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. It is sometimes referred to as standard temperature and pressure (STP), although the complete definition is more complex. 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,² 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.

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.³

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.

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.

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.⁴

See also

• Standard conditions for temperature and pressure

Notes

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 (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. pp. 48–54. Electronic version.
4. ^ 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.