Chemical Reaction End Point Timer. - page 2
Keywords: Physics Chemistry Practical Build device end point timer
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 2 of 9 pages: 1 2 3 4 5 6 7 8 9
to it moves up or down a resistive track varying the amount of output voltage and so changing the volume. If the relationship is linear, e.g. for the volume control and for my potential difference circuit in this experiment, then equal changes in the amount of resistance give equal changes in output. If the relationship is not linear then you need a calibration curve to find the input from the output.
In my experiment, the change in the in the resistance of the LDR is caused by the change in the opacity of the solution in the test tube changing the intensity of the light falling on the LDR.
A LDRs resistance decreases with increasing light intensity. A LDR is made of a high-resistance semiconductor (e.g. cadium sulphide). When light of a high enough frequency falls on it, photons are absorbed by the semiconductor and give the bound electrons enough energy to conduct electricity, so lowering resistance.
The potential difference in a circuit is shared in the same ratio as the resistance, so when the resistance of the LDR changes the voltage across the resistor changes too, and so, by measuring the change in the output voltage of the resistor I can tell how far along the reaction is – I have my sensor!
Setting up & calibrating sensor
To start off I conducted a preliminary experiment; combining 23 ml of hydrochloric acid & 25 ml of sodium thiosulphate & judging by eye when they had finished reacting (become opaque) to gain an approximate idea of how long the reaction takes for when I test my sensor.
End point (judged by eye) 36.73 seconds.
Next I need to pick my apparatus, in particular the LDRs and resistor are important as I need the right sensitivities to maximise the accuracy of my results.
Resistance of LDRs in ambient light – same conditions as experiment (Ω approx.)
LDR A 1400
LDR B 1300
LDR C 1400
Resistance of LDRs when lit by LED –
position for experiment (Ω approx.)
LDR A 1000
LDR B 1150
LDR C 1250
Difference in resistance of LDRs between when lit by LED & when not (Ω approx.)
LDR A 400
LDR B 150
LDR C 150
The LDR that shows the biggest change in the resistance is the most sensitive and the best one for my experiment (ie. the most sensitive has a larger change in resistance for the same change in light level). So I will be using LDR A.
Readings are approximate because I also discovered that the resistance of the LDR has a
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