How will you determine the alkalinity of a water sample containing only hydroxide and carbonate ions? Why can’t the alkalinity of water be due to the simultaneous presence of OH-, CO32-, and HCO3- ions?

To determine the alkalinity of a water sample containing only hydroxide (OH⁻) and carbonate (CO₃²⁻) ions, you can use titration methods. Alkalinity refers to the water's capacity to neutralize acids, typically measured by titrating the sample with a strong acid to a certain endpoint pH, using an indicator or a pH meter.

Here's a general procedure:

Prepare the water sample: 

Ensure the water sample is well-mixed and representative of the source.

Add an indicator: 

Depending on the pH range of your expected endpoint, you can add an appropriate indicator that changes color at or near the endpoint pH. Common indicators include phenolphthalein (for pH around 8.3 to 10), methyl orange (for pH around 3.1 to 4.4), or bromothymol blue (for pH around 6.0 to 7.6).

Titrate the sample: 

Gradually add a standardized solution of a strong acid, such as sulfuric acid (H₂SO₄), to the water sample while stirring. The acid reacts with the hydroxide and carbonate ions present in the water.

Observe the endpoint: 

Continue adding the acid until the indicator changes color, indicating that the endpoint pH has been reached. This change in color signifies that all the hydroxide and carbonate ions have been neutralized.

Calculate alkalinity: 

From the volume and concentration of the acid used, you can calculate the amount of acid required to neutralize the hydroxide and carbonate ions present in the water sample, which corresponds to its alkalinity.

Regarding the simultaneous presence of OH⁻, CO₃²⁻, and HCO₃⁻ ions, the alkalinity of water cannot be solely due to the presence of these ions simultaneously because they are interrelated through chemical equilibrium reactions:Hydroxide ion (OH⁻) + Carbon dioxide (CO₂) ⇌ Bicarbonate ion (HCO₃⁻)

Bicarbonate ion (HCO₃⁻) ⇌ Carbonate ion (CO₃²⁻) + Water (H₂O)

When you have a mixture of these ions, the equilibrium between them will shift to maintain certain ratios according to the water's pH and the concentrations of each species. Therefore, the alkalinity determination considers the total equivalents of hydroxide and carbonate ions present, taking into account their interconversion through these equilibrium reactions. This is why alkalinity is expressed in terms of equivalents of calcium carbonate (CaCO₃) per volume of water.