Acid and Base Chemistry of Aquaponics Brief Overview

It’s all about water

Acid and base chemistry has its roots in the actions of water.  Water (H₂O) does not solely exist as H₂O as it sits in a glass on a table, but rather splits into its ionic (charged) parts.  H-OH breaks into H⁺ and OH^-.  H⁺is a hungry proton, looking for another compound to attack to balance its positive charge and become neutral.  This is what makes acid “acidic”, and why hydrochloric acid (HCl) will eat your flesh…it’s all about those hungry H⁺’s looking to balance their “+” charge.  Similarly, OH^- is a hungry hydroxide looking to balance its charge by attacking other compounds.  This is what makes bases “basic”, and why lye will eat your flesh…it’s all about those hungry OH^-‘s looking to balance their “-” charges.

How water is the acid

Let’s say your fish tank is full of water sitting at room temperature.  Some water will remain as H₂O molecules, while some dissociate (split) into H⁺ and OH^-ions.

Let’s add 4 parts hydrochloric acid (HCl) to the water.  Acid will also dissociate into H⁺and Cl^- ions.

The hungry H⁺’s from HCl will attack the OH^-‘s in the water, turning them back into H₂O.

Since 4 OH^-‘s are removed from the solution, we now have an excess of H⁺.  Thus, by HCl making H₂O less basic…it becomes more acidic. Our extra choride ions are just floating around, doing nothing as spectator ions. 

How water is the base

Now, let’s add 4 parts ammonia (NH₃) to the original neutral water in the fish tank.  Ammonia is uncharged, so it dissolves but doesn’t dissociate in water.  However, nitrogen is famous for having an extra pair of electrons (negative charges) that are hungry for positively charged species.

 

The hungry electrons on ammonia (NH3) will steal 4 H+’s from the water, turning it into the positively charged species ammonium ion (NH4+).  Since 4 H’s are removed from solution, we have an excess of OH-‘s.  Thus, by ammonia making H2O less acidic…it becomes more basic.

 

Where does pH come into play?

pH is based on logarithms.  Specifically, it is the negative log of H+ concentration.  The pH scale only goes from 0-14.  There is nothing more or less.

If concentration of H+ is 10^-14 in a solution, that means its concentration is 0.00000000000001.  This is a VERY weak acid.  Calculate -log(10^-14) = pH of 14.  This VERY basic because not much acid.

If concentration of H+ is 10^-7 in a solution, that means its concentration is 0.0000001.  Calculate -log(10^-7) = pH of 7.  This is neutral, like pure water.

If concentration of H+ is 10^-1 in a solution, that means its concentration is 0.1.  Compared to 10^-14, this is a VERY strong acid.  Calculate -log (10^-1) = pH of 1.  This is VERY acidic. 

So why bother using pH at all?  Consider this:  An H+ concentration of 10^-1is 0.1.  An H+ concentration of 10^-7is 0.0000001, which is 1,000,000 times LESS than 10^-1.  Rather than making it difficult for the layman with calculating exponents and logarithms, pH makes things easy by saying pH 1 and pH 7.  Let’s recap:

Acid = LOW pH (below 7)

Base = HIGH pH (above 7)