Evaporating dish
Encyclopedia
An evaporating dish is a piece of laboratory glassware
used for the evaporation
of solids and supernatant fluidsA fluid containing a saturated solution of a dissolved compound, overlying a forming precipitate of this compound., and sometimes to their melting point
. Evaporating dishes are used to evaporate excess solvents, most commonly water - to produce a concentrated solution or a solid precipitate of the dissolved substance.
Most are of porcelain
or borosilicate glass
. Shallow glass evaporating dishes are commonly termed "watch glasses", from their original use as the front window of a pocket watch
.Although watch glasses were historically used by Victorian chemists, owing to their cheap availability, the chemist's modern "watch glass" is thicker, flatter and of a heat-resistant glass Some used for high-temperature work are of refractory metals, usually of platinum
, owing to its non-reactive behaviour and low risk of contamination.
The capacity of evaporators is usually small - in the range 3-10 ml
. Larger dishes, up to 100 ml, are different in shape, and are more hemispherical.
The evaporator is used most often in quantitative analysis
.
In the determination of silicon
content in an organic sample, a small and accurately-measured quantity of a substance is added to the large amount of sulfuric acid
, then heated in an evaporating dish. The dish is heated with a Bunsen burner
, until only stable precipitate remains, which contains the silica content. The dish is then closed and heated at high temperature until completely clean, fused silica is produced. Comparison of the initial weight of the substance and that of the fused silica allows to the content of silicon in the sample to be determined.
The shape of the evaporating dish encourages evaporation in two ways:
When heating liquid in an evaporating dish, the low walls encourage splashes and so stirring or swirling of evaporating liquids is considered bad practice, owing to the risk of spillage.
Evaporation in a laboratory, especially in production quantities rather than merely for analysis, is now mostly performed in a rotary evaporator
. This is preferred because it works much faster and may be used under vacuum
, avoiding unwanted reactions with the atmosphere and allowing control of noxious fumes. Evaporation under vacuum also reduces the severity of bumping
and violent ebullition.
Laboratory glassware
Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...
used for the evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
of solids and supernatant fluidsA fluid containing a saturated solution of a dissolved compound, overlying a forming precipitate of this compound., and sometimes to their melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
. Evaporating dishes are used to evaporate excess solvents, most commonly water - to produce a concentrated solution or a solid precipitate of the dissolved substance.
Most are of porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
or borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
. Shallow glass evaporating dishes are commonly termed "watch glasses", from their original use as the front window of a pocket watch
Pocket watch
A pocket watch is a watch that is made to be carried in a pocket, as opposed to a wristwatch, which is strapped to the wrist. They were the most common type of watch from their development in the 16th century until wristwatches became popular after World War I during which a transitional design,...
.Although watch glasses were historically used by Victorian chemists, owing to their cheap availability, the chemist's modern "watch glass" is thicker, flatter and of a heat-resistant glass Some used for high-temperature work are of refractory metals, usually of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, owing to its non-reactive behaviour and low risk of contamination.
The capacity of evaporators is usually small - in the range 3-10 ml
ML
ml may refer to:* millilitre , a thousandth of a litre — not to be mixed up with millilambert* Malayalam language ISO 639-1 code* Mali, ISO 3166-1 country code...
. Larger dishes, up to 100 ml, are different in shape, and are more hemispherical.
The evaporator is used most often in quantitative analysis
Quantitative analysis
Quantitative analysis may refer to:* Quantitative analysis , an analysis technique applying mathematics stochastic calculus to finance...
.
In the determination of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
content in an organic sample, a small and accurately-measured quantity of a substance is added to the large amount of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, then heated in an evaporating dish. The dish is heated with a Bunsen burner
Bunsen burner
A Bunsen burner, named after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion.- Operation:...
, until only stable precipitate remains, which contains the silica content. The dish is then closed and heated at high temperature until completely clean, fused silica is produced. Comparison of the initial weight of the substance and that of the fused silica allows to the content of silicon in the sample to be determined.
The shape of the evaporating dish encourages evaporation in two ways:
- The shell is relatively flat. A relatively large liquid surface promotes evaporation.
- If heated in a flask or beaker, a part of the evaporated liquid condenses on the vessel walls and flows back into the solution. This does not happen in a dish.
When heating liquid in an evaporating dish, the low walls encourage splashes and so stirring or swirling of evaporating liquids is considered bad practice, owing to the risk of spillage.
Evaporation in a laboratory, especially in production quantities rather than merely for analysis, is now mostly performed in a rotary evaporator
Rotary evaporator
A rotary evaporator is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation...
. This is preferred because it works much faster and may be used under vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, avoiding unwanted reactions with the atmosphere and allowing control of noxious fumes. Evaporation under vacuum also reduces the severity of bumping
Bumping (chemistry)
Bumping is a phenomenon in chemistry where liquids boiled in a test tube will superheat and, upon nucleation, rapid boiling will expel the liquid from the container....
and violent ebullition.