Gravimetry is a method of measurement based entirely on mass. In chemistry, it means determining how much of a substance is in a sample by isolating that substance and weighing it. In geophysics, it means measuring variations in Earth’s gravitational pull to map what lies underground. The common thread is simple: weigh something precisely, then use that measurement to answer a question.
How Chemical Gravimetry Works
In a chemistry lab, gravimetric analysis starts with a sample of unknown composition. The goal is to figure out exactly how much of a specific substance (the analyte) that sample contains. To do this, a chemist physically or chemically separates the analyte from everything else, weighs it, and then uses the known relationships between the compounds involved to calculate its concentration in the original sample.
What makes gravimetry distinctive is that it requires no calibration against reference standards. The measurement comes directly from mass and the mathematical ratios built into chemical formulas. This is why gravimetric methods were historically used to determine the atomic masses of many elements, in some cases to six decimal places of accuracy. Modern gravimetric analysis routinely achieves precision within a few hundredths of a percent.
The Three Main Types
Chemical gravimetry comes in three forms, each using a different strategy to isolate the substance being measured.
- Precipitation gravimetry is the most common. A chemical reagent is added to the sample solution, causing the target substance to form a solid that drops out of the liquid. That solid is collected, dried, and weighed. For example, adding a barium chloride solution to a sample containing sulfate ions produces a white solid (barium sulfate) that captures all the sulfate. Weighing that solid tells you exactly how much sulfate was in the original sample.
- Volatilization gravimetry works in the opposite direction. Instead of creating a solid, you drive off a gas. The sample is heated or chemically treated so the target substance escapes as a vapor, and you measure either the mass lost by the sample or the mass gained by an absorber that traps the vapor. Measuring moisture content by heating a sample and recording the weight loss is the most familiar example.
- Electrogravimetry uses electrical current to deposit a metal from solution onto a platinum electrode. After the metal has fully plated out, the electrode is dried and weighed. The weight difference before and after tells you how much metal was present. This method works for a wide range of metals, whether alone or in mixtures, and has a typical precision of a few parts per thousand. Like other gravimetric methods, it needs no calibration because the result comes straight from the deposited mass and the known properties of the metal.
Steps in a Precipitation Analysis
Precipitation gravimetry follows a careful sequence designed to ensure nothing is lost or contaminated along the way. First, the sample is accurately weighed and dissolved in water. Then a reagent is added slowly, over several minutes, to the heated solution while stirring. The slow addition matters because it produces larger, purer crystals of the solid that are easier to handle. Once all the reagent has been added, the mixture sits for about 20 minutes so the solid can settle.
The solid is then collected by filtration and washed to remove any residue from the solution. Next comes heating. The collected solid is placed in a crucible and heated intensely, sometimes to the point of glowing, in order to burn off the filter material and drive away any remaining moisture. This step is repeated until the mass stops changing, typically within 0.005 grams between consecutive weighings. That final, stable mass is the number used for the calculation.
The Gravimetric Factor
Converting the mass of the isolated solid into the amount of analyte in the original sample requires a conversion ratio called the gravimetric factor. This factor is built from the molecular weights of the analyte and the precipitate, adjusted for how many molecules of each are involved in the reaction.
The calculation itself is straightforward. You divide the mass of the precipitate by the mass of the original sample, then multiply by the gravimetric factor. The result is the mass fraction of the analyte: essentially, the percentage of your sample that was made up of the substance you were looking for. Because the factor comes entirely from atomic masses and reaction ratios, no external calibration curve is needed.
Precision and Limitations
Gravimetric methods are among the most accurate in analytical chemistry when performed carefully. In one study evaluating the purity of high-purity lead, conventional gravimetry measured purity at 99.98% with an expanded uncertainty of just ±0.24 percentage points at a 95% confidence level. Electrogravimetry produced nearly identical results: 99.97% ± 0.27 percentage points.
The main source of error is practical, not theoretical. Failing to fully dry the precipitate, losing small amounts during filtration, or neglecting to correct for air buoyancy (which introduces roughly a 0.1% error) can all throw off results. The method also takes time. Between dissolving, precipitating, filtering, and repeatedly heating to constant weight, a single analysis can take several hours. This makes gravimetry less convenient than faster instrumental methods for routine work, but its accuracy keeps it relevant for high-stakes measurements like setting reference standards.
Gravimetry in Pharmaceuticals
The pharmaceutical industry uses gravimetric methods to measure volatile content in drugs, a test formally known as “loss on drying.” The United States Pharmacopeia specifies how this works: 1 to 2 grams of the substance is weighed accurately, spread to a depth of about 5 mm in a shallow glass-stoppered bottle, and placed in a drying chamber at a specified temperature (held within ±2 degrees of the target). After drying for the required time, the bottle is sealed, cooled in a desiccator to prevent moisture reabsorption, and weighed again.
For capsules, the test is run on the mixed contents of at least four capsules. For tablets, at least four are ground into a fine powder first. If a substance would melt at the specified drying temperature, it is first held for one to two hours at 5 to 10 degrees below its melting point before being brought up to the full temperature. The weight difference tells manufacturers how much moisture or other volatile material is present, ensuring each batch meets its purity specification.
Gravimetry in Soil Science
Soil scientists rely on gravimetric drying to measure moisture content in soil. The process is conceptually identical to pharmaceutical loss-on-drying: weigh a soil sample, dry it in an oven, then weigh it again. The weight lost equals the water that was present. Standard protocol calls for drying at 105°C for at least 24 hours. If the dried soil will also be tested for carbon and nitrogen content (which would be altered at higher temperatures), a gentler approach is used: 60°C for at least 48 hours.
This gravimetric soil moisture value serves as the baseline against which faster electronic soil moisture sensors are calibrated. It remains the definitive measurement because, like all gravimetric methods, it relies on nothing more than a precise scale and careful technique.
Gravimetry in Geophysics
Outside the laboratory, gravimetry takes on a completely different meaning. Geophysical gravimetry measures tiny variations in Earth’s gravitational field from place to place. These variations, called gravity anomalies, reveal differences in the density of rock and sediment below the surface. Denser materials pull slightly harder, so a sensitive gravity meter (gravimeter) can detect where heavy mineral deposits, underground cavities, or geological faults are located.
Geodesists and geophysicists use gravity anomalies differently. Geodesists use them to define the geoid, the surface that represents what mean sea level would look like if it extended across all continents. This is critical for setting accurate elevation benchmarks worldwide. Geophysicists, on the other hand, strip away broad gravitational effects to isolate local anomalies that hint at subsurface structure. Their reference level can be set at any convenient height, such as the average elevation of the survey area, because they care about relative differences rather than absolute values. Applications range from oil and mineral exploration to monitoring groundwater reserves and volcanic activity.