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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) recommends a conventional set of standard states for general use.<ref name="GB1">IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "standard state".</ref> IUPAC recommends using a standard pressure po = 105 Pa.<ref>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "standard pressure".</ref> Strictly speaking, temperature is not part of the definition of a standard state. For example, as discussed below, the standard state of a gas is conventionally chosen to be unit pressure (usually in bar) ideal gas, regardless of the temperature. However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly {{safesubst:#invoke:convert|convert}} or, somewhat less commonly, {{safesubst:#invoke:convert|convert}}.

The standard state should not be confused with standard temperature and pressure (STP) for gases,<ref>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "standard conditions for gases".</ref> nor with the standard solutions used in analytical chemistry.<ref>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "standard solution".</ref>

At the time of development in the nineteenth century, the superscript plimsoll symbol was adopted to indicate the non-zero nature of the standard state.<ref>Prigogine, I. & Defay, R. (1954) Chemical thermodynamics, p. xxiv</ref> IUPAC recommends in the 3rd edition of Quantities, Units and Symbols in Physical Chemistry a symbol which seems to be a degree sign (°) as a substitute for the plimsoll mark. In the very same publication the plimsoll mark appears to be constructed by combining a horizontal stroke with a degree sign.<ref>E.R. Cohen, T. Cvitas, J.G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H.L. Strauss, M. Takami, and A.J. Thor, "Quantities, Units and Symbols in Physical Chemistry", IUPAC Green Book, 3rd Edition, 2nd Printing, IUPAC & RSC Publishing, Cambridge (2008), p. 60</ref> A range of similar symbols are used in the literature: a stroked lowercase letter O (o),<ref>IUPAC (1993) Quantities, units and symbols in physical chemistry (also known as The Green Book) (2nd ed.), p. 51</ref> a superscript zero (0)<ref>Narayanan, K. V. (2001) A Textbook of Chemical Engineering Thermodynamics (8th printing, 2006), p. 63</ref> or a circle with a horizontal bar either where the bar extends the boundaries of the circle (Expression error: Unrecognized punctuation character "{".) or is enclosed by the circle, dividing the circle in half (Expression error: Unrecognized punctuation character "{".).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Mills, I. M. (1989) "The choice of names and symbols for quantities in chemistry". Journal of Chemical Education (vol. 66, number 11, November 1989 p. 887–889) [Note that Mills (who was involved in producing a revision of Quantities, units and symbols in physical chemistry) refers to the symbol ⊖ (Unicode 2296 "Circled minus" as displayed in http://www.unicode.org/charts/PDF/U2980.pdf) as a plimsoll symbol although it lacks an extending bar in the printed article. Mills also says that a superscript zero is an equal alternative to indicate "standard state", though a degree symbol (°) is used in the same article]</ref> When compared to the plimsoll symbol used on vessels, the horizontal bar should however extend the boundaries of the circle.

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 298.15 K and 105 Pa, although 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.


Standard state sections
Intro  Conventional standard states  See also  References  

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