Chemistry
Sensors:
Loggers:
Drop / Bubble counter, ±1 V Voltage
Any EASYSENSE
Logging time: EasyLog
07 Potentiometric study of a mixture of halide
ions
Read
In this investigation two half cells, one of a silver electrode in a solution of silver ions and a second of
a silver electrode in a solution containing a solution of mixed halide ions are electrically connected by a
salt bridge. The potential of the cell created should be close to the theoretical value +0.7 V.
The potential across the cell will be monitored as a solution of silver ions is titrated into the solution
of mixed halides. The silver ions will react with the halide ions and form the insoluble salts of the
halides, as each halide is converted from its ion to its silver salt the potential across the cell will
change. The potential across the cell will be reducing as the silver concentration in the second half-cell
rises. When both half-cells contain the same concentration of silver, the potential across the cells will
be zero. As each silver halide has a different solubility there should be areas of stable potential
followed by sharp falls in potential as the salt is precipitated out, the supply of the halide ions is
exhausted, and the silver ion concentration can continue to rise.
The Drop counter will measure the volume of the silver nitrate titrated, the quantity of silver ions
added at any point can then be calculated.
Voltage
sensor
If the concentration of the silver ions being titrated into the halides is known the concentrations of
the halide ions should be able to be calculated. The potentiometric titration of silver ions into a
solution of halides is used by many water companies to find the concentration of chloride in the water
supply. It is an accurate and reliable analytical method.
Reagent Reservoir
Drop Counter
L3 Chemistry
07 - 1(V2)
What you need
1.
An EASYSENSE logger.
2. A Smart Q ±1 V Voltage sensor.
3. A Smart Q Drop / Bubble counter with alignment adapter fitted.
4. The Drop counter reagent reservoir fitted with two stopcocks and tip.
5. Magnetic stirrer and follower.
6. Salt bridge.
7. 2 x 250 ml beakers.
8. 50 cm3 0.1 mol dm-3 Silver nitrate (AgNO3).
9. Solution of mixed halides (solution X).
10. Distilled and deionised water.
11. Clean, untarnished silver foil electrodes (approx 4 cm long by 1 cm wide).
12. Small beaker.
What you need to do
1.
Connect the Drop Counter and the Voltage sensor to inputs of the logger. Set up the apparatus
as shown.
2. Make sure the connection between the link wire and the silver electrode in the mixed halide
beaker will not be covered when the silver nitrate is added. If the junction comes into contact
with halide solution the characteristics of the cell and its potential will be altered.
3. From the EasySense software’s Home screen select EasyLog.
4. Select Test Mode from the Tools menu. Check the voltage across the two half-cells (remember
to have the salt bridge in place). The voltage when the cells are connected should be +0.66 V to
+0.70 V. If the voltage is negative, swap the connections with the Voltage sensor. If the
voltage is very low, check the salt bridge is making contact with both solutions and has no
breaks. Turn Test Mode off.
5. The label at the top of the y-axis for the Drop Counter will indicate which calibration range is
currently selected e.g. 27 Volume for the 27drops/cm3 range. If this is not correct, change
the range of the sensor.
6. Fill the reagent reservoir with the 0.1 mol dm-3 AgNO3 solution. Open both stopcocks to allow a
small amount of AgNO3 to pass through. Use the top stopcock to adjust the rate of flow e.g. to
about 1 drop per second, leave in that position and close the lower stopcock. Pour the AgNo3
from the beaker back into the reagent reservoir.
7. Pour solution X into another beaker and place under the reservoir. Add the magnetic stir bar
to solution X and turn on the stirrer to agitate the stir bar, check the stir bar is free to
rotate and is rotating slowly and evenly.
8. Press the button on the Drop Counter to set the counter back to zero.
9. Select the Start icon and open the lower stopcock to let the silver nitrate drop into solution X.
The LED on the Drop counter will blink as a drop passes through the infrared beam. Top up the
AgNO3 in the reagent reservoir if necessary.
