What elements are considered when measuring alkalinity in a water sample?

When measuring alkalinity in a water sample, the focus is on the capacity of water to neutralize acids, often expressed in terms of the presence of specific ions. The correct answer is the combination of carbonate, bicarbonate, and hydroxide ions, as these species directly contribute to the buffering capacity of water.

Carbonate ions (CO₃²⁻) and bicarbonate ions (HCO₃⁻) are the primary components of alkalinity. Bicarbonate is formed in natural water from the dissolution of carbon dioxide, while carbonate can form at higher pH levels. Hydroxide ions (OH⁻) also contribute to alkalinity, particularly in more basic environments, and enhance the solution's ability to resist changes in pH when acids are introduced.

These three ions together help maintain the pH balance in aquatic systems, making it essential to include them when evaluating alkalinity. Other ions, such as nitrate, ammonium, phosphate, sulfate, and chloride, may be important in different environmental contexts and water quality assessments, but they do not play a pivotal role in the measurement of alkalinity.