The acidification of thiosulfate solutions leads to the formation of colloidal sulfur. This sulfur scatters light in the Blocktronic colorimeter, and permits a quantitative study of the reaction. A hands-on "microscale" chemical version of this experiment is included.
This experiment involves the reaction of thiosulfate ion to produce sulfite ion and sulfur. The reaction is catalyzed by acid:
S2O32- --> SO32- + S
The Blocktronic I may be used to study the rate of this reaction. In this case, however, absorbance of the light is not measured. The solid sulfur produced in the reaction scatters light. This phenomenon does not follow Beer's law, and the quantitative analysis of the data is not straightforward.
Hydrochloric acid is corrosive to skin; it may cause blindness.
Handle the dilute hydrochloric acid with caution. Wear Eye protection. Employ cautions to protect computers when using them in the laboratory.
**Visual here.
The kinetics may be studied with microscale visual observations or with a computer interfacing procedure.
++Microscale:
Prepare two 12-well strips as follows:
Strip A Strip B
| drops 0.15 M Na2S2O3 | drops 1 M HCl | drops water | |
| Well 1 | 10 | 2 | 0 |
| Well 2 | 9 | 2 | 1 |
| Well 3 | 8 | 2 | 2 |
| Well 4 | 7 | 2 | 3 |
| Well 5 | 6 | 2 | 4 |
| Well 6 | 5 | 2 | 5 |
| Well 7 | 4 | 2 | 6 |
Add the components dropwise using thin-stem storage pipets.
**Visual here.
When one strip is inverted and stacked on the second strip, capillary action keeps the liquid in the upper wells.
**Visual here.
Place an X-mark on a piece of paper. To mix the chemicals, hold the stacked strips in an elevated position and quickly flick them in a downward direction. Note the time to the nearest second.
**Visual here.
Place the well with the most concentrated mixture over the X-mark. Note the time at which the X-mark disappears.
**Visual here.
Slide the well containing the next most concentrated mixture over the mark, and note the time when it is no longer visible. Repeat for all wells.
**Visual here.
++Computer Interfacing:
++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.
Place 5 mL of the sodium thiosulfate solution in a 13- x 100-mm test tube. Cover with a light shield.
**Visual here.
Use this solution to zero the blocktronic.
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.
Select option 4, SAMPLE LIGHT LEVELS.
**Visual here.
Enter a time for sampling. Try 1 second for the first experiment. Select choice 2, Graphic Display.
**Visual here.
Prepare the reaction mixture. Place 1 mL of 1 M HCl in a Pasteur pipet. Remove the light shield. Use the pipet to "inject" the acid into the tube of sodium thiosulfate solution in the Blocktronic. Replace the light shield.
**Visual here.
Immediately, select choice 3, Accept Values-Start Collection. To save the data, press "s. " Pressing any other key will return you to the main menu. If you press "s," there are two options. Pressing "c" will give a list of the files already stored on the disk. Press "c," or type in a file name.
**Visual here.
This completes the experiment. More experiments can be performed. Follow the screen instructions.
++Setting up Macintosh with ULI:
Connect a blocktronic (See DCExperiment 120 for Mac modifications. Click here to see experiment) or a colorimeter to Port 1 of the ULI. "Data Logger" may be used with a blocktronic.
Place 5 mL of the sodium thiosulfate solution in a 13- x 100-mm test tube. Cover with a light shield. Use this solution to zero the 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 2 points per second to set the data rate.
**Visual here.
Select "Averaging..." from the "Collect" menu. Pick "3".
Prepare the reaction mixture. Place 1 mL of 1 M HCl in a Pasteur pipet. Remove the light shield. Use the pipet to "inject" the acid into the tube of sodium thiosulfate solution in the Blocktronic. Replace the light shield.
**Visual here.
Save the data for analysis with "Save Experiment" from the "File" menu. Select "Analyze Data A" from the "Data" menu. Click the "Tangent" box in the lower right. Move the cursor along the curve to see the rate at any point. Record the initial rate of the reaction.
(Microscale)
The rate of reaction can be represented by the following equation:
Rate = k[S2O32-]a[H+]b
The concentration of H+ is held constant in the procedure; all wells in strip B were filled with the same amount of acid. We may write the rate:
Rate = k'[S2O32-]a
Where [H+]b has been absorbed into the pseudo-rate constant, k'.
