Alkalinity and its determination
Definition
Alkalinity is the acid-neutralizing capacity of water, mainly caused by the presence of hydroxide ions (OH⁻), carbonate ions (CO₃²⁻), and bicarbonate ions (HCO₃⁻). It is usually expressed as mg/L of CaCO₃ because that standard makes comparison and calculation easier in water analysis.
Main Content
1. Nature and causes of alkalinity
Primary sources of alkalinity
Alkalinity in natural water mainly comes from the dissolution of minerals in soil and rocks, especially limestone, dolomite, and other carbonate-bearing materials. Carbon dioxide dissolved in water reacts with these minerals and forms bicarbonates and carbonates. Wastewater from industries, detergents, and cleaning agents can also contribute to alkalinity.
Chemical species responsible
The major species contributing to alkalinity are hydroxide, carbonate, and bicarbonate ions. In some cases, borates, phosphates, silicates, and organic bases may also contribute, especially in industrial water and wastewater. However, for most routine water analysis, alkalinity is considered mainly in terms of OH⁻, CO₃²⁻, and HCO₃⁻.
2. Types and forms of alkalinity
Hydroxide alkalinity
This is present when the pH is above 10.3. It indicates the presence of free hydroxide ions and is usually found in strong alkaline waters or treated industrial waters.
Carbonate and bicarbonate alkalinity
Carbonate alkalinity predominates in water with pH between about 8.3 and 10.3, while bicarbonate alkalinity is common in waters with pH below 8.3. Bicarbonate is usually the most common form of alkalinity in natural waters. The relative amounts of hydroxide, carbonate, and bicarbonate can be estimated from titration results.
3. Determination of alkalinity by titration
Principle of the method
Alkalinity is determined by titrating a known volume of water sample with a standard acid, usually hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), until specific indicator end points or pH end points are reached. The amount of acid required shows how much alkalinity is present.
End points and indicators used
Two important end points are commonly used:
- Phenolphthalein end point (pH 8.3): This measures hydroxide and half of the carbonate alkalinity.
- Total alkalinity end point (pH 4.5): This measures all forms of alkalinity, including bicarbonate.
Phenolphthalein indicator turns pink in alkaline solution and becomes colorless at about pH 8.3. Methyl orange or methyl red is often used for the final end point around pH 4.5, where alkalinity is completely neutralized.
Calculation of alkalinity
Alkalinity is calculated using the formula:
where:
A = volume of acid used in mL
N = normality of acid
V = volume of sample in mL
50,000 = equivalent weight factor of CaCO₃ in mg per equivalent
For example, if 25 mL of water sample requires 10 mL of 0.02 N acid, then:
This numerical value helps compare water samples and assess treatment requirements.
Working / Process
1. Prepare the sample and reagents
Measure a known volume of the water sample in a conical flask. Take standard acid such as 0.02 N HCl in a burette. Add a few drops of phenolphthalein indicator to the sample and note the initial color. If the sample is pink, titration begins until the pink color just disappears, which indicates the phenolphthalein end point. If no pink color appears, phenolphthalein alkalinity is zero.
2. Titrate to the phenolphthalein and total end points
Continue the titration carefully. If the sample has phenolphthalein alkalinity, record the acid volume used up to pH 8.3. Then add methyl orange or methyl red indicator and continue titrating until the color changes to the final acid end point, usually orange to pink or yellow to red depending on the indicator used. In instrumental methods, a pH meter may be used and titration is stopped at pH 4.5 for total alkalinity.
3. Calculate and interpret the alkalinity
Use the titre volume and the formula to calculate alkalinity as CaCO₃. From the phenolphthalein and total alkalinity values, determine the presence and relative amounts of hydroxide, carbonate, and bicarbonate ions. This information is then used to judge water quality, calculate chemical dosage for treatment, and understand the buffering capacity of the water.
Advantages / Applications
Useful in water treatment
Alkalinity data help in deciding lime-soda dosing, coagulation conditions, and pH adjustment requirements in water purification plants.
Important in boiler and industrial water
Proper control of alkalinity prevents scale formation, foaming, and corrosion in boilers, cooling systems, and process water circuits.
Indicator of buffering capacity and water stability
Water with suitable alkalinity resists sudden pH changes, which is important for aquatic life, wastewater treatment, and maintaining stable chemical conditions in distribution systems.
Summary
- Alkalinity is the acid-neutralizing capacity of water, mainly due to bicarbonates, carbonates, and hydroxides.
- It is determined by titration with standard acid using phenolphthalein and methyl orange or pH meter end points.
- The result is commonly expressed as mg/L as CaCO₃ and is essential for water treatment and industrial applications.
- Important terms to remember: alkalinity, phenolphthalein end point, total alkalinity, bicarbonate, carbonate, hydroxide, titration, CaCO₃ equivalence