Potassium metabisulfite (K2S2O5), also known as potassium pyrosulfite is a preservative commonly used in winemaking and food processing. It works by releasing sulfur dioxide, which inhibits the growth of bacteria and yeasts, prolonging the shelf life of products.
| IUPAC name | Potassium pyrosulfite |
| Molecular formula | K2S2O5 |
| CAS number | 16731-55-8 |
| Synonyms | Potassium disulfite, Pyrosulfurous acid, dipotassium salt, Potassium metabisulphite |
| InChI | InChI=1S/2K.H2O5S2/c;;1-6(2)7(3,4)5/h;;(H,1,2)(H,3,4,5)/q2*+1;/p-2 |
Potassium Metabisulfite Properties
Potassium Metabisulfite Formula
The chemical formula for potassium metabisulfite is K2S2O5. This means that each molecule of potassium metabisulfite contains two potassium ions, two sulfur atoms, and five oxygen atoms. The formula is important in determining the chemical properties and behavior of K2S2O5 in different reactions and processes.
Potassium Metabisulfite Molar Mass
K2S2O5 has a molar mass of 222.31 g/mol. This is calculated by adding up the atomic masses of its constituent elements: two atoms of potassium (39.10 g/mol each), two atoms of sulfur (32.06 g/mol each), and five atoms of oxygen (16.00 g/mol each). The molar mass is important in determining the amount of K2S2O5 needed in a particular reaction or process.
Potassium Metabisulfite Boiling Point
K2S2O5 does not have a defined boiling point because it decomposes before it reaches a boiling point. At high temperatures, K2S2O5 decomposes to release sulfur dioxide gas, which is the active ingredient that acts as a preservative.
Potassium Metabisulfite Melting Point
K2S2O5 has a melting point of approximately 190 °C (374 °F). At this temperature, the solid compound melts and turns into a clear liquid. The melting point is important in determining the appropriate conditions for handling and storing K2S2O5.
Potassium Metabisulfite Density g/mL
The density of K2S2O5 is approximately 2.34 g/mL. This means that a given volume of K2S2O5 will weigh 2.34 times more than the same volume of water. The density is important in determining the appropriate amount of K2S2O5 to use in a particular process.
Potassium Metabisulfite Molecular Weight
The molecular weight of K2S2O5 is 222.31 g/mol. This is calculated by adding up the atomic masses of its constituent elements: two atoms of potassium, two atoms of sulfur, and five atoms of oxygen. The molecular weight is important in determining the amount of K2S2O5 needed in a particular reaction or process.
Potassium Metabisulfite Structure
K2S2O5 has a molecular structure that consists of two potassium ions (K+) and a pyrosulfite ion (S2O5^2-). The pyrosulfite ion is formed from two sulfur atoms and five oxygen atoms. The two potassium ions are attracted to the negatively charged pyrosulfite ion, forming a stable ionic compound.
Potassium Metabisulfite Solubility
K2S2O5 is soluble in water, alcohol, and glycerol. Its solubility in water varies depending on the temperature and concentration. At room temperature, it is highly soluble in water, with a solubility of approximately 70 g/100 mL. However, at higher concentrations, it can become less soluble and form precipitates.
| Appearance | White or pale-yellow crystalline powder |
| Specific Gravity | 2.34 at 20 °C |
| Color | White to slightly yellow |
| Odor | Sulfurous, pungent |
| Molar Mass | 222.31 g/mol |
| Density | 2.34 g/mL at 25 °C |
| Melting Point | 190-240 °C |
| Boiling Point | Decomposes before boiling |
| Flash Point | Not applicable (solid) |
| Water Solubility | Soluble in water |
| Solubility | Soluble in ethanol and glycerol, slightly soluble in ether |
| Vapor Pressure | Negligible at room temperature |
| Vapor Density | Not applicable (solid) |
| pKa | 4.34 |
| pH | 4.5-5.5 in a 10% solution |
Potassium Metabisulfite Safety and Hazards
K2S2O5 can pose safety hazards if not handled properly. It can cause skin irritation, eye irritation, and respiratory problems if inhaled. Direct contact with the substance can cause redness, itching, and burning sensations. It can also cause digestive problems if ingested. The substance can release toxic sulfur dioxide gas if heated, so proper ventilation is necessary during handling and storage. It is important to wear appropriate personal protective equipment such as gloves and safety glasses when handling K2S2O5. Proper storage is also crucial, as it can react with acids, metals, and other chemicals.
