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. IUPAC recommends using a standard pressure p⦵ = 105 Pa. 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 298.15 K (25.00 °C; 77.00 °F) or, somewhat less commonly, 273.15 K (0.00 °C; 32.00 °F).
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.
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.
The standard state for a gas is the hypothetical state it would have as a pure substance obeying the ideal gas equation at standard pressure (105 Pa, or 1 bar). No real gas has perfectly ideal behavior, 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 subjected to a total pressure of 105 Pa. For most elements, the reference point of ΔHf⦵ = 0 is defined for the most stable allotrope of the element, such as graphite in the case of carbon, and the β-phase (white tin) in the case of tin. An exception is white phosphorus, the most common allotrope of phosphorus, which is defined as the standard state despite the fact that it is only metastable.
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 behavior. The reason for this unusual definition is that the behavior 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 behavior 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.
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. 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. A range of similar symbols are used in the literature: a stroked lowercase letter O (
o), a superscript zero (0) or a circle with a horizontal bar either where the bar extends beyond the boundaries of the circle (U+29B5 ⦵ CIRCLE WITH HORIZONTAL BAR) or is enclosed by the circle, dividing the circle in half (U+2296 ⊖ CIRCLED MINUS). When compared to the plimsoll symbol used on vessels, the horizontal bar should extend beyond the boundaries of the circle; care should be taken not to confuse the symbol with the Greek letter theta (uppercase Θ or ϴ, lowercase θ ).
- 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" (PDF). Pure Appl. Chem. 54 (6): 1239–50. doi:10.1351/pac198254061239.
- IUPAC–IUB–IUPAB Interunion Commission of Biothermodynamics (1976). "Recommendations for measurement and presentation of biochemical equilibrium data" (PDF). J. Biol. Chem. 251 (22): 6879–85.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard state". doi:10.1351/goldbook.S05925
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard pressure". doi:10.1351/goldbook.S05921
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard conditions for gases". doi:10.1351/goldbook.S05910
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "standard solution". doi:10.1351/goldbook.S05924
- Housecroft C.E. and Sharpe A.G., Inorganic Chemistry (2nd ed., Pearson Prentice-Hall 2005) p.392
- Prigogine, I. & Defay, R. (1954) Chemical thermodynamics, p. xxiv
- 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
- IUPAC (1993) Quantities, units and symbols in physical chemistry (also known as The Green Book) (2nd ed.), p. 51
- Narayanan, K. V. (2001) A Textbook of Chemical Engineering Thermodynamics (8th printing, 2006), p. 63
- "Miscellaneous Mathematical Symbols-B" (PDF). Unicode. 2013. Retrieved 2013-12-19.
- 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 https://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]