Experiment 23 Spectrophotometric Determination of Lead on Leaves Using Solvent Extraction

EXPERIMENT 23 SPECTROPHOTOMETRIC DETERMINATION OF LEAD ON LEAVES USING SOLVENT EXTRACTION  

Lead on the surfaces of leaves is dissolved by shaking with nitric acid solution. The lead is extracted as the dithizone complex into methylene chloride at pH above 9. The intensity of the color of the complex is measured spectrophotometrically and compared to a calibration curve prepared similarly from lead standards to calculate the amount of lead. Cyanide and sulfite may be added as masking agents to eliminate most interference from other metals. 

Provided. 1 M HNO3, 0.1 M HNO3, thymol blue indicator solution (0.1% in water), 2 M NH3 solution, ammonia-cyanide-sulfite solution (350 mL cone. NH3 solution, 30 mL 10% NaCN, and 1.5 g Na2S03 diluted to 1 L the pH is about 11). 

Caution Do not discard any of the solutions used in this experiment down the drain. The aqueous solutions may contain cyanide and should be added to a reservoir provided for collection that contains FeS04 [to convert CN to Fe(CN)6 ]-No acid must ever be added to this reservoir All glassware should be rinsed with an alkaline solution and the washings added to the reservoir. The methylene chloride solutions should also be added to a reservoir provided. 

Collect leaf samples. These can be from trees near a road or highway and from some that are more isolated for comparison. Collect at least two large leaves from each tree and place each in a clean plastic bag and seal. The leaves selected should be reasonably free from dirt or other visible contamination. Record the location of the leaves. Your instructor will advise you of the number of trees you should sample. 

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