Dissolved chemical elements may occur as different species in an aqueous solution. Nitrate (N) is a common example which may occur as NO3, NH3-, NO2 or Si, occuring as H2SiO3 or H3SiO4. Since laboratories often report results in mg/L or µg/L concentration, values vary depending on which species is used to express the element of concern. The conversion for a is calculated as follows:
$$
\frac{n\ X\ as\ Species\ A\text{ (mg/L)}}{\text{FMW(A)}} \times \text{FMW(B)}
$$
where FMW(A) is the formula weight of A, FMW(B) is the formula weight of species B in g/Mol and n is the number of mols of element X in formula A.
The figure below shows the example of 10 mg/L N as N which is to be expressed as NO3. The guidline value for nitrate is commonly set to 50 or 11 mg/L depending on which formula is used for the nitrate-nitrogen.
$$
\frac{11}{\text{62}} \times \text{14.01} = 48.7
$$
