Chemistry
Sensors:
Loggers:
Conductivity
Any EASYSENSE
Logging time: SnapShot
56 Conductivity and concentration
Read
Conductivity in a solution depends upon the presence of ions in the solution. Pure water (which contains
no or very few ions), for example will not conduct electricity. Add a few drops (or grains) of salt to
pure water and it will start to conduct well. If you measure conductivity with increasing additions of
ions the conductivity will increase.
In the experiment salts will be added to samples of water and the increase in conductivity recorded.
Three solutions of ions will be used; the ions will be produced according to the equations shown below;
NaCl(aq) Î Na+(aq) + Cl-(aq)
CaCl2(aq) Î Ca+2(aq) +Cl-(aq)
AlCl3(aq) Î Al+3(aq) +Cl-(aq)
Conductivity is the inverse of resistance; it measures the willingness of a material to let a current flow
through it. In most chemical studies using solutions micro Siemens is used, the base unit of the
Siemens is much too large, it represents a resistance of 1 Ohm
What you need
1.
An EASYSENSE logger
2. A Smart Q Conductivity sensor set to 1,000 μS range (see notes below)
3. Pure water (deionised or distilled)
4. Dropper pipettes
5. 1.0 mol dm-3 Sodium chloride, NaCl (2 ions formed)
6. 1.0 mol dm-3 Calcium chloride, CaCl2 (3 ions formed)
7. 1.0 mol dm-3 hydrated Aluminium chloride, AlCl3 (4 ions formed)
8. 100 ml beaker
9. Measuring cylinders
10. Magnetic stirrer and follower (optional)
Before starting work check the value of the water, for deionised water the conductivity should be
50 μS or less. If the value given is above this the sensor may need cleaning or the water has been
contaminated.
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What you need to do
1.
Connect the Conductivity sensor to an input of the logger.
2. From the EasySense software’s Home screen select SnapShot.
3. Place the sensor in the water and leave it to stabilise for a few minutes.
4. Select Start, click in the graph area to take a reading of the conductivity of the water without
any added ions.
5. Using a clean dropping pipette, take some of the first solution of ions to test. Add one drop of
the test solution to the water sample in the beaker and stir the water to spread the added
ions evenly through the water.
6. When the sensor reading appears to have stabilised, click in the graph area to record the new
conductivity reading.
7. Repeat until you have added 10 drops of the test solution (11 readings in total).
8. Use Save As to save the results. Select Overlay.
9. Discard the water sample and wash the electrode in deionised water (or the water being used
to give the water samples).
10. Test the electrode in the next water sample and check the reading is similar to the first
sample.
11. Repeat the addition of ions using another test solution.
12. Use Save As to save the results.
13. Repeat until all three solutions have been tested.
14. Use the Add Text tool to label each line with the ion solution name that produced it.
Analysis of data
To help in the analysis of the data you may want to rename the data sets with the name of the solution
that produced them. To do this, select the Sensor settings tab from Options and alter the channel
name.
Sample graph of
the data collected,
2 sets of ions +
water shown.
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You need to find the gradient of the slope and then create a ratio of the slopes to each other using
the NaCl slope as equal to gradient = 2.
1. Use the table to find the conductivity (C1) after addition of 0 drop of the ion solution.
2. Use the table to find the conductivity (C2) after addition of 10 drops of the ion solution
(reading 11).
3. Subtract C1 from C2 to find the change in conductivity.
4. Divide the change by 11; this is the gradient of the slope.
Worked example using sample data
Note the worked example used 9 drops and had 10 readings.
For NaCl
Starting conductance = 107
Final conductance = 293
Difference = 186
Gradient = 186 / 10 = 18.6 (19)
If this is the slope created by 2 ions, then the number of times 16 goes into any other slope multiplied
by 2 = the number of ions
For AlCl3
Starting conductance = 107
Final conductance = 508
Difference = 401
Gradient = 401 / 10 = 40.1
Divide slope by 19 = 2.11
Multiply by 2 to correct for ions involved = 4.22
We have a ratio of 2 : 4.22 or 1 : 2.11.
Questions
1.
2.
3.
4.
5.
6.
7.
8.
For each of the compounds used indicate the number of ions produced when they are dissolved
in water.
1.0 mol dm-3 NaCl
1.0 mol dm-3 CaCl2
1.0 mol dm-3 AlCl3
How does the slope of conductivity change with drop number relate to the number of ions
produced by each compound?
In the experiment you added 10 drops of each compound to the test water. What would the
conductivity reading have been (for each compound) if you had added 15 drops?
Why were you able to make the prediction for 15 drops? Explain what you did to find the
value?
What conductivity reading would you expect if you added 5 drops of 1.0 mol dm-3 KCl
(potassium chloride) to the same volume of water as used in the test? (Assume the water had
the same starting conductivity as you found in the experiment).
For extension work, plot the conductivity vs. drops graph for 1.0 mol dm-3 Al2(SO4)3(aq).
Write out the Ionic equation for Al2(SO4)3(aq).
How many ions would Al2(SO4)3(aq) have made when dissolved in water?
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