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EXPERIMENT6‐8

Liquid‐LiquidExtractionandSpectrophotometricDeterminationofCopperfromAlloyswithNeocuproine

(Groupsof3‐4students)

BackgroundReading:Christian, G. D., Analytical Chemistry; 6th ed.; Wiley & Sons: New York, 2003; Ch. 18: pp 541-546.

Introduction:To analyze a metal in alloy samples (such as cast iron, solder, and steel), first the alloy is digested

with a concentrated acid (HNO3) to dissolve the metal in water. Since the alloy contains considerable amounts of other metals besides the analyte, a method to separate the analyte from the aqueous solution of other metals is the extraction. In order to be extracted into an organic solvent the metal may be first chelated. A specific chetating reagent for copper is neocuproine (2, 9-dimethy-l, l0-phenanthroline). But in order to form a complex with neocuproine, copper has to be in the oxidation state Cu+ and not Cu+2. The complex formed between Cu (I) and neocuproine, abbreviated [CuI(neocuproine)2]

+, is slightly

soluble in cold water but much more soluble in propylene carbonate in which absorbs strongly at 457 nm. In this experiment, the brass sample is first digested to generate Cu (II), then Cu (II) is reduced to

Cu (I) with hydroxylamine hydrochloride (NH2OH * HCl). Sodium citrate is used as a masking agent to prevent interference of other metals. Solutions containing Cu (I) ions are colorless, but with the addition of neocuproine, the copper (I) ions in the sample are immediately complexed to produce colorful species. The equation for the formation of copper-neocuproine complex ion is:

2 C14H12N2 + Cu+ [(C14H12N2)2 Cu)]+ The Cu-neocuproine complex is extracted into a propylene carbonate solvent, giving a yellow-

reddish solution. Full color development occurs when the sample’s pH is weak acidic. The light absorbance of the propylene carbonate solutions containing Cu (I) will be measured to build up a calibration curve and after to determine the concentration of Cu in a brass sample. The procedure for this experiment consists of 3 major parts:

1. Preparation of sample and standards, 2. Extraction of the Cu-neocuproine complex into propylene carbonate, 3. Determination of the absorbance of the propylene carbonate/ [CuI(neocuproine)2]

+ solution.

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Procedure:

Part(I):PreparationofSampleandStandards(performthispartinthehood)

 Weigh approximately 353 mg of a dried brass sample into a 250 ml Erlenmeyer flask. Add about 20

mL of H2O and 10 mL of concentrated HNO3. Digest this sample over a hot plate until it dissolves, but be careful not to let the samples go to dryness.

IMPORTANT: Use fume hood only to boil or heat samples containing acid. Safety glasses are protection from you and your neighbor.

Quantitatively transfer the dissolved sample to a 100 mL volumetric flask and dilute to the mark. To

complete a "quantitative" transfer, wash the Erlenmeyer flask many times with small volumes of DI water and pass the washings into the volumetric flask. Transfer 5 mL of this solution to another 100 mL volumetric flask and diluting to the mark. This will be the unknown sample solution from which copper will be extracted and quantified.

Prepare a stock copper solution by weighting approximately 39.3 mg of copper sulfate, CuSO4

.5H2O and quantitatively transferring the weighed sample to a 100-mL volumetric flask and then fill the flask with water to the 100.00 mL mark. Calculate the concentration of copper expressed in ppm, in the stock solution, (please check your answer with the instructor). From the stock solution, prepare 25.00 mL each of copper standard solutions using 1.00, 3.00, 4.00 and 5.00 mL aliquots of Cu2+stock solution and diluting to the mark. Calculate the concentration of copper expressed in ppm of each diluted solution and show the calculations to your instructor.

 

Part(II):ExtractionoftheCu‐neocuproinecomplexwithpropylenecarbonate.

  NOTE: You will need a total of six 25-mL volumetric flasks to carry out this part of the experiment (4 standards + 1 unknown + 1 blank = 6). The reagent blank is prepared by using distilled water in place of standard or unknown. IT IS VERY IMPORTANT TO LEAVE THE EXTRACTED SOLUTIONS IN THE VOLUMETRIC FLASKS (i.e. do not transfer the extracted solutions from the volumetric flask to any other containers) because you have to fill up to the mark of the container in Part (III) [see the description in Part (III) for details).

To determine the total copper in the sample, copper must be completely in the Cu2+ state.

Hence, a mild reducing agent (e.g. hydroxylamine, as its hydrochloride salt) is added, before the complex is formed, in order to reduce Cu2+ present in solution to Cu+. The reaction equation is:

4 Cu2+ + 2 NH2OH 4 Cu+ + N2O + 4H+ + H2O

The pH is adjusted to a value of about 4 by addition of an ammonia or sodium citrate buffer. The extraction step usually takes a full lab period, as 6 solutions (a blank, 4 standards and the unknown sample) must be extracted in the same day so that color development for all solutions is allowed to proceed for approximately the same length of time.

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The extraction procedure is the same for the sample, standard and blank (water):

With a volumetric pipette, transfer 10.00 mL aliquots of the copper solution to a separatory funnel.

Add 5 mL of 10% hydroxylamine hydrochloride solution and 10 mL of 30% (w/w) sodium citrate solution. Adjust the pH to about 4 (add 6M HCl to bring the pH down OR add 6 M NH4OH to bring the pH up, use the provided pH paper to test the pH).

IMPORTANT: 1. A stable complex of [Cu(neocuproine)2]

+ can only be formed under the pH = 4. Failure to adjust the pH to the correct value, will lead to significant error in the final result, since some of the copper will not be reduced.

2. Use accurate transfer pipettes for all Cu-containing aliquots. The volumes of the other reagent solutions added to the copper solutions are in excess, so they do not need to be measured very accurately. The goal is to make the matrix in all five solutions as similar as possible.

