SI Units: 2019 Redefinition

Measurement is an indispensable part of analytical chemistry. The result of measurement helps quantify a substance's physical property and compare it with the physical property of another substance. Each measurement comprises two components - a number indicating the magnitude and a unit of measurement as a standard for comparison. Further, the same quantity can be measured using different units of measurement, which leads to differences in magnitude.

A standard set of units has been defined to help maintain consistency and avoid errors in scientific communication. These units are the 'Système International d'Unités' or SI units. The SI units for physical quantities such as mass, distance, temperature, time, electric current, luminous intensity, plane angle, solid angle, and amount of substance are termed fundamental units.

Other SI units are called derived SI units, constructed from the abovementioned fundamental SI units. For example, the SI unit for electric charge, Coulomb (C), is a product derived from multiplying the units of electric current (A) and time (s). Expression of quantities in non-SI units is also fairly common. For example, temperature is usually expressed in degrees Celsius (°C) or degrees Fahrenheit (°F) rather than the SI unit for temperature, Kelvin (K).

Quantities with very large or very small magnitude compared to a single SI unit are expressed in powers of ten. These powers of ten have prefixes that make their expression much easier, such as 'milli' for ten to the power of negative three or 'kilo' for ten to the power of positive three. This makes a kilometer the expression of a considerable distance, equivalent to a thousand meters or 10³ meters.