As boiler water of a given Na:PO4 ratio is concentrated at local areas of the boiler (under deposits or in zones of film boiling), the sparingly soluble natures of the various phosphate species at boiler water pH values and temperatures cause precipitation (solid phase formation) to occur. The solid phase Na:PO4 ratio will always be less than 3.0:1, but the actual solid phase stoichiometry will vary with the existing liquid phase Na:PO4 ratio.

When the liquid phase Na:PO4 ratio is greater than 2.85:1, the solid phase formed upon precipitation will have a lower Na:PO4 ratio than the liquid phase. Phosphate precipitation when the liquid phase Na:PO4 ratio is greater than 2.85:1 therefore results in an increase in the liquid phase Na:PO4 ratio and causes a shift towards the 3.0:1 Na:PO4 ratio line, above which free sodium hydroxide will exist, creating potential corrosion problems.

Conversely, if the liquid phase Na:PO4 ratio is less than 2.85:1, the solid phase Na:PO4 ratio is always greater than the liquid phase ratio. Phosphate precipitation when the liquid phase Na:PO4 ratio is less than 2.85:1 therefore depletes the water system of sodium, decreasing the Na:PO4 ratio of the remaining liquid phase. Precipitation thereby moves the water system even further below the 3:1 line, rather than toward it.

A plot of the composition of solid in terms of the Na:PO4 ratio versus the composition of solution phase solid in terms of the Na:PO4 ratio in an equilibrium system of disodium phosphate/water at a given temperature will cross a congruence line (line formed of points representing equal solid and solution Na:PO4 ratios) at what is known as a point of congruency, or simply the congruent point. 

For instance, 

• at a temperature of 572 DEG F (300 DEG C) the congruent point is where both the solution phase and the solid phase have an identical Na:PO4 of 2.85:1. 
• at 689 <0>F (365 DEG C), the congruent point is about 2.6:1. 

Since these congruent points form a line of demarcation between the zones where you are either moving towards or away from the 3:1 line under conditions of precipitation, these points become the upper control boundary for the Congruent Control method. 

Typically under Congruent Control the phosphate-pH relationship is controlled to maintain a solution phase (boiler water) Na:PO4 ratio range of 2.6:1 to 2.3:1. The 2.3:1 ratio is chosen as the bottom of the control range to avoid the formation of acid phosphate which can occur below 2.13:1.