Aluminum is present in nearly all natural water as a soluble salt, a colloid, or an insoluble compound due to its vast occurrence in minerals, rocks, and clays. Aluminum's occurrence in natural waters is controlled by pH and by very finely suspended mineral particles. The cation Al3+ predominates at pH less than 4. Above neutral pH, the predominant dissolved form is Al(OH)4−. Aluminum is nonessential for plants and animals. Aluminum and its alloys are also used in a variety of products, such as heat exchangers, aircraft parts, building materials, containers, etc.

Aluminum potassium sulfate (alum) is commonly used during water treatment processes to flocculate suspended particles, but it can leave a residue of aluminum in the finished water. When concentrations are greater than 0.2 mg/L, water has color, but does not cause significant human health effects. For this reason the U.S. EPA has set a secondary minimum contaminant level (SMCL) of 0.05 mg/L to 0.2 mg/L for aluminum.

Hanna Instrument meters use an adaptation of the Aluminon Method to measure aluminum levels in water. The Aluminon indicator combines with aluminum in the sample, forming a red-orange color proportional to the aluminum concentration.

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