What is the relationship between pH and alkalinity when pH is less than 4.5?

When the pH of a solution is less than 4.5, the alkalinity becomes effectively zero. Alkalinity is a measure of the capacity of water to resist changes in pH, which is primarily due to the presence of bicarbonate, carbonate, and hydroxide ions. At low pH levels, specifically below 4.5, the concentration of hydrogen ions is so high that it leads to the dominance of the dissociated forms of these buffers. As a result, bicarbonate and carbonate ions are converted to carbon dioxide in solution and the buffering capacity is diminished.

At this acidic range, water can no longer adequately promote safe neutralization or resist further changes in pH because there are insufficient alkaline constituents to balance the acid. Consequently, the concept of alkalinity becomes negligible or zero, making it the correct interpretation when pH is below 4.5.