What Is the Difference Between Standard Conditions and Standard State?

Understanding Standards of Temperature and Pressure

STP and standard state conditions both specify 1 atmosphere of pressure, but not the same temperature.
STP and standard state conditions both specify 1 atmosphere of pressure, but not the same temperature. Dennis van Zuijlekom/Flickr

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° Celsius) and 1 atm pressure (or 105 Pa). STP describes Standard Conditions. STP is often used for measuring gas density and volume using the Ideal Gas Law. Here, 1 mole of ideal gas occupies 22.4 L. Note: An older definition used atmospheres for pressure, while modern calculations are for pascals.

Standard State conditions are used for thermodynamics calculations. Several conditions are specified for standard state:

  • The standard state temperature is 25°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° Celsius), so standard state calculations may be performed at STP. However, unless specified, assume standard state refers to the higher temperature.

Comparing STP and Standard State Conditions

Both STP and Standard State specify a gas pressure of 1 atmosphere. However, standard state isn't usually at the same temperature as STP, plus standard state 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°C. SATP may be used, which means Standard Ambient Temperature and Pressure. SATP is at 25°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 20oC (293.15 K, 68oF) and 1 atm.

There is also ISA or International Standard Atmosphere, which is 101.325 kPa, 15oC and 0% humidity, and ICAO Standard Atmosphere, which is atmospheric pressure of 760 mm Hg and a temperature of 5oC (288.15 K or 59oF.

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 discipline. Remember, the standards are close to actual values, but won't exactly match real conditions.

Key Takeaways

  • Both STP and standard state conditions are commonly used for scientific calculations.
  • STP stands for Standard Temperature and Pressure. It is defined to be 273 K (0° Celsius) and 1 atm pressure (or 105 Pa).
  • The definition of standard state conditions specifies 1 atm of pressure, that liquids and gases be pure, and that solutions be at 1 M concentration. Temperature is not specified, although most tables compile data at 25°C (298 K).
  • STP is used for calculations involving gases that approximate ideal gases.
  • Standard conditions are used for any thermodynamic calculation.
  • Values cited for STP and standard conditions are based on ideal conditions, so they may deviate slightly from experimental values.