Standard state

In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990. [GoldBookRef | file = S05910 | title = standard conditions for gases] The standard state of a material can be defined at any given temperature, most commonly 25 degrees Celsius, although quite a few texts (especially in related disciplines such as physics and engineering) use 0 degrees Celsius for Standard Temperature and Pressure (STP). The standard state is an "arbitrarily chosen non-zero value", not a natural zero point.

For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, and for many other material standards. The standard enthalpy of formation for an element in its standard state is 0.

When the standard state is referred to a solute in a solution, or to a chemical reaction, it also includes the condition that the concentrations of all solutes are at unity (or another designated quantity) for whatever measure of concentration is specified. If that is molarity that would be 1 mol·dm-3 and for molality 1 mol·kg-1 assuming the solution infinite-dilution behaviour. If mole fraction is used, the pure liquid or solid is the standard state ("x"=1). As it is possible (and in principle legitimate) to take a different unit for each of the species in the reaction, the nature of the standard state needs to be "specified" when reporting or tabulating. Although a definition involving 1 mol/L (molarity at unity) of A in combination with pure B (mole fraction at unity) is clearly a condition that can never be met, such a non-existent standard state nevertheless leads to a consistent system of tabulated values, provided it is used consistently by all. Of course these values are "different" from those where a different standard state is adopted.

Implied in the above is the important but often overlooked concept that many standard states are non-physical states, often referred to as "hypothetical states". Nevertheless, their thermodynamic properties are well-defined, usually by an extrapolation from some limiting condition, such as zero pressure or zero concentration, to a specified condition (usually unit concentration or pressure) using an ideal extrapolating function, such as ideal solution or ideal gas behavior. Thus, the standard state of a gas is usually defined as the hypothetical ideal gas at 1 bar pressure.

The thermodynamic properties of a gas in such a standard are in general not the same as those of a real gas at the same pressure (typically 1 bar pressure by convention), owing to the fact that real gases are non-ideal. Similarly, the thermodynamic properties of a solution in a 1 molar standard state are not the same as thermodynamic properties of real solution at 1 molar concentration.

The usual choice of standard state for a pure solid is the solid with unit total pressure applied to the substance (typically 1 bar) with the further stipulation that the substance be in its most stable form at that pressure. Pure liquids are treated the same way.


In the time of their development (the 19th century) the Plimsoll - symbol was adopted as a superscript o to indicate the non-zero nature of the chosen reference state. For typographic reasons this symbol is often abridged to a rather misleading zero superscript 0 in later texts.

At elevated temperature and pressure

In chemistry of solutions at elevated temperatures and pressures, the term "standard state" often denotes the hypothetical standard concentration, typically:
* 1 mol/kg for solutes assuming an ideal behaviour (i.e., an infinite dilution),
* a unity molar fraction (for pure substances),
* a partial pressure of 100 kPa for gaseous reagents.

It does not imply any particular temperature or total system pressure because, although contrary to IUPAC recommendation, it is more convenient when describing solutions over a wide temperature and pressure ranges. [V. Majer, J. Sedelbauer and Wood, "Calculations of standard thermodynamic properties of aqueous electrolytes and nonelectrolytes." Chapter 4 in: "Aqueous Systems at Elevated Temperatures and Pressures. Physical Chemistry of Water, Steam and Hydrothermal Solutions", D.A.Palmer, R. Fernandez-Prini, and A.Harvey (editors), Elsevier, 2004.]

ee also

* Standard conditions for temperature and pressure


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