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What is Loss on Drying? and Determination of Loss on Drying


Loss on drying procedure and determination of loss on drying by drying in oven and thermogravimetry methods and their calculation as per USP.
Loss on drying is the loss of weight expressed as percentage w/w resulting from water and volatile matter of any kind that can be driven off under specified conditions. The test is carried out on a well-mixed sample of the substance. If the substance is in the form of large crystals, reduce the size by rapid crushing to a powder.
Where the drying temperature is indicated by a single value other than a range, drying is carried out at the prescribed temperature ± 2°.
Unless otherwise specified in the individual monograph, use Method A.

Related: Difference Between Water (Moisture) Content and Loss on Drying (LOD)

Method A

Determination and Calculation of LODWeigh a glass-stoppered, shallow weighing bottle that has been dried under the same conditions to be employed in the determination. Transfer to the bottle the quantity of the sample specified in the individual monograph, cover it and accurately weigh the bottle and the contents. Distribute the sample as evenly as practicable by gentle side wise shaking to a depth not exceeding 10 mm.
Dry the substance by placing the loaded bottle in the drying chamber as directed in the monograph, remove the stopper and leave it also in the chamber. Dry the sample to constant weight or for the specified time and at the temperature indicated in the monograph. Dry by one of the following procedures.
After drying is completed, open the drying chamber, close the bottle promptly and allow it to cool to room temperature (where applicable) in a desiccator before weighing. Weigh the bottle and the contents.
a) "in a desiccator": dry over phosphorus pentoxide at atmospheric pressure and at room temperature;
NOTE - Care must be taken to keep the desiccant fully effective by frequent replacement.
b) "in vacuo": dry over phosphorus pentoxide, at a pressure of 1.5 kPa to 2.5 kPa at room temperature;
c) "in vacuo within a specified temperature range": dry over phosphorus pentoxide, at a pressure of 1.5 kPa to 2.5 kPa within the temperature range given in the monograph;
d) "in an oven within a specified temperature range": dry in an oven within the range given in the monograph;
NOTE - Where the drying temperature is indicated by a single value, dry at the prescribed temperature ± 2°.
e) "under high vacuum": dry over phosphorus pentoxide, at a pressure not exceeding 0.lkPa, at the temperature given in the monograph.

Method B

Thermogravimetry

Thermogravimetry is a technique in which the weight of a sample is recorded as a function of temperature according to a controlled temperature program.

Apparatus

A thermobalance consisting of a device for heating or cooling the substance being examined according to a given temperature program, a sample holder in a controlled atmosphere, an electrobalance and a recorder. The instrument may be coupled to a device permitting the analysis of volatile products.
Temperature verification. Check the temperature scale using nickel or other suitable material according to the manufacturer's instruction.

Calibration of the electrobalance

Place a suitable quantity of calcium oxalate monohydrate RS in the sample holder and record the weight. Set the heating rate according to the manufacturer's instructions and start the temperature programme. Record the thermogravimetric curve as a graph with temperature on the abscissa, increasing from left to right, and weight on the ordinate, increasing upwards. Stop the rise in temperature at 230°. Measure the distance on the graph between the initial and final weight temperature plateaux that corresponds to the loss of weight.
The declared loss of weight for calcium oxalate monohydrate RS is stated on the label.
Note - If the apparatus is in frequent use, carry out temperature verification and calibration regularly. Otherwise, carry out such checks before each measurement.

Procedure

Apply the same procedure to the substance under examination, using the conditions prescribed in the monograph. Calculate the loss of weight of the substance under examination from the distance measured on the graph obtained and express as a percentage w/w of the substance taken.
The actual procedure and the calculations to be employed are dependent on the particular instrument used. Consult the manufacture's literature and/or the thermal analysis literature for the most appropriate technique for a given instrument. In any event, it is imperative to keep in mind the limitations of solid solution formation, insolubility in the melt, polymorphism and decomposition during the analysis.

Calculation:

Loss on drying (%) = initial weight of sample - weight of sample after drying x 100
                                                                Initial weight of sample
Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
Email: .moc.enilediugamrahp@ofni Need Help: Ask Question


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