Diphenylcarbazide Spectrophotometry for Chromium Detection in Water

Chromium in natural waters exists mainly as trivalent chromium (Cr³⁺) and hexavalent chromium (Cr⁶⁺). Hexavalent chromium is highly toxic, carcinogenic, and mobile, making its accurate determination essential for environmental monitoring. Among various analytical methods, the diphenylcarbazide spectrophotometric method is widely adopted due to its simplicity, high sensitivity, and low equipment cost.

Principle

In an acidic medium, hexavalent chromium reacts with 1,5‑diphenylcarbazide to form a violet‑red complex. The colour intensity is proportional to the Cr⁶⁺ concentration. Absorbance is measured at 540 nm, and quantification is achieved using a calibration curve. The method has a detection limit of approximately 0.004 mg/L for a 50 mL sample.

Determination of Hexavalent Chromium

For clean surface water or industrial effluent, a 50 mL neutral sample is placed in a colour comparison tube. A series of standards is prepared similarly. After adding diphenylcarbazide solution and adjusting the pH to 0.05–0.3 mol/L H⁺ with sulfuric acid, the mixture is allowed to stand for 5–15 minutes (colour develops fully at room temperature). The absorbance is then measured at 540 nm against a reagent blank. The Cr⁶⁺ concentration is read from the standard curve.

Determination of Total Chromium

To measure total chromium, trivalent chromium must first be oxidized to hexavalent chromium. An appropriate volume of sample is digested with nitric and sulfuric acids to remove organic interference. After neutralisation, potassium permanganate is added to oxidise Cr³⁺ to Cr⁶⁺ under boiling conditions. Excess permanganate is reduced by a urea‑sodium nitrite mixture until the violet colour disappears. The solution is then treated as for hexavalent chromium, and the result gives total chromium content.

Interference Removal

Several ions can interfere. Ferric iron (＞1 mg/L) forms a yellow colour with diphenylcarbazide; this is masked by adding phosphoric acid, which forms a colourless Fe³⁺‑phosphate complex. Vanadium (＞4 mg/L) produces colour that fades after 10 minutes, so delayed measurement eliminates its effect. 

Coloured or turbid samples require a blank correction: one portion is developed normally, and another portion is measured without the chromogenic reagent. Reducing substances (e.g., Fe²⁺, sulphite) may reduce Cr⁶⁺ to Cr³⁺, causing low results; such samples are pre‑treated with permanganate to destroy reductants. High organic content (>500 mg/L COD) demands more vigorous oxidation, e.g., with acidic permanganate followed by clarification.

Quality Assurance

All glassware must be cleaned with nitric‑sulfuric acid mixture – never with chromic acid, which leaves chromium residue. The diphenylcarbazide reagent is dissolved in acetone and stored in a brown bottle at low temperature; a pink colour indicates degradation. The calibration curve should have a correlation coefficient ≥0.995. Routine analysis of certified reference materials and frequent re‑calibration ensure data reliability.