COPLOT module is used to draw
(also called phase stability or Pourbaix diagrams) which show
the chemical behaviour of one or more elements as a function
of the logarithm of the molality, activity or partial
pressure of others. The lines on the diagram, which separate
areas of predominance, are straight provided certain
simplifying assumptions are made. This makes predominance
area diagrams relatively easy to prepare and they are much
used by corrosion scientists and engineers and by
hydrometallurgists because of the broad insight they provide
into the chemistry of the system.
has been designed as a partner to the other
calculation modules of MTDATA, particularly THERMOTAB
and MULTIPHASE, and it is able to use the
same databases. Data
can be retrieved from the databases incorporated in MTDATA
either by specifying a system within COPLOT itself
previously preparing a purpose built datafile by means of the ACCESS
module. This is the recommended
procedure, since it
allows more control over the substances involved in the
calculations and the preservation of the file for future use
by COPLOT or MULTIPHASE.
has some features not found in other computer based
systems for drawing predominance area diagrams.
The volume of the gas phase can be set by the user, thus
enabling the ratio of "space" occupied by the gas and
any aqueous phase to be varied.
(b) The components whose chemical behaviour is being studied
are fixed by amount. As a result it is possible to
explore the mutual interaction between these components.
(c) Completely inaccessible regions of the diagram can be
excluded from further consideration.
these purposes three types of component are defined.
1: these are set by amount and must be predefined as
components during the setting up of the data input
Type 2: these have a fixed molality or partial pressure.
Type 3: these are the two components for which the
logarithm of the partial pressure, activity or
molality is varied along the two axes of the plot.
initial pass (pass zero) optionally seeks out and excludes from
further consideration any region of the diagram where condensed
compounds of only type 2 and 3 components exist or where gaseous or
aqueous species of these components would form with excessive partial
pressures or activities.
reason why COPLOT has been provided with the
means for dealing with more than one type 1 component can be
illustrated by reference to calculated diagrams for the Mo-O-N and
Mo-Si-O-N systems respectively. Molybdenum is the only type 1 component
and the diagram shows the regions of formation of molybdenum nitride,
molybdenum dioxide, Mo3O9
and molybdenum itself as a function of nitrogen and oxygen pressure at
1400 K. If the amount of molybdenum were raised, the region of Mo3O9
would decrease. If the amount were reduced, the region of gas phase
predominance would increase but Mo3O9
would eventually be replaced by the monomeric form MoO3.
more than one type 1 component is present, COPLOT
treats them in order of decreasing amount. In the diagram for the
Mo-Si-O-N system molybdenum and silicon are type 1 components and there
is more silicon than molybdenum. The diagram for the behaviour of
silicon as a function of the pressures of oxygen and nitrogen is
calculated first as shown by the dotted lines. A molybdenum diagram is
then calculated for each of the areas of the silicon diagram. The
component representing silicon in any particular area of the silicon
diagram is not normally silicon itself but is the compound of silicon
that is stable in that area. This makes the behaviour of the molybdenum
depend on the chemical state of the silicon, as indeed it would in
practice, since the formation of silica, silicon nitride and silicon
oxynitride reduces the thermodynamic activity of silicon and leads to
the formation of a sequence of molybdenum silicides. It is this ability
of COPLOT to predict behaviour in multicomponent
systems that makes the specification of the type 1 components by amount
that the molybdenum silicides are considered to belong to the
component molybdenum, rather than to silicon, because molybdenum is
present in the smaller amount. The logic of this is that, when there is
an excess of silicon over molybdenum, the molybdenum silicides can
coexist with other silicon compounds over a range of conditions but not
with other molybdenum compounds.
formation of gaseous species is investigated by varying the amount
of the type 1 components in relation to the volume of the gas. The
default volume of the gas is set to the volume that would be occupied
by one mole of ideal gas at the set temperature and standard pressure
of 101325 Pa. This makes the partial pressure of molecular species
containing one atom of the type 1 component numerically equal to the
amount of that component in their regions of predominance. The same
process applies to calculation of speciation in aqueous systems but, in
this case, the "space" occupied by the aqueous phase is fixed at one
behaviour of the system can be investigated in much greater detail
as a function of a single variable by means of the MULTIPHASE
module. A single example is shown in the diagram below, which explores
the effect of adding oxygen to molybdenum, silicon and nitrogen. The
same sequence of MoSi2, Mo5Si3,
Mo3Si, Mo and gaseous polymers of MoO3
is quantitatively determined. Where pairs of condensed compounds from
the above series coexist, the oxygen partial pressure (not shown on the
diagram) becomes constant in agreement with the diagram above. The
actual partial pressures of the gaseous species including the
molybdenum oxides and SiO are determined. Many plots can be
made from the same output file as described in the information about MULTIPHASE.
Brief summary of commands
allows the user to define the system to be studied. In one mode, the
data are retrieved from specified databases; in the alternative mode,
components and their associated data are read from a previously
LIST displays the
current definition of the problem in terms of:
the status of the elements, components, substances
and phases present
(b) initial and/or equilibrium
quantities of components
SET by the user
(c) temperature and
pressure/volume set by the user.
CLASSIFY allows the status of individual substances to be
classified as normal (present), or absent for the purposes of
SET specifies the temperature, volume of gas, pressure
limit, amounts of type 1 components, and fixed partial pressures,
activities or molalities of type 2 components.
RANGE determines the two type 3 components that define
respectively the abscissa and the ordinate of the diagram and the range
of the logarithm of the partial pressure, activity or molality of each.
COMPUTE initiates the calculation and allows the function
of the "zeroth" pass that determines if any regions of the diagram are
overdetermined, to be controlled.
takes the user out of MULTIPHASE back to the module
runs a macro taken from a file. The name of the file should be
entered in quotes. Commands can also be passed through to the
operating system by prefixing them with a $ (dollar sign). The
dollar sign and operating system command should be entered within
Labelling the diagrams for clarity of
interpretation poses considerable difficulties. Emphasis has been given
to presenting the essential information in the diagrams rather than
drawing them to standards that might have been expected of a drawing
In both MULTIPHASE and COPLOT
substances are labelled by reference to the substance number that
appears in a list beside the plot and corresponds with the numbering
given in the listing invoked by LIST SYSTEM SUBSTANCES !.
In COPLOT substances corresponding to different
type 1 components are given numbers of different sizes: the biggest for
the component in greatest amount and the smallest for that in smallest
amount. Attention is usually focused on the component of smallest
amount, the compounds of which are distinguished by being written in
upright script rather than in italics.
The coexistence lines for the component of
smallest amount are drawn solid, in distinction from those of other
type 1 components which are drawn with various broken lines.
Where a region of the diagram has been excluded
from further consideration as a result of the "zeroth" pass, its
boundary with the valid part of the diagram is drawn with a solid line.
introduction to using
the options available in COPLOT contains annotated
examples, each directed towards achieving a particular outcome
and shows typical combination of commands which would be necessary
to achieve that end.
summary of the commands
available can also be used as an index to obtain more detailed
help about each command.