Stannous Fluoride Gel

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

Stannous Fluoride Gel contains NLT 95.0% and NMT 115.0% of the labeled amount of stannous fluoride (SnF₂) in a suitable medium containing a suitable viscosity-inducing agent.

[NOTE—If Glycerin is used as the medium in the preparation of this Gel, use Glycerin that has a low water content; that is, Glycerin having a specific gravity of NLT 1.2607, corresponding to a concentration of 99.5%.]

2 IDENTIFICATION

Change to read:

A. The retention time of the fluoride peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

(USP 1-Aug-2022)

B.

Sample solution: Nominally 1 mg/mL of stannous fluoride from Gel in water

Analysis: On a spot plate, mix 2 drops of Sample solution with 2 drops of silver nitrate TS. Acceptance criteria: A brown-black precipitate is formed.

C.

Sample solution: Nominally 1 mg/mL of stannous fluoride from Gel in water Analysis: Add 1 drop of Sample solution to 2 drops of mercuric chloride TS.

Acceptance criteria: A white, silky precipitate is formed. On further addition of the Sample solution, a brown-black precipitate is formed.

3 ASSAY

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PROCEDURE

[NOTE—Store all solutions in plastic containers. It is recommended to use plastic HPLC vials. Use water with a resistivity of NLT 18 megohm- cm to prepare the solutions.]

Mobile phase: 15 mM potassium hy droxide in water. [NOTE—Mobile phase can be generated electrolytically using an automatic eluant generator.]

System suitability solution: 2.0 µg/mL of USP Sodium Fluoride RS and 1.0 µg/mL of USP Sodium Acetate RS in water Standard solution: 2.0 µg/mL of USP Sodium Fluoride RS in water

Sample solution: Nominally 3.7 µg/mL of stannous fluoride in water prepared as follows. Transfer an accurately weighed portion of Gel to a suitable volumetric flask, add about 60% of the flask volume of water, and dissolve with shaking. Dilute with water to volume.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: Conductivity with suppression

Columns

Guard: 4.0-mm × 5-cm; 13-µm packing L120. [NOTE—Alternatively, a 4.0-mm × 0.5-cm column that contains 4.6-µm packing L91 may be used.]

Analytical: 4.0-mm × 25-cm; 7.5-µm packing L113. [NOTE—Alternatively, a 4.0-mm × 25-cm column that contains 4.6-µm packing L91 may be used.]

Column temperature: 40° Flow rate: 1.0 mL/min Injection volume: 20 µL

Run time: NLT 6 times the retention time of fluoride System suitability

Samples: System suitability solution and Standard solution

[NOTE—The relative retention times for the fluoride and acetate ions are 1.0 and 1.1, respectively.]

Suitability requirements

Resolution: NLT 1.5 between the fluoride and acetate ions, System suitability solution Tailing factor: NMT 2.0 for the fluoride ion, Standard solution

Relative standard deviation: NMT 1.0% for the fluoride ion, Standard solution Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of stannous fluoride (SnF₂), P_SnF2, in the portion of Gel taken:

Result = (r_U/r_S) × (C_S/C_U) × (M_r1/M_r2) × 100

r_U = peak response of the fluoride ion from the Sample solution

r_S = peak response of the fluoride ion from the Standard solution

C_S = concentration of USP Sodium Fluoride RS in the Standard solution (µg/mL)

C_U = nominal concentration of stannous fluoride in the Sample solution (µg/mL)

M_r1 = molecular weight of stannous fluoride, 156.71

M_r2 = molecular weight of sodium fluoride, 41.99 (USP 1-Aug-2022)

Acceptance criteria: 95.0%–115.0%

4 SPECIFIC TESTS

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4.1 CONTENT OF TOTAL TIN

Potassium chloride solution: 19.1 mg/mL of potassium chloride in water

Standard stock solution: 1.000 (USP 1-Aug-2022) mg/mL of tin prepared as follows. Transfer an appropriate amount of tin (Sn) to a suitable volumetric flask. Add hy drochloric acid to about 20% of the final volume of the flask, and swirl to dissolve. Add 20% of the flask volume of water into the flask, and allow to cool. Dilute with water to volume. (USP 1-Aug-2022)

Standard solution A: 50.0 µg/mL of tin and 191 µg/mL of potassium chloride in water from Standard stock solution and Potassium chloride solution

Standard solution B: 100.0 µg/mL of tin and 191 µg/mL of potassium chloride in water from Standard stock solution and Potassium chloride solution

Standard solution C: 150.0 µg/mL of tin and 191 µg/mL of potassium chloride in water from Standard stock solution and Potassium chloride solution

Sample solution: Nominally 0.132 mg/mL of stannous fluoride from a portion of Gel prepared as follows. Transfer an accurately weighed quantity of Gel, equivalent to about 132 mg of stannous fluoride, to a plastic beaker. Add 80 mL of water and 20 mL of hy drochloric acid, and mix. Transfer this mixture to a 1000-mL volumetric flask, add 10.0 mL of Potassium chloride solution, and dilute with water to volume.

