揮発

Volatilization converts a desired component, called an analyte, into a volatile species through controlled heating or chemical reactions. The volatile substance is then completely removed from the sample by heating or bubbling an inert gas through the mixture. This separation allows measurement of the analyte's original amount by determining what remains or what was removed, similar to how states of matter and phase changes govern evaporation.

Volatilization gravimetry measures the mass difference of a sample before and after the volatile component evaporates. The weight loss directly indicates the amount of volatile substance present. This mass difference is then used to calculate the analyte's concentration in the original sample, provided no other constituents volatilize during the process.

A sample can be treated with a chemical reagent in a closed vessel to transform the analyte into a chemically different volatile species with known composition. The volatilized substance is then selectively absorbed in a pre-weighed trap containing an appropriate absorbent. The weight gain of the trap reveals the amount of volatile substance released, which is stoichiometrically related to the original analyte.

Pre-weighed traps filled with suitable absorbents selectively capture volatilized substances. For example, soda-lime absorbs carbon dioxide from carbonate compounds, while non-fibrous silicate traps absorb sodium hydroxide. Weighing the trap before and after volatilization determines the mass of absorbed material, which indicates the amount of analyte in the original sample.

Heating a known amount of hydrated sample in a crucible causes water to evaporate and escape. The mass difference between the original sample and the remaining non-volatile components equals the water mass. This approach directly measures the water of crystallization present in hydrated compounds through simple mass measurement.

The sample must be heated to a sufficiently high temperature in a suitable vessel, or an inert gas stream must be bubbled through the mixture to completely expel vapors. These conditions ensure all volatile material is removed from the reaction vessel or crucible, preventing incomplete separation that would compromise the accuracy of the mass-based measurement.

If other constituents in the sample also volatilize, the measured mass loss will not accurately represent only the analyte. The weight difference would include contributions from multiple volatile components, making it impossible to determine the true amount of the target analyte. Selective volatilization ensures measurement accuracy and reliable quantitative results.