Standard Conditions Versus Standard State

Standard conditions, or STP, and standard state both are used in scientific calculations, but they don't always mean the same thing.

STP is short for Standard Temperature and Pressure, which is defined to be 273 K (0 degrees Celsius) and 1 atm pressure (or 10⁵ Pa). STP describes standard conditions and is often used for measuring gas density and volume using the Ideal Gas Law. Here, 1 mole of an ideal gas occupies 22.4 L. An older definition used atmospheres for pressure, while modern calculations are for pascals.

Standard state conditions are used for thermodynamic calculations. Several conditions are specified for the standard state:

• The standard state temperature is 25 degrees C (298 K). Note that temperature is not specified for standard state conditions, but most tables are compiled for this temperature.
• All gases are at 1 atm pressure.
• All liquids and gases are pure.
• All solutions are at 1M concentration.
• The energy of formation of an element in its normal state is defined as zero.

Standard state calculations may be performed at another temperature, most commonly 273 K (0 degrees Celsius), so standard state calculations may be performed at STP. However, unless specified, assume standard state refers to the higher temperature.

Standard Conditions Versus STP

Both STP and standard state specify a gas pressure of 1 atmosphere. However, the standard state isn't usually at the same temperature as STP. The standard state also includes several additional restrictions.

STP, SATP, and NTP

While STP is useful for calculations, it's not practical for most lab experiments because they aren't usually conducted at 0 degrees C. SATP may be used, which means Standard Ambient Temperature and Pressure. SATP is at 25 degrees C (298.15 K) and 101 kPa (essentially 1 atmosphere, 0.997 atm).

Another standard is NTP, which stands for Normal Temperature and Pressure. This is defined for air at 20 degrees C (293.15 K, 68 degrees F) and 1 atm.

There is also ISA, or International Standard Atmosphere, which is 101.325 kPa, 15 degrees C and 0 percent humidity, and ICAO Standard Atmosphere, which is atmospheric pressure of 760 mm Hg and a temperature of 5 degrees C (288.15 K or 59 degrees F).

Which One to Use?

Usually, the standard you use is either the one for which you can find data, the one closest to your actual conditions or the one required for a specific discipline. Remember, the standards are close to actual values, but won't exactly match real conditions.