Materials Included In Kit

Additional Materials Required

Prelab Preparation

Hydroquinone Solution: Add 1 g of hydroquinone to 100 mL of distilled or deionized water. Mix, then store in the hydroquinone-labeled amber bottle.

1,10-Phenanthroline Solution: Add 10 mL of ethyl alcohol and 40 mL of distilled or deionized water to a 1,10-Phenanthroline-labelled amber bottle. Add 0.25 g of 1,10-phenanthroline to the bottle, cap and mix. Add 50 mL of distilled or deionized water to the bottle. Cap and mix.

Sodium Citrate Solution: Add 25 g of sodium citrate to 1 L of distilled or deionized water.

Standard Iron Solution, 40 μg Fe/mL (40 ppm): To a 1-L volumetric, add 50 mL of distilled or deionized water, followed by 25 mL of 1 M sulfuric acid solution. Swirl to mix. Add 0.28 g of ferric ammonium sulfate to the 1-L volumetric. Fill to the 1-L mark with distilled or deionized water.

Alternately, add 2.81 g of ferric ammonium sulfate to the 100-mL volumetric. Dilute to mark with distilled or deionized water. Add 50 mL of distilled or deionized water, followed by 25 mL of 1 M sulfuric acid solution to a clean 1-L volumetric. Swirl to mix. Pipet 10.0 mL of the ferric ammonium sulfate solution in the 100-mL volumetric to the 1-L volumetric. Fill to the 1-L mark with distilled or deionized water.

Sample Preparation

For classes not experienced in the sample preparation technique of digestion, we recommend the teacher prepare the samples for analysis. The following procedure can be incorporated into the student procedure if you choose to do so. Sufficient materials are included in the kit to allow for this. However, each digestion calls for three volumetric flasks. If choosing to have the students perform the digestion, graduated cylinders may be substituted for these flasks.

The overall scheme for the digestion and analysis is

Safety Precautions

Hydrochloric acid solution (6 M) is toxic and severely corrosive to eyes and skin. 1,10-Phenanthroline is toxic by ingestion. Hydroquinone solution is slightly corrosive to skin and eyes. Avoid contact of all chemicals with eyes and skin. Consumer chemicals such as iron supplements that are brought into the lab are considered laboratory chemicals and may not be consumed or removed from the laboratory afterwards. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All leftover solutions may be neutralized and disposed of according to Flinn Suggested Disposal Method #24b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment should be completed before coming to lab, and the data compilation and calculations can be completed the day after the lab.
• One way to check the accuracy of an analytical method is to add a known amount of the chemical being tested to a sample. This is usually referred to as a spiked sample. In this experiment, the students are adding 5 mL of the 40 ppm iron standard solution to the final sample dilution step. The sample now has a known amount of extra iron concentration over that of its unspiked counterpart. Each sample is analyzed and its total iron content determined. If the original sample contains x ppm iron, then the spike sample contains [(5 mL x 40 ppm iron)/100 mL] + x ppm iron, or (2 + x) ppm iron. Students can then calculated a percent recovery.
  {12682_Hints_Equation_4}
  A percent recovery between 90 and 110 percent would indicate that nothing in the sample preparation interferes with the determination of iron. Note: If sample concentration is greater than 3 ppm, use only 5 mL of the sample with 10 mL of the iron(III) standard solution to make the final dilution.
• The use of a color comparison chart, while more subjective than the use of a spectrophotometer, yields surprisingly accurate results, similar to those from water pollution color comparator kits.

Teacher Tips

• Quantitative analysis represents a nearly invisible application of chemistry in our daily lives. To illustrate the hidden importance of quantitative analysis, ask students how they would feel if they could not trust that the water they drink or the medicines they take had been tested to assure their quality and safety.

