Chemical Analysis Debriefing

This problem has been designed to introduce you to the actual decisions chemists must make in the laboratory when carrying out a chemical analysis. You will be asked to make decisions about various reaction conditions, such as what reagents to use and how much.

Iron plays an important role in oxygen transport and storage in your blood cells. A person whose diet is iron-deficient will become anemic, a condition causing fatigue and decreased resistance to infection. Iron deficiency has been estimated to be as high as 20% in the United States. Iron-fortified foods (such as breads and cereals) and dietary supplements containing iron are commonly used to offset a diet deficient in iron. However, excess iron can be toxic so caution must be taken when supplementing your diet.

Below is the ingredient label from a dietary supplement. As part of a laboratory assignment, you have been asked to determine how much iron is contained in each tablet and compare your value to the manufacturer's reported value.

Move mouse over ingredients to get more information. Click on ingredients to get chemical reactivity data.

You are also given the following information regarding iron analysis:

Click on steps to see each step in the analysis of Iron.

Your instructor suggests first grinding the tablets to a fine powder to increase their reactivity. You grind up 100 tablets. The mass of the resulting powder is 101.7 g.

When you are designing a chemical analysis there are four questions you should ask yourself before beginning. In answering these questions, your experimental design will improve, making the analysis more accurate.

Question 1: Will the added reagents react with anything else present?

In the iron analysis, a precipitate (Fe₂O₃) is formed by creating a basic solution. If any of the other ingredients form precipitates in basic solutions, the mass of the resulting precipitate not be due only to the iron present. This would make the analysis inaccurate.

Click on the metal ions that will form precipitates when in a basic solution.

Fe2+	Ca2+	Cu2+	Zn2+

Since more than one metal will precipitate when base is added, a possible solution is to add a reagent that will partially dissovle the tablet. For example, if adding acetone (a common solvent) dissolves only the calcium sulfate and zinc oxide, the resulting mixture could be filtered. The remaining solid would contain the binder, ferrous sulfate and cupric oxide while the liquid would contain the calcium sulfate and zinc oxide. This is a simple method to separate components of mixtures.

The following reagents are available to you at this point:

Which reagent would you like to add to the powdered tablets?

Question 2: Does the reaction go to completion?

Suppose the reaction that produces the precipiate we are to measure usually has an acual yield of 65-75%. If this is the case, the measured precipitate will only be representative of 65-75% of the original iron present. While this can give us a rough estimate of how much iron was originally present in the tablet, we usually want more precise results. Thus the reactions should have actual yields of close to 100% (or go to completion).

Fortunately, the oxidation of Fe²⁺ is a very favorable reaction as is the formation of Fe(OH)₃. It is very difficult to have a solution of Fe²⁺ without this series of reactions happening. In fact, this is a common problem for swimming pool owners. If there is Fe²⁺ in the water supply (which is very common), it is easily oxidized to Fe³⁺ which then will form a fine brick-red precipitate in the pool (not very desirable!).

Question 3: Will the measured product be pure and easy to isolate?

If the resulting precipitate contains impurities, it will have a larger mass, resulting in an inaccurate analysis.

If the Fe₂O₃ is formed quickly (by quick increase of pH), the precipitate will form too quickly, becoming clumpy and including impurities. If on the other hand, the pH of the solution is increased very slowly, the precipitate will form slowly, resulting in a finer, more pure precipiate.

The following bases are available to you:

Which base will you use?

Question 4: How much reagents should be added?

It is essential that the nitric acid and hydroxide are added in excess; if there is more Fe²⁺ in solution than nitric acid added, not all of the Fe²⁺ will be converted to Fe³⁺ and the resulting Fe₂O₃ produced will not be representative of all the Fe²⁺ present.

The bottle reports that each tablet contains 27 mg of iron.

How much nitric acid (available as a 2.5 M solution) will you add?