Measurement of hexavalent chromium in water

Summary

The common valence states of chromium compounds are trivalent and hexavalent. In water bodies, hexavalent chromium generally exists in the form of CrO42-, Cr2O72- and H CrO4- anions, and trivalent chromium and hexavalent chromium compounds can be transformed into each other under the influence of pH, organic matter, redox substances, temperature and hardness in water. The toxicity of chromium is related to its valence state, and it is generally believed that the toxicity of Cr6+ is 100 times higher than that of Cr3+, which is more easily absorbed by the human body and accumulates in the body, leading to liver cancer. When the concentration of Cr6+ in water is 1 mg/L, the water is yellowish and has astringent odor; when the concentration of Cr3+ is 1 mg/L, the turbidity of water increases significantly.

Operation method

Dibenzoyl dihydrazide spectrophotometry

Principle

Under acidic conditions, hexavalent chromium ions can interact with dibenzoyl dihydrazide to produce a violet-red complex for colorimetric determination. Mercurial and high mercury can produce blue or violet-blue compounds with diphenylcarbonyldihydrazide to interfere with the determination, but the reaction is not sensitive under the controlled acidity. The minimum detectable amount of this method is 0.2 ug, and if the volume of water sample is 50 ml, the minimum detectable concentration is 0.004 mg/L.

Materials and Instruments

Dibenzoyl dihydrazine Alcohol Potassium dichromate
Colorimetric tubes Pipettes

Move

I. Instrumentation
Colorimetric tubes, 2 mL, 10 mL pipettes.II. Pharmaceuticals
Dibenzoyl dihydrazide, 95% alcohol, potassium dichromate.III. Reagents
1. Diphenylcarbonyldihydrazide solution: weigh 0.1 g of diphenylcarbonyldihydrazide, dissolve in 50 mL of 95% ethanol, add 200 mL of 1:9 sulfuric acid solution and mix well. This reagent should be a colorless liquid.
2. Chromium standard stock solution ( ^CCr6+ = 0.100 mg/mL): weigh 0.1414 g of potassium dichromate that has been baked at 105°C for 2 h and cooled, dissolve it in distilled water, and then dilute it with water to 500 mL.1.00 mL of this solution contains 0.100 mg of Cr(VI).
3. Chromium Standard Use Solution ( ^CCr6+ = 1.00 ug/mL): Dilute the above solution 100 times.
IV. Experimental steps
1. Take a 50 mL sample of clarified neutral water (or take an appropriate amount of water and distilled water to 50 mL) and place it in a 50 mL cuvette.
2. Take 9 50 mL cuvettes and add 0, 0.20, 0.50, 1.00, 2.00, 4.00, 6.00, 8.00, 10.00 mL of Chromium Standard Liquid respectively, and add distilled water to the scale.
3. Add 2.5 mL of diphenylcarbonyldihydrazide solution to each of the water and standard sample tubes and mix well. Let stand for 10 min, then perform colorimetric analysis.
4. Select a 3 cm cuvette and measure the color at 540 nm on a spectrophotometer.
V. Calculation of results
Hexavalent chromium ( ^Cr6+, mg/L) = Amount of chromium standard use solution (mL)/V water sample

Caveat

1. Diphenylcarbonyldihydrazide, also known as diphenylaminourea, is unstable when prepared as a solution, and should be stored in a refrigerator; if the color becomes darker, it cannot be reused.

2. The color reaction is most stable at 15℃, and the color will become darkest after 2-3 min of color development, and remain unchanged within 5-15 min.

Common Problems

I. Introduction

The common valence states of chromium compounds are trivalent and hexavalent. In the water body, hexavalent chromium generally exists in the form of ^{CrO42-,Cr2O72-,} H ^{CrO4- three} anions. Affected by the pH, organic matter, redox substances, temperature and hardness of the water, trivalent chromium and hexavalent chromium compounds can be transformed into each other. The toxicity of chromium is related to its valence state, and it is generally believed that the toxicity of ^Cr6+ is 100 times higher than that of ^Cr3+, which is more easily absorbed by the human body and accumulates in the body, leading to liver cancer. When the concentration of ^Cr6+ in the water is 1 mg/L, the water is light yellow and has an astringent odor; when the concentration of ^Cr3+ is 1 mg/L, the turbidity of the water increases significantly.

Chromium can be determined by dibenzoyl dihydrazide spectrophotometry, atomic absorption spectrophotometry, plasma emission spectrometry and titration. Clean water samples can be used directly to determine hexavalent chromium by dibenzoyl dihydrazide spectrophotometry. If total chromium is measured, potassium permanganate is used to oxidize trivalent chromium to hexavalent chromium, and then diphenylcarbonyldihydrazide is used for spectrophotometric determination. Water samples containing more chromium

When the chromium content of water samples is high, titration method of ammonium ferrous sulfate is used.

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