Standard Temperature and Pressure (STP)
09th Apr 2019 @ 8 min read
Standard Temperature and Pressure defined by NIST
Standard temperature is 0 ℃ or 273.15 K or 32 ℉.
Standard pressure is 1 atm or 101.325 kPa or 760 mmHg (or torr) or 14.6959 psi.
Standard Temperature and Pressure defined by IUPAC
Standard temperature is 0 ℃ or 273.15 K or 32 ℉.
Standard pressure is 1 bar or 100.000 kPa or 750.6 mmHg (or torr) or 14.5038 psi.
Why do we need STP
Let say a chemist in a laboratory is performing a chemical experiment in London. Mean room temperature and pressure in London is 12 ℃ and 1.015 bar. If the same experiment is conducted by him/her in Austin, where mean room temperature and pressure is 21 ℃ and 1.014 bar, the result of the experiments might significantly vary. This is because many scientific experiments, particularly of chemistry, are influenced by temperature and pressure. Hence, to avoid such deviations we have adopted some standard conditions so that experiments all over the world are performed in similar laboratory conditions. One of such standards is STP (Standard Temperature and Pressure).
Variation in STP
The National Institute of Standards and Technology (NIST) is an American physical sciences laboratory agency that defines standard temperature as 0 ℃ and standard pressure as 1 atm. While the International Union of Pure and Applied Chemistry (IUPAC) defines standard temperature as 0 ℃ but standard pressure as 1 bar or 105 Pa. Earlier (before 1982), IUPAC used to define standard temperature as 0 ℃ and standard pressure as 1 atm. But this definition of IUPAC is discontinued since 1982.
Volume at STP
The volume of gases is usually defined at standard conditions. As volume is a function of pressure and temperature, it is necessary to state the conditions (pressure and temperature) at which it is defined. The same also applies to volume dependent quantities like molar volume, density, volumetric flow etc. In professional work as well as in publications, it is very helpful to state standard conditions with volume (or volume dependent quantities) to avoid confusion. A wrong assumption of standard conditions can lead to errors in calculations which can drastically affect outcomes.
The molar volume Vm of an ideal gas is defined as:
where R is ideal gas constant, its value is 8.314 J K−1 mol−1.
At STP (NIST),
At STP (IUPAC),
With the help of molar volume, we can also calculate the density of air.
where M is the molar mass of air (M = 28.84 g mol−1).
For air at STP (NIST),
For air at STP (IUPAC),
Other Standard Conditions for Pressure and Temperature
There are many organisations who have developed their own standards based on their own needs. Some of them are described below.
Normal Temperature and Pressure (NTP)
Temperature is 20 ℃ and pressure, 1 atm.
Standard Ambient Temperature and Pressure (SATP)
Temperature is 25 ℃ and pressure, 1 bar.
International Standard Atmosphere (ISA)
Temperature is 15 ℃, pressure is 1 atm, and relative humidity, 0 %.
Comparison of Various Standards
The table below lists different standard conditions (pressure and temperature) adopted by different institutes.
| Institute | T (℃) | T (K) | T (℉) | P (mmHg) | P (kPa) | P (psi) | RH (%) | Vm (dm3 mol−1) |
|---|---|---|---|---|---|---|---|---|
| NIST (STP) | 0 | 273.15 | 32 | 760 | 101.325 | 14.696 | 22.41 | |
| IUPAC (STP) | 0 | 273.15 | 32 | 750.06 | 100.000 | 14.504 | 22.71 | |
| NIST (NTP) | 20 | 293.15 | 68 | 760 | 101.325 | 14.696 | 24.05 | |
| IUPAC (SATP) | 25 | 298.15 | 77 | 750.06 | 100.000 | 14.504 | 24.79 | |
| ICAO (ISA) | 15 | 288.15 | 59 | 760 | 101.325 | 14.696 | 0 | 23.64 |
| EACSMC | 20 | 293.15 | 68 | 760 | 101.325 | 14.696 | 0 | 24.05 |
| OSHA | 16 | 789.15 | 60.8 | 760 | 101.325 | 14.696 | 23.72 | |
| OPEC | 16 | 789.15 | 60.8 | 762 | 101.591 | 14.734 | 23.66 | |
| U.S. ASM | 15 | 288.15 | 59 | 750 | 99.991 | 14.502 | 78 | 23.96 |
| ISO-2314, ISO-3977-2 | 15 | 288.15 | 59 | 760 | 101.325 | 14.696 | 60 | 23.64 |
| AMCA* | 21 | 294.15 | 69.8 | 760 | 101.325 | 14.696 | 0 | 24.14 |
| US FAA | 15 | 288.15 | 59 | 760 | 101.325 | 14.696 | 23.64 | |
| US CRSC | 15.6 | 288.75 | 60.08 | 760 | 101.325 | 14.696 | 23.69 | |
| Note 1: * indicates air is at 1.20 kg m−3. Note 2: T, Temperature; P, Pressure; RH, Relative Humidity; Vm, Molar Volume; National Institute of Standards and Technology, NIST; International Union of Pure and Applied Chemistry, IUPAC; International Civil Aviation Organization, ICAO; Euro-Asian Council for Standardization, Metrology and Certification, EACSMC; Occupational Safety and Health Administration, OSHA; Organization of the Petroleum Exporting Countries, OPEC; Army Standard Metro, ASM; Air Movement and Control Association, AMCA; Federal Aviation Administration, FAA; Common Refinery Standard Condition, CRSC | ||||||||
Example
Consider 12 kg of nitrogen gas at STP (T = 273.15 K, P = 1 atm). Calculate the volume of the gas if it behaves like an ideal gas?
The molar mass (M) of the gas is 28 g mol−1. The number of moles of nitrogen for a given weight (W) is calculated as:
Molar volume of the gas at STP (T = 273.15 K, P = 1 atm) is 22.41 dm3 mol−1.
The volume of the gas is molar volume multiple by the number of moles.
Therefore, the volume of the gas of 12 kg is 9.60 m3.
Associated Articles
Copy Article Cite
