Fe3+ + SCN- 
FeSCN2+
One useful application of Beer's Law is to determine the equilibrium constant of a reaction. Reactants (iron (III) and thiocyanate) are mixed in a ratio such that one reactant is presumed to be nearly 100% converted into a colored product. The product is assumed to follow Beer's Law, and the relationship between relative absorbance and concentration is determined from the first experiment. After that, the reactant originally in excess is reduced in concentration. From the colorimetric determination of product concentration, the concentrations of reactant remaining at equilibrium is determined and the quantitative relationships among these concentrations are studied. One of these relationships, the mass action expression, is found to be nearly constant.
In an equilibrium reaction, the constant is calculated as the product of the concentrations of the products, each raised to a power equal to its coefficient in the reaction equation, divided by the similar expression formed by the reactants. For example, in this case the equation is:
Fe3+ + SCN-
The equilibrium expression is:
Kc = [FeSCN2+ ] / ( [Fe3+ ] [SCN- ] )
The concentration of the reactants is based upon dilution from stock solutions.
A large excess is added to one system, and it is assumed that all of one reacting species is shifted to the product. From the color intensity of this solution, the factor between the solution color intensity and concentration of the product is established.
Using this factor the concentration of the colored species in the other mixtures is determined. That concentration, together with the initial concentrations of the reaction, allows one to determine all of the equilibrium concentrations. From these, a numerical value of an equilibrium constant can be estimated.
Thiocyanate is poisonous if ingested. Concentrated nitric acid will stain or eat away hands and clothes and is toxic.
Do not ingest chemicals. Wash your hands before leaving class. Wear older clothing or lab aprons.
**Visual here.
Use a calibrated plastic pipet to place 4 mL of 0.3 M nitric acid into each of six labeled 13- x 100-mm test tubes (Tube 1 to Tube 6).
**Visual here.
Use a calibrated plastic pipet to place 8 mL of 0.2 M iron (III) nitrate into another test tube (Tube 0). Use the pipet to withdraw 4 mL of iron (III) solution from Tube 0 and place it in the first labeled tube of acid. Use the pipet to mix carefully the iron and acid. Then withdraw 4 mL from the first tube and add to the second tube. Repeat for all tubes. Discard the 4 mL removed from the sixth tube.
**Visual here.
Use a fresh plastic pipet to add 4 mL of 0.001 M potassium thiocyanate to each of the seven tubes.
**Visual here.
Select a small square of waxed paper. Use the thumb to cover a tube with this paper. Invert to mix tube contents. Repeat for all tubes.
**Visual here.
Place distilled water in a clean test tube. Insert the tube into the Blocktronic. Cover with a light shield. Use this solution to zero the blocktronic.
**Visual here.
++Data Collection with Apple II
Attach the Blocktronic I through the 9-pin connector to the back of an Apple® IIe computer.
**Visual here.
Insert the GENERAL LABORATORY INTERFACING software in Drive A and turn on the computer and monitor.
**Visual here.
**Visual here.
A title screen will appear. Follow the instructions at the bottom of the screen.
**Visual here.
Select the GENERAL LABORATORY INTERFACING program.
**Visual here.
Select 2, the BLOCKTRONIC option.
**Visual here.
Select option 1, Game Control Port.
**Visual here.
Indicate that you are using port GC0. Follow instructions.
Select option 2, Read Solution Blank. An instruction screen appears. Follow the instructions. The computer will "work" for a while.
**Visual here.
A value between 50 and 70 is appropriate. Turn the potentiometer dial on the Blocktronic to adjust the number to within this range.
**Visual here.
Remove the light shield. Discard the water from the Blocktronic test tube.
Select the most dilute solution. Rinse the Blocktronic test tube with a small portion of this solution. Discard the rinse. Pour the remaining solution into the tube. Replace the tube in the Blocktronic. Cover the tube with a light shield.
**Visual here.
Select option 3, Continuous Light Sensing. Wait while the computer "works."
**Visual here.
Record the relative absorbance reading from the computer screen.
**Visual here.
Repeat the measurement for each of the solutions proceeding from the least to the most colored.
