Advanced Water Chemistry of Freshwater Aquariums

Advanced Water Chemistry of Freshwater Aquariums

How Do I Fix The pH In My Aquarium?

For many beginner aquarium keepers pH seems to be an easy to understand concept; test your pH and add one chemical if its low or another if its high. However, for some pH can end up being an uphill battle, chasing an optimal level with a seemingly phantom force driving it away. There are a lot of misconceptions available online, which can often steer many hobbyists in the wrong direction. It is often best to start from the basics...

Understanding pH

To define pH simply put; pH is a measurement of the concentration of hydronium ions (hydrogen ion activity) in a solution, and is a way of representing how relatively acidic or basic that solution is. A solution of pure water will have a pH value of 7; which is considered neutral because it has no hydrogen ions to give and no space to accept them. Acids are solutions with a pH less than seven, and means these solutions have an excess of hydronium, while bases have a pH greater than 7, meaning they are eager to accept hydrogen ions. The pH scale is logarithmic, meaning each unit represents a 10-fold concentration difference! For example, the distance between 5 and 6, is 10 times larger than between 6 and 7; so each degree of pH is a big jump.

Change The Way You Think About Aquarium Water

When talking about water, most people only think of H20 but are really talking about the entire solution of H2O and salts. Even the H2O molecule can change states and dissociate, a process where one molecule of water will act as an acid and donates a hydrogen ion to another creating hydronium and hydroxide. (2 H₂O <==> H₃O⁺ + OH¯) This dissociation is an equilibrium process, which means it juggles the hydrogen ion constantly at the same time as other H2O molecules. Because the solution is constantly in equilibrium, there isn't an opening for a hydrogen ion or an excess of them so the solution maintains its pH of 7.

pKa, The Secret Force Behind pH!

pKa, or just pK is closely tied to pH. Now that we understand that every solution of water is a constantly changing mix of its contents, we can start to learn how to effectively change its pH. Every solution has a pK value, which is a way of measuring how strong an acid is, but is not the same thing as the acidity (or basicity) which refers to pH. For example, if we make a dilute solution of HCl and another much more concentrated solution of acetic acid, the more concentrated acetic acid could be more acidic(pH), even though it is a weaker acid and has a higher pK value (lower pK values correlate to a stronger acid). Because of the relationship between pH and pK, buffers will work most effectively when their pK value is in line with the desired pH. This is the reason that some buffers cannot maintain pH levels in aquariums: improper pK!

The Most Misunderstood Concept of Aquarium Chemistry: Alkalinity

Alkalinity is simply a measure of the ability of a solution to resist change in pH when an acid is introduced. Alkalinity measures the total acid binding ions in a solution which may include carbonates, borates, phosphates, sulfates, and hydroxides. To avoid confusion, alkalinity should be measured in meq/L which is the proper scientific unit. Alkalinity is often confused with carbonate hardness, since both participate in acid neutralization and many test kits may incorrectly express their results in either dKH or ppm CaCo₃. Alkalinity is not carbonate hardness and it makes no sense to measure it in terms of carbonate hardness. To add to the confusion, often in aquariums alkalinity is the same as carbonate hardness. This is only the case when all of the alkalinity comes from carbonates as opposed to any of the other buffers.

What Buffers Should I Use?

Simply put, you should use a buffer that will leave your aquarium in equilibrium at your desired pH. As we discussed earlier, equilibrium is a state of a solution when chemical activities or concentrations of buffers don't change overtime often affecting pH. Just how H2O molecules can disassociate, every complete buffer is in fact an entire system of molecules which will reach equilibrium and determine the pH of your aquarium. The major buffering system in aquariums is the carbonate buffering system. Within this system, your aquarium will find its equilibrium by balancing carbon dioxide, carbonic acid, bicarbonate, and carbonate. (CO₂ + 3 H₂O <===> H₂CO₃ + 2 H₂O <==> HCO₃₋ + H₃O⁺ + H₂O <==> CO₃²⁻ + 2 H₃O⁺) We recommend using Seachem's Acid and Alkaline Buffer. These products use components from the carbonate buffering system to raise and lower the pH of your aquarium through chemical reactions rather than relying on adding acids or bases.

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