- Chemical structure
- Properties
- Physical appearance
- Melting point
- Boiling point
- Water solubility
- Density
- Stability
- pH
- Synthesis
- Applications
- Foods
- Detergents
- Medicinal
- References
The sodium phosphate is an inorganic salt having the chemical formula Na 3 PO 4. It is also called trisodium phosphate (TSP), or tribasic sodium phosphate, because it completely lacks the ability to donate hydrogen H + ions.
When it is referred to in a plural way, sodium phosphates, it refers to the entire compendium of salts, hydrated or not, originating from the electrostatic interactions between Na + and PO 4 3-. Some of these salts even consist of sodium pyrophosphates, or inorganic polymers that have PO 4 3- as a monomer.
Watch glass with sodium phosphate crystals. Source: Ondřej Mangl
That is why, unless a specification is made, it will always refer to Na 3 PO 4, and not to NaH 2 PO4, Na 2 HPO 4, Na 4 P 2 O 7 or any other salt.
The image above shows the appearance of sodium phosphate, which sports white crystals similar to common or table salt, or is purchased as an amorphous-looking white powder. Na 3 PO 4 is capable of forming various hydrates, being Na 3 PO 4 · 12H 2 O the one that is most commercialized.
It finds use mainly in the food industry, in detergents, sports supplements, and serves as a hyperosmotic laxative.
Chemical structure
Main ions of sodium phosphate. Source: Smolamj via Wikipedia.
The same Na 3 PO 4 formula indicates that for each PO 4 3- anion there are three Na + cations. These ions can be seen in the image above. Note that PO 4 3- has tetrahedral geometry, with its oxygen atoms carrying a resonant negative charge, which attracts Na + from the environment.
Likewise, the Na 3 PO 4 formula indicates that it is the anhydrous salt, so there are no water molecules included in its crystals. The electrostatic attractions between Na + and PO 4 3- end up defining a white trigonal crystal. Its ionic bond is such that Na 3 PO 4 has a melting point of 1583 ºC, considerably higher than that of several salts.
Both Na + and PO 4 3- are ions that can be highly hydrated, without negatively affecting the stability of the crystals.
It is for this reason that hydrates can also crystallize; however, its melting points are lower than those of the anhydrous salt, since the intermolecular hydrogen bonds in water separate the ions and weaken their interactions. In Na 3 PO 4 · 12H 2 O, for example, its melting point is 73.4 ºC, much lower than that of Na 3 PO 4.
Properties
Physical appearance
It can appear as defined white crystals or as a powder or granules of the same color. All the physical quantities expressed below refer only to the anhydrous Na 3 PO 4 salt.
Melting point
1583 ºC
Boiling point
100 ºC
Water solubility
14.5 g / 100 mL at 25 ºC, which is why it is considered to be quite soluble in water. However, it is insoluble in organic solvents such as ethanol and carbon disulfide.
Density
2.536 g / cm 3 at 17.5 ° C.
Stability
It is a hygroscopic solid. Therefore, it must be stored in such a way that it cannot be exposed to moisture.
pH
It is a strongly alkaline salt. For example, a 0.1% aqueous solution thereof already has pH 11.5. This property serves to be used as an additive in detergents.
Synthesis
How is sodium phosphate produced or synthesized? To do this, we start with phosphoric acid, H 3 PO 4, or the sodium diacid phosphate salt, NaH 2 PO 4.
In the vast majority of syntheses, H 3 PO 4 is preferred. Thus, H 3 PO 4 is reacted with a base that neutralizes its three OH groups, taking into account that this acid can also be written as PO (OH) 3.
The preferred base is sodium hydroxide, NaOH (in Germany), but it is also feasible (and cheaper) to use sodium carbonate or soda ash, Na 2 CO 3 (in the United States). The equations for both neutralization reactions are:
Na 2 CO 3 + H 3 PO 4 → Na 2 HPO 4 + CO 2 + H 2 O
Na 2 HPO 4 + NaOH → Na 3 PO 4 + H 2 O
Note that when using Na 2 CO 3, Na 3 PO 4 is not obtained, but Na 2 HPO 4, which will require a subsequent neutralization. As the reaction medium is aqueous, Na 3 PO 4 · 12H 2 O crystallizes, so it must be calcined to remove its water content and thus give rise to the anhydrous salt.
Na 3 PO 4 · 12H 2 O is characterized by "fusing" its crystals into a compact mass that is difficult to break. The smaller the size of your crystals, the faster they will be calcined.
Applications
Foods
Sodium phosphate is used as an effervescent agent to expand cinnamon rolls or cakes. Source: Pixabay.
In the uses of sodium phosphate, it is inevitable to mix Na 3 PO 4 or Na 3 PO 4 · 12H 2 O, since the latter is the hydrate that is most commercialized. For example, either of the two contributes harmless ions (in theory) to the foods to which they are added, enriching them with phosphates and increasing their salinity at the same time.
In the salt of meats, Na 3 PO 4 · 12H 2 O is included to keep them fresh for longer during storage. Likewise, it is added as an emulsifying agent so that the oil-water mixture does not “cut”, but rather remains united; and as a pH regulator, as it helps reduce the acidity of foods (including cereals).
In addition, it is used to thicken some tomato sauces, and for baking cakes or breads (top image). The latter is due to the fact that Na 3 PO 4 · 12H 2 O tends to release water vapor, which increases the size of the pores in the flour dough and its volume.
Detergents
Sodium phosphate is basic enough to saponify some fats or oils with which it comes in contact. Its alkalinity is then used to improve the action of detergents, as long as the surface to be degreased is not metallic, as they can deteriorate it.
However, the production of phosphate-enriched detergents has been reduced due to the eutrophication of lakes and rivers and, with it, the proliferation of algae that coat their surfaces.
It has also been used to make a disinfectant: Na 3 PO 4 · 1/4 NaOCl · 11H 2 O, which is prepared using sodium hypochlorite, NaOCl, instead of NaOH or Na 2 CO 3 to neutralize H 3 PO 4.
Medicinal
Na 3 PO 4 induces defecation by acting as a hyperosmotic laxative, necessary to cleanse the colon before the patient undergoes a colonoscopy. On the other hand, it has also been used (added) in energy supplements.
References
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- National Center for Biotechnology Information. (2019). Trisodium phosphate. PubChem Database. CID = 24243. Recovered from: pubchem.ncbi.nlm.nih.gov
- Wikipedia. (2019). Trisodium phosphate. Recovered from: en.wikipedia.org
- Curran MP & Plosker GL (2004). Oral sodium phosphate solution: a review of its use as a colorectal cleanser. DOI: 10.2165 / 00003495-200464150-00009
- Corey Whelan. (December 22, 2017). Sodium Phosphate. Recovered from: healthline.com
- Jacobs, Joel B. Taborosi, Steve. (2019). Process for the production of trisodium phosphate. Recovered from: freepatentsonline.com
- MaryAnn Marks. (2018). Why Is Trisodium Phosphate (TSP) Added To Our Food? Recovered from: theingredientguru.com