What is Sodium Metabisulfite? Composition, Preparation & Key Applications
Introduction
Sodium Metabisulfite has the chemical formula Na2S2O5 and its molecular mass is 190.107g/mol. It is a sulfur-containing ionic compound of sodium. Sodium has a positive charge while metabisulfite (S2O52-) has a negative charge. It is called Metabisulfite due to the presence of two S atoms. Meta term refers to the direct bond present between two S atoms. It is a well-known antioxidant being used in pharmaceuticals. It is white in color and solid powder form.
In this article, We will discuss the structure, chemical and physical properties, uses of Sodium metabisulfite.
Structure
We know that the chemical formula of Sodium Metabisulfite is Na2S2O5. It has two sodium atoms, two sulfur, and five oxygen atoms. Na+ is balancing the charge in the structure. Two sulfur atoms are directly attached to each other through a single bond. A sulfur atom which has 3 Oxygen atoms, has an oxidation state of +5. Another sulfur atom has an oxidation number of +3. The structure of this compound is shown below:
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Preparation
There are a number of methods available to synthesize sodium metabisulphite. One of them is by using Sodium sulfite and sodium bisulfite solution. Sulfur Dioxide is added to this solution. It maintains the pH of the solution. NaOH is added to the above solution.
In the solution, There is an equilibrium between the ions as shown below:
2HSO32- ⇌ S2O2-5 + H2O
When NaOH is added to the above solution, free Na+ ions form with bisulfite ions and Sodium metabisulfite is achieved. It enhances the crystallization process per cycle and more yield of Sodium Metabisulfite is achieved.
Properties
Physical Properties
It is white in color, solid, and in powder form.
It has a similar odor to Sulfur dioxide.
The molar mass of sodium metabisulfite is 190.107 g/mol.
It has a melting point of 1800C, but it decomposes around 1500C.
On boiling, It decomposes readily.
Chemical Properties
sodium metabisulfite pH is maintained at 4.6 while preparing the solution. So, it is acidic in nature.
It is non-combustible in nature. When heated at high temperature, it releases toxic flames of oxides of sulfur
When it is mixed in water, it releases sulfur dioxide. It is very harmful to the lungs. It has a pungent strong smell.
When it is treated with strong acids, it releases SO2. The reaction is shown below:
2HCL + Na2S2O5 ⇌ 2SO2 + 2NaCl + H2O
Sodium Metabisulfite Uses
It is the most commonly used preservative for baked goods, wines, jams, cookies.
It acts as a reducing agent in the dough, it helps in making batter softer, easier to knead.
It is a well-known antioxidant used in pharmaceuticals.
It is helpful in tests of sickle cell anemia.
It is used in industries as a corrosion inhibitor because it is an oxygen scavenger.
It is used in wastewater treatment as it is a good reducing agent.
It is used as a sanitizing ingredient, to clean the tools of winemaking.
Did You Know?
It is extensively used in makeup and skincare products. It has antioxidant properties, so it is added to skin care products to increase their shelf life. It is safer to use on the skin. Because of its disinfectant properties, it is used in sanitizers. Sanitizers are in high demand nowadays because of the Corona pandemic. So, the usage of sodium metabisulfite is also increased.
FAQs on Sodium Metabisulfite: Structure, Uses, and Importance
1. What is Sodium Metabisulfite and what is its chemical formula?
Sodium Metabisulfite is an inorganic salt composed of sodium, sulfur, and oxygen. It is primarily used as a preservative, antioxidant, and disinfectant. Its chemical formula is Na₂S₂O₅. It is also sometimes referred to as sodium pyrosulfite.
2. What are the primary uses of Sodium Metabisulfite across different industries?
Sodium Metabisulfite has a wide range of applications due to its chemical properties. Its main uses include:
Food Industry: As a preservative in foods like jams, dried fruits, and baked goods to prevent microbial growth and spoilage.
Beverage Industry: Used in winemaking and brewing to act as a steriliser and antioxidant.
Pharmaceuticals: As an antioxidant to stabilise certain medications, including injectables like adrenaline.
Water Treatment: For dechlorination, to remove excess chlorine from treated water.
Textile Industry: As a bleaching agent for wool and jute.
3. How does Sodium Metabisulfite work to prevent food spoilage?
Sodium Metabisulfite acts as a preservative primarily by releasing sulfur dioxide (SO₂) gas when it comes into contact with moisture or acidic components in food. This SO₂ gas is a potent antimicrobial agent that inhibits the growth of common spoilage organisms like bacteria, yeast, and moulds. Additionally, it functions as an antioxidant, preventing enzymatic browning in fruits and vegetables and preserving their colour and freshness.
4. What is the difference between Sodium Sulfite and Sodium Metabisulfite?
The main difference between Sodium Sulfite and Sodium Metabisulfite lies in their chemical composition and structure. Sodium Sulfite has the chemical formula Na₂SO₃, while Sodium Metabisulfite's formula is Na₂S₂O₅. Although both are used as preservatives, Sodium Metabisulfite is often preferred in acidic conditions as it is more stable and yields more sulfur dioxide per mole, which is the active preserving agent.
5. Are there any health risks or side effects associated with Sodium Metabisulfite?
While generally considered safe for consumption in regulated amounts, Sodium Metabisulfite can cause adverse reactions in some people. Individuals with asthma or sulfite sensitivities are particularly at risk. Inhaling or ingesting it can trigger allergic reactions such as:
Wheezing and shortness of breath
Skin irritation, rashes, or hives
Headaches and nausea
Because of these risks, its use is banned in raw fruits and vegetables in some countries.
6. What happens when Sodium Metabisulfite reacts with an acid?
When Sodium Metabisulfite reacts with a strong acid, such as hydrochloric acid (HCl), it undergoes a decomposition reaction. This reaction produces sulfur dioxide gas, a corresponding salt, and water. The balanced chemical equation for this reaction is: Na₂S₂O₅(s) + 2HCl(aq) → 2NaCl(aq) + 2SO₂(g) + H₂O(l). This release of SO₂ is fundamental to its role as a preservative in acidic foods.
7. Why is Sodium Metabisulfite a key ingredient in the winemaking process?
In winemaking, Sodium Metabisulfite (often in the form of Campden tablets) serves two critical functions. First, it acts as a sterilising agent to eliminate wild yeast and bacteria from the grape must before fermentation begins. This allows the winemaker to introduce a specific, cultured yeast strain for controlled fermentation. Second, it serves as an antioxidant throughout the process, protecting the wine from oxidation that could otherwise spoil its colour, aroma, and flavour.
8. What is the chemical structure of the metabisulfite ion?
The metabisulfite ion (S₂O₅²⁻) has a distinct structure that features a direct sulfur-sulfur (S-S) bond. In this anion, one sulfur atom is bonded to three oxygen atoms and the other sulfur atom. The second sulfur atom is bonded to two oxygen atoms and the first sulfur atom. This asymmetric arrangement differentiates it from other sulfur-oxygen anions like sulfate (SO₄²⁻) or sulfite (SO₃²⁻).
