The Purple Signal: Spectrophotometric Determination of Chromium in Water

Chromium in water exists primarily in two valence states: trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. While Cr(III) is relatively less toxic and even essential in trace amounts, Cr(VI) is a known carcinogen and poses serious risks to human health and aquatic life. 

Distinguishing between these species—and quantifying total chromium—is therefore a critical task in environmental monitoring. Among the various analytical techniques available, the spectrophotometric method using 1,5-diphenylcarbazide (DPC) remains the most widely adopted approach, prized for its sensitivity, simplicity, and cost-effectiveness.

The Chemistry Behind the Color

The method exploits a highly specific chromogenic reaction. In an acidic solution, hexavalent chromium reacts with diphenylcarbazide to form a vivid purple-red complex. This compound exhibits maximum light absorption at a wavelength of 540 nm, and its absorbance obeys the Beer-Lambert law, meaning the intensity of the purple color is directly proportional to the concentration of Cr(VI) in the sample. The blank—a reagent solution without the sample—serves as the reference, and the difference in absorbance yields the chromium content.

Determining Hexavalent Chromium Directly

For water samples that only require Cr(VI) analysis—such as those from industrial effluents or groundwater where Cr(III) is not a concern—the procedure is straightforward. A measured volume of the water sample is placed in a colorimetric tube, acidified to the proper pH, and treated with the DPC reagent. 

After a brief color development period—typically around 10 minutes—the sample is transferred to a cuvette and its absorbance is read at 540 nm against a reagent blank. The concentration is then derived from a pre-established calibration curve prepared from known Cr(VI) standards.

Determining Total Chromium

When the goal is to measure total chromium—the sum of both Cr(III) and Cr(VI)—an additional pretreatment step is required. Since Cr(III) does not react with DPC, it must first be converted to Cr(VI). This is achieved by adding an oxidizing agent, typically potassium permanganate, to the acidified sample. The excess permanganate is then carefully reduced with sodium nitrite, and any residual nitrite is decomposed with urea to avoid interference. 

After this oxidation sequence, the DPC reagent is added, and the absorbance is measured as before. The result represents total chromium, and the Cr(III) concentration can be obtained by difference if the Cr(VI) level is known from a separate, unoxidized run.

Interferences and Their Management

No analytical method is free from interferences, and DPC spectrophotometry is no exception. The most common interfering species is iron: when iron concentrations exceed approximately 1 mg/L, it reacts with the reagent to produce a yellowish coloration that overlaps with the purple chromium complex. This interference can be mitigated by adding masking agents such as sodium pyrophosphate or by precipitating iron with sodium hydroxide before analysis. 

Other metal ions—including mercury, molybdenum, and vanadium—may also form colored complexes with DPC, though their effects are generally less pronounced at typical environmental levels. Additionally, strong oxidizing or reducing agents in the sample can alter the valence state of chromium during analysis, leading to inaccurate results; these should be neutralized or removed prior to measurement.