MTDATA – Phase Diagram Software from the National Physical Laboratory







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The COPLOT module is used to draw predominance area diagrams (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.

COPLOT 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 or by 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.

COPLOT has some features not found in other computer based systems for drawing predominance area diagrams.

(a) 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.

For these purposes three types of component are defined.

Type 1: these are set by amount and must be predefined as components during the setting up of the data input file.
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.

An 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.

The 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.

When 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 so useful.

Note 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.

The 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 kilogram.

The 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

DEFINE 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 compiled file.

LIST displays the current definition of the problem in terms of:

(a) 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 subsequent calculations.

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.

RETURN takes the user out of MULTIPHASE back to the module level.

<"Macro name"> 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 quotes.

Further Information

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 office.

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.

A gentle 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.

A simple summary of the commands available can also be used as an index to obtain more detailed help about each command.


Updated 6 July 2010