Spectrophotometric Determination of Aluminum in Water

Aluminum is a common constituent of natural waters, but elevated concentrations—arising from industrial discharge or the use of aluminum-based coagulants in water treatment—can harm aquatic life and pose health concerns. Spectrophotometry offers a practical, cost-effective approach for routine monitoring of aluminum in water samples.

Principle and Reagent Selection

The method relies on the formation of a colored complex between aluminum ions and a chromogenic reagent, with the color intensity proportional to aluminum concentration. Several reagents are available for this purpose. The Eriochrome Cyanine R method, a standard approach, produces a red to pink complex with maximum absorption at 535 nm when dilute aluminum solutions are buffered to pH 6.0. The aluminon method generates a red-orange complex measured at 522 nm, while Chrome Azurol S forms a green-colored lake in weakly acidic acetate-buffered solutions. 

Pyrocatechol violet, widely used in international standards, reacts with aluminum at pH between 5.8 and 6.0, with the resulting colored complex measured at 580 nm. The choice of reagent depends on the expected concentration range, available instrumentation, and required sensitivity.

Typical Procedure

Water samples should be collected in clean, acid-rinsed plastic or glass bottles. For dissolved aluminum determination, samples are filtered through a 0.45-μm membrane filter, discarding the first portion of filtrate. Before analysis, the sample pH is adjusted to the appropriate range for the chosen method—typically 3.5 to 4.5 when using the aluminon method, or pH 6.0 for Eriochrome Cyanine R.

The procedure generally involves adding a buffer solution, a masking agent such as ascorbic acid to suppress iron interference, and the chromogenic reagent to a measured aliquot of sample. After a defined reaction period—often 25 minutes for full color development—absorbance is measured at the specified wavelength against a reagent blank. A calibration curve is prepared using aluminum standard solutions treated identically. For samples containing colored or turbid matter, a blank correction is performed by complexing the aluminum in a separate portion with EDTA and measuring its absorbance to compensate for background interference.

Interference Control

Several substances interfere with aluminum determination. Fluoride and polyphosphates cause negative errors by competing with the chromogenic reagent for aluminum. Iron, even at low concentrations, produces positive interference. Ascorbic acid is commonly added to reduce iron to its colorless ferrous form, while thioglycollic acid has also been used for this purpose. 

Alkalinity inhibits color development and must be eliminated by acidifying the sample just beyond the neutralization point of methyl orange. For samples with high organic content, predigestion with acid is recommended to release bound aluminum. Glassware should be rinsed with dilute hydrochloric acid and metal-free water to avoid contamination.