10. Close the lower stopcock and select Stop to finish logging data. Save the data.
L3 Chemistry
07 - 2(V2)
Results and analysis
The data will be displayed as Volume and Voltage vs. Time graph. To alter the display to a Voltage vs.
Volume graph select Options and then the X-Axis tab, select Channel this will make the X-axis one of
the data channels not time.
If necessary click to the left or below the graph area, as appropriate, to alter the data channel
displayed so that Voltage is on the Y-axis (vertical) and Volume is on the X-axis (horizontal).
Use Values to find the end points (or points of neutralization).
Typical data as collected
Advanced
The a dx/dt function can be applied to the Voltage data to produce a first derivative curve.
Any slight variations in the graph will be exaggerated by applying a function. Select Smoothing (Tools
menu) and smooth the data by no more than 3 units.
If the X-axis is currently one of the data channels, select Options, the X-Axis tab, and select Time.
From the Tools menu select Post-log Function. Choose the formula function a dx/dt and set the
following parameters:
•
•
•
•
•
•
Make channel x = Voltage sensor
a = 10 (this is the multiplier - the rate of change data may produce small numbers, this
multiplier is used to make the numbers large enough to be shown clearly on the graph axis)
Number of decimals = 4
Y axis Limits = Manual, Max = 10, Min = 0
Name = Rate of change
Units = DmV
Setting a to 10 makes the peaks produced large enough to be seen without having to reset the sensor
axis limits. If the trace is still too small try altering a to 100 or alter the axis limits.
L3 Chemistry
07 - 3(V2)
The first derivative plot should reveal three well-defined peaks as illustrated in the example data
shown below.
Use the Values tool to intersect the voltage and volume lines at each peak and record the information
in the table below.
Peak number
Voltage
Volume of AgNO3
1
2
3
The derivative allows us to find the steepest part of the slope, which corresponds to the removal of
effectively all of the ions of each species. The first step is removal of iodide, the second step
bromide, and the last chloride ions.
The number of moles of Ag+ used to precipitate all of the iodide ions from the 20 cm3 of the solution X
added to beaker K is equal to:Calculation 1
(Volume of silver nitrate added to the beaker x molarity of the silver nitrate) / 1000
The moles of Ag+ = the moles of IThe moles of I- per litre of solution =
Calculation 2
(Volume of litre / Volume of solution X) x the moles calculated from calculation 1
The process is completed for each peak to find the concentration in mol dm-3 for Br- and Cl-.
L3 Chemistry
07 - 4(V2)
Find the moles of the ions in the mixture and calculate the mol dm-3 value for the original mass of each
ion used. Compare the values from the calculation and the practical, how do they compare? What
percentage error is there?
Extension
During precipitation of any silver halide, the product of the silver ion and the halide ion concentrations
will be equal to the solubility product of the silver halide. As the halide ion concentration falls, the
silver ion concentration must rise. The solubility product can be calculated using values calculated and
derived from the experimental data.
Step 1
Calculate the concentration of the halide ion when half of them have been precipitated. To do this
take the molar concentration value calculated above and simply divide by 2. Express as an exponential
value (e.g. 1.0 x 10 –3).
Step 2
Using the data collected find the mid point of the first step and record the voltage. Use the voltage
reading as volts and not millivolts. This will be Emeas.
Step 3
L = the beaker with the silver / silver nitrate half cell
K = the beaker with the mixed halides
Emeas is the voltage at the mid point.
[Ag+] is the concentration of silver, [Ag+] L is therefore the concentration of silver in L
The molarity of the original silver solution is 0.1 mol dm-3 which is 10-2 mol dm-3
The electrode potential for the silver / silver half-cell is calculated at 0.058 lg[Ag+]
The measure emf of the cell is represented by the equation;
Emeas = 0.058lg
lg
[ Ag + ]L
[ Ag + ]K
E
10−2
= meas
+
[ Ag ]K 0.058
Step 4
Convert the answer in step 3 to a non-log format.
Multiply this answer by the step 1 answer to give the solubility product.
Similarly, it can be calculated for the bromide and chloride ions. How do the calculated values compare
to published values?
L3 Chemistry
07 - 5(V2)