Applied Technology: Measurement Combined with Control

Applied Technology: Measurement combined with Control Driving question: What is the iron content of steel wool? Student Level: High school level

Duration: 1 to 2 lesson periods

Recommended Setting: Student Investigation

Learning objectives Setting up and performing an automatic titration Interpreting a titration curve and determining the equivalence point Performing chemical calculations using acid-base reactions

Didactical approach In this activity students will carry out a titration to determine the iron content in steel wool. The equivalence point can be determined from the titration curve. Compared to a “regular” titration, the use an automated step motor burette adds an extra dimension to the experiment, allowing for addition of the base using constant speed. In that regard, this experiment focusses on setting up the experiment and analyzing the results. The actual execution of the experiment is done automatically once the student presses Start. By using a solution of potassium permanganate as the oxidizing agent, the equivalence point can be determined both digitally and optically. The moment the purple colour of the solution no longer disappears (the steel wool solution turns purple/pink) the equivalence point is reached. It is very interesting to compare the observation to the equivalence point measured with the ORP-sensor. Do the students’ observation match the measured equivalence point? Concepts learnt in this activity: – Redox reactions – Oxidizing/reducing agents – Equivalence point – Titration as method for analysis – Chemical calculations Materials In this experiment you will use the following materials:

 Interface with output for actuators;

Iron content in steel wool – Teacher notes  ORP-sensor;

 Step motor burette;

 Erlenmeyer flask (100 mL);

 Beaker (50 of 100 mL);

 Volumetric flask (100 mL);

 Volumetric pipette (10.00 mL);

 Propipetter bulb for manual pipetting;

 Hot plate or Bunsen burner with tripod and gauze;

 Scale;

 Magnetic stirrer and stir bar;

 Stand and clamps. In this experiment you will use the following chemicals:

 Distilled water;

 0.200 M potassium permanganate solution;

 2 M sulphuric acid solution;

 Steel wool.

Procedure You might choose to build the setup beforehand. Connect the ORP-sensor to an interface. Connect the step motor. 1. Then again, doing this themselves is important for the development of students’ practical skills. Especially older students should be capable of handling this themselves. It would take them about two lesson periods to complete it. Demonstrate how to use and clean the ORP-sensor. Proper handling and care extend the lifespan of the electrode. Demonstrate the functionality of the step motor burette and how it should be connected. Discuss the appearance of air bubbles. In a sense, the step motor burette should be discussed similarly to a “normal” burette Have the students perform the experiment. If they get a “wrong” answer for the iron content, have them discuss what might have gone wrong.

CMA Learning and Teaching Resources Discuss the results.

Questions and assignments What can you observe during the heating of the mixture? Watch closely! Write down the reaction equation for the reaction that takes place. Describe the mixture after all the steel wool has dissolved. What observations can you make during the titration? Write down the reaction equation for the reaction that takes place during the titration. Why did you use an excess of sulphuric acid to dissolve the steel wool? How can you determine the equivalence point of the titration? Name two ways. Bepaal nauwkeurig het gemiddelde aantal mL kaliumpermanganaatoplossing dat je hebt moeten toevoegen om het equivalentiepunt te bereiken. Accurately determine the average amount of potassium permanganate solution that was needed to reach the equivalence point. Calculate the amount of mole of permanganate-ions that was added. Calculate the iron content (in mass percentage) in steel wool What kind of mass percentage would you expect to find? Does this match the value you found experimentally? Explain any differences between the expected value and your experimental value.

Data Analysis An example of a titration curve can be found in figure 1.

Figure 1: Example titration curve for this redox titration. The cmr7-file contains two results of two measurements. Noise was filtered out using “smoothing” in Coach. The titration was carried out according to the description in the student sheet. The equivalence point is clearly visible. Right after the equivalence point the graph remains horizontal. This is a result of the limited range of the sensor. The sensor can measure the

Iron content in steel wool – Teacher notes potential of the solution between -450 and +1100 mV in reference to a Ag/AgCl electrode. - The standard electrode potentiaal for MnO4 in acidic conditions is so high (+1.51 V, Wikipedia) that it falls outside of the range of the sensor. This might be prevented by choosing a different, less potent oxidizing agent. - Given the limitations, however, MnO4 under acidic conditions is still the preferable choice. The oxidizing agent needs to be strong enough to react with Fe 2+ (+0.77 V), yet also easily workable and produce relatively safe substances. Especially the latter is a good reason to - choose MnO4 , since other oxidizing agents produce toxic vapors or otherwise unwanted products. The diagram in figure 1 can be derived to produce the graph in figure 2.

Figure 2: Derivative of the example titration curve in figure 1 The derivative shows two peaks in the equivalence area. See also figure 3. This might have been caused by the smoothing operation. On the other hand: the other result (see .cmr7 file) shows a similar effect. For this example, the middle between both peaks was taken as the equivalence point. In this case: 9.39 mL

Figure 3 (right) : Derivative of the example titration curve in figure 1 (zoom) The equivalence point of the other measurement is at 9,72 mL. The average of these values is 9.56 mL. This value was used in the calculations: 1. During heating with sulphuric acid, the following reaction occurs: + 2+ Fe (s) + 2 H (aq) → Fe (aq) + H2 (g) 2. During the titration, the following reactions occurs: - + 2+ 2+ 3+ MnO4 (aq) + 8 H (aq) + 5 Fe (aq) → Mn (aq) + 4 H2O (l) + 5 Fe (aq) - 3. During the experiment 9,56 * 0,200 = 1,912 mmol MnO4 was added 4. This reacted with 5 * 1,912 = 9,56 mmol Fe2+ 5. This was formed from 9,56 mmol Fe

CMA Learning and Teaching Resources 6. This weighs 9,56 * 55,85 = 533,926 mg 7. The mass percentage of iron in steel wool is 533,926 / 500 = 107 %

Clearly, 107 mass-% is unrealistic. Your students may perform the experiment better or more accurately. Nonetheless, a critical review of the results is always important. In theory, the amount of iron in steel wool should be well above 95 mass percent. Some differences between different types/brands of steel wool may occur. Resources Coach activity: Iron content in steel wool.cma7 Coach result: Iron content in steel wool.cmr7 Coach activity: Volume calibration for step motor burette.cma7

Copyright Authors: CMA Team

Iron content in steel wool – Teacher notes