Free mineral acidity (FMA) refers to the acidity resulting from the exchange of cations (like calcium, magnesium) with hydrogen ions (H+), forming acids like hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).

 In the context of water treatment using strong acid cation exchange resins, free mineral acidity (FMA) refers to the acidity resulting from the exchange of cations (like calcium, magnesium) with hydrogen ions (H+), forming acids like hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). 

Here's a more detailed explanation:

What is FMA?

Free mineral acidity (FMA) is the measure of the total concentration of strong acids in the effluent (outlet water) of a strong acid cation exchanger. 

How it's formed:

When water containing dissolved salts (containing cations like calcium, magnesium) passes through a strong acid cation exchange resin, the resin exchanges the cations for hydrogen ions (H+). 

Examples of acids:

These hydrogen ions (H+) combine with the anions (like chloride, sulfate, nitrate) originally present in the water, forming acids like hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). 

Significance:

FMA is a crucial parameter in water treatment, especially in demineralization processes, as it indicates the effectiveness of the cation exchange resin and the need for further treatment (e.g., anion exchange) to remove these acids. 

FMA and pH:

FMA is measured by titrating the sample with a standard solution of sodium carbonate to a methyl orange end point (approximately pH 4.3). 

FMA and water quality:

The presence of FMA can contribute to the corrosiveness of water and influence chemical and biological processes.