# What are the terms of STP

## Standard conditions

The expression "Standard conditions"Is used in technical and scientific fields and basically has two meanings:

• In gas volume measurement to define gas volumes and gas volume flows
• In chemistry, biology, medicine and process engineering (this article) for the definition of process conditions to be observed in order to be able to compare the result or behavior of one process with another.

### Standard conditions

The Standard conditions, also as Normal conditions or STP (from the English term Standard Temperature and Pressure) are according to DIN 1343:[1][2]

• Temperature \$ T_n \$ = 273.15 K corresponding to 0 ° C and
• Pressure \$ p_n \$ = 101325 Pa = 101325 N / m² = 1013.25 hPa = 101.325 kPa = 1.01325 bar = 0.101325 MPa (= 1 atm)

### Standard chemical conditions

In chemistry, as determined by IUPAC in 1982[3][4] Are defined:

• Temperature \$ T_ \ mathrm {n} \$ = 273.15 K (0 ° C) for gases and
• Pressure \$ p_ \ mathrm {n} \$ = 100000 Pa = 100000 N / m² = 1000 hPa = 100 kPa = 1 bar

A slightly lower pressure is therefore specified here than in the standard conditions.

While the standard conditions are used as reference values ​​from which one converts, standard conditions are often used in order to be able to avoid conversions. In this sense, the IUPAC definition of exactly 1 bar is more modern and is particularly preferred for specifying thermodynamic material properties. In Germany, the DIN variant applies to the specification of a gas quantity in stores, see standard cubic meters.

### Further definitions

In scientific laboratories, measurement data under "Normal conditionsThe physical-chemical reference temperature is 20 ° C for density, rotation value and refractive index measurements. Refractive indices are consistently with the NaD.Line (589 nm) measured as a light source. 760 Torr (1013 mbar) is used as the reference air pressure for boiling point information.

#### Electrochemistry

In electrochemistry, when specifying the standard redox potential, one refers to the standard condition that all substances involved have an activity of 1. In the case of very dilute solutions, the activity is approximately equal to the concentration; in this case the standard state of a substance in solution is 1 mol / L. In acidic solutions, potentials are related to the potential of H3O+Ions, in basic solution to that of OH--Ions.

In contrast to this, information is often given in biochemistry that relates to the standard condition “pH 7”.

#### Others

Other common pressure / temperature conditions:

• SATP conditions ("Standard Ambient Temperature and Pressure")
T = 298.15 K corresponding to 25 ° C and
p = 101,300 Pa = 1013 hPa = 101.3 kPa = 1.013 bar

In medicine and physiology (especially respiratory physiology) a distinction is made between the following conditions:[5]

• STPD conditions ("standard temperature, pressure, dry"):
T = 273.15 K (0 ° C), p = 101 kPa, water vapor partial pressure p(H2O) = 0 kPa
• BTPS conditions ("body temperature, pressure, saturated"):
T = 310.15 K (37 ° C), p actual air pressure, water vapor partial pressure p(H2O) = 6.25 kPa (water vapor saturation at 37 ° C)
• ATPS conditions ("ambient temperature, pressure, saturated"):
the actual measurement conditions outside the body: T Room temperature, p Air pressure, p(H2O) water vapor saturation

In aviation, the International Standard Atmosphere (ISA) is defined as 15 ° C, 1013.25 hPa (29.92 inHg) at sea level with further standard values ​​for temperature and pressure changes with increasing altitude.