CHEM 181 Chapter : woc 7- cod liver oil and vitamin d.docx

Lab 5. Determination of iron in a supplement tablet by atomic Prelab Helpful Reading: Discussion of atomic absorption spectrometry: The introduction to Chapter 21, section 21-1 and the part describing flames at the beginning of section 21-2 In this lab a flame atomic absorption spectrometer will be used to determine the amount of Fe in a vitamin pill. You may select your sample from one of the pills containing -9 mg/pil, Women's spectrometer detects following three types of pills: Energy Vitamin s Vitamin pills containing 18 mg/pill. The atomic absorption Fe well when the Fe concentration is between 1.25ppm and 5ppm Answer the following prelab questions: 1. A. How will you prepare your calibration solutions? You will use five calibration solutions in this lab, including the blank. A stock iron solution for atomic absorption spectroscopy will be made available to you that has a concentration of 1000 ppm (w/v) Fe. For an iron analysis the Varian atomic absorption spectrometer in the quant lab-requires samples with concentrations between 1.25 and 5 ppm (w/v) and a blank. You will prepare 5 calibration solutions, having respectively, the following final concentrations of Fe: 0, 1.25, 2.5, 3.5 and 5 ppm (w/v) Fe. These solutions will be prepared by diluting an appropriate number of microliters of 1000ppm (w/v) Fe with DI water in a 100 ml volumetric flask. Calculate how many microliters of stock iron solution are needed to prepare calibration solutions at the concentrations mentioned above. 2. How will you prepare your sample for analysis? Draw up one experimental plan for diluting your chosen pill to an appropriate concentration for the atomic absorption instrument using a micropipette. Specifically a. Which pill, from the two types mentioned above do you propose to analyze? How many mg of Fe is the pill expected to have? Assume that you will digest a vitamin pill as you did last week, and will dilute the digest to 100ml-this solution will be called vitamin solution 1 (VS1). You willuse a micropipette to transfer some liquid from VS1 to another 100mL volumetric flask and b. will ilute this to the mark. This second solution, vitamin solution 2 (VS2) will be the sample that you measure. What volume should be transferred from sample solution 1 to sample solution 2 with the micropipette? Draw a picture to illustrate your dilution scheme. Write out a sample preparation equation relating the estimated concentration in the mg/pll (based on the label) to the estimated concentration (in units of ppm) of Fe in the sample that will be measured. Do this in your lab notebook. c. d. Lab 5 Determination of iron/atomic absorption spectroscopy 31

Spectrophotometric Determination of Iron in a Vitamin Pill Introduction Iron is one of the essential elements in the human diet. In the body, iron is present in the form of dietary iron results in the condition of anemia. Many people take vitamin pills containing iron to supplement their dietary intake ofiron. The technique of is method of instrumental analysis. Many different types spectrophotometers are available that a of the electromagnetic radiation transmit light in different regions spectrum. In is experiment, a visible spectrophotometer will be used. A visible spectrophotometer of a measures the amount of light that a colored solution absorbs. When a beam o the sample depends on wavelength passes through a colored solution, the amount of light absorbed by the concentration of what the substance is that is absorbing the light. The absorbance also depends on the the path through the light passes. The relationship between absorbance, concentration, and pathlength is expressed by the equation known as Beers la gte where A is the al of the solution, E is a constant called absorptivity, which is a characteristic of the substance and the wavelength of the absorption, c is the concentration of the absorbing substance in is a unitless the solution, and e is the length of the path through which the light passes. Abe quantity, concentration can be expressed in any convenient concentration unit, and the pathlength is measured in centimeters. The units of absorptivity depend on the concentration units used. In this experiment concentration is measured in mg/mL Therefore, the units of e are mL/mg cm To determine the absorptivity of a particular substance, a calibration curve is constructed. A series of standard solutions in which the concentration of the absorbing species is known are prepared and their absorbances absorbance as a function concentration is made. If the absorbin species obeys Beer's law, the plot will be a straight line, y mr b. When the pathlength is 1.0 cm, the slope of the line is the absorptivity. The absorbance of a solution of unknown concentration can be read and its concentration determined from the calibration curve. In this experiment, a commercial vitamin pill will be analyzed for its iron content. The form of iron present in the vitamin pill, most likely iron(T) sulfate, has a negligible visible absorbance. The iron(T) will be converted to a species that has a large absorbance, allowing the absorbances of solutions of low iron(T) concentration to be measured. The reagent 1.10-phenanthroline, Figure 1, reacts with is red-orange and absorbs visible ligh most strongly at 508 nm. Iron(ll) is easily oxidized to iron(TI) in aqueous solution. A reducing agent, hydroquinone, is added to the solution to prevent this oxidation from occurring. In addition, the acidity of the solution must be controlled to ensure the formation of the complex. In this experiment, sodium citrate is added to maintain the proper pH of the solution A series of standard solutions with known iron concentrations is prepared, the absorbances of the solutions are read, and a calibration curve constructed. From the calibration curve, the concentration of iron in a solution made from the dissolved vitamin pill will be determined and the total amount of iron in the vitamin pill calculated.

Determine why this titration works to determine the concentration of calcium ions. The titration works because the EDTA causes a chemical reaction with the analyte. Write the chemical equation for the titration reaction. YH2/2 + Ca^2+ rightarrow YCa^2+ + 2H+ Write the chemical equation for the reaction that indicates the endpoint of the titration. Use these equations to explain how and why this particular complex metric titration works for the determination of Ca2+. For this experiment, we did three different titrations where we mixed the known solution the titrant with the unknown analyte to create a chemical reaction. To make the titrant we weigh out 1.489g of disodium EDTA needed to make the 250mL of 0.016M EDTA solution. Next, we mixed 100mL of warm distilled water until the EDTA has dissolved. Then we added 9mL of 0.105 M MgCl_2 solution, and 2.8 mL of 3 M NH_4OH to the EDTA solution. For the analyte, we obtain a calcium supplement tablet (Cultrate Calcium and vitamin supplement) and placed a total of 25mL of 3M HCL to it until it dissolved. Before we began with the titration we added 10mL of ammonia/ammonium chloride buffer solution to make the solution basic, and added 13 drops of calamite indicator. Once the experiment began we titrated with the disodium EDTA from the burette until the last traces of red were gone and the solution became a pure blue color. The volumes of disodium EDTA solution were recorded in a table (Figure 1).