Particle Size Determination of Chemical Products: Testing Methods, Standards, and Applications

Particle size determination is a critical analytical process for evaluating the quality, performance, and processability of chemical products, including powders, pigments, fillers, catalysts, emulsions, and nanomaterials. This technical article provides a comprehensive overview of particle size testing samples, measurable parameters, internationally recognized methods such as laser diffraction and dynamic light scattering, and the relevant ISO, ASTM, and GB standards that govern accurate measurement. Whether for quality control, research and development, or regulatory compliance, understanding particle size determination helps manufacturers optimize product performance and consistency.

Technical Overview

Particle size determination refers to the quantitative measurement and characterization of particle dimensions and particle size distribution (PSD) in chemical products. Particle size is one of the most influential physical properties in the chemical industry, directly affecting dissolution rate, chemical reactivity, flowability, bulk density, packing behavior, pigment color strength, suspension stability, dusting tendency, and downstream processing performance. Even when the chemical composition of two batches is identical, differences in particle size distribution can lead to dramatically different behavior in application.

Chemical products span an extremely broad size range, from sub-10-nanometer colloidal dispersions to millimeter-scale granules and pellets. Because no single analytical technique can cover this entire range with adequate resolution, several complementary measurement principles have been standardized, including laser diffraction, dynamic light scattering, sieve analysis, sedimentation, and image analysis. These methods are governed by international and national standards such as ISO 13320, ISO 22412, ISO 13317, ISO 13322, ASTM B822, ASTM E11, ASTM D1921, GB/T 19077, and GB/T 15445, which specify instrument requirements, dispersion procedures, measurement conditions, and data reporting formats.

Modern particle size determination is typically performed in accredited third-party laboratories or in-house quality control laboratories, providing traceable, reproducible data that supports product release, process optimization, research and development, and regulatory submission. Correct sampling, sample dispersion (avoiding both agglomeration and particle breakage), appropriate optical model selection, and validated data interpretation are essential for obtaining results that truly represent the bulk material.

Testing Samples

Particle size determination applies to a wide variety of chemical product categories. Typical sample types submitted for testing include the following:

Powder Chemical Products

Fine powders represent the largest category of samples for particle size testing. These include inorganic and organic pigments such as titanium dioxide and iron oxide, mineral fillers such as calcium carbonate, talc, kaolin, barium sulfate, and silica, as well as specialty chemicals, dye stuffs, flame retardants, and fine abrasive materials. Powders are usually measured either as dry dispersions or as wet suspensions in a suitable carrier liquid, depending on their solubility and agglomeration tendency.

Catalysts and Adsorbent Materials

Catalysts, molecular sieves, activated carbons, and carrier materials require careful particle size control because size influences surface area exposure, pressure drop in fixed-bed reactors, and mechanical strength. Both the fresh material and attrited or crushed samples after strength testing are frequently analyzed.

Emulsions, Suspensions, and Polymer Dispersions

Liquid and semi-liquid chemical products, including polymer latexes, emulsion polymers, waterborne resins, agrochemical emulsions and suspension concentrates, and colloidal silica, are analyzed as liquid dispersions. These samples often require dilution in an appropriate medium to achieve the correct measurement concentration while maintaining colloidal stability.

Nanomaterials and Ultrafine Products

Engineered nanomaterials such as nano-calcium carbonate, nano-zinc oxide, carbon black, and colloidal metals demand specialized techniques with nanometer-scale resolution, most commonly dynamic light scattering, complemented by electron microscopy for morphological confirmation.

Granules, Pellets, and Coarse Materials

Fertilizer granules, plastic pellets and masterbatches, detergent granules, and crystalline products with larger particle sizes are typically characterized by mechanical sieve analysis or dynamic image analysis, which provide robust results for particles above approximately 38 to 75 micrometers.

Testing Items

Depending on the client's requirements and the product specification, particle size determination reports may include the following measurable parameters:

• Particle size distribution (PSD) on a volume, number, or mass basis, presented as differential and cumulative distribution curves
• Percentile diameters D10, D50, and D90 (and Dv10, Dv50, Dv90 for volume-based results), indicating the diameter below which 10%, 50%, or 90% of the particles are contained
• Mean particle size, including number-weighted mean diameter, volume-weighted mean diameter (D43), and Sauter mean diameter (D32)
• Mode size and distribution width indicators such as span, defined as (D90 minus D10) divided by D50
• Uniformity coefficient and distribution sharpness for granular products
• Top-cut size, maximum particle size, oversized fraction, and fines content below a specification threshold
• Sieve residue at specified mesh apertures (for example, residue on 45-micrometer or 325-mesh sieves)
• Estimated specific surface area derived from size distribution, complementing direct BET gas adsorption measurements when required

Results are reported in micrometers (µm) or nanometers (nm) with clear indication of the measurement basis, optical model applied, dispersion medium, and measurement standard followed, ensuring full traceability and comparability between laboratories.

Testing Methods

Method selection depends on the expected particle size range, sample state (dry powder or liquid dispersion), chemical properties, and the applicable product specification. The principal standardized techniques are described below.

Laser Diffraction

Laser diffraction, standardized in ISO 13320, GB/T 19077, and ASTM B822, is the most widely used method for chemical products, covering a typical range from approximately 0.1 micrometers to 3,500 micrometers. The technique measures the angular scattering pattern of a dispersed sample illuminated by a laser beam and converts it into a particle size distribution using Mie or Fraunhofer diffraction theory. Both wet dispersion (circulating the sample in a liquid cell) and dry dispersion (airborne delivery using compressed gas) configurations are available, making the method suitable for the majority of powders, granules, suspensions, and emulsions. Key advantages include rapid measurement, broad dynamic range, excellent repeatability, and well-established instrument qualification procedures.

