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pH & Alkalinity – How are they related?

Improving the Sustainability of Water Treatment

DOUG KELLEY \ December 17, 2020
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A primer on the relationship between the bicarbonate system & pH

The relationship between pH & alkalinity is intimately connected with the chemistry of the aqueous bicarbonate equilibrium system.  An understanding of this will help us to communicate with & understands our customer’s needs.

CO2 +   H2O     <=>     CO2(aq)    <=>     H2CO3        <=>         H+             +              HCO3

                                                                                                   Carbonic acid        Hydrogen ion              Bicarbonate ion

Where <=>  represents a partial (equilibrium) reaction or dissociation

For H2CO3 dissociation constant, ka  = [H+].[HCO3]/[H2CO3]   is a measure of how much it dissociates as a function of [H+] concentration or pH

[H+]         = concentration of H+

[HCO3]  = concentration of bicarbonate ion (alkalinity)

[H2CO3] = concentration of carbonic acid (incl. dissolved CO2)

 

Taking log of both sides   =>  log10ka = log10[H+] + log10{[HCO3]/[H2CO3]}

Reorganizing               -log10[H+] = -log10ka   + log10{[HCO3]/[H2CO3]

And by definition         pH = constant + log10{alkalinity/(carbonic acid+ dissolved CO2)}

ACTI-Mag for alkalinity & pH control

A safe and cost effective alternative to the use of sodium hydroxide (caustic soda) or other chemicals for wastewater alkalinity and pH adjustment.

Find out more about ACTI-Mag
What does this mean for pH control?   Take home messages:
  1. There are only 2 ways to move pH:

Add or remove alkalinity

  •  to raise pH add an alkali (caustic or MHL – Mg(OH)­2 + CO2(aq) => Mg++ + 2HCO3)
  •  to lower pH add an acid (H+ + HCO3 => H2O + CO2^(g)) remove alkalinity & also adds CO2

Add or remove dissolved CO2

  •  dissolve or strip CO2 to decrease or increase pH (increase or decrease bottom line)

Note: in a treatment plant CO2 generation is a ‘given’ as BOD/COD is oxidized to CO2

=> pH will always come down spontaneously even if we dose all influent to >pH 8.5

=> system self-corrects – high pH influent can’t upset the plant unless greatly overdosed

=> excess CO2 generation is normally stripped by aeration

  1. The higher the water alkalinity
  •  the greater the acid or alkali dose required to move pH up or down
  •  the more stable the pH (that’s why you need to maintain adequate alkalinity in your pool)

pH  vs Alkalinity presentation

There is a direct correlation, but pH and alkalinity are not the same.

pH is a measure of the amount of hydrogen ions (H+, acid) in the water.

M Alkalinity is a measure of the bicarbonate (HCO3) concentration.

Think of M Alkalinity and bicarbonate as dissolved limestone (CaCO3).

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