Chemical Impurity

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Impurities
)

In

homogeneous (i.e., a uniform substance that has the same composition throughout the material[2]). The perfect pure chemical will pass all attempts to separate and purify it further. Thirdly, and here we focus on the common chemical definition, it should not contain any trace of any other kind of chemical species. In reality, there are no absolutely 100% pure chemical compounds, as there is always some small amount of contamination
.

The levels of impurities in a material are generally defined in relative terms. Standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, a material's level of purity can only be stated as being more or less pure than some other material.

Impurities are either naturally occurring or added during synthesis of a chemical or commercial product. During production, impurities may be purposely or accidentally added to the substance. The removal of unwanted impurities may require the use of separation or purification techniques such as distillation or zone refining. In other cases, impurities might be added to acquire certain properties of a material such as the color in gemstones or conductivity in semiconductors. Impurities may also affect crystallization as they can act as nucleation sites that start crystal growth. Impurities can also play a role in nucleation of other phase transitions in the form of defects.   

Unwanted Impurities

Impurities can become unwanted when they prevent the working nature of the material. Examples include ash and

Zone refining
, another purification method, is an economically important method for the purification of semiconductors.

A basic distillation set up

However, some kinds of impurities can be removed by physical means. A mixture of water and salt can be separated by distillation, with water as the distillate and salt as the solid residue. This is done by heating the water so it boils and leaves behind the salt. The water is cooled and the gas turns back to a pure liquid.[3] Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids.

No matter what method is used, it is usually impossible to separate an impurity completely from a material. The reason that it is impossible to remove impurities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing their concentration to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.[4]

Impurities in pharmaceuticals and therapeutics are of special concern and the last couple of decades have witnessed a fair number of scandals, from insecure ingredients and incorrect dosage forms to intentionally fortified medications and accidental contaminations.[4]

Wanted Impurities

Occasionally, we may want to include impurities in a material to change its properties. These impurities can be naturally occurring and left unaltered in a material or be intentionally added during synthesis. These types of impurities can show up in our day to day lives such as different colors in gemstones or by doping to tune the conductivity of semiconductors.[5][6]

An example of when impurities are wanted is shown in gems. These gems have slight impurities that act as chromophores and give the stone its color. An example is the gem family beryl witch has the base chemical formula of Be3Al2(SiO3)6. Pure beryl will appear colorless but this rarely occurs and the presence of trace elements change its color. The green of emeralds are from impurities such as chromium, vanadium, or iron. A manganese impurity will give a pink gem called morganite and iron creates the blue gem aquamarine.[5]

Three gems from the beryl family with different colors.

Doping is a process where impurities are purposefully added to semiconductors to increase electrical conductivity and improve a semiconductors function. The dopants, the elements added to the original crystal structure, contain a different number of electrons then the base formula. Semiconductors that are p-doped contains a small amount of elements that have less valence electrons then the other elements in the crystal. N-doping is the opposite and the dopant contains more valence electrons.[6]

Impurities and Nucleation

When an impure liquid is cooled to its

long-range order in the structure.[7]

Impurities play an important role in the nucleation of other phase transitions. For example, the presence of foreign elements may have important effects on the mechanical and magnetic properties of metal alloys. Iron atoms in copper cause the renowned

ferromagnets and dramatically affect their coercivity. In general impurities are able to serve as initiation points for phase transitions because the energetic cost of creating a finite-size domain
of a new phase is lower at a point defect. In order for the nucleus of a new phase to be stable, it must reach a critical size. This threshold size is often lower at an impurity site.

See also

References

  1. ^ "Definition of IMPURITY". www.merriam-webster.com. 2024-03-24. Retrieved 2024-04-01.
  2. ^ "Definition of HOMOGENEOUS". www.merriam-webster.com. 2024-03-10. Retrieved 2024-03-27.
  3. ISBN 978-0-429-10729-0
    .
  4. ^
    PMID 32041209
    .
  5. ^ a b Smith, George Frederick Herbert. "Gem-Stones and Their Distinctive Characters". gutenberg.org. Retrieved 2024-04-15.
  6. ^
    ISBN 978-1-118-57380-8
    .
  7. PMID 32905065
    .
  • Cheng, E. et al., Chemistry – A Modern View, Aristo-Wilson, Hong Kong, 2004
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