Water from most sources has some level of mineral salts dissolved in it. Among these dissolved salts are certain specific minerals that strongly affect the pH of your water. The concentration of these minerals in your water is often described by the term “hardness”. The higher the concentration of these minerals, the “harder” your water is. There are two types of hardness in water – “carbonate hardness”, which is sometimes also referred to as the water’s “buffering capacity” or “alkalinity” and “general hardness” which refers to the concentration of calcium and magnesium ions in the water.

Alkalinity measures the ability to neutralize acid (base). Alkalinity provides a buffering capacity to aqueous system. The higher the alkalinity is, the higher the buffering capacity against pH changes.

The buffering capacity acts like an invisible sponge that soaks up whatever acid or base is in your system – or that you add to your system – until the capacity of the buffer is “used up”. With this sponge-like behavior in mind, imagine trying to adjust your pH. Let’s say you have a pH of 8.0 in your water system and you would like to bring it down to 7.0. You start adding an acid…and adding…and adding…and little or nothing changes. And then all of a sudden the pH plummets. You had a strong buffer capacity in your water which you eventually “overwhelmed”. Another way of thinking about this is that you “filled up” the sponge.

@Duried Alwazeer thank you Professor. Your answer is really helpful.

About the pH of (pure) H2SO4 aq. solutions. ― cf. my posts at: https://www.researchgate.net/post/Do-you-know-a-method-for-titration-of-HNO3-H2SO4-mixtures”