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pH & Alkalinity – How are they related?

Improving the Sustainability of Water Treatment

DOUG KELLEY \ September 29, 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]}

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

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

AMALGAM 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 AMALGAM
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 oxidised 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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