If you have been looking for it, here’s a detailed comparison of sulfate vs sulfite. Mistaking one for the other is quite easy because they contain sulfur and oxygen atoms and an anionic charge of -2. However, there are so many differences between both polyatomic anions.
Also, there are some similarities, and this article fully explains all these. While each compound exists independently, they share a relationship. There is so much to know about these two compounds – their preparation, properties, and application in everyday life.
Properties of sulfates
- Sulfates are compounds of sulfur that contain the sulfate ion, SO42-. They have a molar mass of 96.06 g/mol
- They are products of the neutralization of sulfuric acid (H2SO4) with a base
- These ions have a boiling point of 1,155°F (623.89°C) and a melting point of 518.8°F (270.47°C)
- Sulfates are the conjugate base of hydrogensulfate (HSO4–)
- Sulfates are readily soluble in water except for calcium sulfate, lead(II) sulfate, and barium sulfate. They also tend to form white precipitates in water
- The sulfate ion behaves like a ligand. It easily links with metals using one or two of its oxygen atoms
Properties of sulfites
- Sulfites are compounds that contain the sulfite ion (SO32-) produced from a reaction of sulfur dioxide (SO2) and a base
- The sulfite ion is a conjugate base of bisulfite (HSO3–)
- It has a molar mass of 80.07 g/mol
- Most sulfite salts are stable and insoluble in water. There are a few soluble like the sulfites of sodium and potassium
Sulfate vs sulfite: Similarities
- Both polyatomic ions are anions
- They also both have sulfur and oxygen atoms with a negative -2 charge
- The oxidation state of all oxygen atoms in both anions is -2
- Sulfates and sulfites show resonance in their chemical structures
- The sulfur in both anions can undergo reduction reactions
Sulfate vs sulfite: Differences
Physical & chemical properties
While both anions contain sulfur and oxygen atoms, sulfates have one oxygen more than sulfites. They both have one sulfur atom. The sulfate ion has two double-bonded oxygen atoms while sulfites have one double-bonded oxygen atom.
Also, sulfates have specific boiling and melting points, but sulfites do not.
Lewis structure
According to the Lewis structure, there are two S=O bonds and two S-O bonds. The sulfur atom has 6 valence electrons, the four oxygen atoms have a total of 24, and there are two additional electrons that it accepts to attain an octet. This gives it a total of 36 valence electrons.
Each oxygen atom has a charge of -1, the central sulfur atom has a +2 charge, and the sum of all charges gives the anion its overall charge of -2.
For sulfite, its Lewis structure shows one S=O bond and two S-O bonds. The total number of valence electrons in the sulfite ion is 26. They are from the 6 valence electrons of the central sulfur atom, 18 electrons from the three oxygen atoms, and the two electrons it gained (the charge).
Two oxygen atoms have a -1 charge, the central sulfur has no charge, and the anion has an overall charge of -2.
Hybridization & molecular geometry
Sulfate ion has a tetrahedral molecular and electron pair geometry, according to the VSEPR model theory. The central sulfur atom shares four bonds with four oxygen atoms, which means there are four electron regions around it. To accommodate these electrons, it must form four hybrid orbitals.
It forms one s-hybrid orbital and three p-hybrid orbitals, resulting in an sp3 hybridization. The s-orbital accommodates two electrons, and the p-orbital accommodates 6 electrons. The bond angle of this molecule is 109.5° due to its shape.
Following the prediction of the VSEPR model theory, the shape and molecular geometry of the sulfite ion is a trigonal pyramid. But, it has a tetrahedral electron pair geometry.
The difference between the molecular and electron pair geometry is a result of the lone pair-bond pair electronic repulsion that exists within the molecule. The central sulfur atom has a lone pair of electrons that causes this repulsion and alters its molecular geometry.
Furthermore, the sulfite ion would have a 109.5° bond angle, typical of tetrahedral molecules, but the lone pair on the sulfur atom changes its shape to a trigonal pyramid, decreasing its O-S-O bond angles to 106°.
