ISO 31-8

ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to physical chemistry and molecular physics.

Quantities and units

Quantity Unit Remarks
Name Symbol Definition Name Symbol
relative atomic mass Ar Ratio of the average mass per atom of an element to 1/12 of the mass of an atom of the nuclide 12C one 1 Formerly called atomic/molecular weight.
Example: Ar(Cl) = 35.453.
Both quantities depend on the nuclidic composition.
relative molecular mass Mr Ratio of the average mass per molecule or specified entity of a substance to 1/12 of the mass of an atom of the nuclide 12C
number of molecules or other elementary entities N Number of molecules or other elementary entities in a system one 1
amount of substance n, (ν) mole mol The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg of 12C. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specific groups of such particles. The definition applies to unbound atoms of 12C, at rest and in their ground state.


In the tables of quantities and their units, the ISO 31-8 standard shows symbols for substances as subscripts (e.g., cB, wB, pB). It also notes that it is generally advisable to put symbols for substances and their states in parentheses on the same line, as in c(H2SO4).

Normative annexes

Annex A: Names and symbols of the chemical elements

This annex contains a list of elements by atomic number, giving the names and standard symbols of the chemical elements from atomic number 1 (hydrogen, H) to 109 (unnilennium, Une).

The list given in ISO 31-8:1992 was quoted from the 1998 IUPAC "Green Book" Quantities, Units and Symbols in Physical Chemistry and adds in some cases in parentheses the Latin name for information, where the standard symbol has no relation to the English name of the element. Since the 1992 edition of the standard was published, some elements with atomic number above 103 have been discovered and renamed.

Annex B: Symbols for chemical elements and nucleides

Symbols for chemical elements shall be written in roman (upright) type. The symbol is not followed by a full-stop.


H He C Ca

Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions:

Annex C: pH

pH is defined operationally as follows. For a solution X, first measure the electromotive force EX of the galvanic cell

reference electrode | concentrated solution of KCl | solution X | H2 | Pt

and then also measure the electromotive force ES of a galvanic cell that differs from the above one only by the replacement of the solution X of unknown pH, pH(X), by a solution S of a known standard pH, pH(S). Then obtain the pH of X as

pH(X) = pH(S) + (ESEX) F / (RT ln 10)


F is the Faraday constant;
R is the molar gas constant;
T is the thermodynamic temperature.

Defined this way, pH is a quantity of dimension 1, that is it has no unit. Values pH(S) for a range of standard solutions S are listed in Definitions of pH scales, standard reference values, measurement of pH, and related terminology. Pure Appl. Chem. (1985), 57, pp 531–542, where further details can be found.

pH has no fundamental meaning; its official definition is a practical one. However, in the restricted range of dilute aqueous solutions having amount-of-substance concentrations less than 0.1 mol/L, and being neither strongly alkaline nor strongly acidic (2 < pH < 12), the definition is such that

pH = −log10[c(H+) y1 / (1 mol/L)] ± 0.02

where c(H+) denotes the amount-of-substance concentration of hydrogen ion H+ and y1 denotes the activity coefficient of a typical uni-univalent electrolyte in the solution.

ISO/IEC 80000

ISO 80000 or IEC 80000 is an international standard promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

The standard introduces the International System of Quantities (ISQ). It is a style guide for the use of physical quantities and units of measurement, formulas involving them, and their corresponding units, in scientific and educational documents for worldwide use. In most countries, the notations used in mathematics and science textbooks at schools and universities follow closely the guidelines in this standard.The ISO/IEC 80000 family of standards was completed with the publication of Part 1 in November 2009.

ISO 31

ISO 31 (Quantities and units, International Organization for Standardization, 1992) is a deprecated international standard for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents. It is superseded by ISO/IEC 80000.

Index of physics articles (I)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.


In chemistry, pH () is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7.

The scale is logarithmic. It is approximately the negative of the base 10 logarithm of the molar concentration (measured in units of moles per liter) of hydrogen ions. More precisely it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. The neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Pure water is neutral (pH 7) at 25 °C. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively.Measurements of pH are important in agronomy, medicine, chemistry, water treatment, and many other applications.

The pH scale is traceable to a set of standard solutions whose pH is established by international agreement.

Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.

The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or an indicator.

There are three current theories used to describe acid–base reactions: Arrhenius, Bronsted-Lowry and Lewis when determining pH.

ISO standards by standard number

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