POTASSIUM SULFATE (K2SO4): STRUCTURE, PROPERTIES, USES, SYNTHESIS - CHEMISTRY - 2025

The potassium sulfate is an inorganic salt white or colorless with the chemical formula K ₂ SO ₄. It was already known in the fourteenth century, being called in the seventeenth century salt duplicatum, since it is a combination of an acidic salt and an alkaline salt.

Potassium sulfate is found in mineral form in arcanite, but its presentation is more common in the so-called Stassfurt salts. These are co-crystallizations of potassium, magnesium, calcium and sodium sulfates, observable in minerals such as leonite and polyhalite.

Structural formula of potassium sulfate. Source: Kemikungen

Potassium sulfate is a low-toxic salt and only causes irritation on contact with the eyes, the respiratory tract, or the digestive tract. There is no evidence of a carcinogenic or mutagenic action.

Potassium sulfate is used as a fertilizer, especially in crops that are susceptible to chlorides; such is the case of tobacco and potatoes. The compound provides potassium, one of the three main nutrients in plants, and sulfur, which is present in their proteins.

Structure

Crystalline phases

Tangled crystalline structure of potassium sulfate. Source: Ra'ike (Wikipedia)

In the first image the structural formula of potassium sulfate was shown. For each SO ₄ ^2- anion, of tetrahedral geometry, there are two K ⁺ cations, which can be represented by purple spheres (upper image).

Thus above we have the orthorhombic crystal structure of K ₂ SO ₄, with the SO ₄ ² anions represented by yellow and red spheres; while the K ⁺ cations, already mentioned, are the purple spheres (a bit robust).

This representation can cause confusion if you think that the bars actually correspond to coordination links. Rather, they indicate which ion interacts directly or closely with another around it. That is why each oxygen is "connected" with five K ⁺ (O ₃ SO ^2- - K ⁺), and these in turn with ten oxygens from other surrounding sulfate anions.

There is then a somewhat "rare" coordination sphere for potassium in K ₂ SO ₄:

Coordination sphere for potassium ions in its sulfate salt. Source: Smokefoot

This crystalline structure corresponds to the polymorph β-K ₂ SO ₄. When heated to 583 ºC, a transition occurs to the α-K ₂ SO ₄ phase, which is hexagonal.

Tangle of ions

Certainly the structure of K ₂ SO ₄ is unusually complicated for an inorganic salt. Its ions are positioned forming a kind of meaningless tangle and, at first glance, lacking periodicity.

Those who are dedicated to crystallography can approach this tangle in a more appropriate and descriptive way, observing it from the three spatial axes.

Note that its tangled structure could explain why K ₂ SO ₄ does not form hydrates: H ₂ O molecules have no way to penetrate the crystal lattice to hydrate potassium cations.

Also, with so many interactions encompassed in the tangle of ions, it is to be expected that this crystal lattice will have quite a stability against heat; and in fact this is the case, since the melting point of K ₂ SO ₄ is 1,069 ºC, which shows that its ions are strongly cohesive.

Physical and chemical properties

Potassium sulfate appearance

Names

-Potassium sulfate

-Sulfate of potash

-Arcanite

-Sulfur potassium

Molar mass

174.259 g / mol

Physical appearance

White, odorless, bitter-tasting crystalline, granule or powder solid.

Density

2.66 g / cm ³

Melting point

1,069 ºC

Boiling point

1,689 ºC

Water solubility

111 g / L at 20 ºC

120 g / L at 25 ºC

240 g / L at 100 ºC

The solubility in water decreases due to the presence of potassium chloride, KCl or ammonium sulfate, (NH ₄) ₂ SO ₄, due to the effect of the common ion.

Solubility in organic solvents

Slightly soluble in glycerol, but insoluble in acetone and carbon sulfide.

Refractive index (nD)

1,495

Reactivity

Potassium sulfate can react with sulfuric acid, acidifying to form potassium bisulfate (KHSO ₄). It can be reduced at high temperatures to potassium sulfide (K ₂ S).

Synthesis

First method

Potassium sulfate is synthesized by reacting potassium chloride with sulfuric acid. The synthesis of potassium sulfate occurs in two steps. The first step involves the formation of potassium bisulfate.

This is an exothermic reaction as it releases heat and therefore does not require an external heat supply. The reaction is carried out at room temperature.

KCl + H ₂ SO ₄ => HCl + KHSO ₄

The second step of the reaction is endothermic, that is, it requires the supply of heat for it to occur.

KCl + KHSO ₄ => HCl + K ₂ SO ₄

Second method

Potassium sulfate can be synthesized by the neutralization reaction of sulfuric acid with a base, potassium hydroxide:

H ₂ SO ₄ + 2 KOH => K ₂ SO ₄ + 2 H ₂ O

Third method

Potassium sulfate is produced by the reaction of sulfur dioxide, oxygen, potassium chloride, and water.