**Visual here.
++ Macintosh with ULI
Connect a blocktronic (See DCExperiment 120 for Mac modifications. Click here to see the experiment.) or a colorimeter to Port 1 of the ULI. "Data Logger" may be used with a blocktronic. Load the calibration file prepared when the blocktronic was built. Select "Display Inputs" from the "Collect" menu. The value, which is displayed at the lower center as P1, should be close to 0. Recalibrate if necessary.
To recalibrate, select "Calibrate volts..." from the "Collect" menu. When the reading stabilizes, click "OK," and type 0 for the "real world reading." Remove the test tube, and block the light path with several folds of paper. When the reading stabilizes, click "OK," and type 1.5 for the "real world reading." Save the calibration file with the "File" menu item, Save Calibration.
Select "Data Rate" from the "Collect" menu. Type 0.02 points per second (50 seconds between readings) to set the data rate slow enough to allow you to rinse the test tube and insert a new solution. If you need more time, pick a smaller number.
**Visual here.
Select "Averaging..." from the "Collect" menu. Pick "None".
Click start to begin data collection for analysis.
Select the most dilute solution. Rinse the Blocktronic test tube with a small portion of this solution. Discard the rinse. Pour the remaining solution into the tube. Replace the tube in the Blocktronic. Cover the tube with a light shield. Wait until the next data point is recorded.
**Visual here.
Repeat the measurement for each of the solutions proceeding from the least to the most colored.
**Visual here.
Select "Data A Table" from the "Window" menu. Copy the data to the clipboard with command-C or the "Edit" menu. Paste the data into a spreadsheet program for analysis.
Each initial concentration, before any reaction takes place, would be given as half of the concentration before dilution. Identical concentrations are shown in the data sheet, together with the measured absorbances.
In the first case, assume that all of the thiocyanate is converted into FeSCN+ complex. (This happens to be a fairly good assumption; about 95% of the thiocyanate ion is converted into complex.)
Using the concentration of SCN- (0.00050 M), and the absorbance (0.804), determine a constant for estimating the concentration of the colored complex (0.0005 / 0.804) = 6.22 x 10-4.
The product of this constant and the absorbance gives the [FeSCN2] for the other tubes. Enter this in the computation table. Then, for both the thiocyanate and ferric ions, compute the remaining, unreacted material by the following relationships:
[Fe3+ ]eq = [Fe3+ ]init - [FeSCN2 ]eq
[SCN- ]eq = [SCN- ]init - [FeSCN2+ ]eq
Finally, for each data set, compute the numerical value of the expressions:
ratio = [FeSCN2+ ]eq / ( Fe3+ ]eq x [SCN- ]eq )
Name ___________________________ Class ________
Teacher__________________________
DoChem 133 Equilibrium Constant
Initial Concentrations Absorbance
| [SCN- ] | [Fe3+ ] |
| 0.0005 | 0.10 |
| 0.0005 | 0.025 |
| 0.0005 | 0.0125 |
| 0.0005 | 0.00625 |
| 0.0005 | 0.00313 |
| 0.0005 | 0.00156 |
1. Predict the effect of a 1% error in the reading of the absorbance of the first tube. Say the true reading should have been 0.812. Show the impact of that error on the equilibrium constant calculation.
2. Calculate the value of the following expressions for each data set:
I. [FeSCN2+ ]eq / ( [Fe3+ ]eq + [SCN- ]eq )
II. [FeSCN2+ ]eq / ( [Fe3+ ]eq )2
III. [FeSCN2+ ]eq / ( [Fe3+ ]eq x [SCN- ]eq )
IV. [Fe3+ ]eq x [SCN- ]eq x [FeSCN2+ ]eq
Calculated Ratios from Different Expressions
| [Fe3+ ] total | I | II | III | IV |
| 0.10 | ||||
| 0.05 | ||||
| 0.025 | ||||
| 0.0125 | ||||
| 0.00625 | ||||
| 0.00313 | ||||
| 0.00156 |
Name ___________________________ Class ________
Teacher__________________________
DoChem 133 Equilibrium Constant
Watch the movies, and use this sample data to answer the questions.
