Three Common Methods for Arsenic Determination in Water

Arsenic is a naturally occurring metalloid whose compounds are notorious for their high toxicity. Chronic ingestion of arsenic‑contaminated water can lead to skin lesions, cardiovascular diseases, and various forms of cancer, making accurate determination of arsenic concentrations a fundamental requirement for drinking water safety and environmental quality assessment. 

Over the years, a range of analytical techniques have been developed, each with distinct principles, performance characteristics, and operational demands. Among them, three methods stand out as the most commonly applied in routine and regulatory water analysis.

Atomic Fluorescence Spectrometry

Atomic fluorescence spectrometry (AFS), particularly when coupled with hydride generation, has become one of the most widely adopted techniques for arsenic determination in water laboratories. The underlying principle involves the generation of arsine gas from the sample: after appropriate pre‑treatment, all arsenic species are reduced to the trivalent state, which then reacts with sodium borohydride or potassium borohydride to form volatile arsine. 

This gaseous hydride is swept by an inert carrier gas into an atomiser, where it is decomposed into ground‑state arsenic atoms. Under illumination from a high‑intensity hollow cathode lamp, these atoms absorb radiation and become excited; when they return to the ground state, they emit fluorescence at a characteristic wavelength. The fluorescence intensity is directly proportional to the arsenic concentration in the original sample. 

AFS offers excellent sensitivity, with detection limits well below the regulatory maximum contaminant level for drinking water. Its operation is relatively straightforward, and the equipment is less expensive than mass spectrometric instruments, making it a practical choice for both routine monitoring and compliance testing of surface water, groundwater, and wastewater.

Silver Diethyldithiocarbamate Spectrophotometry

The silver diethyldithiocarbamate (SDDC) spectrophotometric method is a classical technique that has served as a standard for total arsenic determination for decades. In this procedure, arsenic in the sample is first reduced to arsine gas by metallic zinc in the presence of potassium iodide and stannous chloride. 

The generated arsine is then passed through a solution containing silver diethyldithiocarbamate dissolved in pyridine or triethanolamine, where it reacts to form a red‑coloured colloidal silver complex. The absorbance of this red colour is measured at a wavelength of approximately 530 nm using a conventional spectrophotometer, and the arsenic concentration is derived from a calibration curve. The main advantage of this method lies in its modest equipment requirements—any routine laboratory with a spectrophotometer can perform the analysis. 

It is particularly suitable for laboratories with limited budgets or for on‑site applications where sophisticated instrumentation is unavailable. However, the procedure is relatively time‑consuming, requires careful control of reaction conditions, and its sensitivity is lower than that of AFS or ICP‑MS, so it is best applied to samples with moderate to high arsenic levels or as a screening tool in less demanding contexts.