TASK 4 - POTENTIOMETRY

Potentiomery is the method of quantitative measurement of chemical agents by a titration. The Equivalence point (end of titration) is detected by measurement of the electric voltage of a galvanic cell consisting of 2 electrodes: Metal indicator electrode + referential calomel electrode, which are immersed in the solution measured. After gradual addition of the titration agent into titration solution we measure the potential of the galvanic cell by a voltmeter (in mV). Then we create a titration chart on which we find a potential jump (= rapid increase in potential value), and finally we estimate the equivalence point by a graphic or numerical method.

1. Pipette 10 ml of 0.1 M FeSO₄ to a beaker, supplement with 80 ml of deionized water, and add 10 ml of H₂SO₄ (use the loader). Add in a stirring bar and put the beaker on magnetic stirrer; then immerse the platinic indicator electrode (connect to G on the instrument) + referential calomel electrode (connect to R on the instrument).
2. Fill a buret with 0.05M KMnO₄ solution (factor f = 1,000) and place the buret above titration solution..
3. Titrate with this titration agent while stirring:
        3.1. by step of 2 ml up to 14 ml,
        3.2. by step of 1 ml up to 19 ml,
        3.3. by step of 0.1 ml up to 21 ml,
        3.4. by step of 1 ml up to 25 ml.
   After addition of the titration agent wait always for cca 10 seconds, then read the value E [mV].
4. Create the titration curve (x = V KMnO₄ [ml], y = E [mV]).

   Note: While configuring the instrument red button is pressed (STDBY), during measurements - pressed are blue and white buttons.

5. From the titration curve read the equivalence point (in ml) in the moment of potential jump (see the following chart).
   

   
   

6. calculate the concentration of FeSO₄ (on 3 decimal places!):
   

   c(FeSO₄) = [c(KMnO₄) * V(KMnO₄)] / V(FeSO₄)

   c(KMnO₄) = 0,05 mol . l^-1
   V(KMnO₄) = ml added = equivalence point
   V(FeSO₄) = 10 ml 0.1 M FeSO₄.

1. Put into a beaker 10 ml of cca 0.1 M KI (potassium iodide) + 90 ml of deionized water, supplement with a stirring bar, and put on the magnetic stirrer.
2. Immerse the corresponding silver indicator electrode (G-connector) + calomel referential electrode (R-connector).
3. Titrate with 0.1 M AgNO₃:
        3.1. by step of 1ml up to 5 ml,
        3.2. by step of 0.5 ml up to 9 ml,
        3.3. by step of 0.1 ml up to 11 ml,
        3.4. by step of 1 ml up to 15 ml.
4. Create the titration curve (x = V AgNO₃ [ml], y = E [mV]).
   

   ---

5. Create new chart using values E [mV] for V AgNO₃ from 6 to 9 ml (by step of 0.5 ml ): x = V AgNO₃ added, y = (V₀ + V) . 10^-E/g), and supplement with Linear regression line (trendline).
   
   V₀ - total volume of KI used for titration = 10 ml
   V - total volume of 0.1 ml/litre AgNO₃ added (from 6 to 9, in steps of 0.5 ml)
   E - corresponding voltage [mV]
   g - use value 59 mV (the value for 25 ⁰C).
6. read the equivalence point [in ml] at the point in which the Linear regression trendline crosses the axis x.
   
   
   
   
   Example - how to read the equivalence point.
   
   
7. calculate the concentration of KI (on 3 decimal places!):

c(KI) = [c(AgNO₃) * V(AgNO₃)] / V(KI) (mol/litre)

c(AgNO₃) = 0.1 mol . l^-1
V(AgNO₃) = ml added = equivalence point
V(KI) = 10 ml.

8. Calculate factor KI = experimental concentration (see paragraph 6) / theoretical concentration (= 0.1 M).