MTDATA – Phase Diagram Software from the National Physical Laboratory







Training and Support 




Other Services 







allows the user to specify the temperature, gas volume, the identity and amounts of Type 1 components and the identity and partial pressures, activities or molalities of Type 2 components.

There are three types of effective components:
Type 1 - SET by amount; these must be predefined components,
Type 2 - SET by potential (partial  pressure, activity or molality)
Type 3 - defined by the RANGE command for both the abscissa and ordinate in that order.
Note that the effective components are defined by assigning settings or ranges to them. There must be as many effective  components as predefined components.



volume n(*)








volume = <ideal gas volume>

temperature = 298.15 K


set n(Nb) = 1e-4 !

set temperature = = 1023 !

set volume = 1.0 n(Fe) = 1 !

set lgt_pressure(HCl<gas>) = -7.0 !

set lgt_activity(H2O<aq>) = 0 !

set pH = 7 !

Parameter values:

TEMPERATURE         takes the value <number>

The units of temperature are KELVIN.


takes the values <volume>/ UNDEFINED

<volume> sets the equilibrium volume of the system in metres cubed. If the volume is set to zero, the default setting is restored. A volume that is effectively zero can be set by entering a very small positive value, eg 1e-10.

DEFAULT The default volume is that occupied by 1 mol of ideal gas at the set temperature.

LGT_PRESSURE(*) (ie log10(partial pressure)), LGT_MOLALITY(*) (ie log10(molality)), LGT_ACTIVITY(*) (ie log10(activity)), N(*), Eh, pH all take the values <user-defined>/ UNDEFINED


is used to set a fixed partial pressure (in dimensionless units numerically equal to atmospheres) of a substance, specified by *, which can be a valid substance number or a valid substance formula and phase label, for example CO2<gas>, as displayed in LIST SYSTEM SUBSTANCES.


is used to set the equilibrium molality in aqueous solution of a substance, specified by *, which can be a valid substance number or a valid  substance formula and phase label, as displayed in LIST SYSTEM SUBSTANCES.


See general comments. N(*) is used to define the Type 1 components, which must be predefined components, and the amount in moles assigned to those components, where * corresponds to a substance formula without a phase label. In practice for all reasonable purposes, Type 1 components must be elements.


used to set the notional activity, mainly of condensed substances, specified by *, where * is a valid substance number or a valid substance formula and phase label, as displayed in LIST SYSTEM SUBSTANCES.


sets the Eh. The units are volts


set the pH in aqueous systems

General comments:

The commands SET and RANGE (qv) define the effective components of the system as distinct from the predefined components. Effective  components may be set by amount or by their potential expressed in terms of the logarithm of the molality, activity or partial pressure. Components set by amount, of which there must be at least one, are called Type 1 components and must be predefined. The effective components must be orthogonal; i.e. it must not be possible to generate the formula of one component by a combination of other components.

Setting potentials of substances can result in part of the diagram being overconstrained, i.e. experimentally inaccessible. However,there is no absolute need to make certain in advance that this problem will not occur, as COPLOT will reveal the existence of overconstrained regions.

See also RANGE.


Updated 6 July 2010