A situation in Chemistry where monitoring and measuring devices are used:
- In measuring the rate of a chemical reaction.
The reaction between Sodium Thiosulphate and hydrochloric acid can be represented in the following equation:
Sodium thiosulphate + hydrochloric acid --- sulphur dioxide + water + sulphur
Na S O + 2HCl --- NaCl + SO + H O + S
The rate at which sulphur is produced cam be used as a measure of the rate of the chemical reaction. The products of the reaction are soluble or gaseous so only the sulphur is insoluble. As the sulphur is produced it makes the solution cloudy. The usual way in which the rate of reaction is measured is by noting the time taken for enough sulphur to be produced to hide a dark cross on a piece of paper under the reaction container. This is taken as a measure of the speed of the reaction under the given conditions.
Limitations of this Method:
- If different people have to read the results, the accuracy of the results will vary as each person has a different level of eyesight.
- It is hard to stand exactly the same distance away from the container each time you look to see when the cross has disappeared.
- You cannot keep the temperature of the solution constant.
Alternative to the Method:
The availability of sensors has meant that it is possible to get improved results. Instead of measuring the time for the cross to hidden, it is possible to measure the light intensity passing through the mixture over the time of the reaction by using a light sensor.
Diagram to show how the equipment was set up:
This is a diagram to show how the equipment needed to measure the light intensity was set up:
Photograph to show the equipment being used:
This is a photograph to show how the equipment needed to measure the light intensity was set up. I have identified each piece so that it can be matched up with the diagram.
Method.
- A clean test tube was put into the light chamber and the chamber was sealed so that it was light proof.
- Using a syringe, 2ml of sodium thiosulphate was put into the test tube.
- Using a different syringe, 2ml of hydrochloric acid was added to the sodium thiosulphate, the light sensor put in place and the light intensity was logged for about 5 minutes.
- The results were output on the screen as a graph of light intensity (%) against time (min).
- The graph was printed out as a record of the experiment.
Printout of graph (from INSIGHT2) produced:
The graph shows clearly the trends that would be expected.
- At the start of the reaction (point A on the graph), the amount of light that passes through the reaction mixture is at it’s maximum.
- As the reaction continues, the amount of sulphur produced starts to increase and this makes the mixture cloudier and the amount of light that gets through to the sensor begins to get less (between points B and C).
- As the reaction continues it starts to slow down as the reactants get used up and no more sulphur is produced. At point D the reaction is effectively over and the amount of light reaching the sensor stays at a constant value.
The graph produced by the INSIGHT2 software is a bit limited:
- It is difficult to calculate values since it does not produce gridlines of sufficient accuracy
- The Datalogger sometimes produces ‘odd’ results (like dropping to 0 output for no reason)
- It is possible to produce trend lines and the equation of the trend line can be calculated by the software
To get over the problems above, I saved the data as a CSV file that I then exported to a spreadsheet application (EXCEL).
Printout of the graph (from EXCEL) produced:
