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Determine the concentration of lead in a given solution by gravimetric analysis, EDTA titration, spectrophotometry, and atomic absorption, and compare the advantages and disadvantages of each method.

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Introduction

Rhiannon Edwards 27/02/2004 Determination of Lead Objectives: To determine the concentration of lead in a given solution by gravimetric analysis, EDTA titration, spectrophotometry, and atomic absorption, and compare the advantages and disadvantages of each method. Calibration of Pipette Apparatus: * 20ml pipette * Stoppered flask Method: 1) A 20ml pipette was cleaned with a dilute solution of detergent, and rinsed thoroughly with distilled water. 2) The pipette was then filled with distilled water. 3) The water is then run out into a weighed flask. 4) The flask is then re-weighed. Results: Weight of Flask (g) Weight of Flask and Water (g) Weight of water (g) 29.9529 49.8378 19.8849 29.9521 49.9329 19.9808 Difference of the weight of water: 19.9808 - 19.8849 = 0.1ml Average weight of water: 19.9808 + 19.8849 = 19.9329 2 Temperature of water = 25oC True volume of water contained by the 20ml pipette = 19.9329 x 0.99609 = 19.8550 Gravametric Analysis: Apparatus: * 2 x 400ml beaker * 20ml calibrated pipette * Hot plate * Oven (120oC) * 2 x Sintered glass crucibles Method: 1) Two sintered glass crucibles with dilute HCl, followed by dilute NH3, followed by distilled water. 2) They are dried over night in an oven. 3) Duplicate solutions are prepared by pipetting 20ml samples of the unknown lead solution into two 400ml beakers, using the calibrated pipette. ...read more.

Middle

4) The absorbance of each solution was measured. Preparation of lead nitrate solution: 2000ppm = 2000mg in 1000g = 2g in 1000g = 0.5g in 250 ml 0.5g of lead is required, to obtain this using the lead nitrate: weight of lead required x FW lead nitrate = weight of lead nitrate FW lead required 0.5 x 331.2 = 0.799 g 207.2 0.799g of lead nitrate was placed in a 250ml volumetric flask and 1ml 2M nitric acid and made up to 250ml with distilled water. Preparation of standard solutions: ppm Volume of 2000ppm sol (ml) Volume of water (ml) 5 0.25 100 10 0.50 100 20 1.00 100 30 1.50 100 40 2.00 100 50 2.50 100 Dilution of Unknown: 25ppm = 25mg in 1000g = 0.025g in 1000g From the EDTA titration the concentration was calculated as 7459.2ppm, using this we can calculate the 25ppm: 7459.2 in 1000ml = 7459.2 ppm 0.3ml in 100ml = 25 ppm Results: ppm Abs Value 5 0.064 10 0.129 20 0.232 30 0.352 40 0.433 50 0.506 Unknown 0.281 Calculation of Concentration: From the calibration curve the concentration of the unknown lead solution is observed as 25ppm. As you can see from the results the unknown lead solution was diluted by: 0.3ml in 100ml = 1ml in 300ml The lead solution was diluted by a factor of 300 which would calculate the concentration as 300 x 25 ppm = 7500 ppm 7500 ppm = 7500mg ...read more.

Conclusion

EDTA Titration: Advantages - This is a very simple procedure and fairly accurate as the burette can be measured to ?0.02ml, and many titrations can be carried out to obtain an accurate value. It is also a very cost efficient process as only a burette and a conical flask is required. It is also a very quick and time efficient process. Disadvantages - there would be human error due to the solutions being made up by hand. Atomic Absorption: Advantages - high precision can be obtained with this method due to the use of computerised equipment to analyse the solutions. It is also a fairly simple and fairly quick procedure. Disadvantages - the accuracy is not great due to the human error when calculating and making up the dilutions. It is also quite expensive as of the use of the computerised equipment and absorbance spectrometer. Spectrophotometry: Advantages - This is a fairly quick and simple procedure and high precision can be obtained due to the values are measured by the UV/VIS spectrometer. Disadvantages - The accuracy is not great as there is a lot of human error due to the calculations and measurements of dilutions. Conclusion: In my opinion I would say the best method is the EDTA titration as it is very cost efficient and the procedure is very simple and straightforward. The procedure is also very quick and although the solutions are made up by hand the human error is minimal as there are few measurements to make compared with the other methods. ...read more.

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