standard temperature and pressure (STP)

What is standard temperature and pressure (STP)?

Standard temperature and pressure (STP) refers to the nominal conditions in the atmosphere at sea level. These conditions are 0 degrees Celsius and 1 atmosphere (atm) of pressure. The STP value is important to physicists, chemists, engineers, pilots and navigators, among others.

Standard conditions for temperature (T) and pressure (P) refers to a specific pressure and temperature used to report on the properties of matter. STP values are commonly used in experiments involving gases.

In the past, the International Union of Pure and Applied Chemistry (IUPAC) defined STP as the following:

• Temperature: 0 degrees Celsius (273.15 degrees Kelvin or 32 degrees Fahrenheit)
• Pressure: 1 atm (101.325 kilopascal or 760 Torr).

This definition is now discontinued. Nonetheless, these conditions are still commonly used to define the volume (V) term normal cubic meter. Since 1982, IUPAC has applied a more stringent definition of STP:

• Temperature: 0 degrees Celsius (273.15 degrees Kelvin or 32 degrees Fahrenheit)
• Absolute pressure: 100,000 pascals or 10⁵ Pa (1 bar, 14.5 pounds per square inch, 0.98692 atm).

Thus, standard temperature is defined as 0 degrees Celsius, which translates to 32 degrees Fahrenheit or 273.15 degrees Kelvin. This is essentially the freezing point of pure water at sea level in air at standard pressure. The National Institute of Standards and Technology defines STP differently as absolute pressure of 1 atm (101.325 kPa, 14.696 psi) and 20 degrees Celsius (293.15 degrees Kelvin, 68 degrees Fahrenheit).

The need for standard temperature and pressure

Both temperature and air pressure vary from one place to another. To test, compare and document chemical and physical processes where temperature and pressure play a role, including data centers or anywhere computers are used, a standard reference of both is required. Certain properties of matter also vary with changes in temperature or pressure. These include the following:

• density
• viscosity
• melting point
• boiling point

A reference value of temperature and pressure accommodates comparisons and measurements of processes. It also enables better understanding and comparison of various properties of matter. STP provides such a reference.

STP conditions are important to calculate and express fluid flow rates and the volumes of liquids and gases when standard state conditions are applied. These properties are also highly dependent on temperature and pressure conditions and changes. Adopting and stating standard conditions enable similar experiments to occur in similar laboratory conditions and to generate similar and comparable results. It also makes it easier to compare different measurements for gases, such as the moles (mol) of gas in a given volume.

Standard temperature and pressure for gases

STP values are most commonly specified for gases because their characteristics tend to change dramatically with changes in temperature and/or pressure.

STP effect on oxygen

For example, at STP, the volume of oxygen (O₂) in 1 milliliter of atmospheric air is 210 microliters (μl). One micromole (μmol) of gas occupies 22.414 μl, so at 210, this volume of O₂ contains the following:

• 210 / 22.414 = 9.37 μmol

At any other non-STP temperature, the amount of oxygen can be obtained by multiplying it by 273 / (273 + T). Thus at 20 degrees Celsius, the corresponding value is the following:

• = 9.37 x (273 / 273 + 20)
• = 8.73 μmol

STP effect on carbon dioxide

At STP, carbon dioxide (CO₂) usually behaves as a gas. When it is frozen, it behaves as a solid called dry ice. If the temperature and pressure are both increased from STP above the critical point, CO₂ adopts properties that lie midway between a gas and a liquid. In this state, it behaves as a supercritical fluid, where it expands like a gas but has a density similar to that of a liquid.

Supercritical CO₂ is an important commercial and industrial solvent since it enables chemical extraction at low temperatures while remaining stable. It also presents low toxicity and minimal environmental impact.

Volume of gases at STP

The volume of a gas is a function of both pressure and temperature. At STP, 1 mol of gas occupies 22.4 liters (L) of volume. In other words, the molar volume of a gas at STP is 22.4 L. This volume can be found using the ideal gas law: PV=nRT (n = number of moles, R = gas constant).

In addition to the ideal gas law, other laws and formulas used in standard conditions for temperature and pressure are the following:

• Gay-Lussac's law: P₁T₂ = P₂T₁
• Charles's law: V₁T₂ = V₂T₁
• Boyle's law: P₁V₁ = P₂V₂

Incorrect assumption of standard conditions can result in calculation errors, which can drastically affect the final outcome of the experiment. That's why, when defining the volume, it is necessary to state its pressure and temperature conditions. These conditions must also be stated by defining volume-dependent quantities, like molar volume, density and volumetric flow.

Properties of water at STP

At STP, the density of pure water (H₂O) is 62.4 pounds per cubic foot (lb/ft³). Produced water contains numerous impurities, such as salts, which is why its density is higher than the density of pure water. Water-specific gravity is defined as the ratio of density of the produced water to that of pure water.

Another property of water is its formation volume factor. The formation volume factor of produced water is the volume occupied in a reservoir at its prevailing pressure and temperature divided by the water volume plus its dissolved gas at STP. Thus, it is expressed as the following:

• Bw=Vres / Vst (B_w = formation volume factor of water, V_res = water volume in reservoir condition, V_st = water volume at STP)

The density and formation volume factor of water, along with other properties like specific gravity, salinity, viscosity and compressibility, are all frequently used in oil and gas field management.

STP vs. NTP vs. SATP

Laboratory conditions rarely involve STP, so other reference conditions are also used for measurements and calculations of physical and chemical processes and properties of matter.

Normal temperature and pressure (NTP) is one such standard reference for temperature and pressure. It is defined as air at the following conditions:

• Temperature: 20 degrees Celsius (293.15 degrees Kelvin, 68 degrees Fahrenheit)
• Pressure: 1 atm (101.325 kilonewton per square meter, 101.325 kPa, 14.7 PSI absolute, 0 PSI gauge, 29.92 inches of mercury, 407 inches of H₂O, 760 Torr)
• Density: 1.204 kilogram per cubic meter (0.075 lb/ft³)

At these conditions, the volume of 1 mol of a gas is 24.0548 L.

Like STP and NTP, standard ambient temperature and pressure (SATP) is also used in chemistry as a reference standard. It is defined as the following:

• Temperature: 25 degrees Celsius (298.15 degrees Kelvin)
• Pressure: 1 atm (101.325 kPa)

At these conditions, the volume of 1 mol of a gas is 24.4651 L.

In addition to STP, NTP and SATP, two other common standards for pressure and temperature are the International Standard Atmosphere (ISA) and the U.S. Standard Atmosphere.

Under ISA, standard temperature is 15 degrees Celsius, standard pressure is 1 atm and relative humidity is 0%. U.S. Standard Atmosphere is defined as temperature of 288.15 K (15 degrees Celsius, 59 degrees Fahrenheit) at a sea level 0 kilometer geopotential height and pressure of 1 atm (101.325 kPa, 1,013.25 hectopascals, 1,013.25 millibars, 760 millimeters of mercury). The two standards have the same definition of standard temperature and pressure at altitudes up to 65,000 feet above sea level. At higher altitudes, they differ slightly in the temperature ranges.

Many other organizations set their own standards for temperature and pressure. These include the following:

• International Organization for Standardization;
• United States Environmental Protection Agency;
• U.S. Occupational Safety and Health Administration;
• Organization of the Petroleum Exporting Countries; and
• U.S. Federal Aviation Administration.

The precise definition of STP depends on the organization. That's why it's best to explicitly state the temperature and pressure reference conditions instead of simply stating that a measurement was performed at STP or standard conditions.