Description: About H3PO4

Phosphoric acid is an inorganic acid that is commonplace in industry. At SATP or STP, this acid is, when pure, a crystalline solid (US Cancer Institute, 2016). The numerous applications of its salts (phosphates) makes it a very versatile chemical that is used in a variety of chemical reactions. Some of its applications include:

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Addition of Phosphate in Fertilizer

The formation of fertilizer relies heavily on the presence of phosphate, which is essential for plant development and growth. This nutrient is irreplaceable as it is integral in the conversion of other nutrients into usable building blocks (Rhoades, 2016). A common, cheap way to effectively increase the presence of such phosphate in fertilizing compounds is the creation of calcium dyhidrogenphosphate. This compound is produced through the mixing of phosphoric acid with phosphate rock in a rotating drum (University of York, 2013). The presence of phosphates in phosphoric acid is primarily the reason this compound is used in such a process.

Phosphoric acid is used extensively in the manufacturing and betterment of commercial NPK fertilizer. The phosphates present in H3PO4 are integral in plant development and, the use such phosphates, can expedite their growth to increase crop yield and health. A common form of fertilizer is calcium dihydrogenphosphate, as this form lacks the relatively high acidity and corrosiveness phosphoric acid is known for (Encyclopedia Britannica, 2014). Widely used throughout agricultural industry, this compound is created through the reaction of phosphoric acid with phosphate rock:

Ca3(PO4)2 + 4H3PO4 → 3Ca(H2PO4)2

*It is important to note that this reaction takes place as a side reaction during the wet process between sulfuric acid and fluorapatite, however, the use of phosphoric acid concentrates the amount of P2O5 present. This ultimately leads to a higher concentration of phosphates, creating a superphosphate.

Rust Removal/Conversion

Phosphoric acid can be applied to iron oxide (rust) to essentially convert the rust into a more easily removable compound, ferric phosphate. The following reversion of a corrosion reaction describes this interaction (National Science Foundation, 2015):

2 H3PO4 + Fe2O3 → 2 FePO4 + 3 H2O

As demonstrated above, water is a product of this chemical reaction. A common process of rust removal involves the submersion of rusted metal in a dilute, 85% solution of phosphoric acid. The phosphoric acid reacts with the ferric oxide until there is no more left. As the metal is submerged in water, the remaining byproduct (ferric phosphate) is easily removed from the metal. The hydrogen atoms present in H3PO4 are primarily responsible, as these bond with the oxygen in ferric oxide in the process of a double replacement reaction, to convert rust and form water (HowStuffWorks, 2000).

Phosphoric acid can be used as a rust removal asset and when applied directly onto iron, steel and other metal surfaces it can act as a rust convertor. This conversion process that takes place changes ferric oxide (rust) to ferric phosphate which is black in color. Once there is a new ferric phosphate layer it can be easily removed to reveal a fresh new layer of metal. (Khan, 2011). A dilute form of phosphoric acid is needed to achieve such a result, as a high enough concentration of phosphoric acid could dissolve the metal itself. Rust removal, through the use of phosphoric acid, plays to the substances corrosiveness, attributed to its acidic nature.

Left side: Metal Object After Submersion in Phosphoric Acid Solution

Right side: Metal Object Before Submersion in Phosphoric Acid

Uses in Medicine and Dentistry

Phosphoric acid is used as an ingredient in anti-nausea medicines. In the dentistry field phosphoric acid is combined with zinc powder in order to create a cement that can be used for the filling of a tooth. Also it can be used in an etching solution, this etching solution is used to clean and roughen the surface of teeth for the placement of brackets and other dental appliances. (Khan, 2011). Its ability to break down and clean teeth, stems primarily from the acids acidity (caused by the presence of H+ ions). Its safe application in the human body is also related to phosphoric acids weak acidity and easy dilution. In the dentistry field it has been discovered that phosphoric acid lowers bone density and in regards to organs that are affected by the acid it is suggested that phosphoric acid increases risk of kidney damage

Phosphoric Acid Additives in Soda

The addition of phosphoric acid is common in the manufacturing of soft drinks, largely due to the enhancement of the flavour that it provides. When added, the acid gives the drink a sharper taste, which appeals to consumer desire (Senese, 2010). Additionally, the phosphoric acid acts as a preservative as it is capable of slowing the growth of both mold and bacteria. A common characteristic of acidic substances (that is those with a pH below 7), is there actual taste. Acids, generally, are sour when in a watery solution, similar to the type of environment phosphoric acid would be in soda (Ophardt, 2003). This being said, the sharpness in taste that H3PO4 creates is attributed to its acidic nature.

CocaCola is known for its addition of Phosphoric Acid

Additional Notes on Characteristics Of Phosphoric Acid

Phosphoric acid, evidently, is diversely applicable in various applications pertaining to everyday life. The functions and roles that this substance is able to carry out are due to the properties this acid possesses. One of these is the chemicals low pH level. Due to the presence of H+ ions when dissociated in water, this acid is defined as an acid (Durland, 2010). The ions are responsible for the many interactions, such as its ability to conduct electricity and its removal of certain metal oxides. An acid, due to its presence of hydrogen ions, is also known for its sour taste. Phosphoric acid is, however, considered to be a weak acid as it does not fully dissociate in water (Chieh, NA). The not all H3PO4 molecules fully ionize, designating it as weak in nature. This property can be related to the acids use’s in diluted concentrations in consumer products, as it is relatively nonacidic. Phosphoric acid's presence of phosphates also allots its heavy usage in agricultural and biological-industrial applications such as food additives. This is due to the importance phosphorous plays in organism growth and cellular function, more specifically, in ATP.