SI Units

Scientists throughout the world have adopted a standardized system of units known as the International System of Units (SI). This system is based on the seven fundamental base units shown in Table 1.1. Numerous other useful units, such as volts, hertz, coulombs, and joules, are derived from these base units.

To express small or large measured quantities in terms of a few simple digits, prefixes are used with these base units and other derived units. As shown in Table 1.2, these prefixes multiply the unit by various powers of 10. For example, the wavelength of yellow radiation used for determining sodium by flame photometry is about 5.9 x 10^-7 m, which can be expressed more compactly as 590 nm (nanometers); the volume of a liquid injected onto a chromatographic column is often roughly 50 x 10^-6 L, or 50 μL (microliters); or the amount of memory on some computer hard disks is about 20 x 10⁹ bytes, or 20 Gbytes (gigabytes).

Ta ble 1.1

In analytical chemistry, we often determine the amount of chemical species from mass measurements. For such measurements, metric units of kilograms (kg), grams (g), milligrams (mg), or micrograms (μg) are used. Volumes of liquids are measured in units of liters (L), milliliters (mL), microliters (μL), and sometimes nanoliters (nL). The liter, the SI unit of volume, is defined as exactly 10^-3 m³. The milliliter is defined as 10^-6 m³, or 1 cm³.

Ta ble 1.2