(USP 1-Aug-2022)

Blank: Potassium chloride solution, hydrochloric acid, and water (1:2:97)

Instrumental conditions

(See Atomic Absorption Spectroscopy 〈852〉.) Mode: Atomic absorption spectrophotometer Analytical wavelength: 235.5 nm

Lamp: Tin hollow-cathode

Flame: Nitrous oxide-acetylene oxidizing flame Analysis

Samples: Standard solution A, Standard solution B, Standard solution C, Sample solution, and Blank

Use water to adjust the instrument to zero. Concomitantly determine the absorbance of Standard solution A, Standard solution B, Standard solution C, Sample solution, and Blank. Aspirate water before and after each determination. Correct the absorbances of Standard solution A, Standard solution B, Standard solution C, and the Sample solution by subtracting the absorbance of the Blank. Plot the corrected absorbances of Standard solution A, Standard solution B, and Standard solution C versus concentration, in µg/mL, of tin, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration (C), in µg/mL, of tin in the Sample solution.

Calculate the concentration of tin (C_Tin), in µg/mg, in the labeled amount of stannous fluoride: (USP 1-Aug-2022)

Result = C/C_U

C = concentration of tin in the Sample solution (µg/mL)

C_U = nominal concentration of stannous fluoride in the Sample solution (mg/mL)

[NOTE—Use this value to calculate the percentage of stannous tin (USP 1-Aug-2022) for the test for the Content of Stannous Tin. (USP 1- Aug-2022) ]

Change to read:

4.2 CONTENT OF STANNOUS TIN (USP 1-A -2022)

Solution A: 5 mg/mL of sodium hy droxide and 50 mg/mL of potassium iodide in oxygen-free water

Potassium iodide–iodate titrant: 0.1 N potassium iodide-iodate solution prepared as follows. In a 1000-mL volumetric flask, dissolve 3.567 g of potassium iodate, previously dried at 110° to constant weight, in 200 mL of Solution A. Dilute with oxygen-free water to volume, and mix. Standardize this solution by titrating a solution prepared from an accurately weighed quantity of reagent tin (Sn) and hy drochloric acid.

Each milliliter of 0.1 N Potassium iodide-iodate titrant is equivalent to 5.935 mg of tin (Sn).

Sample: A suitable amount of Gel, nominally (USP 1-Aug-2022) equivalent to 80 mg of stannous fluoride Titrimetric system

(See Titrimetry 〈541〉.) (USP 1-Aug-2022)

Mode: Direct titration

Titrant: Potassium iodide–iodate titrant Endpoint detection: Visual

Analysis: Transfer the Sample to a capped plastic vessel equipped for titration in an inert atmosphere. Add a plastic coated stirring bar, 20 mL of recently boiled 3 N hydrochloric acid, and 5 mL of potassium iodide TS. Close the vessel, purge the system with an oxygen-free inert gas, and titrate immediately with Potassium iodide–iodate titrant, adding 2 mL of starch TS as the endpoint is approached.

Calculate the percentage of stannous tin (Sn²⁺) relative to the amount of stannous fluoride in the portion of Gel taken:

Result = [(V_S × N_A × F₁)/(P_SnF2 × F₂ × W)] × 100

V_S = volume of Titrant consumed by the Sample (mL)

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

F₁ = equivalency factor of stannous tin, 59.35 mg/mEq

P_SnF2 = percentage of the labeled amount of stannous fluoride, as determined in the Assay (%)

F₂ = conversion factor from percent to decimal, 0.01

W = nominal weight of stannous fluoride in the Sample taken (mg)

Calculate the percentage of stannous tin (Sn²⁺) relative to the amount of total tin in the portion of Gel taken:

Result = [(V_S × N_A × F₁ × F₂)/(C_Tin × W)] × 100

V_S = volume of Titrant consumed by the Sample (mL)

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

F₁ = equivalency factor of stannous tin, 59.35 mg/mEq

F₂ = conversion factor, 1000 µg/mg

C_Tin = concentration of total tin, as determined in the Content of Total Tin test (µg/mg)

W    = nominal weight of stannous fluoride in the Sample taken (mg) (USP 1-Aug-2022)

Acceptance criteria: NLT 68.2% of stannous fluoride and NLT 90.0% of total tin (USP 1-Aug-2022)

4.3 VISCOSITY—ROTATIONAL METHODS 〈912〉

Sample: A portion of Gel

Analysis: Transfer the Sample to a suitable plastic container, insert the stopper securely, and allow to stand until the Sample is free from air bubbles. Place it in a water bath maintained at a temperature of 25 ± 0.5° until it adjusts to the temperature of the water bath (4 h or longer). Do not stir the Sample while it is in the bath. Remove the Sample from the bath, stir the Sample gently for 2 min, and without delay, using a rotational viscometer, determine the viscosity by using a spindle having a cylinder 1.27 cm in diameter and 0.16 cm high that is attached to a shaft 0.32 cm in diameter, with the distance from the top of the cylinder to the lower tip of the shaft being 2.54 cm and the immersion depth being 5.00 cm (No. 3 spindle). Operate the viscometer at 12 rpm, and record the scale reading at 1-min intervals for 4 min. Calculate the viscosity, in centipoises, by multiplying the scale reading by 100.

Acceptance criteria: 600–170,000 centipoises

4.4 PH 〈791〉

Sample solution: A freshly prepared mixture with water (50:50) Acceptance criteria: 2.8–4.0

5 ADDITIONAL REQUIREMENTS

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PACKAGING AND STORAGE: Preserve in tight (USP 1-Aug-2022) containers. Store at 15°–30°. (USP 1-Aug-2022)

Change to read:

USP REFERENCE STANDARDS 〈11〉

USP Sodium Acetate RS (USP 1-Aug-2022)

USP Sodium Fluoride RS