Fourth method

Potassium sulfate is produced by extracting the potassium sulfate present in a brine from the Loop Nur basin, China. The potassium sulfate is separated from the insoluble components of the brine by adding the acidifying compound sodium tripolyphosphate / urea phosphate.

This compound increases the difference between the solubility of potassium sulfate and the solubility of the other less soluble compounds, achieving, according to the creators of the method, a 100% pure potassium sulfate. In a strict sense it is not a synthesis method, but it is a novel extraction method.

Applications

Fertilizer

Potassium sulfate is used in tobacco crops. Source: Pxhere.

The use of potassium sulfate as a fertilizer is its main application. 90% of its total production is used for this purpose. Its use is preferred to that of potassium chloride in those crops that are sensitive to the presence of chloride in the soil; for example that of tobacco.

Potassium sulfate has a potassium content of 40-44%, while its sulfur concentration represents 17-18% of the compound. Potassium is necessary to perform many essential functions for plants, since it activates enzymatic reactions, protein synthesis, starch formation, etc.

In addition, potassium is involved in regulating the flow of water in the leaves. Sulfur is necessary for protein synthesis, since it is present in amino acids that possess it; such is the case of methionine, cysteine ​​and cystine, and it is also involved in enzymatic reactions.

Even potassium sulfate is used by spraying on the leaves in potassium sulfate particles smaller than 0.015 mm.

Industrial use and as raw material

Crude potassium sulfate is used in the manufacture of glass and in the manufacture of alum and potassium carbonate. It is used as a reagent in the manufacture of cosmetics. It is used in the manufacture of beer as a water corrective agent.

Medicine

It is used to correct a severe decrease in plasma potassium concentration (hypokalemia), caused by excessive use of diuretics that increase urinary potassium excretion.

Potassium is the main intracellular ion of excitable cells, including heart cells. Therefore, a severe decrease in potassium in the plasma compromises cardiac function, and must be corrected immediately.

Potassium sulfate has a cathartic action, that is, it favors the expulsion of feces from the colon. For this reason, a mixture of potassium, magnesium and sodium sulfates is used to clear the colon of stool before performing a colonoscopy, which allows a better visualization of the colon by the doctor.

Veterinary

Potassium sulfate has been used to decrease the requirement of methionine in the feed of poultry. The presence of 0.1% of potassium sulfate in the feed of laying hens is associated with a 5% increase in egg production.

Food flavoring

It is a flavoring agent that gives foods a bitter and salty taste that is desirable in some of them. In addition, it is noted that potassium sulfate is endowed with the four basic flavors: sweetness, bitterness, acidity, and salinity.

Salinity, acidity, and bitterness increase with the concentration of potassium sulfate, while sweetness decreases.

Other uses

Potassium sulfate is used as a pyrotechnic, in combination with potassium nitrate, to generate a purple flame. It is used as a flash reducer in the charges of artillery thrusters.

In addition, it is used as a viscosity-increasing agent in cosmetic products such as face creams.

Risks

Potassium sulfate is a low toxic compound with a very low lethality. The LD50 for the oral dose in mice is 6,600 mg / kg of animal weight, which indicates that a high dose is required to cause the death of the mouse. The same LD50 value occurs in rats.

In the eyes, on contact, potassium sulfate can cause mechanical irritation. On the skin, potassium sulfate causes little damage to industrial handling.

If swallowed, potassium sulfate can cause gastrointestinal irritation with nausea, vomiting, and diarrhea. And lastly, inhaling the potassium sulfate dust causes irritation to the respiratory tract.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2019). Potassium sulfate. Recovered from: en.wikipedia.org
3. National Center for Biotechnology Information. (2019). Potassium sulfate. PubChem Database. CID = 24507. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Brian Clegg. (July 5, 2018). Potassium sulfate. Royal Society of Chemistry. Recovered from: chemistryworld.com
5. Marie T. Averbuch-Pouchot, A. Durif. (nineteen ninety six). Topics in Phosphate Chemistry. World Scientific. Recovered from: books.google.co.ve
6. Chemical Book. (2017). Potassium sulfate. Recovered from: chemicalbook.com
7. Shoujiang L. et al. (2019). Purification and rapid dissolution of potassium sulfate in aqueous solutions. DOI: 10.1039 / C8RA08284G
8. DrugBank. (2019). Potassium sulfate. Recovered from: drugbank.ca
9. The Mosaic Company. (2019). Potassium sulfate. Crop Nutrition. Recovered from: cropnutrition.com
10. Drugs. (2018). Sodium sulfate, potassium sulfate, and magnesium sulfate (Oral). Recovered from: drugs.com