Electrochemistry: Redox Titrations

Computer-Interfaced Experiments - Redoxpotential Measurement
Elektrochemistry
Redox Titrations: Cerimetry, Dichromatometry, Manganometry

Objectives: Determination of the Equivalence Point by Measuring the Redox Potential

Peter Keusch

Datalogging and data analysis using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
0.1 M cerium (1V) sulfate solution
0.1 M potassium dichromate solution
0.02 M potassium permanganate solution
ca 0.02 M iron (II) ammonium sulfate solution
ca 0.1 M iron (II) ammonium sulfate solution
ca 0.004 M hydrogen peroxide solution
ca 0.1 M potassium hexacyanoferrate (II) solution
hydrogen peroxide 30 %
conc. sulfuric acid

Apparatus and glass wares:
magnetic stirrer
magnetic stirring bars
stirring bar remover
contact thermometer
10 beakers 250 mL
burette 50 mL
Eppendorf pipette (100 ml)
volumetric pipette 10 mL
volumetric pipette 20 mL
volumetric pipette 50 mL
volumetric pipette 100 mL
4 pipet bulbs
redox sensor (IBK electronic + informatic)
drop counter (IBK electronic + informatic)

Hazards and safety precautions:

Potassium dichromate: Hexavalent chromium compounds are generally more toxic than trivalent chromium compounds. May be fatal if absorbed through the skin, if swallowed or inhaled. Contains chromium (VI), a known cancer hazard. Allergen. Skin eye and respiratory irritant. May act as a sensitizer.

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Safety glasses and protective gloves required.

Theoretical background

Calibration of the redox sensor:

The redox sensor is calibrated at two points: at 0.16 V (redox potential of the potassium hexacyanoferrate (II) solution) and at 1.184 V (redox potential after the titration with the 0.1 M Cerium (IV)sulfate solution). The relationship between chemical potential and concentration is presented in the relevant literature.

Fig. 1: Experiment set-up

10 mL of water are added to a solution of 4.222 g potassium hexacyanoferrate trihydrate in 90 mL of
0.1 M sulfuric acid.

The redox sensor (1) is attached to the pH-input of the CHEMBOX and the drop counter (2) is connected to the event-input 7 (Fig. 1).

The PC is switched on and the program Chemex is started.

'Options - Calibration - pH-Value' is selected from the menu.

The channel 4 is made functional for the voltage measurement using the redox probe:
ˇ A voltage range suitable for the experiments (for example: 0.1 V to 1.5 V) is selected in the channel program.
ˇ The fixed range for the pH value must be deleted (the checkmark is to be removed). After the values are confirmed with a click on the 'OK' button, one turns back to the calibration program.

Fig. 2: Calbration dialogue 'Redox probe' is entered into the field Pool and the unit 'V' is written into the field Value.

The redox probe is immersed in the beaker containing the potassium hexacyanoferrate(II) solution up to the complete wetting of its electrodes. The position of the sensor is adjusted to the outside of the beaker so that it is not stuck by the stirring bar.

The reference points are set at which the probe needs to be calibrated.

ˇ First calibration point: The appropriate voltage (0.16 V) is entered into the field of Ref1 and afterwards the check box near Set is marked.

Now the potassium hexacyanoferrate (II) solution is titrated with 0.1 M cerium (1V) sulfate solution up to the endpoint (consumption: 40 mL), at which the literature value of the redox potential is reached.

ˇ The second reference point is determined by entering of the appropriate voltage (1.184 V ) near Ref2 and by clicking on Set 2 . The boxes of the two buttons (Ref1, Ref2) must show a green tick. If the check mark does not appear, the calibration must be repeated.

The program 'Chemex' calculates the calibration formula. According to the formula the entering signals are converted into voltage.

Calibration of the drop counter:

Because of the different viscosities of the standard solutions the dropper is to be calibrated in each case again. It is advisable to store each calibration using an appropriate file name (Fig. 3).

Fig. 3: Calibration procedure

Experimental procedure:

The burette is filled with the standard solution. The sample solution is pipetted into the beaker and the stirrer is started. A redox sensor (Fig. 1) connected to the CHEMBOX (input: pH / mV) is immersed in the solution. The standard solution is added dropwise to the sample and immediately the sensing software is started.

