Standard temperature and pressure

Standard temperature and pressure (also used as STP), more specifically standard conditions for gases, are defined as
 * temperature 273,15 K, i.e. 0 °C,
 * pressure 105 Pa, i.e. 100 kPa.

The earlier definition worked with a pressure of 1 atm., i.e. 101,325 kPa.

The U.S. National Institute of Standards and Technology (NIST) is introducing a different standard, used in a number of countries, sometimes referred to as normal conditions or normal temperature and pressure - operating at 20 °C and 101,325 kPa.

You need to distinguish the terms standard state, standard reaction conditions and standard thermodynamic quantities that are used in thermodynamics.

Some areas of chemistry also work with otherwise defined "reference" temperatures and pressures. The temperatures used are 0, 15, 16, 20, 21, 22, 23 and 25 °C, with pressures of 99.99; 100; 101.3; 101,325; 101.33 and 101.6 kPa. For example, most analytical tables are designed for a temperature of 25 °C, the same temperature is most commonly used in pH calculations in aqueous solutions.