Beer’s Law

In this lab, you will combine aqueous solutions of iron(III)chloride and sodium thiocyanate to form an orange-red iron(III)isothiocyanate complex. When iron(III)chloride is dissolved in water, the iron center is surrounded by six water molecules. This acidic complex easily loses protons from its water molecules, which can lead to iron hydroxides precipitating from solution.

To prevent this, your solutions will include enough nitric acid to keep the iron in solution-primarily as near-colorless hexaaquairon(III)and yellow pentaaquahydroxidoiron(III)For this lab, we'll refer to the starting iron complexes collectively as iron(III)The isothiocyanate complex forms when thiocyanate exchanges with another group on iron, with nitrogen interacting with iron. This exchange is reversible, so there's an overall equilibrium between the formation and loss of the isothiocyanate complex. The thiocyanate-iron interaction gives the isothiocyanate solutions their intense color, so you'll ultimately relate the UV-Vis absorbance intensity to the concentration of the isothiocyanate complex using Beer's law.

This will let you experimentally measure the equilibrium concentration of the isothiocyanate complex from which you can estimate an equilibrium constant for the overall process. Before starting this lab, make these tables in your lab notebook. You'll use the first table when you collect data for a calibration curve relating absorbance to concentration.

The second table is for the solutions with iron(III)isothiocyanate at equilibrium. You'll measure out nitric acid and sodium thiocyanate with burettes. The volume dispensed by a burette is tracked by the difference between the starting and final volume readings.

The solutions that you will use are highly corrosive, so be careful when handling them. Before beginning, put on a lab coat, splash-proof safety glasses, and nitrile gloves. To start, label three 150-milliliter beakers as 0.5 molar nitric acid'1 molar iron(III)chloride in 0.5 molar nitric acid'and 0.5 millimolar sodium thiocyanate in 0.5 molar nitric acid'Label a 400-milliliter beaker as waste'Then, label one burette as thiocyanate'and the other as nitric acid'Clamp the burettes high enough so that the 400-milliliter waste beaker can fit under them.

Now, bring a 50-milliliter graduated cylinder and the 150-milliliter nitric acid beaker to the dispensing hood. Measure 100 milliliters of 0.5 molar nitric acid into the beaker. Make sure that the stock solution bottle is capped when you are finished.

Then, cover the beaker with a watch glass and bring it back to your fume hood. Thoroughly wash the 50-milliliter graduated cylinder, rinse it with deionized water, and dry the outside with paper towels. Next, use a 10-milliliter graduated cylinder to measure 20 milliliters of 1 molar iron(III)chloride into the corresponding labeled beaker.

Make sure that the stock bottle is capped before bringing your portion of the solution back to your hood. After that, use the 50-milliliter graduated cylinder to measure 40 milliliters of 0.5 millimolar sodium thiocyanate and pour it into the corresponding labeled beaker. Label and fill a 100-milliliter beaker with deionized water.

Then, ensure the stopcocks of both burettes are closed. Place a funnel in each burette and pour deionized water into both of them. Place the waste beaker under one burette and open the stopcock.

Once the water has drained, close the stopcock, and repeat the process with the other burette. Now, pour about 5 milliliters of 0.5 molar nitric acid into the corresponding burette and place the waste beaker under the spout. Open the stopcock to fill the tip with nitric acid and allow a few milliliters to drain before closing it.

Then, add about 40 milliliters of nitric acid to the burette. Remember that the graduations measure from top to bottom, so you should fill it to about the 10-milliliter mark. Now, place the waste beaker under the thiocyanate burette and fill the tip with sodium thiocyanate solution in the same way as the nitric acid.

Then, pour the rest of your thiocyanate solution into the burette. Gently tap the burette to dislodge any air bubbles. If there are bubbles in the burette tip, briefly open the stopcock to flush them out.

Obtain four squares of plastic paraffin film and two sheets of aluminum foil to finish setting up. You are now ready to create and analyze the blank and calibration curve solutions. You need to know the precise iron(III)isothiocyanate concentrations in your standard solutions to make an accurate calibration curve.

However, the complex is in equilibrium with iron(III)and thiocyanate, so only the isothiocyanate complex itself counts for this purpose. Thus, your standard solutions will use 0.2 molar iron(III)chloride so that iron will be in 500 to 2, 000-fold excess. Any thiocyanate that leaves one iron will find another one so quickly that there will be, effectively, no free thiocyanate in these solutions.

You can, therefore, assume that the iron(III)isothiocyanate concentration is equal to the initial sodium thiocyanate concentration of the standard solution. Now, label the 50-milliliter volumetric flask as 0.2 molar iron(III)chloride'Attach a pipette controller to a 10-milliliter volumetric pipette. Fill it with 1 molar iron(III)chloride and dispense it into the volumetric flask.

Then, add a funnel to the flask and pour about 30 milliliters of deionized water into it. Seal the flask with a square of plastic paraffin film and invert it several times to thoroughly mix the solution. Then, remove the film and discard it.

Use a disposable pipette to fill the flask to the line with deionized water and keep the pipette for later. Seal the flask and invert it several times to mix the solution. Now, turn on your handheld spectrophotometer and ensure that it is set to measure absorbance.

While the light source warms up, wrap five 50-milliliter beakers in aluminum foil to protect the light-sensitive iron(III)chloride solution during the experiment. Label the beakers 1 through 5 along with five clean disposable pipettes. Lay down paper towels as a clean surface for the glassware that you will reuse.

Then, attach a pipette controller to a 5-milliliter volumetric pipette. Fill the pipette with 0.2 molar iron(III)chloride and dispense it into beaker 1. Place beaker 1 under the burette of nitric acid.

