Download Related concepts Concentration cells without transport

Determination of the solubility products of
silver halides with Cobra4
TEC
Related concepts
Concentration cells without transport, electromotive force, salt bridge, liquid junction and diffusion potentials.
Principle
A concentration cell is constructed from two half-cells which are identical, except that the concentration of the ionic species to
which the electrode is sensitive is different on the two sides of the cell. Such a cell can be used to measure the solubility product
of a sparingly soluble salt. In one half-cell the concentration of these ions is known, in the other it is determined by the solubility
product of the salt under investigation. The ratio of the two concentrations (more accurately, activities) determines the e.m.f.
of the cell.
Tasks
Use a concentration cell made from two Ag(s) | Ag+ (aq) electrodes to determine the solubility product of the three silver halides
AgCl, AgBr and AgI.
Equipment
Cobra4 Mobile-Link set, incl. rechargea1
ble batteries, SD memory card, USB cable and software "measure"
1
Cobra4 Sensor-Unit Chemistry
1
Connecting cord, l = 500 mm, red
1
Connecting cord, l = 500 mm, blue
1
Crocodile clips, bare
1
Silver foil, 150 x 150 x 0.1 mm, 25 g
1
Immersion probe NiCr-Ni, teflon, 300 °C
1
Adapter, BNC-plug/socket 4 mm.
1
Temperature probe Pt1000
Set of Precision Balance Sartorius CPA
1
623S and measure software, 230 V
1
Weighing dishes, 80 x 50 x 14 mm
1
Support for two electrodes
1
Spring balance holder
1
Salt bridge
1
Clay pins, d = 8 mm, l = 15 mm
1
Retort stand, h = 750 mm
1
Right angle clamp
1
Silicone tubing, di = 7 mm
1
Rubber caps
1
Syringe, 10 ml
1
Cannula, 0.6 x 60 mm
12620-55
12630-00
07361-01
07361-04
07274-03
31839-04
13615-05
07542-26
13702-01
49224-88
45019-05
45284-01
03065-20
37684-00
32485-01
37694-00
37697-00
39296-00
02615-03
02590-03
02599-04
12
2
1
12
8
1
1
1
1
4
1
1
1
1
1
1
1
1
1
1
1
1
1
Glass beaker, 50 ml, tall
Glass beaker, 150 ml, tall
Glass beaker, 250 ml, tall
Volumetric flask, 250 ml
Volumetric pipette, 25 ml
Graduated pipette, 5 ml
Pipettor
Pipette dish
Graduated cylinder, 100 ml
Funnel, glass, do = 55 mm
Spoon
Pasteur pipettes
Rubber bulbs
Wash bottle, 500 ml
Glass rod, l = 200 mm, d = 5 mm
Scissors, straight, l = 180 mm
Nitric acid, 65 %, 1000 ml
Silver nitrate, 15 g
Potassium nitrate, 250 g
Potassium chloride, 250 g
Potassium bromide, 100 g
Potassium iodide, 50 g
Water, distilled, 5 l
36001-00
36003-00
36004-00
36550-00
36580-00
36598-00
36592-00
36589-00
36629-00
34457-00
33398-00
36590-00
39275-03
33931-00
40485-03
64798-00
30213-70
30222-00
30106-25
30098-25
30258-10
30104-05
31246-81
Fig. 1: Experimental setup.
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TEC
Determination of the solubility products of
silver halides with Cobra4
Set-up and procedure
Set up the experiment as shown in Fig. 1. Prepare the solutions required for the experiment as follows:
– 0.1 molar KCl solution: Weigh 1.8638 g of potassium chloride into a 250 ml volumetric flask, dissolve it in distilled water, and make up to the mark with distilled water.
– 0.01 molar KCl solution: Pipette 25 ml of 0.1 molar KCl solution into a 250 ml volumetric flask and make up to the mark
with distilled water.
– 0.001 molar KCl solution: Pipette 25 ml of 0.01 molar KCl solution into a 250 ml volumetric flask and make up to the
mark with distilled water.
–
0.1 molar KBr solution: Weigh 2.9751 g of potassium bromide into a 250 ml volumetric flask, dissolve it in distilled water, and make up to the mark with distilled water.
– 0.01 molar KBr solution: Pipette 25 ml of 0.1 molar KBr solution into a 250 ml volumetric flask and make up to the mark
with distilled water.
– 0.001 molar KBr solution: Pipette 25 ml of 0.01 molar KBr solution into a 250 ml volumetric flask and make up to the
mark with distilled water.
– 0.1 molar KI solution: Weigh 4.1500 g of potassium iodide into a 250 ml volumetric flask, dissolve it in distilled water,
and make up to the mark with distilled water.
– 0.01 molar KI solution: Pipette 25 ml of 0.1 molar KI solution into a 250 ml volumetric flask and make up to the mark
with distilled water.
– 0.001 molar KI solution: Pipette 25 ml of 0.01 molar KI solution into a 250 ml volumetric flask and make up to the mark
with distilled water.
– 0.1 molar AgNO3 solution: Weigh 4.2469 g of silver nitrate into a 250 ml volumetric flask, dissolve it in distilled water,
and make up to the mark with distilled water.
– 0.01 molar AgNO3 solution: Pipette 25 ml of 0.1 molar AgNO3 solution into a 250 ml volumetric flask and make up
to the mark with distilled water.
– 0.001 molar AgNO3 solution: Pipette 25 ml of 0.01 molar AgNO3 solution into a 250 ml volumetric flask and make up
to the mark with distilled water.
