Sodium Bisulfite vs Sodium Metabisulfite
Both these chemicals are salts of sodium. Sodium bisulfite and sodium metabisulfite are used as preservatives, disinfectants and food additives.
Sodium bisulfite is the compound having the chemical formula NaHSO3. It is a salt of bisulfite. Its structure is as follows, and it has a molar mass of 104 g mol-1.
Sodium bisulfite is present as a white solid, and when it is in the liquid form, it acts as a corrosive liquid. Melting point of the solid is 150 oC. The solid form of the molecule is soluble in water. Due to the proton donation ability, it is slightly acidic. The sulfur is in the +4 oxidation state. Sodium bisulfite can be prepared by bubbling sulfur dioxide gas into carbonated water. Sodium bisulfite will release sulfur dioxide gas when reacting with mild acidic conditions. This molecule can be used in organic chemistry for the purification purposes. It forms a bisulfite adduct with aldehyde, which is in solid form. Therefore, aldehyde can be precipitated from a solution by making it a bisulfite adduct and then can regenerate the aldehyde group by removing the bisulfite. Sodium bisulfite is also used as a mild reducing agent, discoloration agent in organic synthesis. When sodium bisulfite reacts with oxygen gas, it is converted to sodium bisulfate.
Among the several usages of sodium bisulfite, most notable one is the usage as a food additive. It plays a key role in wine production. In the early stages, sodium bisulfite is added to fruit juice. Then sulfur dioxide gas will be released from it, and it will help to sanitize the liquid by killing molds, bacteria, germs and other unwanted organisms. Moreover, it is important 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.
This is the compound with chemical formula Na2S2O5. It is also known as sodium pyrosulfite. Molecular weight of this molecule is 190 g mol-1. This is a sodium salt of divalent ion and has the following structure.
In the above structure, resonance stabilized anion is shown. Two negatively charged oxygen atoms are bonded to each sulfur atom, which allows the anions to form resonance structures. Sodium metabisulfite is a white crystalline powder with the melting point 150 oC. This is freely soluble in water, and can release sulfur dioxide gas. Sodium metabisulfite is used as a preservative agent and a disinfectant like sodium bisulfite.
What is the difference between Sodium Bisulfite and Sodium Metabisulfite?
• Sodium bisulfite has only one sulfur atom and three oxygens, and the bisulfite anion is monovalent. Sodium metabisulfite has two sulfur atoms, five oxygens, and the anion is divalent.
• Sodium bisulfite is a food additive having E222 number. Sodium metabisulfite is also a food additive, and the number is E223.
• Sodium metabisulfite converts to sodium bisulfite when it is dissolved in water. When sodium metabisulfite is in a solution, it gives more sulfite than the normal sodium bisulfite does.