Additionally, sulfate ions are nonpolar molecules. But, they dissolve well in polar solvents. Their nonpolar nature is due to the absence of dipole moments. Although the S-O bonds are polar, the symmetric arrangement of the oxygen atoms nullifies the individual dipole moments.
On the other hand, the sulfite ion is a polar molecule. Each S-O bond has an electronegativity difference of 0.86 unit which makes them polar.
The asymmetric trigonal pyramidal shape of the sulfite ion increases its polarity because the individual dipole moments do not cancel out each other, unlike the sulfate ion.
Chemical reactions
Sulfites undergo oxidation reactions to form sulfates. They react with strong oxidizing or reducing agents such as hydrides or alkali metals. On the other hand, sulfates cannot undergo oxidation reactions.
Test
To test for the presence of sulfate ions, add drops of HCl to the unknown solution and follow up with barium chloride. A formation of white precipitates of BaSO4 signifies the presence of sulfate ions.
You can test for the presence of sulfite ions by carrying out a solubility test. Sulfite salts, except for sodium, potassium, and ammonium are usually insoluble.
Add HCl to an unknown potassium solution. The formation of a sulfite salt and the evolution of sulfur dioxide gas signifies the presence of sulfite ions.
Test the gas further with potassium dichromate paper. Sulfite ions will not make the paper turn green. Also, the iodine test. This test specifically determines the presence of sulfite ions as the acid radical of a salt. The sulfite salt reduces the iodine solution to hydrogen iodide.
Health effects and hazards
Sulfites are not environmental pollutants. They have adverse effects on human health. They usually trigger allergic reactions in people who are sensitive to them.
These allergies manifest with symptoms like wheezing in people with asthma, hay fever, and hives. It could also cause stomach pain, and chronic skin and respiratory conditions.
Sulfates are produced from fossil fuel and biomass combustion. A buildup of these molecules in the air leads to an increase in the acidity of the atmosphere and the formation of acid rain.
Products made with sulfates cause dryness of the skin, leave the hair with a negative charge, and ruin hair cuticles.
Applications: Sulfate vs sulfite
Sulfates are used as additives in detergents, fertilizers, and some medications. They are also used in water treatment, the production of algaecides, and the manufacture of some skincare products.
Sulfites are commonly used as preservatives on food and beverages, and some medications. They are naturally occurring compounds in some wines and fruits.
Examples
Common examples of sulfate salts are sodium sulfate, magnesium sulfate, copper sulfate, calcium sulfate, sodium lauryl sulfate, lead sulfate, iron(II) sulfate, and gypsum.
Examples of sulfite salts are potassium sulfite, sodium sulfite, calcium sulfite, ammonium sulfite, copper(II) sulfite, and barium sulfite.
Can sulfite be converted to sulfate?
Exposure to air converts sulfite to sulfate. This is a reverse reaction of the reduction of sulfates to sulfites.
SO32- + O2 ———> SO42-
FAQs
What is a sulfite sulfate?
A sulfite sulfate is a rare compound of sulfur that contains both sulfate and sulfite anions. In this compound, sulfur has +4 and +6 oxidation states.
Do wines also contain sulfates?
No, wine producers do not use sulfates, unlike sulfites that are naturally present in wines as preservatives to stop bacterial growth.
However, some winemakers use brewer’s gypsum, calcium sulfate, to fix mineral deficiencies in the water.
Is sulfide a sulfite?
A sulfide is an inorganic anion of sulfur, not a sulfite. It is formed by the sulfur atom’s reduction (acceptance of electrons). Sulphide is represented by the chemical formula, S-2 and the sulfur atom has an oxidation number of -2.
Conclusion
Sulfate and sulfite are often used interchangeably but they are two distinct sulfur-based compounds with some similarities and a lot of differences. The oxidation of sulfur forms sulfates, while sulfites are products of reduction of sulfates. This is the reason for the lower oxidation state of sulfites.
Also, sulfates are more abundant in nature and find more applications in biological processes. A comparison of sulfate vs sulfite is important and ensures proper usage in science laboratories and industries.
Also, learn about the differences between chloroplasts and chlorophyll.
Thanks for reading.