Further Extensions

Sample Procedure

1. Place the iron supplement tablet in a clean 100-mL beaker and carefully add 25 mL of 6 M hydrochloric acid.
2. Place the beaker on a hot plate and bring the solution to a slow boil. Continue heating for 15 minutes.
3. Using beaker tongs, remove the beaker from the hot plate and place on a ceramic pad to cool.
4. Set up a filtering funnel and volumetric flask with a ring stand and ring as shown in Figure 2.
5. Obtain a piece of qualitative filter paper. Fold the filter paper into a cone. First fold the filter paper in half and crease (see Figure 1a).
6. Next, fold the filter paper almost in half again, leaving about a 5° angle between the folded edges.
7. Tear off the corner of the top edge, open the filter paper into a cone shape (see Figure 1b).
8. Place the cone into the filter funnel. Position the paper tight against the funnel walls and moisten the paper with about 5 mL of distilled water from a wash bottle (see Figure 1c). Note: After adding the water, use an index finger to seat the filter paper tightly against the sides of the funnel so that little, if any, air gaps are visible in the stem as the liquid flows through the filter.
   {12682_Extensions_Figure_1}
9. Set up the ring stand and iron ring and place the funnel in the ring. Let the funnel drain into a 100-mL volumetric flask (see Figure 2).
   {12682_Extensions_Figure_2}
10. Use a wash bottle to add about 10 to 15 mL of distilled water to the cooled 100-mL beaker.
11. Using a stirring rod, decant the liquid from the 100-mL beaker into the funnel. Be sure to keep the liquid level below the top of the filter paper cone (see Figure 3).
    {12682_Extensions_Figure_3}
12. When all but approximately 10 mL of the liquid has been transferred, swirl the beaker to suspend the precipitate. Transfer this to the funnel, again making sure not to fill the cone above the top of the filter paper.
13. Rinse the flask with small amounts of distilled or deionized water from the wash bottle and then transfer the washings to the filter.
14. When all the solid has been transferred to the filter paper, rinse the solid with three small portions of distilled or deionized water. Allow the funnel to drain completely.
15. Remove the volumetric flask and dilute the solution in the flask to the 100-mL mark. Cap and mix thoroughly.
16. Pipet 10 mL of this sample solution into a clean 100-mL volumetric. Dilute to the mark with distilled or deionized water.

Answers to Prelab Questions

A dietary supplement claims to contain 50 mg of iron in each pill. The pill was analyzed for iron content. The sample was first acidified, filtered and diluted to 1000 mL. A 10-mL aliquot of this solution was treated with the reducing agent hydroquinone, then o-phenanthroline was added to complex the iron(II) cations to a red color. This aliquot was then diluted to 100 mL. The iron concentration was determined by a color comparison to a standard iron concentration color chart.

Iron concentration = ___2.5 ppm___ (mg/L)

Calculate the amount of iron contained in the dietary supplement pill.

The total amount of iron in the final solution is equal to:
2.5 mg/L x 0.10 L = 0.25 mg Fe
This represents 1⁄100 of the total amount of iron in each pill (10 mL of the original 1000 mL sample volume).
100 x 0.25 mg Fe = 25 mg of iron in the pill.
The claim of 50 mg of iron is not accurate. The pill only contains 25 mg of iron.

Sample Data

Answers to Questions

1. From the data, calculate the total amount of iron in
   1. The final 100-mL sample from step 6.

      2.6 mg Fe/L x 0.10 L = 0.26 mg Fe.

   2. The final 100-mL matrix spike sample from step 8.

      4.4 mg Fe/L x 0.10 L = 0.44 mg Fe.

2. The dietary supplement pill was dissolved in 100 mL of water, then 10 mL of that 100-mL solution was diluted to 100 mL to create the sample solution.
   1. Calculate the total amount of iron in the original solution.
      {12682_Answers_Equation_4}
   2. Calculate the percent recovery for the spiked sample.
      {12682_Answers_Equation_5}
3. Each tablet contains 27 mg of iron. Calculate the percent error in the experimental iron content value.
   {12682_Answers_Equation_6}

Teacher Handouts

12682_Teacher1.pdf

Introduction

Analytical chemistry is the study of the chemical composition of natural and artificial materials. A large variety of analytical techniques and procedures, ranging from instrumental methods such as spectroscopy and chromatography to more classical processes, such as gravimetric analysis, have been developed to accomplish that task. The purpose of this experiment is to determine the amount of elemental iron contained in a dietary supplement pill.

Concepts

• Complex ion
• Oxidation–reduction
• Spectroscopy

Background

Iron is an essential mineral for normal human physiology. Most iron is found in hemoglobin, the part of the red blood cell that transports oxygen to the cells and carbon dioxide away from them. Iron deficiency can result in fatigue and decreased immunity.

