If a water sample's Alkalinity is greater than its Total Hardness, what type of hardness is present?

When the alkalinity of a water sample is greater than its total hardness, it indicates that the hardness is entirely in the form of carbonate hardness. This relationship arises because carbonate hardness is associated with bicarbonates (HCO3-) and carbonates (CO3^2-), which contribute to both alkalinity and hardness levels.

In this scenario, carbonate hardness is what primarily contributes to both measures, and since the alkalinity exceeds the total hardness, it signifies that there is no significant contribution from non-carbonate hardness, which usually comes from sources such as calcium sulfate or magnesium sulfate that do not affect alkalinity.

This means that all of the total hardness in the water sample can be classified as being in the carbonate form, reinforcing that the sample's hardness is effectively due to the presence of bicarbonates and carbonates, leading to the conclusion that all hardness present is in the carbonate form. Therefore, having a greater alkalinity than total hardness clearly points to this specific type of hardness.