Neutralized Conductivity
By James McDonald, PE, CWT
Originally Published: CSTN – February 2003
Have you ever wondered why you run neutralized conductivities on boiler
water samples? It is done to neutralize the hydroxyl ions (OH-). Hydroxyl
ions contribute significantly to the total ionic concentration of the water
sample. This affects both the conductivity and TDS readings when based on
the specific conductance of the sample.
Why do we want to neutralize the effect of hydroxyl ions on our
readings? Hydroxyl ions are not considered a dissolved solid, but instead
part of a disassociated water molecule. When we take our conductivity/TDS
readings on the boiler water and calculate cycles of concentration, we want to
have a truer estimate of the cycles of concentration of dissolved solids
without the added conductance effects of hydroxyl ions.
Where do hydroxyl ions come from? There are a few sources. Water
naturally dissociates to some extent to hydrogen ions and hydroxyl ions.
Sodium and/or potassium hydroxide is sometimes added to boiler waters to
raise alkalinities. This directly increases the amount of hydroxyl ions present
in the water. Also, the chemical reaction of carbonate-based alkalinity
breaking down under the heat and pressure in a boiler results in hydroxyl
ions as a reaction product.
How are hydroxyl ions neutralized? Organic acids are used to drop the pH
of the water sample down to the phenolphthalein endpoint without adding
conductivity themselves. Typical organic acids used are gallic acid and citric
acid. The liquid Conductivity Neutralizing Solution (R-7022) provided by
Taylor Technologies, Inc. is a blend of citric acid, phenolphthalein, isopropyl
alcohol, and deionized water.
Why are neutralized conductivities lower than the reading on a boiler
automatic blowdown controller? Boiler automatic blowdown controllers
do not neutralize their water samples before reading conductivity. If you
have a boiler automatic blowdown controller, you will also need to take an
unneutralized conductivity reading for comparison to the controller reading.
Why do you care? If you plan to use boiler conductivity to calculate cycles
of concentration, feedwater demand, blowdown, and percent blowdown, your
results will be wrong if you don't use neutralized conductivity. The cycle
result will be too high, percent blowdown too low, blowdown flowrate too
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low, and feedwater demand too low. Any other calculations related to an
unneutralized conductivity will be correspondingly skewed too.
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