| Hazard Symbols | Xi (Irritant) |
| Safety Description | Keep container tightly closed and dry; Do not ingest; Wear suitable protective clothing, gloves, and eye/face protection; In case of contact with eyes, rinse immediately with plenty of water and seek medical advice; If swallowed, seek medical advice immediately and show this container or label. |
| UN IDs | UN 2676 |
| HS Code | 2832.10.00 |
| Hazard Class | 8 – Corrosive substances |
| Packing Group | III |
| Toxicity | LD50 (rat, oral) – 2000 mg/kg; LC50 (rat, inhalation) – 17 ppm/4h |
Potassium Metabisulfite Synthesis Methods
Different methods can synthesize K2S2O5.
One common method involves the reaction between potassium hydroxide and sulfur dioxide. The reaction vessel combines the two chemicals and heats them. The resulting gas passes through water, producing potassium sulfite. Hydrogen peroxide oxidizes the potassium sulfite, forming potassium sulfate. Sulfur dioxide reacts with potassium sulfate to yield K2S2O5.
Another method involves the reaction of potassium carbonate and sulfur dioxide. The reaction vessel mixes the two chemicals and heats them. The mixture produces a gas that passes through a solution of potassium sulfite, creating potassium bisulfite. Hydrogen peroxide oxidizes the potassium bisulfite, forming potassium sulfate. Sulfur dioxide reacts with potassium sulfate, producing K2S2O5.
Potassium Metabisulfite Uses
K2S2O5 has a wide range of applications in various industries. Some common uses include:
- Food Preservation: Used as a food preservative to prevent spoilage and extend shelf life. Commonly used in wine, beer, and fruit juice production.
- Photography: Used in photography as a developer and fixative for film and prints.
- Textile Industry: Used in the textile industry to remove excess dye and bleach fabrics.
- Water Treatment: Used as a water treatment chemical to remove chlorine and other impurities.
- Pharmaceutical Industry: Used in the pharmaceutical industry as an antioxidant to prevent the degradation of drugs.
- Gold Mining: Used in gold mining to separate gold from other metals and impurities.
- Chemical Manufacturing: Used in the manufacturing of various chemicals such as dyes, resins, and plastics.
- Pulp and Paper Industry: Used in the pulp and paper industry to bleach paper products.
Questions:
Q: How much potassium metabisulfite per gallon?
A: The amount of K2S2O5 needed per gallon varies depending on the intended use. For winemaking, the recommended amount is 1/16 to 1/4 teaspoon per gallon. For cider making, the recommended amount is 1/4 teaspoon per gallon. It is important to carefully follow the recommended dosage to ensure the desired effect and to avoid overuse, which can be harmful.
Q: When should I use potassium sorbate or potassium metabisulfite?
A: Potassium sorbate and K2S2O5 are both used as preservatives in the food and beverage industry. Potassium sorbate is used to prevent the growth of yeast and mold, while K2S2O5 is used to inhibit bacterial growth and oxidation. Potassium sorbate is typically used in products that are not acidic, such as syrups and jams, while K2S2O5 is used in acidic products such as wine and cider. The choice between the two preservatives depends on the specific product and its requirements.
Q: Where to buy potassium metabisulfite?
A: K2S2O5 can be purchased from chemical supply stores, online retailers, and homebrewing stores. It is important to ensure that the product is of high quality and intended for the intended use.
Q: How much potassium metabisulfite per gallon of cider?
A: The recommended amount of K2S2O5 for cider making is 1/4 teaspoon per gallon. This amount is added after fermentation to prevent oxidation and bacterial growth, and to preserve the flavor and quality of the cider.
Q: What is the difference between K2S2O5 and K2S2O8?
A: K2S2O5 and K2S2O8 are both potassium salts, but they have different chemical structures and properties. K2S2O5 is a white crystalline powder that is used as a preservative and antioxidant, while K2S2O8 is a strong oxidizing agent that is used in chemical reactions such as organic synthesis and wastewater treatment. The main difference between the two is their chemical properties and intended use.