Origin and importance

Magnesium (Mg) is the second element in group IIA of the periodic table. It has an atomic number of 12, an atomic weight of 24.30, and a valence of 2. The average abundance of Mg in the Earth's crust is 2.1%; in soil, it is 0.03 to 0.84%; in running water it is 4 mg/L and in groundwater > 5 mg/L. Magnesium is commonly found in the minerals magnesite and dolomite. Magnesium is used in alloys, fireworks, photographic flash, drying agents, refractories, fertilizers, pharmaceuticals, and foods.

The most common form of magnesium in water is Mg2+. Carbonation equilibrium reactions for magnesium are more complex than for calcium, and the conditions for direct precipitation of dolomite in natural waters are usually not favorable. Among the important factors in causing water hardness are magnesium salts, which break down when heated and form scale in boilers. Chemical softening, reverse osmosis or ion exchange will reduce magnesium and thus reduce hardness to an acceptable level.

Magnesium is an essential element in chlorophyll and in red blood cells. Some magnesium salts are toxic if ingested or inhaled. Concentrations above 125 mg/L can also have a laxative and diuretic effect.

Method Selection

The methods presented are applicable to water and wastewater. Direct measurement can be performed by atomic absorption spectrometry and inductively coupled plasma. Inductively coupled plasma mass spectrometry may also be used successfully in most cases (with a lower detection level), even if magnesium is not specifically mentioned as an analyte in the method. These methods can be used for most concentrations, although sample dilution may be required. The choice of method is largely a matter of personal preference and the experience of the analyst. A calculation method (3500-Mg.B) is also available.

Calculation Method 3500 Mg B.

If interfering metals are present in concentrations that are not effective in the calcium titration (Section 3500-Ca.B) and suitable inhibitors are used in the hardness stabilization (Section 2340C), magnesium may be calculated as the difference between hardness and calcium in the CaCO3 state.

mg Mg/L = [Total hardness (as mg CaCO3/L) – Calcium hardness (mg CaCO3/L)] × 0.243