What is the use of evaporating plate?

Answer 1

An evaporating dish is a piece of laboratory-glasswareused for the evaporationof solids and supernatant fluidsevaporating-dish, 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 porcelainor borosilicate-glass. Shallow glass evaporating dishes are commonly termed "watch glasses", from their original use as the front window of a pocket-watch-2.evaporating-dishSome 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-abbreviation. 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 siliconcontent 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.

A generic evaporator

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. 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-chemistryand violent ebullition.

-Jobelle Flores Pelagio