Method of Analysis for Sodium Methyl Parabean

1. Description

White crystalline, odorless or almost odorless, hygroscopic powder

2. Solubility

Freely soluble in water, sparingly soluble in ethanol 95%, practically insoluble in fixed oils, methylene chloride.

3. Identification

A. The IR absorption spectrum of the precipitate obtained in “Identification test C” is concordant with the methylparaben reference spectrum or with the spectrum obtained from methyl paraben WRS.

B. Reaction of sodium salt (Reaction I)

Reagent required
5 % w/v solution of potassium carbonate
Potassium antimonate solution
Procedure: To 1ml of solution S add 1 ml of water. Add 2 ml of 15 % w/v solution of potassium carbonate and heat to boiling. No precipitate is produced. Add 4 ml of a freshly prepared potassium antimonate solution and heat to boiling. Allow to cool in ice and if necessary scratch the inside of the test-tube with a glass-red a dense white precipitate is formed.

Reaction of sodium salt (Reaction II)

Reagent required
1M Acetic Acid
Magnesium Uranyl acetate solution
Procedure: To 1ml of solution S add 1 ml of water. Dissolve 0.1 g of ignited substance in 2 ml of water. Acidity with 1M acetic acid. Add a large excess of Magnesium Uranyl acetate solution mix. A yellow, crystalline precipitate is formed.

C. Melting point

Limit: Between 125°C and 128°C
Procedure: Dissolve 0.5 g in 50 ml of water. Immediately add 5 ml of hydrochloric acid. Filter and wash the precipitate with water. Dry under vacuum at 80°C for 2 h. Fill the capillary tube to about 4 mm with the precipitate obtained in identification test A. Start the melting point apparatus, when the temperature reaches about 10°C below the expected melting point, Insert the capillary tube into the oil bath. Allow the temperature to rise. Note the temperature at which the last solid particle passes into the liquid phase.

D. By Thin Layer Chromatography

Examine the chromatograms obtained in the test “Related substances”. The principal spot in the chromatogram with test solution (b) is similar in position and size to the principal spot obtained in the chromatogram obtained with reference solution (c).

E. Color Reaction An orange to red color is developed

Reagent required
Sodium carbonate solution
Aminopyrazolone solution
Potassium ferricyanide solution
Procedure: Weigh & transfer about 10 mg. of the sample in a test tube, add 1 ml of sodium carbonate solution, boil for 30 seconds and cool. Add 5 ml of aminopyrazolone solution and 1 ml of potassium ferricyanide solution and mix. An orange to red color is developed.

4. pH

Limit: Between 9.5 and 10.5
Solution S: Dissolve 5 gm in carbon dioxide-free water prepared from distilled water and dilute to 50 ml with the same solvent.
Procedure: Dilute 1 ml of solution S to 100ml with carbon dioxide-free water and measure the pH.

5. Clarity and color of solution

Clarity of solution

Reagent required
10% w/v solution of hexamine
Hydrazine sulfate AR.
Methanol AR
Sample Solution: Weigh accurately about 10.0 g. of the sample and transfer into a clean and dried 100 ml volumetric flask, dissolve in water and makeup to volume to 100 ml with water. (Solution S)
Reference suspension I: Weigh accurately about 1.0 g. of hydrazine sulfate and transfer into a clean and dried 100 ml volumetric flask, dissolve in water and makeup to volume to 100 ml with water and allow standing for 4 to 6 hrs. Add 25 ml of this solution to 25 ml of 10% w/v solution of hexamine mix well and allow to stand for 24 hrs. Mix well. Dilute 15 ml of above solution to 1000 ml with water. Dilute 5.0 ml of this suspension (after shaking) to 100 ml with water.
Procedure: Into separate matched, flat-bottomed Nessler cylinders, 15 to 25 mm in internal diameter and of colorless, transparent, neutral glass, place sufficient quantity of the sample solution, reference suspension and water, such that the Nessler cylinders are filled to a depth of 40 mm. After five minutes, compare the contents against a black background by viewing in diffused daylight down the vertical axes of the cylinders. The sample solution is clearer as compared to the diluent (water) or is less opalescent than reference suspension.

