Common Analytical Methods for Detecting Chromium in Drinking Water

Ensuring the safety of drinking water is a global priority. Chromium, particularly in its hexavalent form (Cr(VI)), is a toxic heavy metal of significant concern due to its carcinogenic properties. Accurate and sensitive detection of chromium levels is therefore crucial for water quality monitoring. This article outlines the most commonly employed analytical techniques for this purpose.

Introduction
Chromium exists in water primarily in two oxidation states: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Cr(III) is an essential nutrient, but Cr(VI) is highly toxic and mobile, posing serious health risks. Regulatory bodies like the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) set strict limits for total chromium or Cr(VI) in drinking water. Consequently, reliable analytical methods are required.

Common Detection Methods

1. Atomic Absorption Spectroscopy (AAS)
   AAS is a widely used, cost-effective technique for metal analysis.

• Flame AAS (FAAS): Suitable for measuring total chromium at higher concentrations (typically above 0.1 mg/L). It is robust and easy to operate but lacks the sensitivity for direct measurement at very low regulatory limits.

• Graphite Furnace AAS (GFAAS): Also known as Electrothermal AAS (ETAAS), this method is significantly more sensitive than FAAS. It can detect chromium at parts-per-billion (µg/L) levels, making it suitable for direct measurement of total chromium in drinking water according to regulatory standards.

2. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
   ICP-MS is the gold standard for multi-element analysis due to its exceptional sensitivity, low detection limits (parts-per-trillion), and ability to analyze samples rapidly. It is the preferred method for accurately determining total chromium at very low concentrations. While primarily used for total metal content, coupling ICP-MS with a separation technique like Ion Chromatography (IC-ICP-MS) allows for the specific quantification of Cr(III) and Cr(VI) species.

3. Spectrophotometry (Colorimetry)
   This traditional method is based on a chemical reaction that produces a colored complex with chromium, typically with diphenylcarbazide for Cr(VI). The intensity of the color, measured with a spectrophotometer, is proportional to the concentration. It is a low-cost and accessible technique, ideal for field testing or routine checks. However, it can be susceptible to interferences from other ions and is generally used for measuring Cr(VI) specifically.

4. Ion Chromatography (IC)
   IC is a separation technique used specifically for ionic species. When coupled with a detection method like a UV-Vis detector (for diphenylcarbazide-complexed Cr(VI)) or ICP-MS, it becomes a powerful tool for speciation analysis—distinguishing and quantifying Cr(III) and Cr(VI) separately in a single sample run. This is critical for accurate risk assessment.