Crucible

  A crucible is a cup-shaped piece of laboratory equipment used to contain chemical compounds when heating them to very high temperatures. Crucibles are available in several sizes and typically come with a crucible cover (or lid).

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Crucible materials and description

Crucibles and their covers are made of high temperature-resistant materials, usually porcelain or an inert metal. One of the earliest uses of platinum was to make crucibles. More recently, metals such as nickel and zirconium have been used. The lids are typically loose-fitting to allow gases to escape during heating of a sample inside. Crucibles and their lids can come in high form and low form shapes (see Ext. Link 2 below) and in various sizes, but rather small 10–15 ml size porcelain crucibles are commonly used for gravimetric chemical analysis. These small size crucibles and their covers made of porcelain are quite cheap when sold in quantity to laboratories, and the crucibles are sometimes disposed of after use in precise quantitative chemical analysis. There is usually a large mark-up when they are sold individually in hobby shops.

      A crucible is also a container in which metals are melted, usually for temperatures above 500 °C. These crucibles are usually made of graphite with clay as a binder. These crucibles are very durable and resist temperatures to over 1600 °C. A crucible is placed into a furnace and, after the melting, the liquid metal is taken out of the furnace and poured into the mold. Some furnaces (usually electric or induction) have an embedded crucible and are tilted when the metal is poured out.

Use in chemical analysis

In the area of chemical analysis, crucibles are used in quantitative gravimetric chemical analysis (analysis by measuring mass of an analyte). Common crucible use may be as follows. A residue or precipitate in a chemical analysis method can be collected or filtered from some sample or solution on special "ashless" filter paper. The crucible and lid to be used are pre-weighed very accurately on an analytical balance. After some possible washing and/or pre-drying of this filtrate, the residue on the filter paper can be placed in the crucible and fired (heated at very high temperature) until all the volatiles and moisture are driven out of the sample residue in the crucible. The "ashless" filter paper is completely burned up in this process. The crucible with the sample and lid is allowed to cool in a desiccator. The crucible and lid with the sample inside is weighed very accurately again only after it has completely cooled to room temperature (higher temperature would cause air currents around the balance giving inaccurate results). The mass of the empty, pre-weighed crucible and lid is subtracted from this result to yield the mass of the completely dried residue in the crucible.

A crucible with a bottom perforated with small holes which is designed specifically for use in filtration, especially for gravimetric analysis as just described, is called a Gooch crucible after its inventor, Frank Austen Gooch.

For completely accurate results, the crucible is handled with clean tongs because fingerprints can add weighable mass to the crucible. Porcelain crucibles are hygroscopic, i. e. they absorb a bit of weighable moisture from the air. For this reason, the porcelain crucible and lid is also pre-fired (pre-heating to high temperature) to constant mass before the pre-weighing. This determines the mass of the completely dry crucible and lid. At least two firings, coolings, and weighings resulting in exactly the same mass are needed to confirm constant (completely dry) mass of the crucible and lid and similarly again for the crucible, lid, and sample residue inside. Since the mass of every crucible and lid is different, the pre-firing/pre-weighing must be done for every new crucible/lid used. The desiccator contains desiccant to absorb moisture from the air inside, so the air inside will be completely dry.

Use in Ash Content Determination

Ash is the completely unburnable inorganic salts in a sample. A crucible can be similarly used to determine the percentage of ash contained in an otherwise burnable sample of material such as coal, wood, or oil. A crucible and its lid are pre-weighed at constant mass as described above. The sample is added to the completely dry crucible and lid and together they are weighed to determine the mass of the sample by difference. The crucible, lid, and sample are then fired to constant mass to completely burn up the sample, leaving behind only the completely unburnable ash. After cooling in dryness, the crucible, lid, and remaining ash are weighed to find the mass of the ash from the sample by difference. The fraction of ash (by mass) in the sample is determined by the dividing the mass of the ash by the mass of the sample before burning, which is done by subtracting the weight of the crucible and lid from the figure of the container, lid, and sample.