Add 10.00 mL of 0.1 % neocuproine and 10 mL of propylene carbonate then shake for 30 seconds. Be sure to vent the stopcock occasionally to release pressure in the funnel. Make sure that when you are shaking the separatory funnel, the cap is pointed away from other people. Allow the layers to separate and draw off the lower propylene carbonate phase into a 25 mL volumetric flask. Repeat the extraction with another fresh 5 mL portion of propylene carbonate twice, add these portions to the same 25 mL volumetric flask that contains the original propylene carbonate phase, and then dilute to the mark with propylene carbonate. Rinse the separatory funnel well between sample extractions with water. The propylene carbonate extracts at this point will normally be cloudy because they are supersaturated with water. By the next lab period the excess water (if any) should separate into a distinct phase from the bulk of the propylene carbonate. Store all the 6 extracted samples in their representative volumetric flasks with caps and wrapped the caps with a layer of paraffin.

Part(III):AbsorbancedeterminationoftheCu‐neocuproinesolutions

Examine carefully each of the 6 extracted samples from the last lab period by following steps (a) – (c) for each extracted sample.

(a) If you see 2 layers inside the volumetric flask that indicates that the excess water was separated from the bulk of the propylene carbonate. The separated water layer (if any) can be removed from the bulk propylene carbonate phase with a Pasteur pipette. After removing the water layer with a Pasteur pipette, add propylene carbonate to fill the mark of 25 mL volumetric flask.

(b) If you do not see 2 layers inside the volumetric flask, but realize that the volume is below the mark of the 25 mL volumetric flask, add propylene carbonate to fill the mark.

(c) If you do not see 2 layers inside the volumetric flask and the volume is exactly 25 mL (i.e. at the mark of the volumetric flask), the solution is fine.

NOTE: The purpose of step (a) – (c) is to make sure that all the excess water was removed from the extracted samples AND all the 6 extracted samples have the exact same volume (25.00 mL) prior to the UV absorbance measurement. (i.e. make sure that the liquid level in each 25 mL volumetric flask that holds the extracted samples reach the mark).

SampleAnalyses:Follow the instruction for the use of UV-VIS spectrometer and measure the absorbance at 457 nm of the

[CuI(neocuproine)2]+

propylene carbonate blank, standard and unknown solutions. If possible, first

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perform a scan of the absorption spectrum of the copper-neocuproine complex to determine its maximum of absorption.

NotesonDeterminationoftheConcentrationofanUnknownSamplefromCalibrationCurve:

In order to determine the concentration of an unknown sample from a calibration curve, it is important to ensure that the “linear part” of the concentration curve was used for the analysis. In other words, the signal (in this case the absorbance) of the unknown sample has to be within the range of the signal of the standard solution (e.g. if the absorbance of the standard with the highest concentration is 1.0 and the absorbance of the standard with the lowest concentration is 0.4, the absorbance of the unknown sample should be between 0.4-1.0).

If the signal of the unknown sample is not within the range of the absorbance of the standard solutions and you had not make any mistake or introduce any error when you carried out the experiments, you have to consider the following adjustments: (1) If the unknown sample’s signal (in this case the absorbance signal) is lower than the signal from the

prepared standard with the lowest concentration, you have to prepare a new standard with a lower concentration that will give a signal that is lower than the signal from the unknown sample.

(2) If the unknown sample’s signal is higher than the signal from the prepared standard with the highest concentration, you have to dilute your unknown sample quantitatively (i.e. by using pipette and volumetric flask), and record the signal of the diluted unknown sample. The concentration of the original unknown sample will be calculated by multiplying with the dilution factor. E.g. if you dilute the unknown sample by a factor of 10, and the concentration of the diluted unknown sample was determined from the calibration curve to be 10 mg/mL, the concentration of the original unknown sample should be 10 x 10 mg/mL = 100 mg/mL.

NOTE: Check the answer by using the dilution equation: Mconc (stock) Vconc (stock) = MdilutedVdiluted

TreatmentofData:a. Prepare a calibration curve (absorbance vs. [Cu (I)], ppm) using Cu standard solutions. b. Record the linear regression equation and the correlation coefficient. c. Use the equation of your calibration curve to determine the concentration of Cu (I) in ppm of the

unknown solution whose absorbance you measured. Make sure that you account for dilutions. Do not forget to attach the calibration curve to your report.

d. Report the mass percentage of Cu (% wt/wt) in the alloy. Compare the results with the one you obtained in the Exp # 5. NOTE: Don't forget all the stuff you learned about significant figures and errors in previous labs.

Report your estimate data with the number of significant figures you can justify.

Post‐LaboratoryQuestions:1. The Distribution constant for a compound X is 197. If the concentration of X in the organic layer is

3.00 x 10-3 M, what is the concentration of X in the aqueous layer? (4 pts)

2. Question # 8, in Christian, G. D., Analytical Chemistry; 6th ed.; Wiley & Sons: New York, 2003, p. 515 (6 pts). (note: you will have to do some independent reading in the textbook in order to answer this question).

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References:

1. http://www.chem.fsu.edu/chemlab/chm3120l/llextraction/Cu_sepn_by_liq_extn.pdf 2. Kvítek, L.; Sichertová, D.; Milde, D.; Skopalová, J., Acta Universitatis Palackianae Olomucensis

Facultas Rerum Naturalium, 2000, Chemica 39, p. 53-59; on line at http://publib.upol.cz/~obd/fulltext/Chemica39/Chemica39_07.pdf

3. Harris, D. C, Exploring Chemical Analysis, Ed. 5th, W.H. Freeman and Co. New York, 2013, p. 406,