Alkalinity

What is Alkalinity?

Alkalinity is a measure of the capacity of water to neutralize acids. This is known as the buffering capacity of water or the ability of water to resist a change in pH when acid is added.

Alkalinity in water (with a pH of 7.0 or higher) is caused primarily by the presence of dissolved acid-neutralizing substances (alkali). It relates to the balance of carbon dioxide in water and is a function of pH.

The three primary alkali ions in water that contribute to alkalinity are:

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

Total Alkalinity

Total alkalinity is a measure in parts per million of all the alkali (all carbonate, bicarbonate and hydroxide ions) present in a sample. Therefore, total alkalinity is a measurement of water’s ability to resist changes in pH.

Total alkalinity is determined by titration to a pH of 4.9, 4.6, 4.5, or 4.3, depending on the amount of carbon dioxide present.

Phenolphthalein Alkalinity

Phenolphthalein alkalinity (p-alkalinity, also Partial or Free alkalinity) is determined by titration to a pH of 8.3 (the end point when phenolphthalein indicator changes color) and it registers the total hydroxide and one half the carbonate present. Phenolphthalein alkalinity is a subset of total alkalinity.

Typically, hard water is alkaline, while soft water is more acidic. However, exceptions exist in unusual circumstances.

Hardness is the sum of the multivalent metal ions in solution, whereas alkalinity is a sum of hydroxide, carbonate and bicarbonate defining the solution’s ability to neutralize acids. In natural water systems, both calcium and carbonate ions are usually present and responsible for these two characteristics of the water. Therefore, both hardness and alkalinity are expressed as a concentration of CaCO₃ for convenience of reporting and for ease in calculating a solution’s carbonate and non-carbonate hardness.

Phenolphthalein alkalinity determination by titration with acid uses a phenolphthalein indicator that changes from pink to colorless at the endpoint (pH = 8.3). If the sample does not turn pink after adding the indicator, the sample does not have phenolphthalein alkalinity. The water  will not have any phenolphthalein alkalinity if its pH is less than 8.3.

Samples with an acid capacity above 8 mmol/L must be diluted with deionized (DI) water that contains no carbon dioxide (CO₂). One can remove CO₂ from deionized water by boiling. The absence of CO₂ in the water used for dilution can be checked by carrying out a blank measurement. CO₂ in the DI water will interfere when diluting samples or standards for this colorimetric test. The CO₂ free water is not as critical in the titration procedure. The colorimetric test is more sensitive to slight changes in CO₂ concentration due to the detection method and small sample volume used.

Some common interferences in alkalinity titration methods include:

• Chlorine – Chlorine at levels above 3.5 mg/L can cause a yellow-brown color when the Bromcresol Green-Methyl Red Powder Pillow is added. To remove the interference, pretreat the sample by adding 1 drop of Sodium Thiosulfate Standard Solution, 0.1 N per 100 mL sample, before adding any other reagents.
• Color or turbidity – Color or turbidity can make it difficult to see the color change at the endpoint. Do not filter or dilute samples with color or turbidity. Use a pH meter and titrate the samples to the specified endpoints:
  
  

  Sample composition	Phenolphthalein alkalinity	Total alkalinity
  Alkalinity approximately 30 mg/L	pH 8.3	pH 4.9
  Alkalinity approximately 150 mg/L	pH 8.3	pH 4.6
  Alkalinity approximately 500 mg/L	pH 8.3	pH 4.3
  Contains silicates or phosphates	pH 8.3	pH 4.5
  Industrial wastes or complex systems	pH 8.3	pH 4.5
  Routine or automated analysis	pH 8.3	pH 4.5

• Soaps, oily matter, suspended solids and precipitates – Oils or solids can collect on the pH probe and cause a slow response. Clean the probe immediately after use.