Loss on Drying (LOD) Testing for Chemical Products: Principles, Methods, and Applications

Loss on drying (LOD) testing is a fundamental gravimetric technique used to determine the moisture and volatile matter content of chemical products, providing essential data for quality control, process optimization, and regulatory compliance. Accredited third-party laboratories perform chemical loss on drying testing in accordance with recognized standards such as GB/T 6284, ISO 3251, and ASTM D6980, delivering accurate and reproducible results across inorganic chemicals, organic intermediates, polymers, and fine chemicals. This article explains the technical principle, applicable sample types, testing items, standard methods, and typical industrial applications of loss on drying analysis.

Technical Overview

Loss on drying (LOD), also referred to as drying weight loss or moisture and volatile matter determination, measures the mass fraction of a chemical sample that is lost when the sample is dried under precisely controlled temperature, pressure, and time conditions. The measured weight loss primarily represents free and adsorbed water, but it may also include residual solvents, water of crystallization released at elevated temperatures, and other low-boiling volatile components.

The test is performed by accurately weighing a homogenized sample before and after drying, then calculating the result using the relationship: LOD (%) = (m1 − m2) / m1 × 100, where m1 is the initial sample mass and m2 is the mass after drying to constant weight. Drying conditions are selected according to the thermal stability and hygroscopicity of the material so that only moisture and volatiles are removed while the matrix remains chemically unchanged.

Key reference standards include GB/T 6284 (chemical products — general method for determination of water content by loss on drying), ISO 3251 (determination of loss on drying and non-volatile matter content for paints, varnishes, and plastics), and ASTM D6980 (determination of water content of plastics by loss on drying). It is important to distinguish LOD from Karl Fischer titration: LOD captures the total volatile mass loss, whereas Karl Fischer methods are selectively specific to water. Moisture levels directly influence product purity, flowability, caking tendency, reaction stoichiometry, storage stability, and commercial value, which is why loss on drying testing is a routine release criterion in the chemical industry.

Testing Samples

Inorganic Chemicals

Inorganic salts, oxides, hydroxides, carbonates, phosphates, and fertilizer-grade materials such as ammonium salts, potassium salts, and urea-based compounds are routinely tested. These materials are often hygroscopic, and moisture ingress during production, packaging, or transport must be quantified against specification limits.

Organic Chemicals and Intermediates

Dyes, pigments, surfactants, organic acids, amines, esters, and process intermediates are tested to verify dryness after synthesis, crystallization, filtration, or drying operations. For heat-sensitive organics, reduced-temperature or vacuum drying conditions are selected to prevent decomposition or sublimation during analysis.

Polymer and Resin Materials

Thermoplastic powders, resin pellets, polymer additives, and masterbatch materials are analyzed because residual moisture can cause hydrolysis, bubbles, or molecular weight degradation during subsequent melt processing. Plastics testing is commonly performed following ASTM D6980 or equivalent rapid moisture balance procedures.

Fine, Specialty, and Pharmaceutical-Grade Chemicals

Catalysts, adsorbents, electronic chemicals, food additives, and pharmaceutical intermediates require tight moisture control to ensure potency, stability, and compliance with customer and regulatory specifications. Micro-scale and high-precision gravimetric procedures are applied for these high-value materials.

Testing Items

Typical determinations offered for chemical products include:

• Loss on drying (total moisture and volatile matter content, expressed as mass percent)
• Free and surface moisture content
• Water of crystallization released under specified drying conditions
• Residual volatile solvents within the scope of gravimetric drying loss
• Dry mass fraction and dry-basis purity correction
• Constant-weight verification and measurement repeatability assessment

Testing Methods

Constant-Weight Oven Drying Method

A weighed sample (typically 2–10 g in a pre-dried weighing bottle) is heated in a gravity-convection or forced-ventilation oven, commonly at 105 ± 2 °C for stable inorganic materials, until constant weight is achieved. Constant weight is generally confirmed when the difference between two successive weighings does not exceed the tolerance defined in the applicable standard (commonly 0.3 mg or a defined relative value). This method offers the highest accuracy and serves as the referee technique.

Vacuum Drying Method

Heat-sensitive, hygroscopic, or readily oxidized substances are dried under reduced pressure, typically at 40–70 °C. Lowering the pressure reduces the boiling point of water and solvents, allowing gentle moisture removal without thermal degradation. This approach is standard for APIs, fine chemicals, and materials containing bound moisture.

Rapid Infrared and Halogen Moisture Analysis

Moisture analyzers combining an infrared or halogen heating source with an integrated precision balance provide results within 5–20 minutes. These instruments are widely used for in-process quality control, incoming material screening, and high-throughput routine testing, and are calibrated against oven-based reference methods.

Thermogravimetric Analysis (TGA)

TGA records sample mass continuously as temperature is ramped in a controlled atmosphere, generating a mass-loss profile. This allows differentiation between moisture loss, solvent desorption, and thermal decomposition events, and is particularly useful for method development and investigating ambiguous weight-loss behavior.

Applications

Loss on drying testing supports a wide range of industrial and commercial purposes:

• Quality control of finished chemical products against internal and customer specifications
• Incoming inspection and supplier verification of raw materials
• Monitoring and optimization of drying, crystallization, and packaging unit operations
• Trade settlement, customs declaration, and dry-basis pricing of bulk chemicals
• Shelf-life evaluation and storage stability studies under accelerated humidity conditions
• Regulatory compliance support and generation of test reports recognized in international trade

FAQ

Q: What is the difference between loss on drying and moisture content?

A: Loss on drying measures the total mass lost upon drying, which includes water plus any volatile solvents and other low-boiling substances. Moisture content determined by Karl Fischer titration is specific to water alone. For materials containing volatile organic residues, LOD results may be slightly higher than Karl Fischer water values.

Q: How much sample is required for testing?

A: Standard oven methods typically require 2–10 g of homogenized sample, while TGA and moisture analyzer procedures may require only a few milligrams to a few grams. Representative sampling is essential, particularly for heterogeneous or segregated bulk materials.

Q: What drying temperature is selected?

A: 105 °C is the general default for thermally stable inorganic chemicals. Heat-sensitive organics, pharmaceuticals, and polymer materials are dried at lower temperatures, often under vacuum, to avoid decomposition, melting, or loss of matrix components.

Q: How is constant weight confirmed?

A: The sample is weighed, re-dried for an additional interval, and weighed again. When the difference between successive weighings falls below the tolerance specified in the standard (for example, 0.3 mg), the sample is considered at constant weight and the final mass is recorded.

Q: How long does a typical test take?

A: Conventional oven drying to constant weight may take from several hours up to 24 hours depending on the material. Rapid infrared or halogen moisture analyzers typically deliver results in 5–20 minutes and are suitable for routine screening when validated against the reference method.

Q: Which standards govern chemical loss on drying testing?

A: Commonly applied standards include GB/T 6284 for chemical products, ISO 3251 for coatings and plastics, and ASTM D6980 for plastics. Product-specific monographs and customer specifications may define particular drying temperatures, durations, and acceptance limits.