[Troy Walker]

Hi,

I'm a water guy.  Water chemistry is a bit challenging because we use funny units to measure and report things.  Bicarbonate (HCO3-) is often reported in water quality reports as "alkalinity".  And that's correct.  Most water between pH of about 4.2 and 8.2 (where our city water usually is) almost all the alkalinity in water is bicarbonate.  Yep, the same thing that's in baking soda.

So I was trained as a chemical engineer.  I learned my periodic table and all that.  Trouble is, water treatment was first done by civil engineers.  Lovely people, don't get me wrong, but chemistry.....wasn't their strong suit.  So instead of using things like "moles" or other chemistry concepts, they decided in water to use a common currency for measurement that makes the math a bit easier.  Actually not a bad idea.  What they landed on was to measure things in terms of the molecular weight of calcium carbonate (CaCO3).  You know why?  Well, the atomic weight of calcium is 40. Carbon is 12.  Oxygen is 16 (and there are three of those).  40 + 12 + 16 + 16 +16 = 100!  So it's a convenient number to use.

Here's the confusing bit.  It isn't actually calcium carbonate.  It's just expressed in those units.  Think of it as converting Euros to dollars.

So yes, you will often see bicarbonate (or sometimes just alkalinity) reported as mg/L CaCO3.  In other words, milligrams per liter as calcium carbonate.  Here's an excerpt from the Water Board of California that gives an explanation of the chemistry.  But yes, HCO3 = 1.22 x bicarbonate expressed as calcium carbonate.

https://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/documents/drinkingwaterlabs/AlkalinityConversions.pdf

Alkalinity is important in water because it acts as a buffer.  What does that mean?  If you have low alkalinity, a small amount of acid or caustic will cause the pH to have big swings low or high.  If you have high alkalinity, that same amount of acid or caustic won't change the pH quite so much.  Because it's buffered.  Buffering happens because pH is actually a balance between bicarbonate and dissolved carbon dioxide in water.  You add acid, you convert some of the bicarbonate to CO2.  You add caustic, you do the reverse.  If there's lots of bicarbonate, then you need to add a lot of acid to make enough CO2 to change the pH.

Anyway, hopefully your head isn't spinning.  Relax, don't worry, and have a homebrew.

[BrewBama]

Good stuff Mr Walker! 

[roger]

+1 Thanks for the explanation.

[majorvices]

Great explanation, thanks!