Why Is Purity Important In Chemistry?

by Kate Onissiphorou

Purity is important in chemistry because it acts as an analytical standard. If you know the purity or concentration of a substance, you can work out the concentration of an analyte by comparing it to the known standard. An unknown sample of a substance can also be identified in a similar way.

What Does Purity Mean in Chemistry?

In chemistry, purity focuses on a particular substance in proportion to other substances. It might refer to a high concentration of an element or compound, or a mixture of the two. Purity can also refer to the absence or negligibly low concentrations of other substances.

For example, we talk about the purity of alcoholic drinks in terms of the percentage of ethanol they contain in relation to water. Aside from ethanol, alcoholic drinks like beer and wine contain impurities because of their other ingredients and traces of suspended particles.

Laboratory water is another example. It’s classified into different levels of purity. Type I grade water is ultrapure and is used for clinical and laboratory applications such as:

  • Culturing cells and tissues in petri dishes
  • Liquid chromatography analysis
  • Gas chromatography analysis
  • Molecular biology experiments
  • Inductively coupled plasma mass spectrometry or ICP-MS

The high purity of water used in these experiments and analyses is crucial to the accuracy of results. Pure water has a neutral pH and almost zero suspended solids. This means it will not interfere with the instrument readings or contaminate the cell cultures.

A substance that has a known purity serves as a standard or benchmark for testing other similar substances. 

For example, the purity of gold is typically measured in karats. This is an old system of measurement that divides the proportional weight of gold into 24. One karat gold is equivalent to 1/24th of the whole or about 4.1667%. Hence, pure gold is 24 karats.

Gold bullion and a smelting crucible.

What are the Chemical Grades of Purity?

Various countries, corporations, and international organisations have their own benchmarks for chemical purity. The specific names for these chemical grades may vary, but they usually refer to common standards or ranges. 

Some common chemical grades used in industries are listed below.

  • ACS grade – the ACS grade is set by the American Chemical Society. Chemicals may either meet the minimum standards set by the society or exceed them. The minimum purity for an ACS grade chemical is 95%, which is typically an acceptable standard for most food and pharmaceutical products.
  • Reagent grade – this is equivalent to the ACS grade, which has a purity standard of at least 95%. The grade is used as a benchmark in analytical chemistry, as well as in various food, clinical and pharmaceutical applications.
  • USP grade – short for United States Pharmacopeia grade, the USP grade is an acceptable standard for many pharmaceutical-related laboratory applications. It encompasses medicinal and first-aid products, as well as many personal care products. Its purity level can be as high as 99.7%. ReAgent's USP water
  • NF grade – this is based on the National Formulary (NF) standards and is equal to or greater than the NF and USP requirements. USP and NF collaborated in writing and publishing a book of public pharmacopeial standards which is used by the pharmaceutical industry.
  • Laboratory grade – this grade is commonly used in school laboratories for educational purposes. It doesn’t have specifications regarding its exact levels of purity and is not sufficiently pure to be suitable for food, drug or clinical applications.
  • Purified grade – as with the laboratory grade, this has not qualified for any official standard and is not recommended for food, drug or clinical applications. 
  • Technical grade – this is widely accepted for commercial and industrial applications, but it is not suitable for food, drug or clinical and medical applications.

How to Test for Purity in Chemistry

There are various qualitative and quantitative methods you can use to test the purity of substances. These methods are often used in industrial quality control, medical diagnostics, pharmaceutical formulations, environmental studies, and to enforce safety standards. 

The various methods of testing can be grouped into the following physical and chemical test categories:

  • Using radiolabeled compounds – this method is commonly used in the pharmaceutical industry to study the biochemical processes involved in drugs. Carbon compounds are tagged using radioactive isotopes, which observe their reactions. The purity of the compounds being tested can be determined by directly measuring the proportions of tagged compounds during a reaction.
  • Using a pure standard sample – this is a purely physical method of testing, which makes it highly inaccurate. It relies on comparing the physical properties of a substance (for example, smell, taste, and appearance), to a standard ‘pure’ sample to determine if there are impurities. Obviously, this method is limited to non-toxic substances.
  • Determining the boiling point and melting point – this is a practical and simple method of testing for purity. Again, this method relies on a standard sample. Substances such as hydrocarbons have specific boiling points and melting points. If the test substance deviates from the known boiling point or melting point of its pure sample, it probably contains some significant impurities.
  • Colourimetric methods – as the name suggests, colourimetric methods use colours to determine the purity of substances. This approach is often used to test biochemical substances that change colours when exposed to certain reagents. One application of this is testing for the presence of illicit drugs.
  • Analytical testing methods – these are the most accurate methods for establishing the purity of a substance. A type of quantitative method, they aim to identify the composition of a substance at the molecular level. Analytical testing methods include titration, spectroscopy, chromatography, and optical rotation.

Calculating the Percentage Purity in Chemistry

Calculating the percentage purity of a substance is a relatively straightforward process. You simply need to divide the mass of the pure substance in the impure sample by the total mass of the impure sample and then multiply it by 100.


Calculating the percentage purity in chemistry

What is the Instrument Used to Measure the Purity of Water?

Various instruments are used to test the purity of water, depending on the conditions and type of impurities. The instruments can either be portable and highly compact for on-site rapid testing or they can be bulky and fixed in a laboratory for more exhaustive testing.

They can feature an analogue, digital or video display and the controls can be manual, digital or have a computer interface. The mount can either be hand-held or modular, bench-top or panel-mounted.

These various instruments can detect or measure the following parameters in water:

  • Oil in water
  • The acidity or basicity of water
  • Conductivity or voltammetry quality of water
  • Dissolved solids
  • Turbidity/suspended solids
  • Dissolved oxygen and biological oxygen demand
  • Chlorophyll or algal pigments.

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