Moisture Content Analysis Methods | Water Content of Soil and Volatile Matter

This method covers moisture content analysis and volatile contents for solids and liquids at (moisture content test) temperatures normally at 105°C although other temperatures may be used if specified in the specification. 

The use of high temperatures of 500°C is not covered by this method but is described in Ash determination. Both the oven method and Infrared drying are included in this method.

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The Test Included in This Method

1. An Oven Method EP & USP:
2. Moisture content
3. Determination of Loss on drying
4. Volatile content of silicone oils
5. Volatile Matter
6. Determination of Volatile Material
7. Water content - Gravimetric

Infra-Red radiation Method: - Drying Loss IR

Calibration of Infrared Desiccation Equipment (Infrared drier) -

REFERENCES

2.1 EP 1997 European Pharmacopoeia

2.2 USP 1997 United States Pharmacopoeia 23

2.3 Operating Instructions for Equipment Used.

2.4 Raw Material and Finished Product Health & Safety Data Sheets for Materials Testing.

Moisture Content

Principle of Moisture Content 

The loss in weight is determined when the powder or liquid is subjected to a specified temperature normally 105°C until at constant weight or for a specified time. For some substances, the temperature and pressure are stipulated in the specification. 

In some cases the drying is done in a desiccator with diphosphorus Pentoxide at room temperature, a specified temperature, and if required under a specified vacuum. The conditions are selected so that the original material is not damaged by the physical conditions required for the drying process.

For some substances, the volatile content is assumed to be moisture, particularly in inorganic salts that absorb water. In other cases, the volatile content may be a solvent or a mixture of solvents and moisture. Care is needed in interpreting the results as contamination from other sources may not always be apparent.

The infrared radiation method in combination with a balance and timer is designed to give a rapid method of determining the moisture content. It is not as accurate as of the oven method as some charring may occur. It is normally used for finished product analysis as a quality control procedure.

REFERENCE MATERIALS

It is possible to obtain reference materials from the Laboratory of the Government Chemists at Teddington, London for samples of known moisture content with a certificate of analysis. As the laboratories are listed it is possible to repeat the conditions they used for oven drying and check the method. This needs to be done locally as the technique and equipment used is important in ensuring accurate results.

CHEMICALS AND REAGENTS

Diphosphorus pentoxide reagent grade is used in the desiccator dish.

EQUIPMENT

1. Volatile matter Oven or Vacuum drying.
2. Chemical Balance weighing 0.1 mg.
3. Oven with fan air circulation set at 105°C ± 2°C or capable of being adjusted to the specified. temperature and controlled to ± 2°C.
4. Desiccator with dry diphosphorus pentoxide contained in a petri dish.
5. Dish to contain 2g of the sample giving a maximum depth of 5 - 10 mm fitted with a lid. containing a capillary of 225 ± 25 μm this is important if using a vacuum. For Vacuum drying, if required.
6. Desiccator with tap to be connected to vacuum pump housed in wire safety cage.
7. Vacuum Oven.
8. Infra-red radiation drying.
9. Balance with infrared drying attachment (Sartorius Model MA 40 or MA 100).
10. Disposable aluminum scale pans.

SAMPLING

As with all testing, the sampling is extremely important and samples taken from the surface or retained in jars with a large headspace may absorb water and not be representative of the whole. 

For sampling from a large consignment, a sampling plan needs to be drawn up and thought given to retain samples to keep moisture pick up or drying to a minimum. The sampling containers need to be airtight, dry and have the minimum headspace

For solids, the particle size needs to be small so large particles should be broken down gently to a maximum of 2 mm avoiding producing heat or allowing moisture pick up, or drying by keeping the time of exposure in the air to a minimum.

INTERFERENCE

Pick up of moisture or drying may occur in the storage of both of the original materials or samples. It is important to keep, containers sealed and use air-tight sample containers with minimum headspace.

If the sample contains large lumps care is required in breaking down the lumps so that it can be dried satisfactorily. Excessive heat or exposure to the air during the breaking down of particles may affect the results of the analysis.

PROCEDURE

Precautions: Read Health & Safety Data Sheets for Raw Materials & Finished Products being samples. Read the Operating Instructions for Ovens or Infra Red Dryers used in this test method before commencing analysis. Observe all Health & Safety Warnings.

