Spectrophotometric Determination of Hexavalent Chromium in Water by Diphenylcarbazide - Master's thesis - Dissertation
**Spectrophotometric Determination of Hexavalent Chromium in Water Using Diphenylcarbazide**
**Keywords:** Spectrophotometry; Hexavalent Chromium; Analytical Instrument; UV-1100; UV-1200
This method is designed for the quantitative determination of hexavalent chromium (Cr(VI)) in surface water and industrial wastewater. It utilizes a spectrophotometric technique based on the reaction between hexavalent chromium and diphenylcarbazide, forming a stable purple-red complex that can be measured at a wavelength of 540 nm.
### **Scope of Application**
1.1 This standard applies to the analysis of Cr(VI) in surface water and industrial wastewater samples.
1.2 The measurement range depends on the cuvette path length. With a 30 mm cuvette, the detection limit is 0.004 mg/L, with a minimum detectable amount of 0.2 µg. For a 10 mm cuvette, the upper limit is 1.0 mg/L.
1.3 Interference from certain ions such as iron, molybdenum, mercury, and vanadium may occur. However, under the specific conditions of this method, their effects are minimized. Iron causes yellowing, while vanadium may cause interference if present above 4 mg/L, but it fades over time.
### **Principle**
In an acidic medium, hexavalent chromium reacts with diphenylcarbazide to form a colored complex. The intensity of the color is directly proportional to the concentration of Cr(VI), which is measured using a spectrophotometer at 540 nm.
### **Reagents**
All reagents must be of analytical grade and free from chromium contamination. Key reagents include:
- Acetone
- Sulfuric acid (1+1)
- Phosphoric acid (1+1)
- Sodium hydroxide (4 g/L)
- Zinc sulfate solution (8%)
- Potassium permanganate (40 g/L)
- Chromium standard stock solution (prepared from potassium dichromate)
- Diphenylcarbazide solutions (developer I and II)
### **Instrumentation**
A spectrophotometer such as the UV-1100 or UV-1200 is used. All glassware should be cleaned thoroughly and not washed with potassium dichromate solutions to avoid contamination.
### **Sampling and Sample Preparation**
Water samples should be collected in glass bottles and stored at pH ~8. They should be analyzed within 24 hours. If necessary, pretreatment steps like zinc hydroxide precipitation or oxidation with potassium permanganate can be applied to eliminate interferences.
### **Procedure**
6.1 Sample Pretreatment: Includes methods for removing suspended solids, correcting chromaticity, and eliminating reducing or oxidizing agents.
6.2 Blank Test: A blank sample is prepared using distilled water instead of the actual sample to correct for background absorbance.
6.3 Determination: The sample is treated with sulfuric acid, phosphoric acid, and diphenylcarbazide. After 5–10 minutes, the absorbance is measured.
6.4 Calibration: Standard solutions of known Cr(VI) concentrations are used to construct a calibration curve, allowing for accurate quantification.
### **Notes**
- When using zinc hydroxide precipitation, the sample must be filtered and diluted before analysis.
- The developer (I) can also be prepared using phthalic anhydride and ethanol, and stored for up to six months.
- Proper handling and storage of reagents and instruments are crucial to ensure accuracy and reproducibility.
This procedure provides a reliable and widely used method for determining hexavalent chromium in environmental and industrial water samples. It is particularly useful in monitoring water quality and ensuring compliance with regulatory standards.
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