CHEM 334 Quantitative Analysis Laboratory Spectrophotometric D etermination o f I ron Introduction 2+
Aqueous iron, in its reduced ferrous form (Fe ), can be determined spectrophotometrically from its intensely colored -­‐1
complex with 1,10-­‐phenanthroline (also o-­‐phenanthroline, C12H8N2, FW 180 g mol ), in acidic (pH 3-­‐4) solution, by the following reaction (note the stoichiometry): !"#$ %&'()*+&,+)+-')*.($/')'*-.01).+0$+2$3*+0$.0$4.)1-.0$5167')8$3+$
(1) In our highly oxidizing environment, iron most often exists as the more oxidized ferric ion (Fe ) and must be reduced in #
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proceed. Hydroquinone (C6)*#
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reductant as seen in the following reaction: AB-7-,#C75'/#$D#)&#'(/#'/?'8--EFG#
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(2) Citric acid, C3H5O(COOH)3, is a weak, triprotic acid and its conjugate base, citrate, as its salt trisodium citrate, C3H5O(COONa)3, serves to neutralize excess aL
cid (vide infra) while the acid-­‐conjugate base pair buffer the solution at the L
required pH value. :;
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This method is well suited to the determination of iron in dietary supplements. It is necessary to release the iron ions from the mix of compounds that makes up the solid or liquid supplement. Digestion with moderately concentrated acid H
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and at elevated temperature is an effective procedure for affecting this release. #
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Procedures !@C(/&5&'(,-7)&/#
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Preparation of #Standard Iron Solution: Prepare 50 mL of solution of approximately 1 mM but accurately known $
9:;<:=5%#
concentration ferrous ammonium sulfate (Fe(NH4)2(SO4)2•6H2O) in approximately 10 mM sulfuric acid. Use the solid salt #
and 1 M sulfuric "#$%!&'()!)*+,,
acid to make up this solution in a volumetric flask. This s,olution can be stored in a polyethylene bottle. 9,/*(7=#+,/+5,/0#*-71')-&#.-&'5)&)&O#$N#OPQ
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Other Reagents: Obtain or prepare 50 mL of a solution of 10 mM hydroquinone in water, 50 mL of a solution of 0.2 M #
sodium citrate in water and 50 mL S)**-76/#:GD#O#)&#$NN#4Q#-3#/'(5&-7#5&0#500#TNN#4Q#-3#<5'/,G##R'-,/#)&#5&#
of a solution of 20 mM phenanthroline in 10 % (v/v) ethanol in water (the solid is first !345*)2)15%!6()*+,,
dissolved in one-­‐tenth volume ethanol then made up to the final volume with water). These solutions are prepared and 548/,#8-''7/G#
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stored in polyethylene bottles. Note the required accuracy of the concentrations of the solutes and recognize that the 712)$2%$,8*,9:;:<,/=,8*>/?@+,,,C,/+5,/#8=#0)**-76)&O#NG:"$#O#-3#,/5O/&'@O,50/#
use of volumetric flasks is not required for the preparation of the solutions. 9/ALI!F:ARJ!F:#UI:J#)&#<5'/,#)&#5#$@Q#6-714/',).#375*E#.-&'5)&)&O#$#4Q#-3#T"#<'#V#I:RJ!G#
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Colorado State University 2014.09.16 Page 1 of 2 #
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Spectrophotometric Determination of Iron CHEM 334 Quantitative Analysis Laboratory Generation of a Calibration Curve: Pipet a 5.0 mL aliquot of the standard iron solution into a 50-­‐mL beaker and add a -­‐1
few drops of methyl yellow (0.1 g L in ethanol) to measure the pH. Add the sodium citrate solution drop-­‐wise with swirling until the pH of the solution is approximately 3.5 as indicated by a peach color of the solution (tens of drops are usually required but this value can vary). Discard this solution. Pipet a fresh 5.0 mL aliquot of the standard iron solution into a 50-­‐mL volumetric flask and add the same number of drops of citrate solution as required in the previous step. Add 1.0 mL of hydroquinone solution and mix thoroughly. Add 1.0 mL of the phenanthroline solution and mix thoroughly. Fill to volume with water. Prepare a blank by preparing a solution with all reagents excepting the iron-­‐containing solution. Allow the solutions to stand at room temperature for at least five minutes (but less than thirty minutes) and measure the absorbance of each solution (using the 0.0 mL solution as a blank) at 508 nm and using the same cuvette. The color complex is stable so several solutions may be prepared at once and absorbances measured subsequently. Prepare four more solutions using this procedure from 4.0, 3.0, 2.0 and 1.0 mL of the standard iron solution. Use a volume of sodium citrate solution in proportion to the volume of iron solution used. Preparation and Assay of Unknown Sample Solution: Accurately weigh approximately one iron-­‐containing supplement tablet. Digest the tablet in a 100-­‐mL beaker in the hood using 10 mL of 6 M hydrochloric acid on a hot plate set at 150°C for ten minutes. The tablet will not dissolve completely. Let the solution cool slightly (in the hood) and then quantitatively transfer (on the bench) all the contents of the beaker into a 100-­‐mL volumetric flask. After cooling the solution completely to room temperature, fill to volume with water. There may be some sediment in the flask; let it settle for a few minutes. Pipet 5.0-­‐mL of the clear (not necessarily colorless) supernatant into a 100-­‐mL volumetric flask and fill to volume with water. Perform the iron assay in triplicate using three separate 5.0-­‐mL aliquots of the diluted unknown solution. Measure the absorbance of the three solutions against a freshly prepared blank solution as described previously. Analysis: The molar absorptivity for the iron-­‐phenanthroline complex at 508 nm is determined by fitting a linear function to the set of absorbance versus concentration measurements. The concentration of iron in the diluted unknown solution is found by the application of Beer's law and the molar absorptivity for the iron-­‐phenanthroline complex. Calculate the mass of elemental iron in the original tablet based on the dilutions performed. Results Report the molar absorptivity for iron-­‐phenanthroline complex at 508 nm. Include a suitable error analysis for this value. Report the values of the determined mass of iron in the tablet as mass elemental iron. Include a suitable error analysis for this value. Discussion Discuss the basis for the upper and lower limits of detection of this method. Estimate both values and justify them. Decide and report whether the tablet manufacturer should be hauled into criminal court for fraud or applauded for "truth in advertising." Justify your decision. References •
Atkins, R. C. J. Chem. Educ. 1975, 52, 550. Colorado State University 2014.09.16 Page 2 of 2