Dibasic Calcium Phosphate Tablets

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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1 DEFINITION

Dibasic Calcium Phosphate Tablets contain NLT 92.5% and NMT 107.5% of the labeled amount of dibasic calcium phosphate dihydrate (CaHPO₄ · 2H₂O).

[Note-An equivalent amount of Dibasic Calcium Phosphate with less water of hydration may be used in place of CaHPO₄ · 2H₂O in preparing Dibasic Calcium Phosphate Tablets.]

2 IDENTIFICATION

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

Sample solution: A filtered portion of the Sample solution from the Assay

Acceptance criteria: Meets the requirements

2.2 B. Identification Tests-General, Phosphate 〈191〉

Sample solution: A filtered portion of the Sample solution from the Assay, neutralized with ammonium hydroxide

Acceptance criteria: Meets the requirements

3 ASSAY

3.1 Procedure

Sample solution: Transfer a portion of powder, equivalent to 1 g of dibasic calcium phosphate dihydrate, from NLT 20 powdered Tablets, to a 100-mL volumetric flask containing 15 mL of hydrochloric acid and 10 mL of water. Heat on a steam bath, with occasional mixing, to dissolve the dibasic calcium phosphate, but not longer than 30 min. Cool, add water to volume, and mix. If the solution is not clear, filter, discarding the first 10 mL of the filtrate.

Blank: Water

Titrimetric system

• (See Titrimetry 〈541〉.)
• Mode: Direct titration
• Titrant: 0.05 M edetate disodium VS
• Indicator: Hydroxy naphthol blue
• Endpoint detection: Visual

Analysis: Transfer 25.0 mL of the Sample solution to a 250-mL beaker equipped with a magnetic stirrer. With constant stirring, add, in the order named, 0.5 mL of triethanolamine, 300 mg of Indicator, and from a 50-mL buret, about 23 mL of Titrant. Add sodium hydroxide solution (45 in 100) until the initial red color changes to clear blue. Continue to add it dropwise until the color changes to violet, and add an additional 0.5 mL. The pH is 12.3–12.5. Continue the titration dropwise with the Titrant to the appearance of a clear blue endpoint that persists for NLT 60 s. Perform a blank determination.

Calculate the percentage of the labeled amount of dibasic calcium phosphate dihydrate (CaHPO₄ · 2H₂O) in the portion of Tablets taken:

Result = {[(V_s − V_B) × M × F]/W} × 100

V_s = Titrant volume consumed by the Sample solution (mL)

V_B = Titrant volume consumed by the Blank (mL)

M = actual molarity of the Titrant (mM/mL)

F = equivalency factor, 172.08 mg/mM

W = nominal weight of dibasic calcium phosphate dihydrate in the Sample solution taken for Analysis (mg)

Acceptance criteria: 92.5%–107.5%

4 PERFORMANCE TESTS

4.1 Dissolution 〈711〉

Medium: 0.1 N hydrochloric acid; 900 mL

Apparatus 2: 75 rpm

Time: 45 min

Standard solution: Solution having a known concentration of calcium in Medium

Sample solution: Filtered portion of the solution under test, suitably diluted with Medium if necessary

Instrumental conditions

• (See Atomic Absorption Spectroscopy 〈852〉.)
• Mode: Atomic absorption spectrophotometry
• Analytical wavelength: 422.7 nm
• Lamp: Calcium hollow-cathode
• Flame: Air-acetylene

Analysis

Samples: Standard solution and Sample solution

Determine the concentration of calcium (Ca) in the Sample solution in comparison with a Standard solution.

Calculate the percentage of the labeled amount of dibasic calcium phosphate dihydrate (CaHPO₄ · 2H₂O) dissolved:

Result = (A_u/A_s) × (C_s × D × V/L) × (M_r/A_r) × 100

A_u = absorbance of the Sample solution

A_s = absorbance of the Standard solution

C_s = concentration of calcium in the Standard solution (mg/mL)

D = dilution factor for the Sample solution

V = volume of Medium, 900 mL

L = label claim (mg/Tablet)

M_r = molecular weight of dibasic calcium phosphate, 172.08

A_r = atomic weight of calcium, 40.08

Tolerances: NLT 75% (Q) of the labeled amount of dibasic calcium phosphate dihydrate (CaHPO₄ · 2H₂O) is dissolved.

4.2 Uniformity of Dosage Units 〈905〉: Meet the requirements

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers.

Labeling: The quantity of dibasic calcium phosphate stated in the labeling is in terms of dibasic calcium phosphate dihydrate (CaHPO₄ · 2H₂O).