In each reaction well we wait until the character is no longer visible. Presumably this requires that the same amount of sulfur be produced. The amounts of reactant used up in causing this to take place is small, so the reactant concentrations remain essentially constant throughout the time of reaction.
The expression for rate is:
(&Mac198;[S2O32-] / &Mac198;t)
But the amount of thiosulfate used up at the time of the endpoint is a constant because the amount (moles) of sulfur is constant for each well. Therefore, the rate is related to a constant divided by the time it takes to reach the end point. Plotting (1 / &Mac198;t) is the same as plotting a constant times the reaction rate.
But the rate is equal to k'[S2O32-]a. Therefore, a plot of the rate versus [S2O32-] gives an indication of the exponent, a. If the slope is zero, a = 0. If there is a straight line through the origin, a = 1. If there is a parabola,
a = 2.
Plot a graph of the results. Use the x-axis for number of drops and the y-axis for the reciprocal of the reaction time. Draw the best fitting curve to this plot.
Based upon the graphs, determine the order of the reaction with respect to S2O32-.
Name ___________________________ Class ________
Teacher__________________________
DoChem 130 Kinetic Study of Thiosulfate in Acid
| drops Na2S2O3 | Time (sec) | |
| Well 1 | ______ | ______ |
| Well 2 | ______ | ______ |
| Well 3 | ______ | ______ |
| Well 4 | ______ | ______ |
| Well 5 | ______ | ______ |
| Well 6 | ______ | ______ |
| Well 7 | ______ | ______ |
Name ___________________________ Class ________
Teacher__________________________
DoChem 130 Kinetic Study of Thiosulfate in Acid
Watch the movie.
| drops Na2S2O3 | Time (sec) | |
| Well 1 | 10 | 30 |
| Well 2 | 9 | 32 |
| Well 3 | 8 | 37 |
| Well 4 | 7 | 42 |
| Well 5 | 6 | 50 |
| Well 6 | 5 | 60 |
| Well 7 | 4 | 78 |
Plot a graph of the results. Use the x-axis for number of drops and the y-axis for the reciprocal of the reaction time. Draw the best fitting curve to this plot.
Based upon the graphs, determine the order of the reaction with respect to S2O32-.
To study the rate of reaction of thiosulfate ion in an acid solution.
**Visual here.
++Microscale
10 12-well strip
5 thin stem transfer pipet filled with 0.15 M Na2S2O3
5 thin stem transfer pipet filled with 1 M HCl
5 thin stem transfer pipet filled with distilled water
5 piece of paper, pencil
5 cotton swab
++Blocktronic
1 13-mm x 100-mm test tube
1 25-mL graduated cylinder
1 50-mL beaker
1 dropping pipet
1 light shield
10 mL 0.15 M Na2S2O3 (3.72 g Na2S2O3•5H2O in 100 mL of solution)
5 mL 1 M hydrochloric acid (8.3 mL concentrated HCl in 100 mL of solution)
distilled water
++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 or colorimeter software
Although you may have students build the Blocktronic during class time, preassembly of this part is recommended.
Teacher preparation: 25 minutes
Class time: 40-50 minutes
All of the solutions used may be safely discarded at the sink. The test tubes and some 12-well strips may need to be brushed to remove sulfur deposits.
++Microscale data
| drops Na2S2O3 | Time (sec) | |
| Well 1 | 10 | 30 |
| Well 2 | 9 | 32 |
| Well 3 | 8 | 37 |
| Well 4 | 7 | 42 |
| Well 5 | 6 | 50 |
| Well 6 | 5 | 60 |
| Well 7 | 4 | 78 |
See Figure V130V2
Since the rate (1/time) is related to the concentration of S2O32- raised to the first power, the exponent of S2O32- in the reaction is one.
++Blocktronic data
See Figure V130V1
The original module was written for Project SERAPHIM by Patricia Barker and Kenneth Hartman.
Project SERAPHIM is supported by the National Science Foundation.
For additional information write:
Project SERAPHIM
Department of Chemistry
University of Wisconsin-Madison
1101 University Avenue Madison, WI 53706
kinetics
light scattering
order
reaction order
colorimeter
Blocktronic
clock reaction