Potassium Hydroxide Flakes, ≥ 90% (Caustic Potash)
Strong inorganic base for pH adjustment, alkaline solutions and analytical work
Potassium hydroxide (KOH) is a strong, highly water-soluble base supplied as white flakes with an assay of at least 90%. It is used to prepare alkaline solutions, adjust pH, saponify fats, absorb carbon dioxide and, in microbiology, for the 3% KOH string test. It is corrosive and must be handled with eye and skin protection.
Also known as: Caustic potash, potash lye, potassium hydrate, KOH.
Overview
Potassium hydroxide dissociates completely in water to potassium and hydroxide ions, giving strongly alkaline solutions (a 0.1 M solution has a pH of about 13). It dissolves with strong heat release and absorbs moisture and carbon dioxide from the air, slowly forming potassium carbonate. Compared with sodium hydroxide, it forms more soluble salts and soaps.
Applications
| Application | How KOH is used |
|---|---|
| pH adjustment | Raising the pH of buffers, culture media and reagents |
| Analytical chemistry | Standard alkaline solutions, saponification value, Kjeldahl and titrations |
| Microbiology | 3% KOH string test to distinguish Gram-negative from Gram-positive bacteria |
| Organic synthesis | Base and catalyst; saponification of esters and fats |
| Gas absorption | Removing CO₂ and H₂S in gas analysis |
| Soap and cleaning research | Preparing potassium (soft, liquid) soaps |
Preparing a KOH Solution
Always add KOH to water, never water to KOH. Add the flakes slowly to cold purified water in a heat-resistant vessel while stirring, and allow the solution to cool before making up to volume.
Example: 3% KOH for the string test — dissolve 3 g in water and make up to 100 mL. Because the assay is about 90%, standardise solutions by titration when an exact concentration is needed.
Frequently Asked Questions
What is potassium hydroxide used for in the laboratory?
Potassium hydroxide is a strong base used to prepare alkaline solutions, adjust pH, saponify fats, absorb carbon dioxide, and as a reagent in titrations and organic synthesis. In microbiology, 3% KOH is used for the string test to distinguish Gram-negative from Gram-positive bacteria.
How do I safely dissolve potassium hydroxide?
Always add the flakes slowly to cold water while stirring, never water to the flakes. Dissolving releases a lot of heat and can cause boiling and splashing. Wear goggles, gloves and a lab coat.
Why is the assay ≥ 90% and not 100%?
Solid potassium hydroxide always contains some water and a little potassium carbonate, because it absorbs moisture and carbon dioxide from the air. Commercial flakes are typically 85–90% KOH.
Is potassium hydroxide a dangerous good?
Yes. It is UN1813, Potassium hydroxide, solid, Class 8 (corrosive), Packing Group II. It is also a Schedule 6 poison in Australia.
How should potassium hydroxide be stored?
Store in a tightly closed plastic or glass container in a cool, dry place, away from acids, aluminium, zinc and tin. Keep the container closed to prevent it absorbing moisture and carbon dioxide.
What is the difference between potassium hydroxide and sodium hydroxide?
Both are strong bases. Potassium hydroxide (KOH, 56.11 g/mol) forms more soluble salts and soaps, and is preferred for liquid soaps, some titrations and the microbiological string test. Sodium hydroxide (NaOH, 40.00 g/mol) is cheaper and used more widely for general pH adjustment.
Product Identification
Specifications
Storage and Shelf Life
Store at 15–25 °C in the original tightly closed plastic container, in a dry place away from acids, aluminium, zinc and tin. Keep closed: the flakes absorb moisture and carbon dioxide. Store locked up. Use before the expiry date on the label.
Hazard Identification
| Signal word | Danger |
|---|---|
| GHS classification | Corrosive to metals 1; Acute toxicity (oral) 4; Skin corrosion 1A; Serious eye damage 1 |
| Hazard statements | H290 May be corrosive to metals. H302 Harmful if swallowed. H314 Causes severe skin burns and eye damage. |
| Key precautions | P260 Do not breathe dust. P280 Wear protective gloves, protective clothing, eye and face protection. P301+P330+P331 IF SWALLOWED: Rinse mouth. Do NOT induce vomiting. P305+P351+P338 IF IN EYES: Rinse cautiously with water for several minutes. P310 Immediately call a POISON CENTER or doctor. P405 Store locked up. |
| Poisons Schedule | Schedule 6 |
First Aid
| Eyes | Rinse immediately with running water for at least 15 minutes, holding eyelids apart. Get medical help without delay. |
|---|---|
| Skin | Remove contaminated clothing. Rinse with plenty of water until advised to stop by the Poisons Information Centre. |
| Ingestion | Rinse mouth. Do NOT induce vomiting. Call 13 11 26 immediately. |
| Inhalation | Move to fresh air. Seek medical advice. |
Transport
| UN number | UN1813 |
|---|---|
| Proper shipping name | POTASSIUM HYDROXIDE, SOLID |
| Class / Packing group | 8 / II |
| Hazchem | 2W |
| Limited quantity (ADG) | 1 kg |
The full 16-section Safety Data Sheet is supplied with every order and is available from support@ausamics.com.