0.3 Determine the value of an equilibrium constant by complex ion  (Page 2/2)

A = $\epsilon$ bc Equation 4

In this equation, the measured absorbance (A) is related to the molar absorptivity constant ( $\epsilon$ ), the path length (b), and the molar concentration (c) of the absorbing species. The concentration is directly proportional to absorbance.

Thiocyanate ( ${\text{SCN}}^{-}$ ) is an interesting ion and is widely used in a variety of industrial processes such as the manufacturing of thiourea, photofinishing, metal separation, and electroplating. It is also found in gold mining wastewater as a result of treating the cyanide rich ore with sulfur dioxide in order to produce the less toxic thiocyanate( ${\text{SCN}}^{-}$ ) ion. Iron, as you will see later on in the semester, has the unique ability to inexpensively clean up and produce drinking quality water in Third World countries.

 procedure

Equipment

• 5 test tubes
• Burette (0.1 ml graduations) filled with 0.0200M $\text{Fe}({\text{NO}}_3{)}_3$ in 0.5 M  ${\text{HNO}}_3$
• Neighboring partners’ burette filled with $6\text{.}\text{00}\times {\text{10}}^{-4}$ M KSCN
• stirring rod
• small labels or china clay pencil
• 5 cuvettes for the spectrophotometers
•  250 ml bottle acetone for rinsing

Hazard: as always wear safety glasses while performing this experiment

Contamination Notes: If your flask is wet before you prepare your standard/sample solutions ensure that the flask is wet with dilutant (in this case it is 0.5 M HNO ₃ ) .

Calibration Of MicroLab/Spectrophotometer

N.B. Do not use test tubes from your drawer!

Find and open the MicroLab program.

1. Find and open the MicroLab program. This brings up a box that will enable you to select from a list of experiments. Select colorimeter. Check that the accompanying box has power and is turned on, and that it is connected to the laptop via the USB plug.
2. In the tab labeled “New” you will find the icon for the “Spectrophotometer”, please double click this. Make sure that you click on the absorbance tab.
3. This brings up the program, at which point you should take a reading of a blank, this is done by filling a vial with 15 mL of 0.5 M  ${\text{HNO}}_3$ (not deionized water in this case) and placing in the appropriate slot. Covering with the film case to exclude ambient light from entering the system. When the blank sample is in place, click the button “Read Blank”. This will generate a series of data points.
4. A solution of 0.0200M $\text{Fe}({\text{NO}}_3{)}_3$ in 0.5 M  ${\text{HNO}}_3$ has been prepared for you.
5. Dilute 1.5, 3.0, 4.5 and 6.0 mL portions of $6\text{.}\text{00}\times {\text{10}}^{-4}$ M KSCN to 20 mL with the 0.0200M $\text{Fe}({\text{NO}}_3{)}_3$ in 0.5 M  ${\text{HNO}}_3$ . For this you can use some of the smaller beakers in your lab drawer. These need to be rinsed and dried thoroughly before each use. Use two clean, dry burettes to dispense the two solutions.
6. This will give you 4 solutions which can be assumed to be $6\text{.}0\times {\text{10}}^{-5}$ M, $1\text{.}2\times {\text{10}}^{-4}$ M, $1\text{.}8\times {\text{10}}^{-4}$ M and $2\text{.}4\times {\text{10}}^{-4}$ M in ${\text{FeSCN}}^{2+}$
7. Take the most concentrated solution and collect a visible spectrum from 400 to 800 nm to determine ${\lambda }_{\text{max}}$ _.
8. Measure the absorbance of all the solutions at ${\lambda }_{\text{max}}$ , using 0.5 M  ${\text{HNO}}_3$ as the blank/reference solution.
9. Measure the absorbance of these solutions again at 430nm, under the same conditions.
10. Plot absorbance vs. [ ${\text{FeSCN}}^{2+}$ ] for both wavelengths using Excel and add a linear trendline passing through the origin (under Options, set intercept equal to 0). Using Beer’s law and the equation of the trendline, find the molar absortivity.

Experimental determination of kc

The mixtures will be prepared by mixing solutions containing known concentrations of iron (III) nitrate, $\text{Fe}({\text{NO}}_3{)}_3$ , and potassium thiocyanate, KSCN. The color of the ${\text{FeSCN}}^{2+}$ ion formed will allow us to determine its equilibrium concentration. Knowing the initial composition of a mixture and the equilibrium concentration of ${\text{FeSCN}}^{2+}$ , we can calculate the equlibrium concentrations of the rest of the pertinent species and then determine $K_c$ .

1. Label five regular test tubes 1 to 5, with labels or by noting their positions in the test tube rack.
2. Dilute 0.02 M $\text{Fe}({\text{NO}}_3{)}_3$ in 0.5 M ${\text{HNO}}_3$ by a factor of 10 using the volumetric flask that you have in your drawer. Remember to do the dilution with 0.5 M ${\text{HNO}}_3$ not water.
3. Pour about 25 mL 0.002 M $\text{Fe}({\text{NO}}_3{)}_3$ in 0.5 M ${\text{HNO}}_3$ into a clean, dry burette.
4. Dispense 5.00 mL of that solution into each test tube.
5. Then pour about 20 mL $6\text{.}\text{00}\times {\text{10}}^{-4}$ M KSCN into another clean, dry burette.
6. Dispense 1,2,3,4, and 5 mL from the KSCN burette into each of the corresponding test tubes labeled 1 to 5.
7. Using a small graduated cylinder, dispense the proper number of milliliters of 0.5 M ${\text{HNO}}_3$ into each test tube to bring the total volume in each tube to 10.00 mL.
8. The volumes of reagents to be added to each tube are summarized in the table.

1. Mix each solution thoroughly with a glass stirring rod. Be sure to dry the stirring rod after mixing each solution to prevent cross-contamination.
2. Place the mixture in tube 1 in a spectrophotometer cell and measure the absorbance of the solution at ${\lambda }_{\text{max}}$ .
3. Determine the concentration of ${\text{FeSCN}}^{2+}$ from your calibration curve. Record the value on your Report form. Repeat the measurement using the mixtures in each of the other test tubes.