Experiment Five: Spectrophotometric Determination of Iron in Vitamin Tablets
November 1, 2018
Lead Chemist: Ariel Carter
Introduction:In the experiment, Spectrophotometric Determination of Iron in Vitamin Tablets, the students or chemists were expected to find the amount of iron that was in a vitamin than what was advertised on the bottle. Many pharmaceutical companies advertise that their vitamins contain the most grams of iron in their tablets, however this may not be the truth. This is where spectrophotometry comes into play. Spectrophotometry is one of the most useful techniques in quantitative analysis and other fields of science that uses chemical compounds (Vo, 2016). Spectrophotometry allows for chemist to test these compounds and find the exact concentrations of the components that make up the compound. In this experiment, in order for the iron to be detected it had to react with phenanthroline. Phenanthroline is a ligand that can illuminate metal complexes or can interact with DNA inducing DNA cleavage (Bencini, 2010). In the case of this experiment the phenanthroline is used to illuminate the iron in the tablet so that it can be detected by the spectrophotometer. The iron in the tablet also had to be reduced to Fe2+ with the hydroquinone and the sodium citrate was used to maintain the pH of the solution.
Spectrophotometry is a technique that measures how much of a substance is in a known chemical. Every chemical transmits light over a particular range of wavelengths (Spectronic 20, 2017). The spectrophotometer measures the intensity of light that is absorbed after it passes through a solution. In order to calculate the absorbance, the transmittance must be known. Transmittance is one of these variables. Transmittance is the fraction of light that passes through the sample or T=ItIo, where It is the intensity of the light after the beam of light passes through the cuvette and Io is the intensity of the light before the beam of light passes through the cuvette. Once the transmittance has been calculated it can be plugged into the equation: A=-log?(T) which will give you the absorbance. After these variables have been calculated the concentration of iron can be determined by using Beer’s Law. Beer’s law, A=?×v×c, states that there is a linear relationship between the absorbance and the concentration of the sample, in this case the iron (Vo, 2016). A, is the absorbance, ?, is the molar absorptivity, v, is the path length, and c is the concentration. However, Beer’s law can only be applied when there is a linear relationship.
The instrument used in this was the Spectronic 20 or more commonly called the Spect 20. A spectrophotometer is an instrument that is designed to measure the amount of light which passes through a solution (Spectronic 20, 2017). The readings from a spectrophotometer may be expressed in terms of absorbance, which is the amount of light the solution absorbs or transmission, the amount of light that passes through a solution (Vo, 2016). The measured absorbance can be used to determine the concentration of a colored material in solution. Spectrophotometers also incorporate the use of a prism in order to create a certain range of wavelengths that can be recognized. This is done because a solution that absorbs light over all of the visible ranges, will result it will appear black.
Data ; Results:
Bencini, Andrea. “1,10-Phenanthroline.” Coordination Chemistry Reviews, Elsevier, 24 Apr. 2010, www.sciencedirect.com/science/article/pii/S0010854510001025.
“Spectronic 20.” What Does a Spectronic 20 Measure?, Purdue University, 29 Oct. 2017, chemed.chem.purdue.edu/genchem/lab/equipment/spec20/whatis.html. Vo, Kevin. “Spectrophotometry.” Chemistry LibreTexts, Libretexts, 21 July 2016, chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Kinetics/Reaction_Rates/Experimental_Determination_of_Kinetcs/Spectrophotometry.
One tablet of the iron tablet was placed in a 125-mL flask and boiled for 15 minutes with 25- mL of 6 M HCl. The solution was filtered into a 100-mL volumetric flask. The flask was rinsed and filtered several times with water to make sure everything was transferred. The solution cooled and was then diluted to the indicated mark and mix well. In a new 100-mL volumetric flask, at 10-mL aliquot was added. In a beaker, 10-mL of the standard Fe solution was added and the pH was measured. Sodium citrate was added one drop at a time until the pH reached about 3.5. Another 10-mL was added to a new volumetric flask and the number of drops used previously was added to the volumetric flask. Additionally, added was 2-mL of hydroquinone and 3-mL of phenanthroline. The solution was diluted to the mark and mixed well. Three more solutions were prepared using 5-mL, 2-mL, and 1-mL of the standard Fe solution. A blank was prepared using no Fe solution. Sodium citrate drops were added in proportion to the volume of the Fe solution. In a new beaker 10-mL of the iron tablet solution was added and sodium citrate was added one drop at a time to get the pH about 3.5. Another 10-mL of the iron tablet solution was added to a 100-mL volumetric flask and the same number of drops from the previous step was added along with 2-mL of hydroquinone and 3-mL of phenanthroline. The solution was diluted to the mark and mixed well. The solutions stood for 10 minutes. Each of the solutions were placed in a cuvette and ran through the spectrophotometer.