Introduction
Hexavalent chromium is a common heavy metal pollutant in wastewater, with high toxicity and carcinogenicity, and its accurate determination is crucial for environmental protection and human health. The colorimetric titration method is based on a specific chromogenic reaction to determine the hexavalent chromium concentration by measuring the change in the absorbance of the solution, which has the characteristics of both ease of operation and reliable results. This specification aims to provide a standardized assay process that covers the entire process from sample preparation to result calculation, ensuring the accuracy and comparability of assay data.
Method principle
In acidic media, hexavalent chromium reacts with diphenylcarbonyl dihydrazine to form a purplish-red complex, the color depth of which is directly proportional to the concentration of hexavalent chromium, and has the greatest absorption at a wavelength of 540 nm. The change of absorbance during the titration process was monitored by a colorimetric titrator, and the titration endpoint was determined, and then the content of hexavalent chromium in the wastewater was calculated. The reaction can be expressed as:
Cr2O72− + 3H4R + 8H+ → 2Cr(HR)2+ + 7H2O + CO2↑
where H4R stands for diphenylcarbonyl dihydrazine.
Instruments and reagents
Colorimetric titrators (equipped with magnetic stirrers, photodetectors and titration devices), pH meters, analytical balances, volumetric flasks and other glassware. The required reagents include: diphenylcarbonyl dihydrazine solution, sulfuric acid solution, phosphoric acid solution, and hexavalent chromium standard solution. All reagents should be analytically pure, and the experimental water should meet the first-class water standard.
Sample preparation
Wastewater samples collected should be avoided from light and analyzed as soon as possible. If the sample is cloudy, it needs to be filtered through a 0.45 μm membrane. Take an appropriate amount of filtrate and adjust the pH to 2.0±0.5 with sulfuric acid or sodium hydroxide solution to eliminate the effect of interfering ions. If the concentration of hexavalent chromium in the sample is outside the measurement range, it should be properly diluted.
Assay steps
Turn on the colorimetric titrator and preheat for 30 minutes. Set the detection wavelength to 540 nm and adjust the zero point. Accurately pipette 50.00 mL of the treated sample into the titration cell, add 5 mL of phosphoric acid solution and 1 mL of diphenylcarbonyl dihydrazine solution, and mix well. The titration cell is placed in the instrument, the magnetic stirrer is started, and the standard titration solution is titrated to record the titration volume and real-time absorbance value. The end of titration is when the absorbance reaches a stable plateau. Parallel assays were performed three times while the blank test was performed.
The result is calculated
The content of hexavalent chromium in wastewater is calculated as follows:
C = (V - V0) × c × M × f / Vs
where C is the hexavalent chromium concentration (mg/L); V is the titrated volume of the sample (mL); V0Titrated volume (mL) for blanks; c is the standard titration concentration (mol/L); M is the molar mass of chromium (51.996 g/mol); f is the dilution factor; Vsis the sample volume (mL). The calculation result retains three significant figures.
Notes:
| Sample preservation | Samples should be refrigerated and protected from light and analyzed within 24 hours |
| Acidity control | The reaction should be performed at pH 2.0±0.5 |
| Interference elimination | Interfering ions such as iron and mercury need to be masked or separated in advance |
| Cleaning of utensils | All glassware should be soaked in nitric acid |
| Safety | Protective gloves and glasses should be worn during operation |
Quality control
Each batch of samples should be accompanied by quality control samples, and the spiked recovery rate should be controlled between 95% and 105%. Calibration with certified reference materials is carried out regularly to ensure stable instrument performance. Experimental records should be complete, including sample information, reagent batch number, environmental conditions, and raw data.
Cited Literature
Water quality Determination of hexavalent chromium Diphenyl carbohydrazine spectrophotometry, national environmental protection standard.
Standard Methods for the Examination of Water and Wastewater, 23rd Edition, APHA.