At what pH level can alkalinity be represented solely by bicarbonate (HCO3-)?

At a pH level of approximately 8.3, bicarbonate (HCO3-) predominates in solution as the dominant alkaline species. This is due to the equilibrium between bicarbonate, carbonate (CO3^2-), and carbonic acid (H2CO3) governed by the carbonic acid equilibrium reactions.

At lower pH levels, such as 4.5, carbonic acid is the predominant species, while at higher pH levels (like 12) carbonate becomes the dominate species. Therefore, around pH 8.3, the balance tips so that bicarbonate is the main form of inorganic carbon present, which is a key factor in determining alkalinity in natural waters. At pH values significantly above 8.3, the concentration of bicarbonate decreases as carbonate starts to dominate.

In practical terms, this means that at pH levels of 8.3 or slightly above, the contribution of hydroxide ions is relatively low, allowing bicarbonate to represent the bulk of the alkalinity in the water. Thus, the correct answer highlights the pH level at which bicarbonate serves primarily as the measure of alkalinity in aquatic systems.