Experimental Procedure

Estimated time to complete the experiment: 2 h

Materials

Reagents

1 (or more, according

0.02, 0.04, 0.06,

to the instructor) 3-mL

0.08 and 0.1M

spectrophotometer cuvettes

CuS04

1 spectrophotometer

1 Zn pellet

This experiment comprises two steps. 1) The cementation of Cu2+ (E®u2+/Cu = 0.34 V) by Zn (£«n2+/Zn = —0.76 V) is accomplished in a small-volume (about 3 mL) semimicro spectrophotometer cuvette (path length = 1 cm), and the concentration c o w

Figure 1. Absorbance spectrum of CUSO4 in solution. (Adapted from Ibanez, 2007).

of Cu2+ is monitored in situ at a fixed, predetermined wavelength. 2) A calibration plot is obtained with known solutions; then, absorbance versus time data points are acquired and transformed into a concentration versus time curve from which removal kinetic data can be analyzed.

a) Calibration curve

Prepare a series of at least five different concentrations of CUSO4 solutions (for example, 0.02, 0.04,0.06,0.08, and 0.1 M). Trace the absorption vs wavelength curve with a scanning spectrophotometer from 400 to 900 nm for each solution. The peak in the vicinity of 800-810 nm is used for the quantification of Cu2+ in this experiment. See Figure 1. A fixed-wave spectrophotometer (or colorimeter)

can also be used at 810 nm. (Although more precise Cu(II) determinations exist in the literature, this strategy dramatically simplifies the experimental procedure). Then, obtain a calibration curve by plotting the absorbance vs concentration points for each concentration.

b) Cementation of Cu2+

Find a Zn pellet of appropriate dimensions such that it cannot fall to the bottom of the spectrophotometer cuvette. Rinse the pellet with acetone or ethanol to remove any grease or other foreign substances from its surface, allow it to dry, and place it in the cuvette as shown in Figure 2.

Next, with a Pasteur or a Beral pipet fill the cuvette with 0.1 M CUSO4 solution as to completely cover the Zn pellet. Insert the cuvette into the spectrophotometer and take an absorbance vs time datum every 5 min. Convert each absorbance point to concentration by using your calibration curve. Allow the experiment to proceed until one half or more of the initial amount of Cu2+ is removed. Plot the results on a concentration vs time curve.

At the end of the experiment, separate the Cu-covered Zn pellet and place it in a labeled container. The instructor may choose to recycle it for future experiments. Collect all the solutions containing Cu(II). The instructor may choose to recover the Cu from these solutions (e.g., by electrolysis).

Figure 1. Absorbance spectrum of CUSO4 in solution. (Adapted from Ibanez, 2007).

200 400 600 800 1000 Wavelength, nm

200 400 600 800 1000 Wavelength, nm

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