Determination of the equivalence point: CHEMEX permits the accurate determination of the titrant volume which has been consumed at the midpoint of the sharply rising vertical portion of the titration curve. Using the tool 'Line' provided by CHEMEX two lines are drawn tangent to the linear parts in the flat regions of the sigmoidal titration curve and one line is fitted the linear portion which exhibits the greatest slope (Fig. 4). This procedure allows to find the equivalence point: The values E_a and E_b at the intersection points between the 'horizontal' lines and the 'vertical' line are displayed in the titlebar by exact positioning of the cursor.

The value of the equivalence point potential E_c is given by:
E_c = (E_b + E_a) / 2

By positioning the mouse cursor at the appropriate point on the graph (Fig. 4 cursor: nw-resize) the equivalence point volume is displayed in the title bar on the top of the screen.

Experiment 1:
Titration of a potassium hexacyanoferrate (II) solution with a 0.1 M cerium (IV) sulfate solution (cerimetry)

Sample (analyte): 10 mL of approx. 0.1 M potassium hexacyanoferrate (II) solution + 90 mL of 0.1 M conc. sulfuric acid

Fig. 4: Realtime graph

Calculation of the potassium hexacyanoferrate (II) concentration:

c = n / V Ž n (cerium (1V) sulfate) = c ˇ V = 0.1 mol ˇ L^-1 ˇ 10.3 mL = 1.03 mmol
according to the equations (1):

n (cerium (1V) sulfate) ş n (potassium hexacyanoferrate (II)) Ž n (Fe²⁺) = 1 ˇ 1.03 mmol = 0.00103 mol

c = n / V Ž c (potassium hexacyanoferrate (II)) = 0.00103 mol / 0.01 L = 0.103 mol ˇ L^-1

Experiment 2:
Titration of a iron (II) ammonium sulfate solution with 0.1 M potassium dichromate solution (dichromatometry)

Sample (analyte): 80 mL of approx. 0.1 M solution of iron (II) ammonium sulfate hexahydrate + 5 mL of conc. sulfuric acid

Fig. 5: Realtime graph

equation

calculation oft the Fe²⁺ concentration:

c = n / V Ž n (K₂Cr₂O₇) = c ˇ V = 0.1 mol ˇ L^-1 ˇ 13.3 mL = 1.33 mmol
according to the equations (2):

6 ˇ n (K₂Cr₂O₇) ş n (Fe²⁺) Ž n (Fe²⁺) = 6 ˇ 1.33 mmol = 0.00798 mol

c = n / V Ž c (Fe²⁺) = 0.00798 mol / 0.1 L = 0.0798 mol ˇ L^-1

Experiment 3:
Titration of a iron (II) ammonium sulfate solution with 0.02 M potassium permanganate solution (manganometry)

Sample (analyte): 95 mL of approx. 0.02 M iron (II) ammonium sulfate solution + 5 mL of conc. sulfuric acid

rRealtime graph

Fig. 6: Real-time graph

equation

Calculation of the Fe²⁺ concentration:

c = n / V Ž n (KMnO₄) = c ˇ V = 0.02 mol ˇ L^-1 ˇ 20.4 mL = 0.408 mmol
according to the equations (3):

5 ˇ n (KMnO₄) ş n (Fe²⁺) Ž n (Fe²⁺) = 5 ˇ 0.408 mmol = 0.00204 mol

c = n / V Ž c (Fe²⁺) = 0.00204 mol / 0.1 L = 0.0204 mol ˇ L^-1

Experiment 4:
Titration of a hydrogen peroxide solution with a 0.02 M potassium permanganate solution (manganometry)

Sample (analyte): 30 mL of 20% sulfuric acid + 70 mL of dist. water + 40.8 ml of hydrogen peroxide solution (30%)

Fig. 7: Realtime graph

Gleichung

Calculation of the H₂O₂ concentration:

c = n / V Ž n (KMnO₄) = c ˇ V = 0.02 mol ˇ L^-1 ˇ 8.62 mL = 0.1724 mmol
according to the equations (4):

5 ˇ n (KMnO₄) ş 2 ˇ n (H₂O₂) Ž n (H₂O₂) = (5 ˇ 0.1724 mmol) : 2 = 0.000431 mol

c = n / V Ž c (H₂O₂) = 0.000431 mol / 0.1 L = 0.00431 mol ˇ L^-1

Index of Chembox Experiments