**The Sulphur system**

– Sulphur system is a one-component, four-phase system.

– The four phases are:

(a) Two solid polymorphic forms :

- Rhombic Sulphur (S
_{R}) - Monoclinic Sulphur (S
_{M})

(b) Sulphur Liquid (S_{L})

(c) Sulphur Vapour (S_{V})

– All the four phase can be represented by the only chemical individual (sulphur) itself and hence one component of the system.

– The two crystalline forms of sulphur S_{R} and S_{M} exhibit enantiotropy with a transition point at 95.6ºC. Below this temperature S_{R} is stable, while above it S_{M} is the stable variety.

– At 95.6^{º}C each form can be gradually transformed to the other and the two are in equilibrium.

– At 120ºC, S_{M} melts.

Thus,

**Phase diagram for the sulphur system**

– The phase diagram for the sulphur system is shown in this figure:

– The salient features of the phase diagram are described below.

(1) The six curves AB, BC, CD, BE, CE, EG

(2) The three Triple points B, C, E

(3) The four areas :

- ABG marked (solid Rhombic)
- BEC marked (solid Monoclinic)
- GECD marked (liquid Sulphur)
- ABCD marked (Sulphur vapour)

– Let us now proceed to discuss the significance of these features :

## (1) The curves AB, BC, CD, BE, CE, EG

– These six curves divide the diagram into four areas.

**Curve AB**

– The Vapour Pressure curve of S_{R}.

– It shows the vapour pressure of solid rhombic sulphur (S_{R}) at different temperatures.

– Along this curve the two phases S_{R} and sulphur vapour (S_{V}) are in equilibrium.

– The system S_{R}/S_{V} has one degree of freedom,

**F = C – P + 2 = 1 – 2 + 2 = 1**

i.e., it is monovariant.

**Curve BC**

– The Vapour Pressure curve of S_{M}.

– It shows variation of the vapour pressure of monoclinic sulphur (S_{M}) with temperature.

– S_{M} and S_{V} coexist in equilibrium along this curve.

– The system S_{M}/S_{V} is monovariant.

**Curve CD**

– The Vapour Pressure curve of S_{L}.

– It depicts the variation of the vapour pressure of liquid sulphur (S_{L}) with temperature.

– S_{L} and S_{V} are in equilibrium along CD.

– The two phase system S_{L}/S_{V} is monovariant.

– One atmosphere line meets this curve at a temperature (444.6^{º}C) which is the boiling point of sulphur.

**Curve BE**

– The Transition curve.

– It shows the effect of pressure on the transition temperature for S_{R} and S_{M}.

– As two solid phases are in equilibrium along the curve, the system S_{R}/S_{M} is monovariant.

– The transformation of S_{R} and S_{M} is accompanied by increase of volume (density of S_{R} = 2.04; S_{M} = 1.9) and absorption of heat i.e.,

**SR + Q (heat energy) ↔ SM**

– Thus the increase of pressure will shift the equilibrium to the left (Le Chatelier’s Principle) and the transition temperature will, therefore, be raised.

– This is why the line BE slopes away from the pressure axis showing thereby that the transition temperature is raised with increase of pressure.

**Curve CE**

– The Fusion curve of S_{M}.

– It represents the effect of pressure on the melting point of S_{M}.

– The two phases in equilibrium along this curve are S_{M} and S_{L}.

– The system S_{M}/S_{L} is monovariant.

– As the melting or fusion of S_{M} is accompanied by a slight increase of volume, the melting point will rise by increase of pressure (Le Chatelier’s principle).

– Thus the curve CE slopes slightly away from the pressure axis.

– The curve ends at E because S_{M} ceases to exist beyond this point.

**Curve EG**

– The Fusion curve for S_{R}.

– Here the two phases in equilibrium are S_{R} and S_{L}.

– The number of phases being two, the system S_{R}/S_{L} is monovariant.

**(2) The Triple points B, C, E**

**Triple point B**

– This is the meeting point of the three curves AB, BC and BE.

– Three phases, solid S_{R}, solid S_{M} and S_{V} are in equilibrium at the point B.

– There being three phases and one component, the system S_{R}/S_{M}/S_{L} is nonvariant.

**F = C – P + 2 = 1 – 3 + 2 = 0**

– At B, S_{R} is changed to SM and the process is reversible.

– Thus the temperature corresponding to B is the transition temperature (95.6^{º}C).

**Triple point C**

– The curves BC, CD, CE meet at this point.

– The three phases in equilibrium are S_{M}, S_{L} and S_{V}.

– There being three phases and one component, the system S_{M}/S_{L}/S_{V} is nonvariant.

– The temperature corresponding to C as indicated on the phase diagram is 120^{º}C. This is the melting point of S_{M}.

**Triple point E**

– The two lines CE and BE, having different inclinations away from the pressure axis, meet at E where a third line EG also joins.

– The three phases S_{R}, S_{M} and S_{L} are in equilibrium and the system at the point E is nonvariant.

– This point gives the conditions of existence of the system S_{R}/S_{M}/S_{L} at 155^{º}C and 1290 atmospheres pressure.

**(3) The Areas**

– The phase diagram of the sulphur system has four areas or regions.

– These are labelled as rhombic sulphur, monoclinic sulphur, liquid sulphur and vapour.

– These represent single phase systems which have two degrees of freedom,

**F = C – P + 2 = 1 – 1 + 2 = 2**

– That is, each of the systems S_{R}, S_{M}, S_{L}, and S_{V} are bivariant.

**(4) Metastable Equilibria**

– The change of S_{R} to S_{M} takes place very slowly.

– If enough time for the change is not allowed and S_{R} is heated rapidly, it is possible to pass well above the transition point without getting S_{M}.

– In that case, there being three phases (S_{R}, S_{L}, S_{V}) only and one component, the phase diagram, like that of water system, will consist of three curves, one triple point and three areas.

**The dashed curve BF**

– The Vapour Pressure curve of metastable SR.

– This is a continuation of the vapour pressure curve AB of stable S_{R}.

– The metastable phases S_{R} and S_{V} are in equilibrium along this curve.

– It is a monovariant system.

**The dashed curve CF**

– The Vapour Pressure curve of supercooled S_{L}.

– On supercooling liquid sulphur, the dashed curve CF is obtained.

– It is, in fact, the back prolongation of DC.

– The curve CF represents the metastable equilibrium between supercooled S_{L} and S_{V}.

– Thus it may be designated as the vapour pressure curve of supercooled S_{L}.

– It meets the dashed curve BF at F.

**The dashed curve FE**

– The Fusion curve of metastable S_{R}.

– The two metastable phases S_{R} and S_{L }are in equilibrium along this curve and the system is monovariant.

– This shows that the melting point of metastable S_{R} is increased with pressure.

– Beyond E, this curve depicts the conditions for the stable equilibrium S_{R}/S_{L} as the metastable S_{R} disappears.

**The metastable Triple point F**

– At this point, three metastable phases S_{R}, S_{L} and S_{V} are in equilibrium.

– The system is a metastable triple point with no degree of freedom.

– The corresponding temperature is the melting point of metastable S_{R} (114ºC).