Sodium 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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NaOH              40.00

Sodium hydroxide CAS RN: 1310-73-2.

1 DEFINITION

Sodium Hydroxide contains NLT 95.0% and NMT 100.5% of total alkali, calculated as sodium hydroxide (NaOH), including NMT 3.0% of sodium carbonate (Na₂CO₃). It also contains NLT 54.0% and NMT 59.8% of sodium.

[CAUTION-Exercise great care in handling sodium hydroxide, because it rapidly destroys tissues.]

2 IDENTIFICATION

A. IDENTIFICATION TESTS GENERAL (191), Chemical Identification Tests, Sodium: A solution (1 in 25) meets the requirements.

B. PH (791)

Sample solution: 0.1 mg/mL of Sodium Hydroxide

Acceptance criteria: NLT 11.0

3 ASSAY

3.1 TOTAL ALKALI

Sample: 1.5 g

Blank: 40.0 mL of carbon dioxide-free water

Titrimetric system

(See Titrimetry (541).)

Mode: Direct titration

Titrant: 1 N sulfuric acid VS

Endpoint detection: Visual

Analysis: Dissolve the Sample in 40 ml of carbon dioxide-free water. Cool the solution to room temperature, 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 Titrant (V_S1). Add methyl orange TS, and continue the titration until a persistent pink color is produced. Record the volume of Titrant (V_S2). Perform a blank determination, and make any necessary corrections.

Calculate the percentage of total alkali, calculated as sodium hydroxide (NaOH), in the Sample taken:

                      Result = {[(V_S2 - V_B) x N x F₁]/W} x 100

V_S2 = volume of Titrant consumed by the Sample to the second endpoint (mL) 

V_B = volume of Titrant consumed by the Blank (mL)

N = actual normality of the Titrant (mEq/mL)

F₁ = equivalency factor, 40.00 mg/mEq

W = weight of the Sample (mg)

Calculate the percentage of sodium carbonate (Na₂CO₃) in the Sample taken:

                      Result = {[(V_S2 - V_S1) x Nx F₂]/W} x 100

V_S2 = volume of Titrant consumed by the Sample to the second endpoint (mL)

V_S1 = volume of Titrant consumed by the Sample to the first endpoint (mL)

N = actual normality of the Titrant (mEq/mL)

F₂ = equivalency factor, 106.0 mg/mEq

W = weight of the Sample (mg)

Acceptance criteria: 95.0%-100.5% of total alkali; NMT 3.0% of sodium carbonate (Na₂CO₃)

3.2 CONTENT OF SODIUM

Diluent: 1% hydrochloric acid solution

Standard stock solution: 25.41 µg/mL of sodium chloride in Diluent. This solution contains 10 µg/mL of sodium.

Standard solutions: Transfer 6.0-, 7.5-, and 9.0-mL portions of Standard stock solution to separate 100-mL volumetric flasks. Dilute the content of each flask with Diluent to volume, and mix to obtain solutions having known concentrations of 0.6, 0.75, and 0.9 µg/mL of sodium, respectively.

Sample stock solution: 1.303 mg/mL of Sodium Hydroxide in Diluent

Sample solution: Transfer 0.1 mL of Sample stock solution to a 100-mL volumetric flask and dilute with Diluent to volume.

Instrumental conditions

(See Atomic Absorption Spectroscopy (852).)

Mode: Atomic absorption spectrophotometry

Analytical wavelength: 589.0 nm (sodium emission line)

Lamp: Sodium hollow-cathode

Flame: Air-acetylene

Blank: Diluent

Standard curve

Samples: Standard solutions

Plot: Absorbance values versus their corresponding concentration (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 Sodium Hydroxide taken:

                      Result = (C_S/C_U) x 100

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

C_U = concentration of Sodium Hydroxide in the Sample solution (µg/mL)

Acceptance criteria: 54.0%-59.8%

4 IMPURITIES

POTASSIUM

Diluent: 1% hydrochloric acid solution

Standard stock solution: 1.907 mg/mL of potassium chloride, previously dried at 105" for 2 h, in water. Transfer 5.0 mL of this solution to a 1.0-L volumetric flask and dilute with Diluent to volume. This solution contains 5 µg/mL of potassium.

Standard solutions: Transfer 2.0-, 5.0, and 10.0-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 having known concentrations of 0.10, 0.25, and 0.50 µg/mL of potassium, respectively.

Sample stock solution: 0.5 mg/mL of Sodium Hydroxide

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

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: Diluent

Standard curve

Samples: Standard solutions

Plot: Absorbance values versus their corresponding concentration (µg/mL) of potassium. The correlation coefficient is NLT 0.99.

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 Sodium Hydroxide taken:

                      Result = (C_S/C_U) x 100

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

C_U = concentration of Sodium Hydroxide in the Sample solution (µg/mL)

Acceptance criteria: NMT 0.5%

5 SPECIFIC TESTS

INSOLUBLE SUBSTANCES AND ORGANIC MATTER: A solution (1 in 20) is complete, clear, and colorless to slightly colored.

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight containers.