Initial Concentrations
| [SCN- ] | [Fe3+ ] | Absorbance |
| 0.0005 | 0.10 | 0.804 |
| 0.0005 | 0.025 | 0.770 |
| 0.0005 | 0.0125 | 0.683 |
| 0.0005 | 0.00625 | 0.511 |
| 0.0005 | 0.00313 | 0.386 |
| 0.0005 | 0.00156 | 0.258 |
1. Predict the effect of a 1% error in the reading of the absorbance of the first tube. Say the true reading should have been 0.812. Show the impact of that error on the equilibrium constant calculation.
2. Calculate the value of the following expressions for each data set:
I. [FeSCN2+ ]eq / ( [Fe3+ ]eq + [SCN- ]eq )
II. [FeSCN2+ ]eq / ( [Fe3+ ]eq )2
III. [FeSCN2+ ]eq / ( [Fe3+ ]eq x [SCN- ]eq )
IV. [Fe3+ ]eq x [SCN- ]eq x [FeSCN2+ ]eq
Calculated Ratios from Different Expressions
| [Fe3+ ] total | I | II | III | IV |
| 0.10 | ||||
| 0.05 | ||||
| 0.025 | ||||
| 0.0125 | ||||
| 0.00625 | ||||
| 0.00313 | ||||
| 0.00156 |
To determine the equilibrium constant of a reaction.
**Visual here.
(10 students working in pairs)
250 mL of 0.2 M iron (III) nitrate (Fe(NO3)3•9H2O, 20.3 g plus 5 mL concentrated HNO3 diluted with enough distilled water to make 250 mL of solution.)
200 mL of 0.0010 M potassium thiocyanate
200 mL of 0.3 M nitric acid (20 mL concentrated nitric acid diluted with 1000 mL of distilled water)
15 50-mL beaker
5 test tubes to fit a colorimeter
35 13- x 100-mm test tube
10 calibrated 4-mL (or 3-mL) plastic dropping pipet (other dilution apparatus will work equally well; this is a particularly effective, low-cost choice)
5 test tube rack
35 3 x 3 cm squares waxed paper
(overhead projector, 100-mL beaker, two 10-mL graduated cylinders with detachable plastic bases, Pasteur pipet)
++either
1 suitable computer
1 suitable SERAPHIM software floppy disk
1 Blocktronic I interface (See DMEX 120)
++or
Macintosh Computer with ULI or serial box interface from Vernier software
1 colorimeter (TPB-DIN) or 1 Blocktronic I interface (See DMEX 120)
1 "Data Logger" software
Check this experiment in advance; many things can go wrong.
This experiment is extremely well-adapted to the Blocktronic I. It is less expensive to add this as an accessory to a computer (if one is available) than to purchase a spectrophotometer.
Make a 0.001 M solution of potassium thiocyanate in two steps. Weigh 9.71 g KSCN, and use distilled water to make 1 L of solution (0.10 M). Take a 10 mL aliquot of this solution, and dilute to 1 L with distilled water (0.0010 M).
Prepare the ferric nitrate solution using the dilute nitric acid as solvent instead of water. The ferric nitrate solution should be nearly colorless.
Disposable calibrated plastic dropping pipets of 3-mL or 4-mL volume are available from some supply houses and work very well for this experiment. They are very low in cost and are reusable many times. Have students practice mixing solutions with the pipets before they begin the experiment.
An alternative to a Blocktronic I involves a Figure comparison of color seen looking through different depths of solution. Use the 10-mL graduated cylinders that come with detachable plastic bases. Support two glass cylinders in a small beaker on an overhead projector. Fill one cylinder with the less intensely colored solution to the 10-mL graduation mark. Place enough intensely colored solution in the other cylinder to obtain a color match. Use a Pasteur pipet to add or remove drops easily. Read and record the volume, x mL. The concentration of the less concentrated is given as (x/10) times that of the more colored solution.
With just a bit of practice, fitting data using a spreadsheet can be accomplished. In the case shown here, data were fitted by using one cell in which the concentration factor was set arbitrarily. The first concentration factor used was 0.0005/ 0.804, the molar concentration of thiocyanate divided by the absorbance. This factor was used to calculate all of the concentrations of FeSCN2+, which in turn was used together with the initial concentrations to determine the equilibrium concentrations. To find a best fit, we averaged the values for the equilibrium constant and added the absolute difference between each individual value and the average. The sum of the absolute differences was minimized by changing the concentration of FeSCN2+ found for the first case. This number can be 0.0005 M as a maximum, but will be less if the reaction is not 100% complete. The minimum value for the sum of the differences was found when the concentration of FeSCN2+ in the first experiment was set to 0.000481 instead of 0.0005 M.
