Determination of Alkalinity of water samples
Theory:
Water's alkalinity, which serves as a gauge of its buffering capability, is its ability to neutralize acids.
Because of the existence of carbonate (CO₃²⁻), bicarbonate (HCO₃⁻), and hydroxide (OH⁻) ions, this
feature enables water to maintain its pH level. The water's capacity to neutralize acids increases with
alkalinity.
Types:
1. Bicarbonate alkalinity: this type of alkalinity is caused by the presence of bicarbonate ions in
water i.e. -, HCO31-
2. Hydroxide/caustic alkalinity: caused by the presence of hydroxide ions i.e. OH1-
Objectives:
• Gaining a clear understanding of the chemical aspects of drinking water.
• Learning the basic concepts and definitions related to water alkalinity.
• Exploring how to measure the alkalinity of water.
• Understanding why measuring alkalinity is essential.
• Discovering how to present alkalinity data effectively and why it matters.
Types of Alkalinities:
• Caustic alkalinity: Caused by the presence of Hydroxides ions. The amount of strong
acid required to decrease the pH of sample to about 10. It gives the alkalinity due to
hydroxyl ions.
• Carbonate Alkalinity /Phenolphthalein alkalinity: Caused by the presence
of carbonate (CO32− ). The amount of strong acid required to lower the pH of sample to
8.3.It is also known as phenolphthalein alkalinity. It gives the alkalinity due to carbonates
and hydroxyl ions.
• Total Alkalinity/methyl orange alkalinity: The amount of strong acid required to lower
pH of sample water to 4.3. Color indicator used is methyl orange hence total alkalinity is
also known as methyl orange alkalinity.
Principle:
Using given pH, volume, and concentration information, you may titrate a sample of water with sulfuric
acid to determine its alkalinity. CaCO3 is commonly used to express alkalinity in mg/L.
To neutralize all bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻) ions in a water sample
with a pH higher than 4.5, you titrate it with acid until the pH falls to 4.5.
Adding one or two drops of phenolphthalein indicator is the first step for samples with a pH greater than
8.3. When hydroxide ions (OH⁻) are present, the solution will turn pink. The pink hue goes away when
you titrate with sulfuric acid, indicating that the hydroxide ions have been neutralized.
Add one or two drops of methyl orange indicator after that. The mixture will become yellow. Keep
titrating.
Apparatus:
➢ Pipette
➢ Conical flask
➢ Burette and retort stand
➢ Dropper
➢ Measuring cylinder
➢ 0.02 N Sulphuric acid
➢ Phenolphthalein and methyl orange indicator
➢ Filler
➢ Sample 50ml
Procedure:
Phenolphthalein Alkalinity:
Add one or two drops of phenolphthalein indicator to 50 milliliters of the sample solution in a conical
flask. The presence of hydroxyl ions is indicated by the solution turning pink if the pH is higher than 8.5.
Slowly add 0.02 N sulfuric acid to the sample in a burette until the pink hue goes away and the solution
turns colorless. Note how much sulfuric acid was used, then mark it with a "P."
Methyl Orange Alkalinity:
The same sample should then be treated with 1-2 drops of methyl orange indicator, which will cause the
solution to turn orange. Until the color shifts from orange to red, indicating the reaction's completion,
keep titrating with 0.02 N sulfuric acid. Note the amount of sulfuric acid used in this stage and mark it
with the letter "M."
• Calculations:
Initial value =
Final value =
(25.8-3.5)∗.02∗500𝟎𝟎
Methyl orange alkalinity = = 446 mg/l of CaCO3
𝟓𝟎
Phenolphthalein alakalinity = 70 mg/l
Total alkalinity = 446+70 = 516 mg/l of CaCO3
Conclusion:
The total alkalinity of the water sample was determined to be 516 mg/L, indicating a high
capacity to neutralize acids. This level suggests the presence of significant concentrations of
bicarbonates, carbonates, or hydroxides in the water, which could affect its suitability for
domestic or industrial use.
❖ Always wear a lab coat, closed-toe shoes, and a mask during the experiment.
❖ Avoid mixing solutions, especially adding water to acids.
❖ Keep personal items like bags away from the workspace.
