Spectrophotometric Determination of Hexavalent Chromium in Water

Hexavalent chromium (Cr(VI)) is a toxic pollutant commonly found in industrial wastewater, such as electroplating and tanning effluents. Accurate on-site or laboratory determination is essential for environmental monitoring. Among various techniques, spectrophotometry using diphenylcarbazide is the most widely adopted method due to its simplicity, sensitivity, and low cost.

Principle

In an acidic solution, Cr(VI) reacts with 1,5-diphenylcarbazide to form a soluble red-violet complex. The intensity of the color is proportional to the Cr(VI) concentration and can be measured at a wavelength of 540 nm.

Reagents and Equipment

Diphenylcarbazide solution (prepared in acetone)

Dilute sulfuric acid or phosphoric acid

Potassium dichromate standard solution (for calibration)

Visible spectrophotometer (with 1 cm cuvettes)

Glassware (volumetric flasks, pipettes)

Procedure

Sample preparation – Collect a water sample in a clean glass bottle. If the sample is turbid, filter it through a 0.45 µm membrane. Adjust the pH to approximately 2 with dilute sulfuric acid if needed.

Color development – Transfer 50 mL of the sample into a 100 mL volumetric flask. Add 2 mL of diphenylcarbazide solution and 2 mL of dilute sulfuric acid. Mix well and dilute to the mark with deionized water. Wait for 5–10 minutes for full color development.

Blank preparation – Prepare a blank using deionized water following the same steps.

Calibration – Prepare a series of standard solutions (e.g., 0, 0.1, 0.2, 0.5, 1.0 mg/L Cr(VI)) from the potassium dichromate stock. Develop color for each standard.

Measurement – Set the spectrophotometer to 540 nm. Zero the instrument with the blank. Measure the absorbance of each standard and construct a calibration curve (absorbance vs. concentration). Then measure the absorbance of the sample.

Calculation – Determine the Cr(VI) concentration in the sample using the calibration curve. Express results in mg/L.

Important notes

The reaction is specific to Cr(VI); Cr(III) does not interfere. However, high concentrations of colored ions (e.g., Fe³⁺, Cu²⁺) or reducing agents may cause errors.

Perform the analysis promptly after sampling because Cr(VI) may be reduced in storage.

Diphenylcarbazide solution is unstable; prepare fresh or store in a dark bottle at 4°C for no more than one week.