Color of solution

Reagent required
Yellow Primary Solution
Red Primary Solution
Blue Primary Solution
1% w/v Hydrochloric acid
Reference solution BY6: In a 100 ml volumetric flask, transfer 24 ml of yellow primary solution, 10 ml of red primary solution and 4 ml of blue primary solution. Make up the volume to 100 ml with 1% w/v of HCl. Dilute 5.0 ml of above solution to 100 ml with 1.0% w/v HCl.
Sample solution: Use solution S
Procedure: Transfer 2 ml each of sample solution and reference solution BY6 in separate identical colorless, transparent, neutral 12 mm in external diameter, clean and dried Nessler cylinders. After 5 minutes, compare the color intensity in diffused daylight by viewing horizontally a white background. The sample solution is less colored as compared to the reference solution BY6.

6. Chloride

Limit: Not more than 350 ppm
Nitric acid AR
0.1M silver nitrate
Chloride standard solution (5 ppm Cl)
Standard solution: Transfer 14 ml of chloride standard solution (5 ppm Cl) into a clean and dried Nessler cylinder, add 1 ml of water.
Sample solution: To 10ml of solution S, add 30 ml of water and 1ml of nitric acid and dilute to 50ml with water. Shake and filter. Dilute 10 ml of the filtrate to 15 ml with water.
Procedure: Add 1 ml of 2M nitric acid, pour the mixture as a single addition into 1 ml of silver nitrate solution R2 and allow standing for 5 minutes protected from light. When viewed transversely against a black background any opalescence produced is not more intense than standard solution.

7. Sulfate

Limit: Not more than 300ppm
Reagent required
25 % w/v solution of barium chloride
Ethanolic sulfate standard solution
Sulfate standard solution (10 ppm SO4)
5M acetic acid
2M hydrochloric acid
Standard preparation: 15 ml of Sulphate standard solution (10 ppm SO4).
Sample preparation: Pipette out 25 ml of solution S into a clean, dried 50 ml volumetric flask, add 5 ml of water and 10 ml of hydrochloric acid, dilute to 50 ml with water and filter. Dilute 10 ml of the filtrate to 15 ml with water.
Procedure: Into two separate Nessler cylinders take 1.5 of ethanolic sulfate standard solution, add 1 ml of 25 % w/v solution of barium chloride. Mix and allow standing for 1 minute. Add 15 ml of sample preparation and 15 ml of the standard preparation. Add 0.5 ml of 5M acetic acid. Allow standing for 5 minutes. Compare the opalescence if any, produced in the sample preparation with that produced with the standard solution. The opalescence produced with the sample preparation is not more intense than that with the standard solution.

8. Related substance

By Thin Layer Chromatography

Stationary phase: TLC plate with octadecylsilyl silica gel
Mobile Phase: Mix 1 volume of glacial acetic acid, 30 volumes of water and 70 volumes of methanol as the mobile phase.
Test solution (a): Dissolve 0.1 g. of the sample in 10 ml of water and transfer the solution in a clean separating funnel. Immediately add 2 ml hydrochloric acid and shake with 50 ml of ether. Discard the lower layer and collect upper layer in the clean beaker. Evaporate to dryness and dissolve the residue obtained in 10 ml of acetone.
Test solution (b): Dilute 1 ml of test solution (a) to 10 ml with acetone.
Reference solution (a): Dissolve 34.3 mg of 4-hydroxy benzoic acid in acetone and dilute to 100 ml with acetone.
Reference solution (b): Dilute 0.5 ml of test solution (a) to 100 ml with acetone.
Reference solution (c): Dissolve 10 mg of methyl parahydroxybenzoate in acetone and dilute to 10 ml with acetone.
Reference solution (d): Dissolve 10 mg of ethyl parahydroxybenzoate in 1 ml of test solution (a) and dilute to 10 ml with acetone.
Procedure: Apply separately to the plate (previously dried at 105°c for at least 30 minutes), 5 µl of each of solutions and dry the plate in air. Introduce the plate in the mobile phase Allow the mobile phase to run about 15 cm from the spot of application. After removal, the plate, dry it in the air and examine in ultraviolet light at 254 nm.
In the chromatogram obtained with test solution (a), any spot due to 4- hydroxy benzoic acid is not more intense than the spot in the chromatogram obtained with reference solution (a) (4%) and any spot, apart from principal and the spot due to 4-hydroxy benzoic acid is not more intense than the spot in the chromatogram obtained with reference solution (b) (0.5 %). The test is not valid unless the chromatogram obtained with reference solution (d) shows two clearly separated principal spots.

