Typically performed in the field of analytical chemistry, gravimetric analysis can help to determine the mass of suspended particles in a solution. The solids can be suspended or dissolved; substances that crystallize usually require compounds called reagents to separate them out of a solution. Another step called digestion is often performed, which re-dissolves the precipitated particles and then filters them into a more pure form. There are several methods of preparing a sample. The total remaining substance, previously suspended in the liquid, can then be weighed to determine its total mass.
When the desired substance, or analyte, is separated from a solution, it is often further isolated by a process called precipitation. The entire substance must be precipitated for gravimetric analysis, and it usually has to be large enough to settle and be filtered. It should generally be pure as well, with no other compounds mixed in, and be in a stable state when dry. The solution can also be vaporized to collect the analyte; cold temperature equipment, such as a cryogenic trap, or an absorbent material like activated carbon are often used to collect and measure the amount of substance present.
Lab equipment such as bottles, beakers, and filters are often used for gravimetric analysis, so more sophisticated, expensive tools are typically not required. If the sample is precipitated, then scientists usually check to see if this step is complete by adding a special liquid. Vacuum filtration can then be used to transfer the solution; a piece of equipment called a rubber policeman is often helpful in checking if the precipitated substance has been completely transferred to a filter. The sample is typically then dried, and can then be weighed.
Precipitated substances sometimes need to be converted during gravimetric analysis, so that they are more chemically stable. The addition of particular compounds, depending on the precipitate, or heating, can complete this step. Once the sample of material cools, its weight is calculated, minus that of the container it is in. The mass of the precipitated substance, which was once suspended inside of a solution, can then be determined.
Gravimetric analysis is typically very accurate. It has also been used by scientists to calculate the atomic masses of most elements. The associated equipment is usually inexpensive, while the procedure can also be applied to calibrating the accuracy of laboratory instruments. It is, however, not as accurate when more than one substance is present in a sample.