Analytical Chemistry Laboratory 2
Spectrophotometric Determination of Iron Mr. *****
Department of Chemical Engineering and Chemistry, Mapua Institute of Technology
A comple complex x of iron iron (II) (II) in the water water sample sample was formed formed with with 1,10-p 1,10-phen henant anthro hrolin line, e, 2+ Fe(C12H8 N N2)3 and the absorbance of this colored solution is measured with a spectrophotometer. The volume reading was recorded at 7.0271 mL. The spectrum was plotted to determine the absorption maximum. Hydroxylamine (as the hydrochloride salt to increase solubility) was added to reduce any Fe 3+ to Fe2+ and to maintain it in that state.
Objective
The main purpose of this experi experimen mentt is to determ determine ine the amount amount of iron in the given sample from reading its absorbance by using a spectrophotometer. A series of solutions with different concentration will be measured to establish a calibration curve. Discussion
Overview The spectrophotometer is a simple instru instrumen mentt for measur measuring ing the the amount amount of light transmitted through solutions. It has a diffraction grating that can be used to adjust the the wave wavele leng ngth th of ligh lightt tran transm smit itte ted. d. In gene generral the ligh lightt used shou should ld be the the complimentary color of the solution being test tested ed.. The The amou amoun nt of ligh lightt at a give given n wavelength that a sample absorbs is often directly proportional to the concentration of the absorbing species in the sample and to the dista distance nce (path (path length length)) the light light must must travel through the sample. Iron Iron is a stab stable le ion. ion. It form formss an inte intens nsee red red colo colorred comp compou ound nd with orthop orthophen henant anthro hrolin linee soluti solution on,, and and is the
bas basis is for for its its dete determ rmin inat atio ion. n. Sinc Sincee the the solutions are reddish orange, a blue-green wavelength of light (508) should be selected for the spectrophotometer. Chemically, iron is an active metal. It combin combines es with with the haloge halogens ns (fluo (fluorin rine, e, chlori chlorine, ne, bromin bromine, e, iodin iodine, e, and astati astatine) ne),, sulfur, sulfur, phosphoru phosphorus, s, carbon, carbon, and silicon. It displaces hydrogen from most dilute acids. It burns burns in oxygen oxygen to form form ferros ferrosofe oferri rricc oxide, Fe3O4. When exposed to moist air, iron becomes corroded, forming a reddish bro brow wn, flak flaky, y, hydra ydratted ferr erric oxide ide commonly known as rust. The formation of rust is an electroche electrochemical mical phenomenon phenomenon in which the impurities present in iron form an electrical “couple” with the iron metal. A smal smalll curr curren entt is set set up, up, wate waterr from from the the atmo atmosp sphe here re prov provid idin ing g an elec electr trol olyt ytic ic solution. Water and soluble electrolytes such as salt accelerate the reaction. In this process the iron iron metal metal is decomp decompose osed d and reacts reacts with oxygen in the air to form rust. The reac reacti tion on proc procee eeds ds fast faster er in thos thosee plac places es where rust accumulates, and the surface of the metal becomes pitted
Methodology Preparation of the Calibration Curve
25 mL of the standard iron solution was transf transferr erred ed to a 100-mL 100-mL volume volumetri tricc flas flask, k, and and 25 mL of dist distil ille led d wate waterr to a second 100-mL volumetric flask. To them, 1 mL of hydrox hydroxyla ylamin mine, e, 10 mL of sodium sodium acetate, and 10 mL of orthophenanthroline were added to each flask. The mixtures were allowed to stand for 5 min; diluted to the mark and mixed. A pair pair of matc matche hed d cell cellss for for the the inst instru rume ment nt was was clea cleane ned. d. Each Each cell cell was was rinsed rinsed with with at least least three three porti portions ons of the solution it is to contain. The absorbance of the standard was determined with respect to the blank. This This proced procedure ure was repeat repeated ed with with five five othe otherr volu volume mess of the the stan standa dard rd iron iron solution (1, 5, 15, 35, and 45 mL). Determination of Iron 10 mL of the unknown was transf transferr erred ed to a 100-ml 100-ml volume volumetri tricc flask flask;; trea treate ted d in the the exac exactt same ame way as the the standards and the absorbance was measured with with respec respectt to the blank. blank. The The volume volume of unkn unknow own n take taken n to obta obtain in abso absorb rban ance ce measu measure remen ments ts for for repli replica cate te samp sample less are are within the range of the calibration curve was altered.
Instrumentation
(orthophenanthroline) is useful in determ determini ining ng iron iron in water water suppl supplies ies.. The reagent is a weak base that reacts to form phenanth phenanthrolin rolinium ium ion, phenH phenH+, in acid acidic ic media. Complex formation with iron is thus best described by the equation: Fe2+ + 3phenH+ ↔ Fe(phen)32+ + 3H+ Iron (II) is quantitatively complexed in the pH range between 3 and 9. A pH of abou aboutt 3.5 is ordi ordina nari rily ly reco recomme mmend nded ed to prevent precipitation of iron salts, such as phosphates. An excess of a reducing reagent, such such as hydr hydrox oxyl ylam amin ine, e, was was need needed ed to maintain iron in the +2 oxidation state. The complex, once formed, is very stable. The determinat determination ion was performed performed with a spectrophotometer set at 508 nm and the data were obtained as shown: V (mL)
Absorbance
1
0.0280
5
0.0731
15
0.3478
25
0.4400
35
0.7679
45
0.9901
Unknown
0.1299
The volume of the unknown sample # 4 was 7.0271 mL, computed by using the method of linear regression. Conclusion
Spectrophotometer is an instrument used to measure the intensity of wavelengths in a spectrum of light compared with the intensity of light from a standard source.
Spectophot Spectophotomete ometers rs are a vital and nece necess ssar ary y inst instru rume ment nt in quan quanti tita tati tive ve analysis. In this experiment the volume of iron in the given solution was determined using this technolog technology. y. The result result obtained obtained using the calibration curve was 7.0271 mL. Several reagent were added to the sample to main mainta tain in its its stab stable le stat statee thro throug ugho hout ut the the procedure. References
Interpretation
The red-orange complex that forms between between iron (II) and 1,10-phen 1,10-phenanthr anthrolin olinee
1. Christian, G.D., Analytical Chemistry, 6 th edition. New Jersey. John Wiley, 2004.
2. Filby, G. “Spreadsheets for Chemists,” VCH Publishers, New York, 1995, p. 210.
3. Schwartz, L.M. “Advances in Acid-Base Gran Plot Technology,” J. Chem. Educ., 64, 947-950 (1987).
