The creation of the Coordinated Phosphate program was the first step in a series of solutions that boiler water treatment history kept seeking for new challenges.
Before this program emerged, some boilers were treated with soda ash, to adjust internal boiler conditions to those that produced the lowest corrosion rate. But as construction methods advanced and the need for greater energy efficiency grew, these newer boilers began to show caustic corrosion.
The Creation of the Program
Against this backdrop, in the 1940s, Purcell and Whirl created the Coordinated Phosphate program. The idea behind this approach was to dose Na₃PO₄, which through hydrolysis would produce NaOH. When evaporation occurred, the reaction would reverse and the soda would form trisodium phosphate again. However, the phosphate compounds generated from Na₃PO₄ hydrolysis had different solubilities, so the soda wasn't fully reversed — but that's the subject of the next article, on Congruent Phosphate.
What was interesting was that the program was controlled through pH and phosphate content analyses. The resulting point was plotted on a chart, and if it fell within a control envelope, the program was on target. The Na/PO₄ molar ratio was expected to stay between 2.3 and 3, and the control envelope compared whether pH and the ratio were within the expected range.
Why Sodium Wasn't Analyzed
The reason sodium concentration wasn't used to control congruency is that the water's total sodium doesn't distinguish its chemical origin. For example, Na₃PO₄ supplies sodium associated with phosphate and participates in the program's equilibrium, while NaCl and other salts also contribute to total sodium but don't take part in the phosphate–hydroxide equilibrium. It's common to use sodium content to control the program in the belief it will be more precise, when in fact that's a mistake.
This program was later replaced by Congruent Phosphate and, after that, by Equilibrium Phosphate.
If your boiler runs a phosphate program today, it's worth asking: what Na/PO₄ ratio is being maintained, and is it still appropriate for your plant's current operating pressure?