The key difference between phosphoric acid and citric acid is that phosphoric acid is a weak mineral acid, whereas citric acid is a weak organic acid.
Phosphoric acid and citric acid are weak acids. Therefore, these acids cannot dissociate completely into ions; they can only partially dissociate into the ions.
CONTENTS
1. Overview and Key Difference
2. What is Phosphoric Acid
3. What is Citric Acid
4. Similarities – Phosphoric Acid and Citric Acid
5. Phosphoric Acid vs Citric Acid in Tabular Form
6. Summary – Phosphoric Acid vs Citric Acid
What is Phosphoric Acid?
Phosphoric acid is a weak mineral acid having the chemical formula H3PO4. The IUPAC name of this compound is orthophosphoric acid, and we can identify it as a non-toxic acid. Moreover, it is an important phosphorous-containing compound from which the dihydrogen phosphate ion (H2PO4–) is derived. Therefore, ions in phosphoric acid are very important for plants because it is the major source of phosphorous.
Figure 01: Structure of Phosphoric Acid
The molar mass of phosphoric acid is 97.99 g/mol. There can be hydrated and anhydrous forms of this compound. Phosphoric acid appears as a white solid that is deliquescent and odorless. Moreover, phosphoric acid production has two pathways: wet process and thermal process. The wet process uses fluoroapetite (phosphate rock) for the production of this acid, along with concentrated sulfuric acid. The chemical reaction is as follows:
Ca5(PO4)3F + 5H2SO4 + 10H2O →3H3PO4 + 5CaSO4.2H2O + HF
In the thermal process, liquid phosphorous (P4) and air undergo a chemical reaction inside a furnace at 1800-3000 K. First, a machine sprays the phosphorus liquid into a furnace chamber, where the phosphorous burns in the air reacting with oxygen (O2). The product from this step reacts with water in a hydration tower to produce the acid.
P4(l)+ 5O2(g)→2P2O5(g)
P2O5(g)+ 3H2O(l)→2H3PO4(aq)
The most common and important application of phosphoric acid is the production of fertilizers containing phosphorus. Furthermore, there are three major forms of phosphate salts that are useful as fertilizers: triple phosphate, diammonium hydrogenphosphate, and monoammonium dihydrogenphosphate.
What is Citric Acid?
Citric acid is a weak organic acid that occurs naturally in citrus fruits. Since there are many applications of this compound, manufacturers tend to produce a high amount of citric acid per year. Some of its important applications include use as an acidifier, as a flavouring, and chelating agent. We can observe this acid occur in two forms as the anhydrous form and the monohydrated form.
The anhydrous form of citric acid is the water-free form. It appears as a colourless substance and is odourless as well. There is no water in its dry, granulated form. We can produce this compound via crystallization from hot water.
Figure 02: Citric Acid in Solid Form
The anhydrous citric acid is formed from the monohydrate form at 78 °C. The density of the anhydrous form is 1.665 g/cm3. It melts at 156 °C, and the boiling point of this compound is 310 °C. The chemical formula of this compound is C6H8O7 , while the molar mass is 192.12 g/mol.
Monohydrate citric acid is the water-containing form of citric acid. It has one water molecule associated with one citric acid molecule. We call this water the water of crystallization. This form of citric acid is formed via crystallization from cold water.
What are the Similarities Between Phosphoric Acid and Citric Acid?
- Phosphoric acid and citric acid are weak acids.
- Both acids cannot dissociate completely; they dissociate partially into ions
- They are non-toxic acids.
What is the Difference Between Phosphoric Acid and Citric Acid?
Phosphoric acid and citric acid are weak acids. Therefore, these acids cannot dissociate completely into ions; they can only partially dissociate into the ions. The key difference between phosphoric acid and citric acid is that phosphoric acid is a weal mineral acid, whereas citric acid is a weak organic acid. Moreover, phosphoric acid appears as a white solid that is deliquescent, while citric acid appears as colorless solid/granules or in liquid form.
The following infographic lists the differences between phosphoric acid and citric acid in tabular form.
Summary – Phosphoric Acid vs Citric Acid
Phosphoric acid is a weak mineral acid having the chemical formula H3PO4, while citric acid is a weak organic acid, and it occurs naturally in citrus fruits. The key difference between phosphoric acid and citric acid is that phosphoric acid is a weal mineral acid, whereas citric acid is a weak organic acid.
Reference:
1. “Citric Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.
Image Courtesy:
1. “Phosphoric-acid-3D-balls” (Public Domain) via Commons Wikimedia
2. “Zitronensäure Kristallzucht” By TipFox – Own work (CC BY-SA 4.0) via Commons Wikimedia