Lactose Monohydrate
(lak' tose mon'' oh hye' drate).
Lactose Monohydrate is a natural disaccharide, obtained from milk, which consists of one glucose and one galactose moiety. [Note—Lactose Monohydrate may be modified as to its physical characteristics. It may contain varying proportions of amorphous lactose. ]
•  B. Thin-Layer Chromatographic Identification Test 201
Adsorbent:  0.25-mm layer of chromatographic silica gel
Diluent:  Methanol and water (3:2)
Standard solution A:  0.5 mg/mL of USP Lactose Monohydrate RS in Diluent
Standard solution B:  0.5 mg/mL each of USP Dextrose RS, USP Lactose Monohydrate RS, USP Fructose RS, and USP Sucrose RS in Diluent
Sample solution:  0.5 mg/mL of Lactose Monohydrate in Diluent
Application volume:  2 µL
Developing solvent system:  Ethylene dichloride, glacial acetic acid, methanol, and water (10:5:3:2)
Spray reagent:  5 mg/mL of thymol in a mixture of alcohol and sulfuric acid (19:1)
Samples:  Standard solution A, Standard solution B, and Sample solution
Allow the spots to dry, and develop the plate in a paper-lined chromatographic chamber equilibrated with Developing solvent system for about 1 h prior to use. Allow the chromatogram to develop until the solvent front has moved about three-quarters of the length of the plate. Remove the plate from the chamber, dry in a current of warm air, and redevelop the plate in fresh Developing solvent system. Remove the plate from the chamber, mark the solvent front, and dry the plate in a current of warm air. Spray the plate evenly with Spray reagent. Heat the plate at 130 for 10 min.
System suitability:  The test is not valid unless the chromatogram of Standard solution B shows four clearly discernible spots, disregarding any spots at the origin.
Acceptance criteria:  The principal spot from the Sample solution corresponds in appearance and RF value to that from Standard solution A.
•  C. Procedure
Sample solution:  50 mg/mL of Lactose Monohydrate
Analysis:  To 5 mL of the Sample solution add 3 mL of ammonium hydroxide, and heat in a water bath at 80 for 10 min.
Acceptance criteria:  A red color develops.
Inorganic Impurities 
•  Residue on Ignition 281: NMT 0.1%, determined on a specimen ignited at a temperature of 600 ± 25
•  Heavy Metals 231: NMT 5 ppm
Sample solution:  4 g in 20 mL of warm water. Add 1 mL of 0.1 N hydrochloric acid, and dilute with water to 25 mL.
•  Clarity and Color of Solution
Analysis:  A solution of 1 g in 10 mL of boiling water is clear and nearly colorless. Determine the absorbance of this solution at a wavelength of 400 nm.
Acceptance criteria:  The absorbance divided by the path length, in cm, is NMT 0.04.
•  Microbial Enumeration Tests 61 and Tests for Specified Microorganisms 62: The total aerobic microbial count does not exceed 100 cfu/g, the total combined molds and yeasts count does not exceed 50 cfu/g, and it meets the requirements of the test for absence of Escherichia coli.
•  Optical Rotation, Specific Rotation 781S
Sample solution:  Dissolve 10 g by heating in 80 mL of water to 50. Allow to cool, and add 0.2 mL of 6 N ammonium hydroxide. Allow to stand for 30 min, and dilute with water to 100 mL.
Acceptance criteria:  +54.4 to +55.9, calculated on the anhydrous basis, determined at 20
•  Acidity or Alkalinity
Sample solution:  Dissolve 6 g by heating in 25 mL of carbon dioxide–free water, cool, and add 0.3 mL of phenolphthalein TS.
Acceptance criteria:  The solution is colorless, and NMT 0.4 mL of 0.1 N sodium hydroxide is required to produce a red color.
•  Loss on Drying 731
Analysis:  Dry a sample at 80 for 2 h.
Acceptance criteria:  The monohydrate form loses NMT 0.5% of its weight. The modified monohydrate form loses NMT 1.0% of its weight.
•  Water Determination, Method I 921: 4.5%–5.5%, determined on a preparation containing lactose monohydrate in a mixture of methanol and formamide (2:1)
•  Protein and Light-Absorbing Impurities 851
Sample solution:  Measure the light absorption of a 1% (w/v) solution in the range of 210–300 nm.
Acceptance criteria:  The absorbance divided by the path length, in cm, is NMT 0.25 in the range of 210–220 nm and is NMT 0.07 in the range of 270–300 nm.
•  Packaging and Storage: Preserve in tight containers.
•  Labeling: Where the labeling states the particle size distribution, it also indicates the d10, d50, and d90 values and the range for each. For modified Lactose Monohydrate, also label it to indicate the method of modification.
•  USP Reference Standards 11
USP Dextrose RS Click to View Structure
USP Fructose RS Click to View Structure
USP Lactose Monohydrate RS Click to View Structure
USP Sucrose RS Click to View Structure
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Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
(EXC2010) Monographs - Excipients
61 Radhakrishna S Tirumalai, Ph.D.
Principal Scientific Liaison
(GCM2010) General Chapters - Microbiology
62 Radhakrishna S Tirumalai, Ph.D.
Principal Scientific Liaison
(GCM2010) General Chapters - Microbiology
Reference Standards RS Technical Services
USP35–NF30 Page 1839