Note the current volume in the burette, and then dispense precisely 5 milliliters of nitric acid into the beaker. Mix the solution with a glass stirring rod. Then, rinse the stirring rod with deionized water and set it aside.

Use pipette 1 to fill a cuvette about 75%full of solution 1 and cap the cuvette. Clean the transparent sides of the cuvette with a lab wipe and place it in the spectrophotometer. Acquire a background measurement, or a solvent blank, and then remove the cuvette.

Empty the cuvette into the waste beaker and rinse it three times with deionized water. The same cuvette will be used for all measurements to minimize error associated with cuvette imperfections. Next, prepare the mid-concentration solution, sample 3.

Use the 5-milliliter volumetric pipette to add 5 milliliters of 0.2 molar iron(III)chloride to beaker 3. Dispense 2 milliliters of sodium thiocyanate and 3 milliliters of nitric acid into the beaker from the burettes. Mix solution 3 with the glass stirring rod.

Then, use pipette 3 to fill the clean, dry cuvette about 75%full of solution 3. As before, cap the cuvette, clean the transparent sides, and place it in the spectrophotometer. Measure the absorbance of the solution for about 5 seconds and identify the wavelength with the maximum absorbance.

Record this wavelength in your lab notebook as the lambda max for the iron(III)isothiocyanate complex. Now, set the spectrophotometer to average readings over 10 seconds and adjust the wavelength to the lambda max. Save a full absorbance measurement of solution 3.

Record the absorbance at lambda max in your lab notebook. Follow the same process for solutions 2, 4, and 5. Make the solutions one at a time or keep the prepared solutions under aluminum foil until you are ready to use them.

Add the appropriate solution to the cuvette and measure the absorbance values. Once you have recorded all of the absorbance values at lambda max, sketch a plot of thiocyanate volume versus absorbance value, which should be linear. If you see any outliers, remake the solution and try again.

In this last part of the lab, you'll prepare four iron(III)isothiocyanate solutions with a 0.02 molar iron(III)chloride solution so that iron is in only 40 to 100-fold excess. At these concentrations, the thiocyanate that leaves an iron center will not necessarily find another iron center immediately. Thus, the isothiocyanate complex will be in equilibrium with iron(III)and thiocyanate, and its concentration will not be equal to the starting thiocyanate concentration.

Now, empty the 50-milliliter beakers into the waste beaker, remove the labels, and rinse the beakers with deionized water. Dry the five beakers with paper towels and relabel them as 6 through 10. Also, label five disposable pipettes as 6 through 10.

Next, empty the volumetric flask of 0.2 molar iron(III)chloride solution into the waste beaker and rinse it with deionized water. Relabel the rinsed flask as 0.02 molar iron(III)chloride. Now, use a 1-milliliter volumetric pipette to transfer 1 milliliter of 1 molar iron(III)chloride to the volumetric flask.

Add about 40 milliliters of deionized water to the flask. Then, seal the flask with plastic film and invert it several times to mix the solution. Then, open the flask and fill it to the line with deionized water.

Seal the flask with plastic film and mix the solution well. Now, prepare solution 6, the new solvent blank. Add 5 milliliters of 0.02 molar iron(III)chloride and 5 milliliters of nitric acid, and then mix with the stirring rod.

Pipette the solution into the cuvette and acquire the background measurement as before. Next, following the table, prepare solutions 7 through 10. Acquire their spectra and record their absorbance values at lambda max as you did before.

Remember to thoroughly rinse the stirring rod and cuvette with deionized water between each solution. When you are finished, empty the cuvette, volumetric flask, and beakers into the waste beaker and rinse them with deionized water. Collect any excess nitric acid and sodium thiocyanate in their respective beakers.

Neutralize the waste and the excess reagent solutions with baking soda and dispose of the waste and the excess iron(III)chloride solution in the appropriate waste container. Flush the neutralized nitric acid and sodium thiocyanate solutions down the drain with tap water. Then, wash your glassware and equipment by your lab's standard procedures.

Lastly, collect paper towels and other trash from your fume hood and put them in the lab trash. Now, we'll use our absorbance data to calculate the equilibrium constant for iron(III)isothiocyanate in solution. First, let's calculate the concentrations of iron(III)isothiocyanate in the standard solutions.

In these solutions, the iron concentration is so high that we can assume that the starting thiocyanate concentration is the same as the iron(III)isothiocyanate concentration. For each trial, multiply the thiocyanate stock concentration by the volume of sodium thiocyanate added and divide that by the total solution volume of 10 milliliters. Repeat for all standard solutions.

Now, create a calibration curve by plotting the absorbance at lambda max for the standard solutions with respect to their iron(III)isothiocyanate concentrations. Find the linear function that fits the data and set the y-intercept to 0, if necessary. Beer's law is expressed by this linear function, which relates absorbance to concentration.

Thus, the slope of your calibration curve is equal to the molar attenuation coefficient times the cuvette width, or pathlength, which was 1 centimeter in this lab. Now that you know the molar attenuation coefficient for iron(III)isothiocyanate, rearrange the linear equation to solve for concentration. Remember that your molar attenuation coefficient and pathlength will be the same for every trial.

For each unknown trial, fill in the absorbance and solve to get the equilibrium concentration of iron(III)isothiocyanate for that solution. Next, let's estimate the equilibrium constant for iron(III)isothiocyanate in solution. For simplicity, we'll express the constant in units of 1 over molarity.

Now, calculate the starting concentrations of iron(III)and thiocyanate for each trial. Then, determine their equilibrium concentrations in each trial by subtracting the equilibrium concentration of the isothiocyanate complex from the starting reagent concentrations for that trial. Fill in the equilibrium concentrations of the product and reactants.

Then, solve the equilibrium equation to estimate the constant for this trial. Lastly, compare the constants for the trials.