is H3PO4 a weak acid?
Phosphoric acid, or H3PO4, is classified as a weak acid due to its limited dissociation in water, resulting in the release of only a small number of hydrogen ions (H+) compared to strong acids.
Here’s why H3PO4 is considered a weak acid:
- Partial dissociation: When H3PO4 dissolves in water, it undergoes partial dissociation into its ions, releasing some H+ ions (protons). The chemical equation for its dissociation is as follows: 2
H3PO4(aq) ⇌ H+(aq) + H2PO4-(aq)
- Limited ionization: In contrast to strong acids, which completely dissociate into ions in solution, weak acids only partially ionize. 3 This means that in the case of H3PO4, not all molecules break apart into H+ and (H2PO4)- ions; a significant portion remains as undissociated H3PO4 molecules.
- Equilibrium favoring reactants: The dissociation reaction is reversible, meaning it can proceed in both forward and reverse directions. In the case of weak acids, the equilibrium position lies more towards the reactants (H3PO4) than the products (H+ and (H2PO4)-). 4 This is why weak acids have lower concentrations of H+ ions compared to strong acids in the same concentration.
- pH value: The pH scale is used to measure the acidity or basicity of a solution. 5 Since H3PO4 only partially dissociates, it produces a lower concentration of H+ ions, resulting in a pH value that is higher (less acidic) compared to strong acids with the same concentration.
It’s important to note that the strength of an acid depends on its ability to donate protons (H+ ions) and how readily it dissociates in water. In the case of H3PO4, it does have acidic properties but does not fully ionize in solution, making it a weak acid.
How does the dissociation of H3PO4 differ from that of a strong acid?
Here’s a comparison of the dissociation of H3PO4 (phosphoric acid) with that of a strong acid, such as HCl (hydrochloric acid).
| Aspect | H3PO4 (Phosphoric Acid) | HCl (Hydrochloric Acid) |
| Degree of dissociation | Partial dissociation 6 7 | Complete dissociation 8 |
| Ionization in water | H3PO4 + H2O ⇌ H2PO4- + H3O+ | HCl + H2O ⇌ H3O+ + Cl- |
| Hydrogen ion release | Releases only a few H+ ions | Releases a large number of H+ ions |
| pH level | pH value remains higher (less acidic) 9 | pH value becomes significantly lower (strongly acidic) 10 |
| Electrical conductivity | Less conductive in solution | Highly conductive in solution 11 |
| Strength | Weak acid | Strong acid |
In the case of phosphoric acid (H3PO4), only a fraction of the molecules dissociate in water, resulting in the formation of dihydrogen phosphate ions (H2PO4)- and hydronium ions (H3O)+. The remaining H3PO4 molecules remain intact.
On the other hand, hydrochloric acid (HCl), being a strong acid, undergoes complete dissociation when dissolved in water. 12 This means that nearly all HCl molecules donate their hydrogen ions (H+) to water, forming hydronium ions (H3O)+ and chloride ions (Cl-).
Due to the difference in the degree of dissociation, strong acids tend to have a much lower pH level (more acidic) compared to weak acids. Additionally, strong acids are excellent conductors of electricity in aqueous solutions, whereas weak acids have lower electrical conductivity.
Applications of H3PO4 based on its weak acidic nature
The weak acidic nature of H3PO4 (phosphoric acid) lends itself to various practical applications in different industries. Some of the key applications based on its properties as a weak acid include:
- Food and Beverage Industry: Phosphoric acid is commonly used as an additive in the food and beverage industry. 13 It is used as an acidulant and flavor enhancer in soft drinks, providing the tangy flavor in colas. Additionally, it acts as a pH regulator and preservative in various processed foods.
- Fertilizers: H3PO4 is a primary ingredient in many fertilizers. 14 It is a source of phosphorus, an essential nutrient for plant growth. The weak acidic nature of phosphoric acid ensures that the nutrients are released gradually, providing sustained nourishment to the plants.
