Loss on Drying Determination for Chemicals: Principles, Standards, and Testing Methods

Loss on drying (LOD) determination is a fundamental gravimetric test that quantifies the moisture and volatile matter released when a chemical product is heated under precisely controlled conditions. This article provides a technical overview of LOD principles, relevant standards such as GB/T 6284, ASTM E1868, and ISO 787-2, typical sample types, drying procedures, and practical applications across the chemical industry. It also answers frequently asked questions to help quality control laboratories achieve accurate and reproducible results.

Technical Overview

Loss on drying (LOD) determination is a gravimetric analytical technique used to measure the mass fraction of volatile substances—primarily moisture, together with residual solvents and other low-boiling components—that are driven off from a chemical product when it is heated under specified conditions. A pre-dried weighing vessel is charged with a accurately weighed test portion, the sample is dried at a defined temperature (commonly 105 °C) either for a fixed time or to constant mass, cooled in a desiccator, and reweighed. The relative reduction in mass, expressed as a percentage, is reported as the loss on drying.

The calculation is performed as follows: LOD (%) = (m2 − m3) / (m2 − m1) × 100, where m1 is the mass of the empty dried vessel, m2 is the mass of the vessel plus the sample before drying, and m3 is the mass of the vessel plus the sample after drying to constant weight.

It is important to recognize that loss on drying is an empirical, non-specific test. The measured mass loss may include not only water but also residual organic solvents, adsorbed gases, and, if the temperature is too high, volatile decomposition products. Where a water-specific result is required, complementary techniques such as Karl Fischer titration should be considered. Key reference methods include GB/T 6284 (general method for determination of water content in chemical products by loss on drying), ASTM E1868 (loss on drying by thermogravimetry), ASTM D6980 (moisture in plastics by loss in weight), and ISO 787-2 (matter volatile at 105 °C in pigments and extenders).

Testing Samples

Inorganic Chemicals and Salts

Common test materials include chlorides, sulfates, carbonates, oxides, and hydroxides in powder, crystal, or granule form. Care must be taken with hydrated salts, because elevated drying temperatures may remove water of crystallization in addition to surface moisture, artificially inflating the result.

Organic and Fine Chemicals

Organic intermediates, dyes, surfactants, catalysts, and specialty chemicals frequently retain process solvents and atmospheric moisture. Loss on drying verifies that residual volatiles remain within the limits defined in the product specification.

Pharmaceutical and Food-Grade Raw Materials

Active ingredients, excipients, and additives are routinely tested for loss on drying because excess moisture can compromise chemical stability, flowability, microbial control, and shelf life.

Polymer and Plastic Materials

Resins, pellets, and polymer powders are tested for moisture and volatile content prior to melt processing. Excessive moisture in polymers such as polyesters and polyamides can cause hydrolytic degradation and surface defects in molded or extruded parts, as addressed in ASTM D6980.

Pigments, Fillers, and Coating Powders

Powder coatings, mineral fillers, and pigments are evaluated for volatile matter at 105 °C in accordance with ISO 787-2, since moisture influences dispersion quality, bulk density, and application properties.

Testing Items

Depending on the product specification and the selected standard, the following parameters are typically reported:

• Loss on drying, expressed as mass fraction (%) of the original sample
• Volatile matter content at a specified temperature and dwell time
• Total moisture content (approximated as total volatiles under the drying conditions)
• Hygroscopic moisture for hygroscopic mineral products
• Dry residue or total solids content, calculated as 100% minus the loss on drying
• Mass-loss profile versus temperature when thermogravimetric analysis is applied

Testing Methods

Convection Oven Drying Method

The classical procedure uses a ventilated drying oven set at 105 °C (or the temperature specified in the product standard). A 1–10 g test portion is spread in a shallow, pre-dried weighing bottle, dried for two to four hours, cooled in a desiccator for approximately 30 minutes, and weighed. Drying cycles are repeated until constant weight is achieved. This method offers the best comparability with regulatory specifications and is the reference technique in GB/T 6284.

