Phenyo Jerry Mmereki 314.docx

Phenyo Jerry mmereki 201602475 Group: Tuesday 3-6pm Date; 26/02/2019 Title: SPECTROPHOTOMETRIC DETERMINATION OF IRON IN VITAMIN TABLETS

AIM; the aim of the experiment is to determine iron content in vitamin supplement tablet INTRODUCTION Organic compounds (vitamins) are essential for proper biological function. Certain minerals are necessary for enzymatic activity, cellular function and structural maintenance. Since the human body cannot synthesize these compounds, we are required to ingest them from exogenous sources. Iron is a common dietary supplement in a variety of foods. Iron is essential for oxygen transport as it is the main component of hemoglobin and for energy storage. The iron present in the vitamins is in the form of soluble iron(ii) , insoluble iron(iii) is generally unusable for living organisms. According to Harvey et al (1955) to determine the total iron in the sample, it must completely be in the ferrous state. However, Fe2+ can readily be air-oxidized to the ferric state, Fe3+. orthoPhenanthroline will form a colored complex with Fe 3+, but its spectrum is different from that of the ferrous complex and the color is not as intense. Thus, one could not determine the total iron present by making measurements at only one wavelength. Hence, in the preparation of standard solutions, a mild reducing agent is added before the color is developed in order to prevent Fe2+ from being oxidized to Fe3+ and to provide a measure of the total Fe present in the solution, hydroxylamine as its chloride salt can be used. The reaction is shown as follows; 2Fe3+ + 2NH2OH-HCl + 2OH  2Fe2+ + N2 + 4H2O + H+ + ClTo the workable standard of Fe(ii) solution, after adding hydroxylamine hydrochloride, 1,10phenthroline is then added to the mixture. This is done to prevent other metals such as silver, bismuth, chromium and copper form causing interferences to the solution. The acetate buffer is also added to keep the pH of solution between 6 and 9. To determine the iron in solution, colorimetric analysis is used. The sample solution must have color and absorb visible radiation to be analyzed by colorimetric analysis. A standard curve of absorbance versus concentration is established to determine the concentration of the complex in the unknown sample solution. The curve adopts the linear nature of Beers law:

A= E b c Beers law states the absorbance of light is directly proportional to the concentration of the absorbing solution when the path length of radiation in the absorbing medium is fixed.

MATERIALS AND METHODS Materials/ apparatus 7* 100mL volumetric flasks Measuring cylinder 100ml beaker Cuvettes Spectrophotometer Pipettes

Reagents/ chemicals Standard iron stock Concentrated sulphuric acid 1,10-phenanthroline Hydroxylammonium chloride solution Sodium acetate

PROCEDURE In order to prepare a standard stock(ii) solution 0.0281g of ferrous ammonium sulphate was weighed, dissolved in 20ml of water and then quantitatively transferred to 100ml volumetric flask. 0.1ml of concentrated sulphuric acid was added and the solution was diluted to the mark using distilled water. In preparation of the sample, the tablet containing iron was dissolved in 25ml of 6M HCL in 100ml beaker and was boiled for 15minutes. The solution was filtered directly into a 100ml volumetric flask and the beaker was washed several times with water to complete the quantitative transfer. The solution was allowed to cool and then diluted to the mark. 1ml, 2ml, 5ml, 7ml and 9ml of the standards of iron solution, 50ml of water and 10ml of the unknown or sample were placed in 100ml volumetric flasks separately. To each of the flasks 1.0ml of hydroxylammonium chloride solution, 5.0ml of the 1,10phenanthroline solution and the each of the solution was then buffered by addition of 8.0ml of sodium acetate to produce red colour of ferrous 1,10-phenanthroline. The solutions were allowed to stand for 15 minutes for color to fully develop and after that each solution was then diluted to the mark using distilled water. The absorption spectrum of the 5ml of stock solution dissolved in 100ml (2ppm standard) was obtained by measuring the absorbance from 400nm to 700nm using scan function of the Evolution 201 spectrophotometer and using the blank solution as a reference solution. The wavelength was then selected at maximum absorbance of the standard. A calibration curve was then prepared by measuring the absorbance of other standards and the selected wavelength. Absorbance of the unknown was also determined in the same manner.