- Rust Removal and Cleaning: Phosphoric acid is used as an ingredient in various cleaning agents and rust removers. Its weak acidic nature makes it effective in removing rust, mineral deposits, and scales from metal surfaces without causing excessive damage to the underlying material.
- Water Treatment: In water treatment, H3PO4 is used to adjust the pH of water and control alkalinity. It helps to prevent scaling and corrosion in water systems and serves as a buffer to maintain stable water conditions. 15
- pH Control in Chemical Processes: The weak acidic properties of phosphoric acid make it useful in chemical processes that require pH control. It is employed as a buffer to stabilize pH levels during reactions and to control acidity in various chemical reactions.
- Pharmaceuticals: Phosphoric acid is used as an excipient in pharmaceutical formulations, primarily as an acidifying agent in oral medications. 16 Its weak acidic nature allows for controlled and gentle acidification when necessary.
- Electroplating: In certain electroplating processes, H3PO4 is utilized as an electrolyte due to its weak acidic behavior, providing a controlled and stable plating environment.
- Beverage Acidifier: Beyond soft drinks, phosphoric acid is used as an acidifier in other beverages, such as fruit juices and flavored water, to enhance the taste and act as a preservative. 17
It’s important to note that while H3PO4 is widely used in various applications, proper handling and safety measures should be observed due to its acidic nature, regardless of its strength.
Further reading
Is Acetic Acid (CH3COOH) a Strong or Weak Acid?
Is H2CO3 (Carbonic Acid) a Strong or Weak Acid?
Is Formic Acid (HCOOH) a Strong or Weak Acid?
Is HCN a Strong or Weak Acid?
Is HBr a Strong or Weak Acid?
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References
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- H3PO4 + H2O (Phosphoric acid + Water). (2020, April 9). YouTube. https://www.youtube.com/watch?v=aJZKOfgXG5g
- Uiuc.edu http://www.chem.uiuc.edu/rogers/Text12/Tx124/tx124.html
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- pH – Wikipedia. (2020, July 15). pH – Wikipedia. https://en.wikipedia.org/wiki/PH
- Weak acid-base equilibria (article) | Khan Academy. (n.d.). Khan Academy. https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/copy-of-acid-base-equilibria/a/weak-acid-base-equilibria
- Utexas.edu https://ch302.cm.utexas.edu/chemEQ/ab-theory/selector.php?name=weak-acid-base
- Ufl.edu https://christou.chem.ufl.edu/wp-content/uploads/sites/62/2017/01/Chapter-18-Acids-and-Bases-Week-1.pdf
- Fsu.edu https://chem.fsu.edu/chemlab/chm1046course/kaconstant.html
- Purdue.edu https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/conjugat.php
- Elmhurst.edu http://chemistry.elmhurst.edu/vchembook/185strength.html
- RLO: Acids Alkalis and Bases: Further application. (n.d.). RLO: Acids Alkalis and Bases: Further Application. https://www.nottingham.ac.uk/nmp/sonet/rlos/science/acid_base_further_app/page_two.html
- Spainhour, C. (2014). Phosphoric Acid. Encyclopedia of Toxicology, 916–919. https://doi.org/10.1016/b978-0-12-386454-3.00904-0
- Johnston, A. E., Poulton, P. R., Fixen, P. E., & Curtin, D. (2014). Phosphorus. Advances in Agronomy, 177–228. https://doi.org/10.1016/b978-0-12-420225-2.00005-4
- Epa.gov https://www.epa.gov/system/files/documents/2023-03/Phosphoric%20Acid%20Supply%20Chain%20Profile.pdf
- Fontana, J. D., Tiboni, M., & SiebertKoop, H. (2017, July 12). Diluted Thermopressurized Phosphoric Acid: A Gentle Proton Donor for Polysaccharide Acid Depolymerization and (Bio)processing. | IntechOpen. https://doi.org/10.5772/intechopen.68236
- Gurtler, J., & Mai, T. (2014). PRESERVATIVES | Traditional Preservatives – Organic Acids. Encyclopedia of Food Microbiology, 119–130. https://doi.org/10.1016/b978-0-12-384730-0.00260-3
