ISO 31-8 is the part of
international standard ISO 31
ISO 31 ( Quantities and units, International Organization for Standardization, 1992) is a superseded international standard concerning physical quantities, units of measurement, their interrrelationships and their presentation. It was revised and ...
that defines names and symbols for
quantities
Quantity or amount is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value multiple of a unit ...
and
units
Unit may refer to:
Arts and entertainment
* UNIT, a fictional military organization in the science fiction television series ''Doctor Who''
* Unit of action, a discrete piece of action (or beat) in a theatrical presentation
Music
* Unit (album), ...
related to ''
physical chemistry
Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistica ...
and
molecular physics
Molecular physics is the study of the physical properties of molecules and molecular dynamics. The field overlaps significantly with physical chemistry, chemical physics, and quantum chemistry. It is often considered as a sub-field of atomic, m ...
''.
Quantities and units
Notes
In the tables of quantities and their units, the ISO 31-8 standard shows symbols for substances as subscripts (e.g., ''c''
B, ''w''
B, ''p''
B). 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''(H
2SO
4).
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 element
A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
s from atomic number 1 (
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, H) to 109 (
unnilennium, Une).
The list given in ISO 31-8:1992 was quoted from the 1998
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
"Green Book"
Quantities, Units and Symbols in Physical Chemistry
''Quantities, Units and Symbols in Physical Chemistry'', also known as the ''Green Book'', is a compilation of terms and symbols widely used in the field of physical chemistry. It also includes a table of physical constants, tables listing the pr ...
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.
Examples:
: H He C Ca
Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions:
* The
nucleon number (
mass number
The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxima ...
) is shown in the left
superscript position (e.g.,
14N)
* The number of atoms of a
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
is shown in the right
subscript position (e.g.,
14N
2)
* The
proton number (atomic number) may be indicated in the left subscript position (e.g.,
64Gd)
* If necessary, a state of
ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
or an
excited state may be indicated in the right superscript position (e.g., state of ionization Na
+)
Annex C: pH
pH is defined
operationally as follows. For a solution X, first measure the
electromotive force ''E''
X of the
galvanic cell
A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
:reference electrode , concentrated solution of KCl , solution X , H
2 , Pt
and then also measure the electromotive force ''E''
S 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) + (''E''
S − ''E''
X) ''F'' / (''RT'' ln 10)
where
:''F'' is the
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
;
:''R'' is the
molar gas constant
The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
;
:''T'' is the
thermodynamic temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
.
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 i
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 = −log
10 +) ''y''1 / (1 mol/L)">'c''(H+) ''y''1 / (1 mol/L)± 0.02
where ''c''(H
+) denotes the amount-of-substance concentration of hydrogen ion H
+ and ''y''
1 denotes the activity coefficient of a typical uni-univalent electrolyte in the solution.
{{ISO standards
Physical chemistry
Molecular physics
#00031-08