Alkalinity represents the amoount of carbonates in water. When I compare concentration of inorganic carbon measured by TOC-analyzer, with concentration of inorganic carbon calculated from alkalinity,they are in pretty good accordance with each other. But, when there is also acidity present, then the better correlation is achieved with adding the amount of acidity to alkalinity value before multiplying with 12.01. pH is around neutral (6.8)
What is the basis of this? Why this adding gives better estimation of inorganic carbon concentration?