1. Why KOH Has So Many "Numbers" 💡

Potassium hydroxide is sold and used across very different industries - soap, biodiesel, water treatment, analytical labs, batteries - and each describes strength in its own language. The same drum of caustic potash might be labelled by weight percent (45%), the same titrant by normality (1N), the same lab reagent by molarity (0.1M), and the same flakes by solid assay (90%).

These are not interchangeable, and confusing them is a common, costly mistake. This guide explains what each one means and how to move between them. If you're new to KOH, our pillar guide covers what potassium hydroxide is, and our KOH solubility guide explains how much will dissolve.

2. Weight Percent: 10%, 45%, 50% 💧

Weight percent (% w/w) is the most common way to describe commercial liquid KOH. It tells you how many grams of pure KOH are in every 100 g of solution. So a 45% KOH solution contains 45 g of KOH and 55 g of water in each 100 g.

• 🔹 10% solution - a dilute working strength used for cleaning and many general applications.
• 🔹 45% / 50% solution - the standard commercial concentrations for liquid caustic potash; concentrated, dense and ready to dilute.
• 🔹 "Concentrated KOH" generally refers to these high-percent solutions; "aqueous KOH" simply means KOH dissolved in water at any strength.

Higher percent means more active KOH per litre but also higher density and viscosity. Our liquid potassium hydroxide product page lists the concentrations we supply.

3. Molarity (M) and Normality (N) ⚗️

Laboratories prefer molarity and normality because they relate directly to the number of reacting particles:

• 🔹 Molarity (M) = moles of KOH per litre of solution. Since KOH has a molar mass of 56.11 g/mol, a 1M solution contains 56.11 g of KOH per litre.
• 🔹 Normality (N) = moles of reactive hydroxide (OH⁻) per litre. Because each KOH releases exactly one OH⁻, for KOH normality equals molarity - 1N = 1M, 0.1N = 0.1M, 0.5N = 0.5M.

So a "0.1N potassium hydroxide" titrant and a "0.1M KOH solution" are the same thing - 5.611 g of KOH made up to one litre. These standardised solutions are the backbone of acid-value and saponification testing.

4. Solid Assay: 90%, 95% & "Anhydrous" 🧱

For solid KOH, the number means something different again. It is the assay (purity) - the percentage of the solid that is actually KOH, with the remainder being mostly absorbed water plus a little potassium carbonate:

• 🔹 90% flakes - the most common commercial solid grade.
• 🔹 95% / 99% - higher-purity grades for demanding uses.
• 🔹 "Anhydrous" KOH - essentially water-free; rare and difficult to keep dry because KOH is so hygroscopic.

For a full breakdown of purity tiers and what separates industrial, electronic and reagent material, see our KOH grades & purity guide.

5. Quick Conversion Reference 🔢

Approximate equivalents for aqueous KOH at room temperature (use for orientation; always standardise for analytical work):

📌 Percent-to-molarity conversions depend on solution density, which rises with concentration; the values above are rounded for guidance. For precise figures consult density tables such as those referenced by the NIST Chemistry WebBook.

6. How to Prepare a KOH Solution 🧪

To prepare a target molarity from solid KOH:

• 1️⃣ Calculate the mass. Grams needed = molarity × 56.11 × litres. For a 90% assay, divide by 0.90 to compensate for non-KOH content.
• 2️⃣ Add KOH to water - never the reverse - slowly and in portions, because dissolving is strongly exothermic (see our solubility guide).
• 3️⃣ Let it cool to room temperature before making up to final volume - hot solutions occupy more space and will read low once cooled.
• 4️⃣ Make up to volume in a volumetric flask and mix thoroughly.

To dilute concentrated liquid KOH, use C₁V₁ = C₂V₂, and again add the concentrate to water with cooling.

7. How to Standardise It Accurately 🎯

KOH solutions cannot be trusted to stay at their exact label strength. Because caustic potash absorbs carbon dioxide from the air, an open or aged solution slowly converts to potassium carbonate and drifts weaker. For any analytical use, the solution must be standardised:

• 🔹 Titrate against a primary standard acid such as potassium hydrogen phthalate (KHP).
• 🔹 Calculate the true (actual) normality from the titration result.
• 🔹 Re-standardise periodically, and store titrants protected from air to limit carbonate drift.

Alcoholic (ethanolic) KOH is often preferred for saponification and acid-value titrations - see our alcoholic potassium hydroxide guide.

8. Which Concentration Do You Need? ✅

For sourcing the right starting material, our KOH forms guide compares flakes, pellets, powder and liquid.

9. Frequently Asked Questions ❓

🔹 Is 1N potassium hydroxide the same as 1M?

Yes. Because each KOH unit releases one hydroxide ion, normality equals molarity for KOH - 1N = 1M, 0.1N = 0.1M.

🔹 What does 45% KOH mean?

It is a weight percent: 45 g of pure KOH in every 100 g of solution, with the remaining 55 g being water.

🔹 How much KOH makes a 1M solution?

About 56.11 g of pure KOH made up to one litre. For 90% flakes, use about 62 g to compensate for the non-KOH content.

🔹 What is the difference between 90% flakes and 45% solution?

90% is the purity of a solid; 45% is the concentration of a liquid. They describe different things and cannot be compared directly without calculation.

🔹 Why does my standardised KOH lose strength over time?

KOH absorbs CO₂ from the air, slowly forming potassium carbonate and drifting weaker. Store titrants sealed and re-standardise periodically.

🔹 What is anhydrous potassium hydroxide?

It is essentially water-free KOH. Because KOH is extremely hygroscopic, truly anhydrous material is hard to obtain and store, so most "solid" KOH still contains some water.

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