Total alkalinity and pH are two important parameters that affect the water quality and chemistry of swimming pools, spas, aquariums, and natural water bodies. They are often confused with each other, but they are not the same. In this article, we will explain the difference between total alkalinity and pH, and how they influence each other.
What is pH?
pH stands for potenz Hydrogen, more commonly referred to as the “power of Hydrogen”. It is the negative logarithm of Hydrogen ion concentration in a solution. The lower the pH, the exponentially more Hydrogen
We measure how acidic or basic (also called alkaline) substances are by using the pH scale. The pH scale goes from 0 to 14 and is relative to pure water, which has a perfectly neutral pH of 7. When the concentration of hydrogen ions goes up, the pH goes down (and vice versa). The scale is logarithmic, which means each whole number on the 1-14 pH scale is 10 times more or less than the numbers around it
The pH of water affects many aspects of water chemistry, such as:
- Chlorine strength: The lower the pH, the more effective chlorine is as a disinfectant. However, if the pH is too low, chlorine can also cause eye irritation, corrosion, and chlorine loss. The optimal pH range for chlorine effectiveness is 7.2 to 7.8
- Bather comfort: The ideal pH range for human skin and eyes is 7.4 to 7.6. If the pH is too high or too low, it can cause skin dryness, itching, burning, redness, and eye irritation
- Etching/Scaling: The pH of water also affects the solubility of calcium and other minerals in water. If the pH is too low, calcium and other minerals can dissolve from the pool surfaces and equipment, causing etching and pitting. If the pH is too high, calcium and other minerals can precipitate out of the water, causing scaling and staining. The optimal pH range to prevent etching and scaling is 7.4 to 7.6
What is Total Alkalinity?
Total alkalinity (TA) is the measure of all alkaline substances in water that can neutralize acids. The most common alkaline substances in water are carbonate (CO3-2), bicarbonate (HCO3-), and hydroxide (OH-). Total alkalinity is expressed in units of concentration, such as milligrams per liter (mg/L) of calcium carbonate (CaCO3)
The total alkalinity of water affects the stability of pH in water, such as:
- TA and pH reduction: Total alkalinity acts as a buffer that resists changes in pH caused by acids. Acids can come from various sources, such as rainwater, organic matter decomposition, bather waste, and chemical additions. When acids are added to water, they react with alkaline substances and reduce both TA and pH. The higher the TA, the more acid it takes to lower the pH by a given amount
- TA and pH increase: Total alkalinity can also contribute to an increase in pH over time. This happens when carbon dioxide (CO2), which is a weak acid in water, escapes from the water surface due to aeration or agitation. When CO2 leaves the water, it shifts the equilibrium between carbonate, bicarbonate, and carbonic acid (H2CO3), resulting in more hydroxide ions being formed. Hydroxide ions raise the pH of water. The higher the TA, the more CO2 it takes to raise the pH by a given amount
How to Adjust TA and pH?
The ideal TA range for most pools and spas is 80 to 120 mg/L CaCO3. The ideal pH range for most pools and spas is 7.4 to 7.6. To maintain these ranges, it is important to test the water regularly and adjust TA and pH accordingly.
To raise TA and pH:
- Add sodium bicarbonate (baking soda) or sodium carbonate (soda ash) to the water.
- Sodium bicarbonate raises both TA and pH moderately.
- Sodium carbonate raises both TA and pH significantly.
To lower TA and pH:
- Add muriatic acid or sodium bisulfate (dry acid) to the water.
- Muriatic acid lowers both TA and pH significantly.
- Sodium bisulfate lowers both TA and pH moderately.
When adjusting TA and pH, it is recommended to adjust TA first, then adjust pH as needed. This is because changing TA can also affect pH, but changing pH does not affect TA significantly.
Conclusion
Total alkalinity and pH are two related but distinct parameters that affect water chemistry and quality. Total alkalinity is the measure of all alkaline substances in water that can neutralize acids. pH is the measure of how acidic or basic water is. Total alkalinity stabilizes pH by buffering against acid additions or losses. However, total alkalinity can also cause pH to rise over time by releasing hydroxide ions when carbon dioxide escapes from the water. To maintain optimal water conditions, it is important to test and adjust TA and pH regularly.