9. Heavy metals

Limit: Not more than 10 ppm
Reagent required
Strong ammonia
Thioacetamide reagent
Acetate buffer pH 3.5
Lead standard solution (10 ppm Pb)
25% w/v Magnesium sulfate in 1M sulphuric acid
2M Hydrochloric acid
Phenolphthalein solution
Sample Preparation: Weigh accurately and transfer 2.0 g. of the sample in a silica crucible with 4 ml of a 25% w/v solution of magnesium sulfate in 1 M sulphuric acid. Mix using a glass rod and heat cautiously to dryness on a water bath. Keep the crucible in the muffle furnace. Progressively heat to ignition, below 800°C and continue heating until a white or at most grayish residue is produced. Allow to cool, moisten the residue with 0.2 ml of 1M sulfuric acid, evaporate, ignite again and allow cooling. The total period of ignition should not exceed 2 hours. Dissolve the residue using two 5-ml quantities of 2M hydrochloric acid. Add 0.1 ml of phenolphthalein solution and add strong ammonia dropwise until a pink color is produced. Cool, add glacial acetic acid until the solution is decolorized and add a further 0.5 ml. Filter and dilute the solution to 20 ml with water.
Standard solution: Take 2 ml of lead standard solution (10 ppm lead) in a silica crucible, add 4 ml of 25% w/v solution of magnesium sulfate in 1 M sulphuric acid and repeat the procedure beginning at the words “Mix using a glass rod………” under sample preparation.
Procedure: Take two Nessler cylinders. In sample cylinder pipette out 12 ml of the sample solution and in standard cylinder pipette out 2 ml of the sample solution and 10 ml of standard solution. Add to both cylinders, 2 ml of acetate buffer pH 3.5, mix and add 1.2 ml of thioacetamide reagent, mix, allow standing for 2 minutes. Any brown color produced in the sample solution is not more intense than that produced in the standard solution.

10. Water

Limit: Not more than 5.0%
Reagent required
Anhydrous methanol AR
KF reagent Pyridine free single solution
Procedure: Take about 40 ml of anhydrous methanol in the titration vessel, neutralize with KF reagent and find out the factor of KF reagent in mg H2O/ 5 ml in triplicate and enter the mean value.
Add accurately about 500 mg of the sample to the titration vessel. Allow titrating with KF reagent to the electrometric endpoint. Record the percentage of water content obtained by Karl Fischer titrator. Find out water content of the sample in triplicate and take mean value.

11. Assay

Limit: NLT 99.0% and NMT 102.0% of Sodium 4-(methoxycarbonyl) phenolate, calculated with reference to the anhydrous substance.
Reagent required
Glacial acetic acid
0.1M Perchloric acid
Procedure: Weigh accurately about 150 mg. of the sample in a titration beaker; add 50 ml of glacial acetic acid and dissolve. Connect the beaker to the Autotitrator and determine the end-point potentiometrically. 1 ml of 0.1M Perchloric acid is equivalent to 17.41 mg. of C₈H₇NaO₃.
Calculation
                                                   V x F x M x 100                 100
% Assay on anhydrous basis = ------------------------ x --------------------
                                                            0.1 x W              (100 - % water)
Where
V = Consumed volume of 0. 1M Perchloric acid
M = Molarity of 0.1 M Lead nitrate
F = Factor
W = Weight of Sample