Difference Between Sodium Bisulfite and Sodium Metabisulfite

The key difference between sodium bisulfite and sodium metabisulfite is that the sodium bisulfite has only one sulfur atom and three oxygens, and the bisulfite anion is monovalent whereas, the sodium metabisulfite has two sulfur atoms, five oxygens, and the anion is divalent.

Both sodium bisulfite and sodium metabisulfite are salts of sodium. We mainly use these chemicals as preservatives, disinfectants and food additives.

CONTENTS

1. Overview and Key Difference
2. What is Sodium Bisulfite
3. What is Sodium Metabisulfite
4. Side by Side Comparison – Sodium Bisulfite vs Sodium Metabisulfite in Tabular Form
5. Summary

What is Sodium Bisulfite?

Sodium bisulfite is the compound having the chemical formula NaHSO₃. It has a molar mass of 104 g mol^-1. Moreover, it exists as a white solid, and when it is in the liquid form, it acts as a corrosive liquid. The melting point of the solid is 150 ^oC. Further, the solid form of this compound is soluble in water.

Due to the ability to donate protons, it is slightly acidic. Moreover, the sulfur atom in this compound is in the +4 oxidation state. We can prepare sodium bisulfite by bubbling sulfur dioxide gas into carbonated water. It will release sulfur dioxide gas when reacting with mild acidic conditions. We can use this compound in organic chemistry for purification purposes. It forms a bisulfite adduct with an aldehyde, which is in solid form. Therefore, we can precipitate aldehyde from a solution by making it a bisulfite adduct and then can regenerate the aldehyde group by removing the bisulfite. Furthermore, sodium bisulfite is useful as a mild reducing agent, discolouration agent in organic synthesis. Above all, when sodium bisulfite reacts with oxygen gas, it converts into sodium bisulfate.

Among the several usages of this compound, a most notable one is using as a food additive. There, it plays a key role in wine production. Earlier, people used sodium bisulfite to fruit juice. Then, the sulfur dioxide gas releases from it, and it will help to sanitise the liquid by killing moulds, bacteria, germs and other unwanted organisms. Moreover, it is essential in preserving wine in the storage. The main purpose of adding sodium bisulfite as a food additive is because of its ability to function as a bacteria inhibitor.

What is Sodium Metabisulfite?

Sodium Metabisulfite is the compound with chemical formula Na₂S₂O₅. We call it also as sodium pyrosulfite. The molecular weight of this molecule is 190 g mol^-1. Moreover, this is a sodium salt of a divalent ion. More importantly, this compound shows resonance stabilization in its anion. There, the two negatively charged oxygen atoms bind with each sulfur atom, which allows the anions to form resonance structures.

Furthermore, sodium metabisulfite is a white crystalline powder with the melting point 150 ^oC. Also, this is freely soluble in water and can release sulfur dioxide gas. Sodium metabisulfite is useful as a preservative agent and a disinfectant like sodium bisulfite.

What is the Difference Between Sodium Bisulfite and Sodium Metabisulfite?

Sodium bisulfite is the compound having the chemical formula NaHSO₃ and Sodium Metabisulfite is the compound with chemical formula Na₂S₂O₅. The key difference between sodium bisulfite and sodium metabisulfite is that the sodium bisulfite has only one sulfur atom and three oxygen atoms, and the bisulfite anion is monovalent whereas, the sodium metabisulfite has two sulfur atoms, five oxygen atoms, and the anion is divalent.

Moreover, sodium sulfite gives less sulfite than the sodium metabisulfite does, when we dissolve it in water. Therefore this is also a difference between sodium bisulfite and sodium metabisulfite. More differences are shown in the infographic of difference between sodium bisulfite and sodium metabisulfite.

Summary – Sodium Bisulfite vs Sodium Metabisulfite

Sodium bisulfite is a food additive having E222 number. Sodium metabisulfite is also a food additive, and the number is E223. However, they are two different compounds. The key difference between sodium bisulfite and sodium metabisulfite is that the sodium bisulfite has only one sulfur atom and three oxygens, and the bisulfite anion is monovalent whereas, the sodium metabisulfite has two sulfur atoms, five oxygens, and the anion is divalent.

Reference:

1. “Sodium Metabisulfite.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine. Available here 
2. “Sodium Bisulfite.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine. Available here  

Image Courtesy:

1.”Sodium bisulfite”By Edgar181 – Own work, (Public Domain) via Commons Wikimedia  
2.”Sodium-metabisulfite-2D” (Public Domain) via Commons Wikimedia