Gravimetric analysis determines the mass of particles in a solution through precipitation and filtration. The sample is then dried and weighed, and the mass of the precipitated substance is calculated. It is accurate and inexpensive but less so with multiple substances.
Typically performed in the field of analytical chemistry, gravimetric analysis can help determine the mass of particles suspended in a solution. Solids can be suspended or dissolved; substances that crystallize usually require compounds called reagents to separate them from a solution. Another step called digestion is often performed, which redissolves the precipitated particles and then filters them back into a purer form. There are several methods for 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 usually must be large enough to settle and be filtered out. It should generally also be pure, with no other compounds mixed in, and be in a stable state when dry. The solution can also be sprayed to collect the analyte; Low-temperature equipment, such as a cryogenic trap, or an absorbent material such as activated carbon is often used to collect and measure the amount of substance present.
Laboratory equipment such as bottles, beakers, and filters are often used for gravimetric analysis, so more sophisticated and expensive instruments are typically not needed. If the sample has precipitated, scientists usually check 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 useful for checking whether the precipitated substance has completely transferred into a filter. The sample is typically then dried and can then be weighed.
Precipitated substances sometimes have 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 has cooled, its weight is calculated, minus that of the container it is in. The mass of the precipitated substance, which was once suspended within a solution, can then be determined.
Gravimetric analysis is usually 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. However, it is not as accurate when there is more than one substance in a sample.
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