Boiler Primer 3: pH, alkalinity and conductivity

Understanding the environment that reactions take place in helps us to determine which are more likely.  Variables like pH and alkalinity do that, because they tell us what form a species is in.  For example carbonates, which are the components of alkalinity have two protonation states, since H2CO3 is diprotic.  It is an acid that is able to give up two protons.

H₂CO₃ <=> HCO₃⁻ + H⁺

K_a1 = 2.5×10⁻⁴;^[1] pK_a1 = 3.6 at 25 °C.

HCO₃⁻ <=> CO₃²⁻ + H⁺

K_a2 = 4.69×10⁻¹¹; pK_a2 = 10.329 at 25 °C and ionic strength = 0.0 (data from wikipedia)

These two pKa values describe the extent to which one of the species is present at a certain pH.  See the speciation plot below for carbonic acid (source):

This means that at low pH, carbonic acid is primarily in its H2CO3 state.  Most of the carbonic acid molecules have given up their first proton by pH 6, and most of those have given up their second proton by pH 9.5 to become CO3.  Thus, above pH 10, CO3 exists as the dominant species.  It is a weak base.  This water is high in alkalinity therefore, but not corrosive.

We like to operate at this pH, because it dramatically reduces the corrosion from oxygen and other sources such as carbonic acid (source):

However, it means that we have a large cohort of CO3 present, which is amenable to reaction with calcium to form calcium carbonate.  Because this is a scale forming environment, we address this with dispersants and inhibitors that get to the calcium before it can form scale.

Conductivity then is a measure of the amount of dissolved solids in water.  This increases as more salts are available in solution to conduct electrons.  This is essentially one number that helps to boil down how full the water is of ions (alkalinity, treatment, hardness).  Water with higher conductivity tends to have more possible reactions as well, shifting equilibrium toward scale formation.  Conductivity is a measure that we use to manage how often blowdown occurs--how often we get rid of water that has many ions, and introduce makeup water with fewer ions, to reduce scale formation.