Drying in an oven or under a vacuum

Weigh accurately between 1-2 g of the sample to the nearest 0.1 mg into a glass or aluminum dish fitted and a capillary vent, which has been previously dried for 30 minutes weighed and cooled. If it is a powder distribute it so it has a depth of 5 to 10 mm, remove the lid, and place it in the dish oven at 105°C or under the conditions specified in the raw material specification.

Maintain the contents for 3 hrs or the time stated in the specification. Then lid and remove to a desiccator and allow to cool. Weigh and place in the oven for 30 minutes and as before cool and reweigh. Repeat until at constant weight. For Cosmetic Proteins, amino acids, and their derivatives dry for 18 hours.

 When drying in a capillary-stoppered bottle follow the conditions laid down in the raw material specification and maintain the heating chamber at the temperature as specified and a pressure of 5 mm of mercury (0.67kPa) or less for the USP or for the EP, Vacuo is 1.5 kPa to 2.5 KPa (11mm - 19 mm Hg) and high vacuum is <0.1 kPa (Less than 0.75 mmHg).

Ensure the diphosphorus pentoxide is fresh. At the end of the heating period, admit dry air to the heating chamber, remove the bottle, and with the capillary stopper in place allow it to cool in a desiccator before weighing.

10B Drying by InfraRed radiant heater

10B.1 Instrument settings

Set the following parameters:

Program: 1 = 45 °C

2 = 70 °C

3 = 105 °C

Drying time: automatic

Display: drying loss of sample as %

10B.2 Measurement Once the unit has been turned on and the desired program has been selected, open the drying hood, place a disposable aluminum pan on the holder- and tare the balance. Distribute roughly 5 g of the sample evenly on the pan.

Close the drying hood; the drying process will start automatically. During the drying process, the measured value will be displayed on the balance. After the drying process has been completed, the displayed value is stored, and "END" is displayed.

To change samples, press the 'CF' button until "SEL" appears in the display. The next sample can then be measured.

Calculated Moisture Content

11A.1 % Volatiles = W2 - W3 x 100/W2 - W1

Where 

W1 = Weight of dried evaporating dish

W2 = Original weight of sample plus evaporating dish

W3 = Final weight of sample plus evaporating dish

ACCURACY

For a loss of 1 mg an accuracy of ± 0.3% is possible if there are no losses or inaccuracies introduced in handling.

The Infrared divisions read to 0.01% moisture giving a precision of 0.01%. The accuracy of the method will have to be determined by comparing it with method A which will vary depending on the substances under test.

REPORTS

If the result is under 0.1% report as less than 0.1%w/w. Quote, the test method used Tex 23647A or 23647B, and temperature and vacuum conditions were used. Record all results and observations.

Calibration of Infrared Drying Equipment

SCOPE: Verifying and calibrating infra-red desiccators.

REFERENCE

Instruction Manual for the Infra-Red Drying Equipment that is in use.

DEFINITIONS

PRINCIPLE

Verification of the heating capacity of the apparatus for a given time.

REFERENCE STANDARD

Calibration kit for the desiccator

Temperature recording strip

The results from the equipment may be verified by determining the moisture on a primary standard where the moisture content is known. Pure inorganic salts with known amounts of water of crystallization may be used if the temperature required to drive off the moisture is known.

REAGENTS - none

EQUIPMENT

The infra-red desiccator is to be verified and/or calibrated

Calibration kit for the desiccator

Temperature recording strip

INTERFERENCE

The instrument should be housed in a draft-free area free from vibration.

SAMPLING - not applicable

PROCEDURE

Precautions, and recommendations: check on the spirit level that the apparatus is level.

(a) set the desiccator to 100°C, 20 minutes

(b) place the disc and temperature recording strip in the desiccator

(c) switch on the desiccator

(d) when the time has elapsed, read off the result.

RESULTS

The 99°C bands on the temperature recording strip must be completely colored and the 104°C bands must be intact. Record the result on the maintenance record sheet.

Settings/Adjustment

Increase or decrease the temperature using the screw on the side. Do the test again and record the results until the appliance conforms to specification.

CALCULATIONS

ACCURACY

Regular calibration following the Guidelines from the Supplier of the instrument ensures that the equipment conforms to the laid down standards for precision. The temperature used and the nature of the sample will influence the accuracy of the result as spitting or lack of complete drying may give a bias.

RECORD

There shall be a record of the calibration showing the person carrying out the operation, the date, the method used, and any problems or deviations from the method. The equipment shall state the next date for calibration. The amount of drift from the true values before the machine is adjusted shall be recorded as this indicates if calibration is frequent enough.