Vacuum Oven Drying Method

For heat-sensitive substances such as certain organic intermediates, enzymes, and hydrates, drying is performed under reduced pressure (for example, 60–80 °C at 5 kPa or below). The lower operating temperature and partial vacuum remove moisture gently while minimizing thermal degradation and avoiding loss of crystal water.

Infrared and Halogen Rapid Moisture Analysis

Rapid moisture analyzers combine a halogen or infrared heating source with an integrated precision balance, delivering results in five to fifteen minutes. They are widely used for in-process control and incoming inspection, but the drying program must be validated against the oven method for each specific matrix.

Thermogravimetric Analysis (TGA)

Thermogravimetry, standardized in ASTM E1868, heats a milligram-scale sample through a programmed temperature ramp while continuously recording mass. TGA distinguishes overlapping mass-loss events, such as surface moisture versus structural water or solvent loss, and provides insight into thermal stability in addition to a quantitative loss-on-drying value.

Desiccator Method

For extremely thermolabile materials, the sample is stored over a strong desiccant such as phosphorus pentoxide or silica gel at ambient temperature until constant weight is reached. The procedure is slow but avoids any thermal stress on the specimen.

Applications

Loss on drying determination supports quality assurance and regulatory compliance throughout the chemical value chain. Typical applications include:

• Incoming raw material inspection and supplier qualification for purchased chemicals
• Batch release testing against internal or contractual moisture specifications
• Endpoint control of drying, crystallization, and spray-drying unit operations
• Long-term and accelerated stability studies to monitor moisture uptake over shelf life
• Verification of dry-basis assay calculations, since analytical results are often corrected to a dry or as-received basis
• Trade settlement and customs declarations where moisture limits are contractually defined
• Research and development studies of hygroscopicity, packaging selection, and formulation behavior

Accredited third-party laboratories routinely perform loss on drying tests in accordance with GB, ISO, and ASTM methods, issuing test reports that are widely accepted for product registration, export documentation, and customer audits.

FAQ

What is the difference between loss on drying and water content?

Loss on drying measures the total mass lost upon heating, which may include water, residual solvents, and volatile degradation products. Water content methods such as Karl Fischer titration are specific to water molecules. For most inorganic and organic chemicals the two values are close, but for solvent-retaining or heat-sensitive materials they can differ significantly.

At what temperature should the test be performed?

The temperature is defined by the applicable product standard; 105 °C is the most common condition. Lower temperatures or vacuum drying are selected for hydrates and thermolabile substances, while some specifications require 200 °C or higher to capture higher-boiling volatiles. Deviating from the specified condition changes the result and must be reported.

What is meant by drying to constant weight?

Constant weight is reached when the difference between two successive weighings, after additional drying cycles of at least 30 minutes, does not exceed a defined tolerance—typically 0.3 mg or the limit stated in the product standard. Reaching constant weight confirms that all volatile matter removable under the chosen conditions has been eliminated.

How much sample is required?

A test portion of 1 g to 10 g is typical, selected so that the absolute mass loss is large enough for the required precision but thin enough to dry uniformly. The sample must be homogeneous and representative, and hygroscopic materials should be handled rapidly in a low-humidity environment.

Can thermal decomposition distort the result?

Yes. If the drying temperature approaches the decomposition point of the sample, degradation products will evaporate and be incorrectly counted as moisture. In such cases, vacuum drying at a reduced temperature, TGA to identify the mass-loss steps, or a water-specific method should be used instead.

Why must the sample be cooled in a desiccator before weighing?

A hot sample warms the air inside the weighing vessel, creating convection currents that cause unstable and erroneously low readings, and a hot hygroscopic surface will rapidly reabsorb atmospheric moisture during cooling. Cooling in a desiccator isolates the dried sample from ambient humidity until an accurate stable weight can be recorded.