RESULTS Calculation of molar absorptivity using 2ppm standard Moles of standard iron= mass/ molar mass= 0.0310g/392.13gmol= 7.906*10-5 moles Concentration= moles/ volume = 7.906*10-5 moles/ 5.00*10-3L = 0.01581 M

Absorbance of the 2ppm standard at 511.69 nm= 0.422 Molar absorptivity= absorbance/(cell path length* concentration) = 0.422 /(1cm * 0.01581M)/ = 26.7M-1 cm-1

Concentration of the unknown Absorbance of unknown= 0.033 Concentration= absorbance / (molar absorptivity* path length) = 0.033/ (26.7M-1 cm-1* 1cm) = 0.001236M Mass of iron in the unknown Volume of solution= 100ml Moles= concentration*volume = 0.001236M * 0.1L = 0.0001236moles Mass= moles*molar mass = 0.00001236moles* 55.84gmol-1 = 0.006902g = 6.902mg of iron in the unknown

Mass of iron from the standard curve Y= 0.211x- 0.0007 Y=0.033 Therefore concentration= x = (0.033+0.007)/ 0.21= 0.190ppm Therefore mass= 0.0190mg

0.9

0.8

y = 0.2109x + 0.0071 R² = 0.9997

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Figure 1: Standard curve

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DISCUSSION The observations made from the graph is that concentration is directly proportional to the absorbance of the solution, when the concentration of the solution increases the absorbance also increases. The more the concentration of solution , the more the number of molecules that interact with photons , thus greater absorbance of light, (Skoog et al ,1998). The graph plotted of absorbance against concentration showed a straight line which indicates that the beer lambert law is obeyed. The maximum absorbance was determined using the absorbance of the 2ppm standard iron solution and was found to be at 0.442 at 511.71nm maximum wavelength, this wavelength was also used to measure other standards and the unknown. The molar absorptivity was found using the 2ppm standard to be 26.7M-1 cm-1. The maximum wavelength of iron(II)-Phen complex is 510nm with molar absorptivity 1.1*104 M-1cm-1 (Oktavia et al ,2008).This is in contrast to the results as the values do not match hence indicating that an error was made during the experiment. The unknown solutions concentration was calculated as 0.001236M and its mass was determined as 6.902mg and therefore the concentration of the iron in the vitamin tablet in terms of milligrams per liter is 69.02mg/L whereas a different mass and concentration was obtained from the graph which are 0.0124mg and 0.124ppm respectively. From the graph the R2 was 0.9944 which is close to 1 which shows precise results since there is less scatter of points. Some errors that might have occurred during the experiment might have been due to the dilution of the solutions due to instrumental uncertainties. The stray of light, noise and effects due to polychromatic radiation may cause spectrophotometers to suffer from non-linearity.

CONCLUSION The concentration of the unknown was determined to be 0.001236M and its mass was determined to be 6.902mg from calculations and graphically the concentration was determined to be 0.0124mg/L. The molar absorptivity of Iron(II)-phenathroline was found to be 26.7M-1 cm-1.

REFERENCE I.

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Harvey, A.E., Smart, J.A., and Amis, E.S. (1955). Simultaneous Spectrophotometric determination of iron(II) and total iron with 1,10-Phenanthroline. Analytical chemistry. Vol 2(1), pp 26-29. Oktavia, B., Lim, L.W., and Takeuchi, T. (2008). Simultaneous determination of Fe(III) and Fe(II) ions via complexation with salicylic acid and 1,10-phenathroline in Microcolumn ion chromatography. Japan society of for analytical chemistry, vol 4(1), pp1488-1489. Skoog, D. A.; Holler, F. J.;Nieman, T. A. “Principles of Instrumental Analysis’’, 5th ed.; Harcourt Brace & Company: Philadelphia, 1998; pp 337-345