3.6 H2O-NaCl Equilibrium Diagram

Figure 3.09 summarizes the equilibrium relations of the phases brine, halite, and ice. It is, therefore, a phase diagram.

Figure 3.09. H2O-NaCl equilibrium phase diagram. Phases at equilibrium for the system H2O-NaCl are shown as a function of chemical composition (weight percent NaCl) and temperature. The blue area indicates temperatures and compositions for which a single phase, brine, is present at equilibrium. Click on the diagram to see a larger version with more information.

The H2O-NaCl phase diagram in Figure 3.09 is divided into 4 areas by thick black lines. The one area that is colored blue idenifies chemical compositions (weight percent NaCl) and temperatures (°C) that yield a single salty liquid (brine) at equilibrium. The 3 areas that are filled with a horizontal line pattern identify chemical compositions and temperatures that yield a mixture of two phases at equilibrium. The identities of the two phases are given by the written labels.

The horizontal line patterns represent tie lines that connect the compositions of pairs of phases at equilibrium at specific temperatures. For the "Brine+Halite" area, the brine composition at each temperature is given by the halite-saturation curve. For the "Ice+Brine" area, the brine composition at each temperature is given by the ice-saturation curve. Notice that both halite and ice have no variation in chemical composition. This is because there is no solid solution of H2O in halite and there is no solid solution of NaCl in ice.

As predicted, the halite-saturation curve and the ice-saturation curve meet. The brine composition at the intersection point (23.20 weight percent NaCl) is for a liquid that is saturated with both halite and ice. If ice crystals, halite crystals, and brine are all present at equilibrium, (1) the brine must be saturated both with halite and with ice, (2) the brine composition must be 23.20 weight percent NaCl, and (3) the temperature must be -21.21°C. This is a special, fixed point that can be reproduced in any laboratory by mixing halite and ice with water and stirring until equilibrium is reached. It is the temperature produced in the tub of an old-style ice cream maker when ice and halite are mixed in excess. It is also the temperature below which rock salt (halite) will not react with (melt) ice. Below -21.21°C, ice and halite can be in contact at equilibrium. "Ice+Halite" is the third two-phase equilibrium area on Figure 3.09.