Standard temperature and pressure (STP) refers to the internationally agreed upon standard of measurement for experiments in chemistry.
According to the International Union of Pure and Applied Chemistry (IUPAC), the currently accepted values for standard temperature and pressure are 273.15 K (0 °C) and exactly 100kPa (0.986923 atm) (kPa = kilopascal). The purpose of STP is to provide chemists with a common experimental baseline from which to interpret and compare data.
The IUPAC definitions of STP are not universally accepted. Most chemistry textbooks, for example, still use a standard pressure value of 1 atm. Different industries use different standards, depending on what exactly their interests are. For instance, the National Institute of Standards and Technology (NIST) defines STP to be 293.15 K (20 °C) and 1 atm of pressure (101.325 kPa), and the International Standard Metric Conditions (ISMC) defines STP to be 288.15 K (15 °C) and 101.325 kPa. Although the exact values of STP might change from context to context, the underlying idea is the same; STP values provide a commonly agreed upon set of experimental conditions in which to observe and describe the behavior of substances.
Why Do We Need Standards?
Chemists require STP definitions because the behavior of a substance varies greatly depending on the temperature and pressure. STP definitions give chemists a common reference point to describe how a gas behaves under “normal” conditions. Scientists use standards like STP definitions for two purposes, to define certain quantitative metrics and to allow for consistent and repeatable experiments.
Imagine someone tells you the molar volume of methane is 22.4 liters (L). The molar volume of a substance is just a measure of how much space one mole of that substance takes up. On its own, this value is not very informative. It is known that the volume of a gas varies greatly with respect to pressure and temperature, so a gas could have multiple molar volumes, depending on the exact temperature and pressure. One needs to specify a temperature and pressure to make a molar volume measurement of 22.4 L a more meaningful quantity. Scientists agree upon a predefined temperature and pressure to report quantitative properties of gases. As it just so happens, one mole of any gas at STP (273.15 K, 100kPa) has a volume of 22.4 L. Quantitative measurements of gas, like volume, volumetric flow, and compressibility, all must be defined with respect to some defined pressure and temperature.
Chemists also adopt experimental standards so that they can be sure that their experimental trials have the same conditions. Differing experimental conditions can change the result of an experiment, so scientists agree on standard conditions to make their results reliable and reproducible. If a scientist does not give enough information about their experimental setup, then other scientists cannot attempt to reproduce their findings. Reproducibility is integral to confirming experimental data.
Imagine Alice performs an experiment on some sample of gas and records its behavior. She does not, however, record the temperatures and pressures at which her experiments were performed. Bob then attempts to reproduce Alice’s experiment, but since he does not know the pressure and temperatures at play in Alice’s experiment, he gets different results. STP definitions exist to prevent situations like this from happening. Standard conditions give scientists a common reference frame from which to perform experiments and compare data.
Uses of STP Values
Accepted STP values can be used to predict the behavior of gases under normal conditions. Using the ideal gas law equation PV = nRT we can calculate the properties that a given gas must have under different pressures and temperatures. In these examples, we will use a standard pressure value of 1 atm for simplicity’s sake and since the most common value of the R constant (0.08206) is expressed in terms of atm of pressure.
Q: What is the volume of a sample of 1.4 moles of hydrogen gas (H2) at STP?
Recall that STP conditions are defined as 273.15 K and 1atm. The value of R is 0.08206. In the ideal gas law equation, P is generally expressed in Plugging these values into the ideal gas law equation gives us:
(1atm)(V) = (1.4mol)(0.08206)(273.15K) = 31.4 L.
1.4 moles of hydrogen gas takes up 31.4 liters of space. Notice how the calculation of the answer did not depend on any specific properties of hydrogen gas, only on the amount of it.
We can also use STP conditions to extrapolate how a gas will behave under a given temperature and pressure. Here is an example:
Q: If a gas has a volume of 0.13 L at 5 atm of pressure and 300 K, what will the volume be at STP conditions?
