Alkalinity in Water

Alkalinity is a measure of the water's ability to neutralize acids. It is an important parameter in water quality assessment as it helps in determining the water's buffering capacity and its resistance to pH changes. Alkalinity is expressed in terms of milligrams per litre (mg/L) and is typically reported as milligrams of equivalent calcium carbonate per litre (mg/L CaCO3).

Calcium Carbonate as Equivalent

The alkalinity in water is expressed as milligrams per litre in terms of equivalent calcium carbonate (CaCO3). This means that the alkalinity concentration is reported as the amount of calcium carbonate that would be required to neutralize the acid in the water sample.

Calcium Carbonate as a Common Standard

Calcium carbonate is chosen as the common standard for expressing alkalinity because it is a common and abundant compound found in nature. It is also the primary component of many minerals, such as limestone and chalk. Calcium carbonate is chemically stable and has a well-defined stoichiometry, making it suitable for use as an equivalent standard.

Calcium Carbonate Equivalent Calculation

To determine the alkalinity in water, a titration method is commonly used. In this method, a known volume of the water sample is titrated with a standardized acid solution until the pH reaches a certain endpoint. The amount of acid required to reach the endpoint is then used to calculate the alkalinity.

The alkalinity is calculated by multiplying the volume of acid used by its normality and dividing by the volume of the water sample. To express the alkalinity in terms of milligrams per litre of calcium carbonate, the result is multiplied by the equivalent weight of calcium carbonate (50.04 mg/mmol).

Advantages of Calcium Carbonate Equivalent

Using calcium carbonate equivalent as the standard for expressing alkalinity has several advantages:

1. Standardization: Calcium carbonate provides a standardized reference point for comparing alkalinity values across different water samples.

2. Abundance: Calcium carbonate is a naturally occurring compound that is widely available, making it easy to obtain and use as a standard.

3. Chemical Stability: Calcium carbonate is chemically stable and does not readily react with other substances in water, ensuring the accuracy and reliability of alkalinity measurements.

4. Stoichiometry: Calcium carbonate has a well-defined stoichiometry, simplifying the calculations involved in determining alkalinity.

In conclusion, alkalinity in water is expressed as milligrams per litre in terms of equivalent calcium carbonate (CaCO3) due to its abundance, chemical stability, well-defined stoichiometry, and suitability as a standard for comparing alkalinity values.