Sodium Alginate

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

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

DOWNLOAD PDF HERE

(C₆H₇NaO₆)_n+2

Alginic acid, sodium salt;

Sodium alginate

CAS RN^®: 9005-38-3.

1 DEFINITION

Sodium Alginate is the purified carbohydrate product extracted from brown seaweeds by the use of dilute alkali. It consists chiefly of the sodium salt of Alginic Acid, a linear, unbranched, amorphous copolymer of β-d-mannuronic acid (M) and α-l-guluronic acid (G) linked to each other by 1→4 glycosidic bonds. The M and G units in the alginates may be randomly or non-randomly arrayed as heterogeneous or homogeneous sequences. It contains NLT 90.8% and NMT 106.0% of sodium alginate, calculated on the dried basis. The typical average molecular weight for Sodium Alginate is in the range of 10,000–600,000 g/mol.

2 IDENTIFICATION

2.1 A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K or 197A

2.2 B.

Sample solution 1: A solution (1 in 100)

Analysis 1: To 5 mL of Sample solution 1 add 1 mL of calcium chloride TS.

Acceptance criteria 1: A voluminous, gelatinous precipitate is formed immediately.

Sample solution 2: A solution (1 in 100)

Analysis 2: To 10 mL of Sample solution 2 add 1 mL of 4 N sulfuric acid.

Acceptance criteria 2: A heavy, gelatinous precipitate is formed.

2.3 C. Identification Tests—General 〈191〉, Chemical Identification Tests, Sodium

Sample solution: Ignite 0.2 g of Sodium Alginate at 700° for 2 h, and dissolve the residue in 2 mL of water.

15% Potassium carbonate solution: Dissolve 7.5 g of potassium carbonate in 50 mL of water and mix well.

Analysis 1: Transfer 2 mL of the Sample solution to a test tube, add 2 mL of 15% Potassium carbonate solution, and heat to boiling.

Acceptance criteria 1: No precipitate is formed.

Analysis 2: To the test tube from Analysis 1 add 4 mL of potassium pyroantimonate TS, heat to boiling, cool in ice water, and rub the inside of the test tube with a glass rod.

Acceptance criteria 2: A dense precipitate is formed.

3 ASSAY

Change to read:

3.1 Procedure

Transfer 200 mL of 0.5 N hydrochloric acid VS to a 400-mL beaker. Place it on a magnetic stirrer and stir. Weigh about 1.0 g of Sodium Alginate. Add the sample to acid with stirring. Stir for 30 min and leave the solution for another 30 min. Filter through a suitable glass crucible. Wash the filter with water until the pH of the filtrate water is neutral. Transfer the contents of the filter crucible to a beaker. Flush the filter crucible with water. Place the beaker on a stirrer and start stirring. Add a few drops of phenolphthalein indicator solution and titrate with 0.5 N sodium hydroxide until it is a stable pink color. Note the amount of sodium hydroxide consumed in milliliters (V).

Analysis: Each milliliter of 0.5 N sodium hydroxide consumed is equivalent to 55.5 mg of Sodium Alginate (equivalent weight 222.0 g/mol).

Calculate the percentage of sodium alginate in the portion of Sodium Alginate taken:

Result = (V₂ × N × W_E × 10 _{(ERR 1-Sep-2023)})/(W × D)

V₂ = volume of sodium hydroxide consumed (mL)

N = normality of sodium hydroxide solution

W_E = equivalent weight of Sodium Alginate, 222.0 g/mol

W = sample weight (g)

D = percent dry matter value for the sample

Acceptance criteria: 90.8%–106.0% on the dried basis

4 IMPURITIES

4.1 Limit of Lead and Arsenic

[Note—When water is specified as the diluent, use deionized ultrafiltered water. Use of glass volumetric flasks is discouraged.]

Diluent: Weigh around 3.5 g of sodium nitrate in a 1000-mL volumetric flask. Add 500 mL of water to dissolve. Add 170 mL of nitric acid, ultratrace, and mix. Cool to room temperature and dilute with water to volume.

Standard stock solution: Into a 100-mL volumetric flask, transfer 100 µL of a standard solution containing 1000 mg/L of arsenic¹ and 500 µL of a standard solution containing 1000 mg/L of lead². Add 1–2 drops of nitric acid, ultratrace. Dilute with water to volume.

Internal standard solution: Transfer 1.0 mL of a standard solution containing 10,000 mg/L of yttrium³ to a 100-mL volumetric flask. Add 1–2 drops of nitric acid, ultratrace, and dilute with water to volume.

[Note—The concentration of the Internal standard solution can be adjusted if a high number of signal counts from the Internal standard solution causes an artifact.]

Calibration standard solution A: Into a separate 100-mL volumetric flask, prepare a solution containing 0.01 µg/mL of arsenic and 0.05 µg/mL of lead from the Standard stock solution. To each flask, add 100 µL of the Internal standard solution and 20 µL of a standard solution containing 1000 mg/L of gold⁴. Dilute with Diluent to volume.

Calibration standard solution B: Into a separate 100-mL volumetric flask, prepare a solution containing 0.02 µg/mL of arsenic and 0.1 µg/mL of lead from the Standard stock solution. To each flask, add 100 µL of the Internal standard solution and 20 µL of a standard solution containing 1000 mg/L of gold. Dilute with Diluent to volume.

