1. The student will use a complexometric titrant, EDTA.
2. The student will use a buffer to control the titration reaction.
3. The student will use a method of aliquots.
1. Preparation of EDTA (0.01M). Dry EDTA for 2 hours at 80o C. GMW = 372.2., or check the reagent bottle. Dissolve needed grams in a 500 mL vol. flask. Store in a conditioned polyethylene container. EDTA solutions attack glass and deteriorate after 1 or 2 days.
2. Preparation of known copper solution. Place 0.2 g to 0.25 g of pure Cu. wire (or shot) (weighed on an analytical balance) in a 100 mL beaker. Add 15 mL conc. HNO3 and 1 mL conc. H2SO4. Place a watch glass over the beaker. Boil until dense white fumes of SO3 cease. Transfer carefully to a 200 mL vol. flask. Remember to rinse the watch glass. Dilute to the mark.
3. Preparation of unknown copper solution. Weigh 0.5 to 0.6 grams of unknown into a 200 mL vol. flask. Add 30 mL of 1:5 nitric acid. Heat until all particles are dissolved. Dilute to the mark.
4. Titration with EDTA - murexide indicator. Do this step for 4 samples of known and 4 samples of unknown.
KNOWN: Place 10 mL of Cu known solution (step 2) in 250 mL Erlenmeyer flask. Add pH buffer drop-wise until the solution turns completely blue and the precipitate first formed dissolves. Add about 100 mL di. Add more buffer if solution becomes cloudy. Add 1 tablet of murexide indicator and titrate with EDTA until color changes from yellow (green) to deep violet. If the volume of EDTA is less than 10 mL, repeat using 25 mL aliquot of Soln. 2.
UNKNOWN: Place 25 mL of Cu unknown solution (step 3) in 250 mL Erlenmeyer flask. Add pH buffer drop-wise until the solution turns completely blue and the precipitate first formed dissolves. Add 1 tablet of murexide indicator and titrate with EDTA until color changes from yellow (green) to deep violet. If the vol. of EDTA solution is less than 10 mL. use a larger aliquot of Soln. 3. Check with Dr. Sherren.
Remarks: The copper-murexide complex is pure yellow, but the color at the starting point of the titration is greenish depending on the amount of copper present. The green is the mixture of yellow (indicator complex) and blue (copper-tetramine complex). As the titration proceeds the tetramine complex is transformed to the less intensely colored EDTA complex and the yellow of the indicator complex becomes permanent.
CALCULATION: Calculate the molarity of the EDTA. Calculate percent copper in the sample using the value of the Molarity of the EDTA found. Include the spread sheet for the calculation.
CAUTION: Remember to treat all chemicals with respect.
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