Teacher preparation: 1 hour
Class time: 40-50 minutes
Thiocyanate is poisonous if ingested. Concentrated nitric acid will stain or eat away hands and clothes and is toxic.
Do not ingest chemicals. Have students wash their hands before leaving class. Warn students to wear older clothing or lab aprons.
All of the chemicals used in this experiment may be disposed of at the sink.
Dispose of the waxed paper with ordinary trash.
Initial Concentrations
| [SCN- ] | [Fe3+ ] | Absorbance |
| 0.0005 | 0.10 | 0.804 |
| 0.0005 | 0.025 | 0.770 |
| 0.0005 | 0.0125 | 0.683 |
| 0.0005 | 0.00625 | 0.511 |
| 0.0005 | 0.00313 | 0.386 |
| 0.0005 | 0.00156 | 0.258 |
Closure Questions:
1. Predict the effect of a 1% error in the reading of the absorbance of the first tube. Say the true reading should have been 0.812. Show the impact of that error on the equilibrium constant calculation.
2. Calculate the value of the following expressions for each data set:
I. [FeSCN2+ ]eq / ( [Fe3+ ]eq + [SCN- ]eq )
II. [FeSCN2+ ]eq / ( [Fe3+ ]eq )2
III. [FeSCN2+ ]eq / ( [Fe3+ ]eq x [SCN- ]eq )
IV. [Fe3+ ]eq x [SCN- ]eq x [FeSCN2+ ]eq
Answers to Closure Questions:
1. If the true value of the absorbance were 0.812, the calculated values of the ratio would be, for the 2nd through 7th sets, 370, 215, 272, 286, 313, and 332. These compare with 457, 230, 284, 294, 320, and 338 respectively. A small change in the measured absorbance causes a fairly large change in the calculated ratios, especially where the amount of iron used is large. The first value is decreased by 19%, and the averages differ by 7%. This large discrepancy results because the computation depends upon a difference between two numbers.
2. A table of values for the expressions I, II, III, and IV, using data for the 1st through 7th experiments based upon best fit for ratio III is:
| I | II | III | IV |
| 4.8E-03 | 0.05 | 254 | 9.10E-10 |
| 9.3E-03 | 0.19 | 236 | 8.98E-10 |
| 1.7E-02 | 0.68 | 182 | 9.18E-10 |
| 3.0E-02 | 2.53 | 241 | 5.75E-10 |
| 5.0E-02 | 8.65 | 265 | 3.53E-10 |
| 7.3E-02 | 27.48 | 296 | 1.80E-10 |
| 8.8E-02 | 78.12 | 318 | 7.50E-11 |
Note that expression I varies over an 18-fold range, expression II over a 1500-fold range, and expression IV over a 12-fold range. These ratios are not constant. Ratio III varies over a 1.75 fold range, essentially constant. You might try to see if you can find other ratios that give a "constant" for these data.
The data analysis associated with this experiment may be done with the aid of a spreadsheet computer applications program. See Lesson 134 for suggestions. Closure Question 2 is also well-suited to computer analysis using a spreadsheet.
See Figure V133V1
The data above are analyzed assuming that all of the SCN- reacts to form FeSCN2+ in the 0.10 M Fe3+ mixture. The absorbance is then used to calculate the [FeSCN2+]eq for the other reaction mixtures. The average ratio found for the mass action expression is 301. There is a two-fold variation between the lowest and highest values.
Using the spreadsheet, it is possible to try different extents of reaction. When the equilibrium concentration of FeSC2+ in the 0.10 M Fe3+ data set is assumed to be 0.000481, the following results are obtained. In this case, the data for all seven tubes may be included.
See Figure V133V2
When one "fits" the data, the ratio has an average of 256 and a 1.75-fold range. This seems to be reasonable. It is the same as saying that only 96.2% instead of 100% of the thiocyanate is tied up in the complex.
For either of these cases (raw or "fitted" data) the other expressions that were tried had a much larger variation.
equilibrium constant
absorbance
colorimeter
spreadsheet
data fitting