Ammonium phosphate is a class of inorganic salts derived from the reaction between ammonia and phosphoric acid. These compounds are used in various industrial processes due to their high content of nitrogen and phosphorus, two macronutrients vital for life. The specific chemical formula for the neutral form, also known as triammonium phosphate, is \((\text{NH}_4)_3\text{PO}_4\). It is a highly soluble crystalline substance, making it easy to incorporate into aqueous solutions for various applications.
Ionic Components and Formula Derivation
The formation of ammonium phosphate involves the combination of two polyatomic ions: the ammonium cation and the phosphate anion, which are attracted to each other through strong ionic bonds. The ammonium ion is written as \(\text{NH}_4^+\) and carries a positive charge of \(+1\). The phosphate ion is written as \(\text{PO}_4^{3-}\) and carries a negative charge of \(-3\).
For an ionic compound to be electrically neutral, the total positive charge from the cations must cancel out the total negative charge from the anions. This principle, known as charge neutrality, dictates the ratio in which the ions must combine. Since the phosphate ion has a \(-3\) charge, it requires a total of three positive charges to achieve a neutral compound.
Each ammonium ion contributes a single \(+1\) charge. Therefore, exactly three ammonium ions (\(\text{NH}_4^+\)) are necessary to balance the \(-3\) charge of one phosphate ion (\(\text{PO}_4^{3-}\)). When writing the resulting chemical formula, the ammonium ion is enclosed in parentheses with a subscript of three outside to indicate that three of these polyatomic ions are present. This results in the formula \((\text{NH}_4)_3\text{PO}_4\) for the neutral salt.
The resulting compound is an ammonium salt of orthophosphoric acid, which can be prepared by treating phosphoric acid with an ammonia solution. This particular triammonium phosphate salt is comparatively unstable at room temperature. It readily evolves ammonia gas, which leads to the formation of other, more stable ammonium phosphate species.
Key Physical and Chemical Properties
Ammonium phosphate is a white crystalline solid that is often odorless. It has high solubility in water, with about 58.0 grams dissolving in 100 milliliters of water at \(25\) degrees Celsius.
When dissolved in water, ammonium phosphate creates a slightly basic solution. This occurs because the ammonium ion acts as a weak acid while the phosphate ion acts as a moderately strong base, causing hydrolysis. The phosphate ion reacts with water to produce hydroxide ions, which slightly increases the solution’s \(\text{pH}\) above \(7\).
The compound is thermally unstable, particularly the triammonium form. When heated, triammonium phosphate does not melt but instead undergoes a multi-stage thermal decomposition. The process begins with the loss of ammonia gas (\(\text{NH}_3\)), leading to the formation of diammonium phosphate (\((\text{NH}_4)_2\text{HPO}_4\)) and then monoammonium phosphate (\(\text{NH}_4\text{H}_2\text{PO}_4\)). At higher temperatures, this decomposition continues, ultimately resulting in the formation of phosphoric acid and various condensed polyphosphoric acids.
Essential Applications of Ammonium Phosphate
The application of ammonium phosphate compounds is in the agricultural industry as a high-concentration fertilizer. These salts simultaneously supply plants with two macronutrients: nitrogen, provided by the ammonium ion, and phosphorus, provided by the phosphate ion. The high water solubility of these compounds allows the nutrients to be readily available for plant uptake immediately after application.
Ammonium phosphates are also used as flame retardants in various industrial settings. When exposed to the heat of a fire, the compound decomposes, resulting in the release of non-flammable ammonia gas and leaving behind a residue of phosphoric acid. This acidic residue reacts with the material, such as cellulose in wood or fabric, promoting the formation of a carbon char barrier that prevents oxygen from reaching the fuel source. This mechanism makes ammonium phosphate a component in many dry-powder fire extinguishers used for Class \(\text{A}\), \(\text{B}\), and \(\text{C}\) fires.
In the food industry, ammonium phosphates serve multiple functions, including use as a leavening agent or a \(\text{pH}\) buffer. These compounds help control the acidity in food products, which is important for flavor, preservation, and chemical reactions during cooking. They are also used in fermentation processes for dairy cultures and dough improvers.
The commercially dominant forms of ammonium phosphate are not the neutral triammonium salt \((\text{NH}_4)_3\text{PO}_4\), but rather monoammonium phosphate (\(\text{NH}_4\text{H}_2\text{PO}_4\)), known as \(\text{MAP}\), and diammonium phosphate (\((\text{NH}_4)_2\text{HPO}_4\)), known as \(\text{DAP}\). \(\text{MAP}\) and \(\text{DAP}\) are more stable and are produced by controlling the ratio of ammonia to phosphoric acid during their manufacture. \(\text{MAP}\) is often preferred as a “starter” fertilizer to reduce the risk of ammonia toxicity to germinating seeds, while \(\text{DAP}\) is one of the world’s most commonly traded phosphorus fertilizers.