The plimsoll symbol as used in shipping

In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry (IUPAC) defines a conventional set of standard states for general use.^[1] IUPAC recommends using a standard pressure p^o = 1 bar (100 kilopascals) and a standard temperature T^o = 298.15 kelvins, although it is common to see values quoted at a pressure of one atmosphere and/or a temperature of 0 ºC, especially in older tables. The standard state should not be confused with standard temperature and pressure (STP) for gases,^[2] nor with the standard solutions used in analytical chemistry.^[3]

In the time of their development in the nineteenth century, the superscript plimsoll symbol ^o was adopted as a to indicate the non-zero nature of the standard state. The superscript circle º is also commonly used, not least for typographical reasons, and both are equally acceptable.^[4]

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 change of formation for an element in its standard state is zero, and this convention allows a wide range of other thermodynamic quantities to be calculated and tabulated. The standard state of a substance does not have to exist in nature: for example, it is possible to calculate values for steam at 25 ºC and 1 bar, even though steam does not exist (as a gas) under these conditions. The advantage of this practice is that tables of thermodynamic properties prepared in this way are self-consistent.

Conventional standard states

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, or by empirical measurements.

Gases

The standard state for a gas is the hypothetical state it would have as a pure substance obeying the ideal gas equation at 298.15 K and 1 bar. No real gas has perfectly ideal behaviour, but this definition of the standard state allows corrections for non-ideality to be made consistently for all the different gases.

Liquids and solids

The standard state for liquids and solids is simply the state of the pure substance at 298.15 K and 1 bar. This convention illustrates the difficulty of using 0 °C as the standard state temperature: in reactions involving water, it would not be clear if liquid water or solid water was being referred to. For elements, the reference point of -_fH^o = 0 is defined for the most stable allotrope of the element at 298.15 K: this is graphite in the case of carbon, and the --phase (white tin) in the case of tin.

Solutes

For a substance in solution (solute), the standard state is the hypothetical state it would have at the standard state molality or amount concentration but exhibiting infinite-dilution behaviour. The reason for this unusual definition is that the behaviour of a solute at the limit of infinite dilution is described by equations which are very similar to the equations for ideal gases. Hence taking infinite-dilution behaviour to be the standard state allows corrections for non-ideality to be made consistently for all the different solutes. Standard state molality is 1 mol„kg^“1, while standard state amount concentration is 1 mol„dm^“3.

See also

• Standard conditions for temperature and pressure

References

• International Union of Pure and Applied Chemistry (1982). "Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions". Pure Appl. Chem. 54 (6): 1239-50, http://media.iupac.org/publications/pac/1982/pdf/5406x1239.pdf. 
• IUPAC„IUB“IUPAB Interunion Commission of Biothermodynamics (1976). "Recommendations for measurement and presentation of biochemical equilibrium data". J. Biol. Chem. 251 (22): 6879-85, http://www.jbc.org/cgi/reprint/251/22/6879.pdf. 

1. ^ International Union of Pure and Applied Chemistry. "standard state". Compendium of Chemical Terminology Internet edition.
2. ^ International Union of Pure and Applied Chemistry. "standard conditions for gases". Compendium of Chemical Terminology Internet edition.
3. ^ International Union of Pure and Applied Chemistry. "standard solution". Compendium of Chemical Terminology Internet edition.
4. ^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. pp. 48-54. Electronic version.