Back to Basics: What is Alkalinity?

Alkalinity is a measure of a water's capacity to neutralize acids. Understanding this critical parameter is important to developing and maintaining successful water treatment programs.

What are the Components of Alkalinity

Naturally occurring alkalinity in water is primarily due to the presence of the following ions:

• Carbonate (CO₃^2-)
• Bicarbonate (HCO₃^-)
• Hydroxide (OH^-)

Why is Alkalinity Important?

In industrial water treatment, alkalinity is a critical parameter to understand and manage because it can impact scale formation, corrosion, steam purity/quality, and the effectiveness of chemical treatments.

• Scale Formation: The alkalinity of water is a key factor in determining the tendency for scale to form. The higher the alkalinity, the greater the potential for scale formation. The alkalinity components themselves may become part of the scale deposit, such as calcium carbonate and magnesium hydroxide, which can reduce the alkalinity as a result.
• Corrosion Control: Low alkalinity water can tend to be more corrosive while higher alkalinity waters can be less corrosive.
• Steam Purity & Quality: In steam boiler systems, excessive alkalinity can lead to carryover of liquid water and impurities with the steam due to foaming, impacting downstream heat exchangers, turbines, and process equipment.
• Water Treatment: In cooling systems, alkalinity impacts the amount of acid needed for pH control to prevent scale. In steam boiler systems, alkalinity can break down and lead to carbonic acid attack of condensate systems, requiring mitigation. A minimum level of alkalinity is required in the boiler water for scale deposit control. High alkalinity can interfere with some industrial processes like beverage production and textile dyeing.

How is Alkalinity Measured?

Alkalinity is measured by titrating a water sample with a standard acid solution down to a set pH. Colored indicators are typically used to indicate when these pHs are reached.

• M Alkalinity measures Total Alkalinity, which represents all titratable species that neutralize acid. The endpoint pH for M alkalinity is 4.5. Common indicators used include bromcresol green-methyl red and methyl orange. The phrases “Total Alkalinity” and “M Alkalinity” are interchangeable.
• P Alkalinity corresponds to the amount of alkalinity present at a pH at and above 8.3, which equates to the disappearance of carbonate from the solution when present. Phenolphthalein is the indicator typically used.
• Hydroxide (OH-) Alkalinity is determined by calculation or by using a barium chloride method. Hydroxide Alkalinity is important to monitor, especially when silica levels are high in boiler water. The equation 2P-M is frequently applicable to boilers (where P is “P Alkalinity” and M is “M Alkalinity” both expressed as CaCO₃). When silica is the limiting factor for boiler cycles of concentration, it is recommended to use the barium chloride test to confirm the appropriate ratio of Hydroxide Alkalinity to silica.

How are Alkalinity and pH Related?

While pH measures the concentration of hydrogen ions and alkalinity measures the acid neutralizing capacity of water, the two are related. Lower alkalinity water tends to have a lower pH and be corrosive. Higher alkalinity water tends to have a higher pH and can be scale forming. The following table illustrates the relationship between the two, but the exact Total Alkalinity to pH relationship may differ significantly and should be regularly confirmed when one is being used to predict the other.

How do You Control Alkalinity?

Alkalinity can be either reduced or increased using multiple ways.

• Reduced: Alkalinity can be reduced through various processes, including:
  • Acid feed: Adding acid to raw water can reduce alkalinity. Downstream decarbonation can be employed to remove the alkalinity species as well.
  • Ion exchange: Methods such as a chloride-cycle dealkalization can be used to reduce alkalinity. Demineralization removes all soluble ionic material from water, including alkalinity ions.
  • Membrane Filtration: Reverse osmosis can effectively remove alkalinity.
• Increased: In some cases, such as steam boilers, alkalinity is managed by raising the pH of the feedwater. Caustic (NaOH) is commonly used in steam boiler applications.

What Other Ions Impact Alkalinity?

Several ions besides carbonate, bicarbonate, and hydroxide can contribute to the alkalinity of water, although they are typically not the primary contributors. Here's a breakdown of other ions that can impact alkalinity:

• Phosphate: Phosphates added to boiler and cooling water can be either acidic or basic and affect alkalinity measurements. Although they may have a small effect when compared to carbonate alkalinity, phosphates can sometimes be a significant factor in total alkalinity. Phosphate can also be a component of scale, which may reduce Total Alkalinity when scale-forming precipitation occurs.
• Ammonia: Ammonia is an ionizing gas found in water that can contribute to alkalinity.
• Borate: Borate, present in water as nonionized boric acid, can contribute to alkalinity at high pH.
• Neutralizing Amines: Neutralizing amines can increase the acid-absorbing ability of water, which might be mistaken as increased alkalinity, particularly in the context of OH-alkalinity measurements.
• Polymers: In boiler applications, polymers may titrate as part of Total Alkalinity, giving a falsely high number.

The impact of these other ions may vary depending on the specific water source, treatment processes, and operating conditions. In some cases, these ions can interfere with the alkalinity test itself, leading to inaccurate readings. For example, high levels of chlorine can alter the test’s colors significantly. Therefore, when assessing alkalinity, it’s important to consider the presence and influence of these additional ions.

Conclusions

In summary, understanding alkalinity and its impact on industrial water systems is crucial for maintaining efficient and reliable operations. Proper management of alkalinity is key to preventing scale formation, corrosion, and other water-related issues. For more information about alkalinity and your water operations, please contact your local Chem-Aqua representative.

Written by:  James McDonald, PE, CWT