Potassium Hydroxide

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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KOH 56.11

Potassium hydroxide CAS RN®: 1310-58-3.

1 DEFINITION

Potassium Hydroxide contains NLT 85.0% of total alkali, calculated as potassium hydroxide (KOH), including NMT 3.5% of potassium carbonate (K₂CO₃). It also contains NLT 59.9% of potassium.

[Caution-Exercise great care in handling Potassium Hydroxide because it rapidly destroys tissues.]

2 IDENTIFICATION

2.1 A. Identification Tests-General 〈191〉, Potassium

A solution (1 in 25) meets the requirements.

2.2 B. pH 〈791〉

Sample solution: 0.1 mg/mL of Potassium Hydroxide

Acceptance criteria: NLT 10.5

3 ASSAY

3.1 Total Alkali

Sample: 1.5 g of Potassium Hydroxide

Titrimetric system

• (See Titrimetry 〈541〉.)
• Mode: Direct titration
• Titrant: 1 N sulfuric acid VS
• Endpoint detection: Colorimetric

Analysis: Dissolve the Sample in 40 mL of carbon dioxide–free water. Cool the solution to 15° and add phenolphthalein TS. Titrate with 1 N sulfuric acid VS. At the discharge of the pink color of the indicator, record the volume of acid solution required, then add methyl orange TS and continue the titration to a persistent pink color. Each milliliter of 1 N sulfuric acid is equivalent to 56.11 mg of total alkali, calculated as potassium hydroxide (KOH), and each milliliter of acid consumed in the titration with methyl orange is equivalent to 138.2 mg of potassium carbonate (K₂CO₃).

Acceptance criteria: NLT 85.0% of total alkali, calculated as potassium hydroxide (KOH), including NMT 3.5% of potassium carbonate (K₂CO₃).

3.2 Content of Potassium

Diluent: 1% hydrochloric acid solution

Sodium chloride solution: 0.2 g/mL of sodium chloride

Blank solution: Transfer 2.0 mL of the Sodium chloride solution to a 100-mL volumetric flask and dilute with Diluent to volume.

Standard stock solution: 57.21 µg/mL of potassium chloride, previously dried at 105° for 2 h, in water. This solution contains 30 µg/mL of potassium.

Standard solutions: Transfer 2.0-, 4.0-, and 6.0-mL portions of the Standard stock solution to separate 100-mL volumetric flasks. To each flask, add 2.0 mL of the Sodium chloride solution. Dilute the content of each flask with Diluent to volume and mix to obtain solutions with known concentrations of 0.6, 1.2, and 1.8 µg/mL of potassium.

Sample stock solution: 0.5 mg/mL of Potassium Hydroxide

Sample solution: Transfer 1.0 mL of the Sample stock solution to a 250-mL volumetric flask. Add 5.0 mL of the Sodium chloride solution and dilute with Diluent to volume.

Instrumental conditions

• (See Atomic Absorption Spectroscopy 〈852〉.)
• Mode: Atomic absorption spectrophotometry
• Analytical wavelength: 766.5 nm (potassium emission line)
• Lamp: Potassium hollow-cathode
• Flame: Air–acetylene
• Blank: Blank solution

Standard curve

• Samples: Standard solutions
• Plot: Absorbance values versus their corresponding concentrations (µg/mL) of potassium. The correlation coefficient is NLT 0.999.

Analysis

Sample: Sample solution

From the Standard curve, determine the concentration of potassium in the Sample solution.

Calculate the percentage of potassium in the portion of Potassium Hydroxide taken:

Result = (C_s/C_u) × 100

C_s = concentration of potassium in the Sample solution from the Standard curve (µg/mL)

C_u = concentration of Potassium Hydroxide in the Sample solution (µg/mL)

Acceptance criteria: NLT 59.9%

4 IMPURITIES

4.1 Limit of Sodium

Diluent: 1% hydrochloric acid solution

Standard stock solution: 12.71 µg/mL of sodium chloride, previously dried at 105° for 2 h, in water. This solution contains 5 µg/mL of sodium.

Standard solutions: Transfer 1.0-, 10.0-, and 15.0-mL portions of the Standard stock solution to separate 100-mL volumetric flasks. Dilute the content of each flask with Diluent to volume and mix to obtain solutions with known concentrations of 0.05, 0.5, and 0.75 µg/mL of sodium.

Sample stock solution: 0.5 mg/mL of Potassium Hydroxide

Sample solution: 50 µg/mL of Potassium Hydroxide in Diluent, prepared from the Sample stock solution

Instrumental conditions

• (See Atomic Absorption Spectroscopy 〈852〉.)
• Mode: Atomic absorption spectrophotometry
• Analytical wavelength: 589.0 nm
• Lamp: Sodium hollow-cathode
• Flame: Air–acetylene
• Blank: Diluent

Standard curve

• Samples: Standard solutions
• Plot: Absorbance values versus their corresponding concentrations (µg/mL) of sodium. The correlation coefficient is NLT 0.995.

Analysis

Sample: Sample solution

From the Standard curve, determine the concentration of sodium in the Sample solution.

Calculate the percentage of sodium in the portion of Potassium Hydroxide taken:

Result = (C_s/C_u) × 100

C_s = concentration of sodium in the Sample solution from the Standard curve (µg/mL)

C_u = concentration of Potassium Hydroxide in the Sample solution (µg/mL)

Acceptance criteria: NMT 1.0%

5 SPECIFIC TESTS

5.1 Insoluble Substances

Sample solution: 1 g of Potassium Hydroxide in 20 mL of water

Acceptance criteria: The Sample solution is complete, clear, and colorless.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.