When dietary supplements of iron are recommended, the supplemental iron is available in two forms—ferrous, iron(II), and ferric, iron(III), salts.

A common method of analyzing the amount of iron in a material involves the reaction of iron(II) ions with an organic compound, phenanthrolene, to give a red complex ion. The amount of iron is determined based on the intensity of the red color. To begin the analysis of iron supplement pills, the pills are first dissolved in an acidic solution. The resulting mixture is filtered and diluted to a fixed volume. The solution is diluted so that the iron concentration—and the red color—of this final solution after reaction with phenanthrolene falls into the range of the calibration standards that can be measured by color comparison. A portion (aliquot) of this solution is removed, its pH is adjusted with the addition of a weak base (sodium citrate), then hydroquinone is added to reduce iron(III) cations to iron(II).

To this a volume of o–phenathroline solution is added. o–Phenathroline reacts with the iron(II) cation to form a red complex ion. The entire mixture is diluted again to its final volume and mixed. The resulting color is compared against known color standards to determine the concentration of iron in the final solution. This value is then used to calculate the amount of dietary iron contained in the original dietary supplement pill.

An additional sample is determined alongside the original sample. In this determination, a known amount of iron(II) is added to the final solution. The will serve as a check on the validity of the iron concentration in the original sample.

Materials

Prelab Questions

1. A dietary supplement claims to contain 50 mg of iron in each pill. The pill was analyzed for iron content. The sample was acidified, filtered, and diluted to 1000 mL. A 10-mL aliquot of this solution was treated with the reducing agent hydroquinone, then phenanthroline was added to give a red color. This aliquot was then diluted to 100 mL and its color was compared using a color comparison chart to determine the iron concentration.
   {12682_PreLab_Equation_3}
   The iron concentration in this final volume was found to be 2.5 ppm (mg/L).
   
   Calculate the amount of iron contained in the dietary supplement pill. Is their claim of iron content accurate?

Safety Precautions

1,10-Phenanthroline is toxic by ingestion. Hydroquinone solution is slightly corrosive to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

The sample was prepared by dissolving one dietary pill in a solution of hydrochloric acid. This solution was filtered and diluted to 100 mL with distilled water. 10 mL of that 100-mL solution was taken and diluted to another 100 mL. This final 100-mL solution is the sample to be analyzed.

1. Using a graduated cylinder, pour a 10 mL portion of the sample solution into a clean 100-mL beaker.
2. Test the solution with the narrow range (~3.0 to 5.5) pH test paper. Use a disposable pipet to add sodium citrate dropwise, counting the number of drops, until the solution reaches a pH of 3.5. Record the number of drops.
3. Pipet 10 mL of the sample solution into a clean 100-mL volumetric. Add to the volumetric flask the same number of drops of the sodium citrate solution used in step 2.
4. Use a transfer pipet to add 2 mL of the 0.1 M hydroquinone solution to the 100-mL volumetric, followed by 3 mL of the 0.5% phenanthroline solution.
5. Dilute the flask to the 100-mL mark with distilled or deionized water and mix thoroughly.
6. Allow the solution color to develop. After 10 minutes pour a portion of the sample into a clean test tube. Compare the solution color to the iron concentration color chart to determine the iron concentration of the solution. Record this value in the Data Table. Empty, then rinse the volumetric flask with distilled or deionized water.
7. Pipet 10 mL of the sample solution into the rinsed out 100-mL volumetric, followed by 5 mL of the standard 40 ppm iron solution. Add to the volumetric flask the same number of drops of the sodium citrate solution used in step 2.
8. Use a transfer pipet to add 2 mL of the 0.1 M hydroquinone solution to the 100-mL volumetric, followed by 3 mL of the 0.5% phenanthroline solution.
9. Dilute the flask to the 100-mL mark with distilled or deionized water and mix thoroughly. This sample is referred to as a matrix spike.
10. Allow the solution color to develop. After 10 minutes pour a portion of the sample into a clean test tube. Compare the solution color to the iron concentration color chart to determine the iron concentration of the solution. Record this value in the data table.
11. Dispose of the waste solutions as directed by the teacher. 
12. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12682_Student1.pdf