Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.
In chemistry, IUPAC changed the definition of standard temperature and pressure (STP) in 1982: [1][2]
Until 1982, STP was defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 1 atm (101.325 kPa).
Since 1982, STP is defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 105 Pa (100 kPa, 1 bar).
STP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.
NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP). Please note that these stated values of STP used by NIST have not been verified and require a source. However, values cited in Modern Thermodynamics with Statistical Mechanics by Carl S. Helrich and A Guide to the NIST Chemistry WebBook by Peter J. Linstrom suggest a common STP in use by NIST for thermodynamic experiments is 298.15 K (25°C, 77°F) and 1 bar (14.5038 psi, 100 kPa).[3][4]
The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) and 101.325 kPa.[5]
In industry and commerce, standard conditions for temperature and pressure are often necessary to define the standard reference conditions to express the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases vary significantly with temperature and pressure): standard cubic meters per second (sm3/s), and normal cubic meters per second (nm3/s).
However, many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them; often substituting the term with older "normal conditions", or "NC". In special cases this can lead to confusion and errors. Good practice always incorporates the reference conditions of temperature and pressure. If not stated, some room environment conditions are supposed, close to 1 atm pressure, 293 К (20 °C), and 0% humidity.
Explanation:
