INTRODUCTION

Alkalinity is the presence of sufficient alkaline ions in water. The absence of alkalinity makes the water acidic. Water is said to be alkaline when pH of water is above 7.00. The determination of alkalinity is very useful in water and waste water because it provides buffering to resist changes in pH value. The alkalinity and acidity in water usually changes in natural waters with large algal growth. The alkalinity of natural waters is primarily due to the salts of week acids and bicarbonates represent the major form of alkalinity. Alkalinity can be expressed as follows:

Alkalinity (mol/l) = [HCO₃^-] + 2[CO₃ ^2-] + [OH^-] – [H⁺]

The presence of alkalinity in surface waters is primarily indicates the concentration of carbonate, bicarbonate and hydroxide contents. Alkalinity in excess of Alkaline earth metal concentrations is significant in determining the suitability of a water for irrigation. Alkalinity measurements are used in the interpretation and control of water and waste water treatment processes. Raw domestic waste water has an alkalinity less than or only slightly greater than that of the water supply.

Alkalinity of water is the capacity of water to neutralize acid. It is usually expressed as Total Alkalinity or Caustic Alkalinity in water. It is significant in many uses and treatments of natural waters and wastewaters. Alkalinity is measured titrimetrically by titrating against Dilute Sulphuric Acid. Phenolphthalein and Methyl Orange are used as indicator to indicate pH 8.3 and pH 4.3. Phenolphthalein process a pink colour when pH is above 8.3 and colourless when pH is below 8.3.

Natural water alkalinity is caused primarily by the presence of weak acid salts, though strong bases (i.e. OH- ) may also contribute in extreme environments. Bicarbonates are the most common type of alkalinity found in natural waters, and they are formed by the partitioning of CO2 from the atmosphere and the weathering of carbonate minerals in rocks and soil. Other salts of weak acids, such as borate, silicates, ammonia, phosphates, and organic bases from natural organic matter, may be present in small amounts. Alkalinity is frequently identified as mg/L CaCO3 because the majority of alkalinity is derived from the weathering of carbonate minerals.

Titration with standardized acid determines the total amount of hydroxyl ions in a solution. This is a well-known water-analysis procedure for estimating the concentrations of hydroxyl, carbonate ion, and bicarbonate ions. This titration has two pH endpoints, P and M, which correspond to the phenolphthalein and methyl orange indicators.

As previously stated, alkalinity in natural waters is primarily caused by carbonate species, and the following chemical equilibria are established in waters.

CO₂ + H₂O <=> H₂CO₃^*

H₂CO₃ <=> HCO₃^- + H⁺

HCO₃^- <=> CO₃^2- + H⁺

where H₂CO₃^* represents the total concentration of dissolved CO₂ and H₂CO₃. The first chemical equation represents the equilibrium of CO₂ in the atmosphere with dissolved CO₂ in the water. The equilibrium constant, Henry’s Law, for this reaction is

The equilibrium relationships for the last two reactions may be expressed as

Relevant Indian Standard for Alkalinity Test :

1. IS 3025-23 (Part 23) - 1986: Methods of Sampling and Test (physical and chemical) for Water and Wastewater, Alkalinity, First Revision.