To figure out this one, we can use a law derived from the ideal gas law that states that the ratio of the product of pressure and volume to temperature stays constant for a fixed amount of gas. Mathematically, this is:
P1V1 / T1 = P2V2 / T2
Using this equation we can find out the new volume. Plugging in the values we get:
(0.15L)(5atm)/300K = V2(1atm)/273.15K)
Solving for V2 gives us:
(0.15L⋅5atm/300K)⋅273.15 = V2 = 0.68 L
Thus, the gas will have a volume of 0.68 liters at STP conditions.
Additionally, assuming STP conditions allows us to simplify the ideal gas formula. Normally the formula is
PV = nRT
Assuming STP values, P=1 and T=273.15. Thus, the general state equation for a sample of gas at STP values can just be written as
V = nR(273.15).
This equation can also be used to give us the molar volume of a gas at STP conditions. The molar volume of a gas is just the volume one mole of gas takes up, so setting n=1 gives us:
V = R(273.15) = ~22.4 L
This equation tells us that one mole of any gas at STP condition has a volume of 22.4 liters. Interestingly, the molar volume of a gas is entirely independent of any specific chemical properties of the gas.
Why Those Values?
It’s one thing to explain why scientists use agreed upon standards. It is another question to ask why scientists use the particular standards that they do. Why does the IUPAC define STP to be 273.15 K and 100 kPa? Why not 100 K and 50 mmHg?
Historically, standard temperatures and pressures were defined to be roughly equal to temperatures and pressures at sea level—about 15 °C and 1 atm of pressure. These values make sense due to the lack of technology to create extremely controlled conditions in the lab. Most people running chemical experiments at the time would find themselves in such an environment, so it made sense to set those values as a standard. At the time, the Celsius temperature scale was based on the freezing and boiling points of water. Water’s boiling point was defined as exactly 100 °C and its freezing point 0 °C.
Originally, 1 atmosphere (atm) of pressure was defined as the pressure exerted by a 760 mm-high column of mercury (Hg) at sea level. Evangelista Torricelli (1608-1647) calculated a value of 760 mm as the expected height a column of mercury would rise to under the influence of the atmosphere. Torricelli derived this value by taking the observed value of the height of a column of water and calculated the expected difference in height due to Mercury’s increased density. This value persisted for quite a while due to the widespread use of mercury barometers.
By the early 20th century, most scientific organizations had shifted to using values of 0 °C and 1 atm of pressure as a standard. Many commercial industries, such as the newly booming oil and gas industries in the U.S., continued to use a temperature of 15–20 °C as a standard simply because the industry and technology had been set up around those values. Scientific organization shifted to a standard of 0 °C as it represented a “cleaner” value that was based on the phase behavior of a well-understood substance (water).
In 1982, the IUPAC changed the definitions of STP conditions to 273.15 K and 100kPa. The change was motivated by a desire to express STP conditions in units that can be entirely expressed in terms of SI units. The Kelvin temperature scale is the SI accepted base unit for temperature and is based on the triple point of water, which is exactly defined as 273.15 K. A pascal is a derived unit of pressure that is expressed in terms of newtons per meter squared (N/m²), both of which are SI quantities. As it turns out, 1 atm of pressure is equal to 101.325 kPa, so a defined value of 100 kPa at standard pressure is continuous with old values. Nowadays, most scientists prefer to use only SI units as SI units are based on invariant physical constants found in nature.
To sum up, standard temperatures and pressures (STP) are agreed upon experimental standards for pressure and temperature. STP definitions exist to set a common baseline from which chemists can perform experiments, interpret data, and communicate results. The behavior of substances is heavily dependent on the temperature and pressure, so standards must exist to allow for accurate and reproducible experiments. The current STP values as defined by the IUPAC in 1982 and used by the majority of academic chemists are 273.15 K and 100 kPa. Other STP definitions exist in different sectors, depending on their interests.