– 0.5 molar HNO3 solution: Pour 150 ml of distilled water into a 150 ml beaker, and pipette 3.5 ml of 65% nitric acid into
it.
– Saturated KNO3 solution: Weigh 20 g of potassium nitrate into a 150 ml beaker, add 50 ml of distilled water, and stir for
some minutes at room temperature. Some potassium nitrate must remain on the bottom of the beaker in the solid state.
If this is not the case, add further potassium nitrate. After the undissolved potassium nitrate has settled to the bottom,
decant the saturated solution so prepared into a second beaker.
Soak the clay pins in the saturated potassium nitrate solution overnight. Pour 40 ml each of the potassium halide and silver nitrate solutions into labelled 50 ml glass beakers. To precipitate a little of the sparingly soluble silver halide, use a Pasteur pipette
to add one drop of the 0.1 molar AgNO3 solution to each of the nine beakers containing potassium halide solutions.
Cut silver electrodes (50 x 10 mm) from the silver sheet and clean them by placing them in the 0.5 molar nitric acid solution
for a few minutes. Use a syringe with a cannula to carefully fill the salt bridge with saturated potassium nitrate solution. Remove
air bubbles by tapping at the arms of the salt bridge. Seal each arm of the salt bridge with a porous clay pin and fix each of them
in place with a short length (20 mm) of silicone tubing. Replace the cap.
Investigate the following concentration cells:
Clamp the silver electrodes with two crocodile clips and fix their ends in the holder for two electrodes. Place one of the silver
electrodes in the silver nitrate solution and connect it to the input socket of the pH meter with a connecting cord. Dip the other
one into the potassium halide / silver halide solution and connect it to ground potential. Switch the pH-meter to voltmeter mode.
Connect the two electrode solutions by means of the salt bridge. Attach the temperature probe to the electrode holder using a
spring balance holder and connect it to the pH meter. Record the cell e.m.f. for the given concentration cells. Before measuring
a new concentration, rinse the electrodes and the salt bridge with distilled water and dry them. Switch the pH meter to the temperature measuring mode to read the temperature.
2
PHYWE Systeme GmbH & Co. KG © All rights reserved
P3061062
Determination of the solubility products of
silver halides with Cobra4
TEC
Theory and evaluation
A single potential cannot be measured, only a potential difference (e.m.f.). To be able to determine a single potential, a galvanic
cell must be set up. The concentration cell used here can be expressed as:
where X refers to the halide ion Cl-, Br- or I-.
The e.m.f. is the difference between the cathode potential (right half-cell R) and the anode potential (left half-cell L) and is always positive, otherwise the process runs in the other direction. The cathode is the electrode with the higher potential. It is the
positive pole in a galvanic cell. The processes which take place at the cathode are always reductions. Cations are deposited,
non-metals go into solution as anions, oxidizing agents are reduced.
The anode is the electrode with the smaller potential. It is the negative pole in a galvanic cell. The processes which take place
at the anode are oxidations. Metals go into solution as cations, anions are deposited, reducing agents are oxidized. The e.m.f. E
is generally calculated from the two standard potentials E by subtracting them, together with a second expression, which takes
the activities a of the participating substances into account. As in the law of mass action, the substance on the right appears in
the numerator, the substance on the left in the denominator. In this case, we have:
where
aAg+,R
aAg+,L
Activity of the silver ions in the silver nitrate solution of known concentration
Activity of the silver ions in the potassium halide / silver halide solution
A galvanic cell which consists of two half-cells, each of which contain the same substance, but at different concentrations, is
called a concentration cell. It can be used to determine solubility products. For this purpose, one half-cell containing a saturated
solution of the substance is connected to a second half-cell containing a solution of known activity. The connection is made
via a switch which is filled with an electrolyte that is inert with regard to the saturated solution.
aAg+,L is controlled by the solubility product Ksp of the silver halide AgX
where aX -,L, the activity of the halide ions in the left half-cell, is assumed to arise from the fully dissociated potassium halide,
the contribution from the sparingly soluble silver halide being negligible in comparison. Substituting equation (2) into equation
(1) allows the cell e.m.f. to be expressed as
which can be rearranged to
The solubility product of the silver halide may therefore be calculated from the e.m.f of the cell E if aAg+,R and aX -,L are known.
The activity of an ion in solution is given by
where ci is the concentration of the species i and ai is the mean activity of the electrolyte. The electrolyte concentrations required
for this experiment are listed in Table 1. Using these values the ion concentrations can be converted to activities and the solubility products calculated from equation (5). The results are summarised in Table 2.
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TEC
Determination of the solubility products of
silver halides with Cobra4
Data and results
Having taken ion activities into account and in the absence of any systematic errors, the measured Ksp are calculated to be:
Ksp (AgCl)
=
1.3 · 10-10 mol2 / l2
(lit. value: 1.8 . 10-10 mol2 / l2)
Ksp (AgBr)
=
4.3 · 10-13 mol2 / l2
(lit. value: 5.3 · 10-13 mol2 / l2)
Ksp (AgI)
=
7.4 · 10-17 mol2 / l2
(lit. value: 8.3 · 10-17 mol2 / l2)
Before comparing the experimental and literature values, the students should be made aware of the effect that an uncertainty
of only 5 mV in the measured e.m.f. of the cell will have on the value determined for Ksp.
4
PHYWE Systeme GmbH & Co. KG © All rights reserved
P3061062
Determination of the solubility products of
silver halides with Cobra4
TEC
www.phywe.com
P3061062
PHYWE Systeme GmbH & Co. KG © All rights reserved
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