Calibration standard solution C: Into a separate 100-mL volumetric flask, prepare a solution containing 0.04 µg/mL of arsenic and 0.2 µg/mL of lead from the Standard stock solution. To each flask, add 100 µL of the Internal standard solution and 20 µL of a standard solution containing 1000 mg/L of gold. Dilute with Diluent to volume.

Blank solution: In a 100-mL volumetric flask, add 100 µL of the Internal standard solution. Add 20 µL of a standard solution containing 1000 mg/L of gold to the same flask. Dilute with Diluent to volume, and mix well.

Sample solution: Weigh about 0.50 g of Sodium Alginate in a Teflon pressure vessel. Add 10 mL of nitric acid, ultratrace. Add 10 µL of a standard solution containing 1000 mg/L of gold. Screw the cap of the pressure vessel and commence digestion in a microwave digester per the program given in Table 1.

Table 1

Once completed, allow it to cool. Rinse the vessel using water and transfer the rinsate to a separate 50-mL volumetric flask. Add 50 µL of the Internal standard solution. Dilute with water to volume, and mix well.

Instrumental conditions

(See Plasma Spectrochemistry 〈730〉.)

Mode: ICP–OES

Emission wavelengths: 189.042 nm for arsenic, 220.353 nm for lead, and 224.306 nm for yttrium. Set the sample read time and other instrument parameters as appropriate or as recommended by the instrument manufacturer.

System suitability

Samples: Calibration standard solutions A–C, Blank solution, and Sample solution

Suitability requirements

[Note—Instrument performance must be verified to conform to the manufacturer's specifications for resolution and sensitivity. Before analyzing samples, the instrument must pass a suitable performance check. Additional system suitability parameters can be used as per the instrument manufacturer's recommendations, along with the internal quality requirements.]

Correlation coefficient: NLT 0.999, determined from the Calibration curve constructed in the Analysis

Analysis

Samples: Calibration standard solutions A–C, Blank solution, and Sample solution.

[Note—The following analysis is described for one type of ICP–OES instrument. If a different ICP–OES instrument is used, follow the instrument manufacturer's recommendations for operation.]

Take 3 replicate scans with the integration set as recommended by the instrument manufacturer. Follow the instrument manufacturer's recommendations for delivering the sample. Flush the samples through the system before analysis. Program a read delay into the sampling routine to allow for fluid flow equilibration after the high-speed flush, before the first analytical read of the sample. Between samples, wash the pumping system by flushing the Blank solution. Analyze the Sample solution on the ICP.

Calibration curve: Generate the calibration curve using the Blank solution and Calibration standard solutions A–C as follows. Scan the Internal standard solution while running the Blank solution to measure the intensity of the yttrium emission. Hold this value constant throughout the remainder of the test. Separately scan the Blank solution, Calibration standard solutions A–C, and Internal standard solution. Normalize the yttrium intensity to the value of the Internal standard solution. Apply this normalization factor to the intensity of the respective elements, which is then referred to as the corrected intensity. Obtain the concentration of arsenic and lead (C), in µg/mL, in the Sample solution, through the calibration curve. Plot the corrected intensity versus the known concentrations, in µg/mL, of arsenic and lead.

Calculate the content in µg/g (ppm) of arsenic and lead in the portion of Sodium Alginate taken:

Result = (C × V)/W

C = concentration of arsenic/lead in the Sample solution obtained from the Calibration curve (µg/mL)

V = volume of the Sample solution (mL)

W = weight of Sodium Alginate taken to prepare the Sample solution (g)

Acceptance criteria: NMT 1.5 µg/g of arsenic and 10 µg/g of lead

5 SPECIFIC TESTS

Microbial Enumeration Tests 〈61〉 and Tests for Specified Microorganisms 〈62〉: The total bacterial count does not exceed 200 cfu/g. The tests for Salmonella species and Escherichia coli are negative.

Loss on Drying 〈731〉

Analysis: Dry at 105° for 4 h.

Acceptance criteria: NMT 15.0%

Articles of Botanical Origin 〈561〉, Methods of Analysis, Total Ash

Analysis: Proceed as directed in the chapter, carefully igniting 3 g in a tared platinum dish until the residue is thoroughly carbonized (5 min), and then igniting in a muffle furnace at a temperature of 800 ± 25° until the carbon is completely burned off (approximately 75 min).

Acceptance criteria: 18.0%–27.0% of ash on the dried basis

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

USP Reference Standards 〈11〉

USP Sodium Alginate RS

¹ Arsenic ICP standard solutions are commercially available. A suitable ICP standard is available from LGC (www.lgcstandards.com) or Millipore Sigma (www.sigmaaldrich.com).

² Lead ICP standard solutions are commercially available. A suitable ICP standard is available from LGC (www.lgcstandards.com) or Millipore Sigma (www.sigmaaldrich.com).

³ Yttrium ICP standard solutions are commercially available. A suitable ICP standard is available from LGC (www.lgcstandards.com) or Millipore Sigma (www.sigmaaldrich.com).

⁴ Gold standard solutions are commercially available. A suitable ICP standard is available from LGC (www.lgcstandards.com) or Millipore Sigma (www.sigmaaldrich.com).