Dynamic Light Scattering (DLS)

Dynamic light scattering, standardized in ISO 22412 and GB/T 29022, measures the Brownian motion of particles in suspension and determines the hydrodynamic diameter via the Stokes-Einstein relationship. It is the preferred technique for nanoparticles, colloidal dispersions, polymer latexes, and fine emulsions in the approximate range of 0.3 nanometers to 10 micrometers. Results are typically reported as intensity-weighted Z-average diameter and polydispersity index (PDI), with optional conversion to volume and number distributions.

Sieve Analysis

Mechanical sieving, standardized in ASTM E11, ASTM D1921, ISO 3310, and GB/T 6003, remains a fundamental reference method for coarser chemical products. A nested stack of calibrated woven-wire sieves is agitated mechanically, and the mass retained on each sieve is weighed to construct a cumulative mass-based distribution. Dry sieving suits free-flowing granules, while wet sieving is applied to powders that agglomerate or contain water-soluble components. Sieve analysis is direct, absolute, and traceable, making it ideal for fertilizer, plastic pellets, and specification of sieve residue.

Sedimentation Methods

Gravitational and centrifugal liquid sedimentation, standardized in ISO 13317, determine particle size from the settling velocity of particles in a fluid according to Stokes' law. The technique is well suited to dense, fine powders such as pigments and ceramic materials in the approximate range of 0.05 to 300 micrometers, and it provides high resolution in the fine fraction where it historically served as a reference method.

Image Analysis and Microscopy

Static image analysis (ISO 13322-1) using optical or electron microscopy and dynamic image analysis (ISO 13322-2) capture direct geometric measurements of individual particles, delivering not only size distributions but also shape descriptors such as aspect ratio, circularity, and elongation. Electron microscopy (SEM/TEM) provides essential verification for nanomaterials and fibrous or acicular particles, and image-based methods serve as valuable arbiters when optical methods disagree.

Electrical Sensing Zone Method

The electrical sensing zone (Coulter principle) technique measures the electrical resistance pulse generated as particles suspended in an electrolyte pass through a small aperture. It delivers number-based size distributions and absolute particle counts over a range of roughly 0.5 to 1,000 micrometers and is particularly useful for narrow distributions and contamination analysis in clean liquid systems.

Applications

Particle size determination data support critical decisions throughout the chemical product lifecycle:

• Quality control and batch release testing against internal or contractual particle size specifications for pigments, fillers, and specialty chemicals
• Process development and optimization of unit operations such as milling, classification, crystallization, spray drying, emulsification, and granulation
• Research and development of new materials, including nanostructured products and functional coatings, where size-property relationships must be established
• Raw material incoming inspection and supplier comparison to ensure consistent feedstock quality
• Stability studies of suspensions and emulsions, detecting agglomeration, creaming, or particle growth over time
• Regulatory compliance, customs inspection, and certification support, including documentation of nano-material classification where applicable
• Failure analysis and troubleshooting of application problems such as poor dispersion, filter blockage, surface defects, and inconsistent reactivity

By correlating particle size data with application performance, manufacturers can establish meaningful specifications, reduce variability, and accelerate product development cycles.

FAQ

How do I choose the right particle size measurement method for my chemical product?

Method selection depends primarily on the expected size range and sample form. Liquid dispersions and emulsions below roughly 10 micrometers are best measured by dynamic light scattering; powders and suspensions between 0.1 and 3,500 micrometers are most efficiently measured by laser diffraction; granular products above 38 to 75 micrometers are typically characterized by sieve analysis or image analysis. Accredited third-party laboratories can advise on the most appropriate technique based on a preliminary evaluation of your sample.

What is the difference between D50 and mean particle size?

D50 is the median diameter, meaning half of the particles by the chosen weighting basis are smaller and half are larger. Mean particle size is an arithmetic calculation, such as D43 (volume moment mean) or D32 (surface area moment mean), and is sensitive to the shape of the distribution. For skewed or multimodal distributions, D50 and the mean can differ significantly, so both should be interpreted together with the full distribution curve.

Why can laser diffraction and DLS results differ for the same sample?

The two techniques measure different size representations: laser diffraction reports a volume-equivalent spherical diameter, while DLS reports an intensity-weighted hydrodynamic diameter. In addition, different weighting bases (intensity, volume, number) emphasize different parts of the distribution. Differences are expected and do not necessarily indicate error; they reflect the underlying physical principles of each method.

How are agglomerated powders handled during sample preparation?

Proper dispersion is essential. Wet measurements use suitable dispersants, surfactants, and controlled ultrasonic energy to break soft agglomerates without fracturing primary particles, while dry measurements optimize dispersion air pressure for the same purpose. Dispersion adequacy is verified by repeating measurements under increasing dispersion energy until the result stabilizes.

What repeatability and accuracy can be expected?

For a well-dispersed sample, laser diffraction typically achieves repeatability of the D50 within approximately 1% relative standard deviation and D10/D90 within about 2–3%, in line with ISO 13320 requirements. Accuracy depends on correct optical properties, sampling representativeness, and instrument qualification using certified reference materials.

How much sample is required and how long does testing take?

Laser diffraction and sieve analysis generally require from a few grams to several tens of grams of representative sample, while DLS requires only a small volume of dilute dispersion. Routine measurements are usually completed within minutes after dispersion development, and most standard tests can be reported within a few working days, including method setup and data verification.