Potentiometric Estimation

Electrochemical analysis is the group of chemical analytical methods in which a potential change is generated by the electrochemical reaction involved.

What is the real picture, in simple words the chemical change here is measured by the change in potential. As we already know about the reference and indicator electrodes, here also we need one of each and while picking the indicator electrode we have to take care of the fact that the sensing of the electrode is based on the component we want to measure.

Let’s the very common example,

Potentiometric Estimation of FAS using standard K2Cr2O7

  • Instrument: Potentiometer
  • Apparatus: burette, 100ml beaker, pipette, straw, and electrode assembly
  • Solutions: FAS, K2Cr2O7 and sulphuric acid
  • Electrodes:  Platinum electrode (indicator) – calomel electrode (reference) assembly

Procedure: –

  1. Pipette out 25ml of FAS into a beaker and for that matter any known amount.
  2. Add 2 test tubes of dil.H2SO4

The acid is added to make the medium acidic so that, the n factor is 5.

  • Immerse calomel electrode-platinum electrode assembly into it. Connect the assembly to a potentiometer and measure the potential.
  1. The cell potential is calculated by the formula below,

Reduced Form ———– > Oxidized Form + electrons

Using the nerst Equation,

ECell = EFe+2 = E0 + 0.0591 log [Fe3+]/[Fe2+]

  1. Now we add the oxidizing agent and will note the reading of electrode potential or we can say the change in the electrode potential of the cell from the potentiometer, we will add 0.5ml of K2Cr2O7 each time here.

Now here is the special feature, Numbers playing a simple trick here,

Let’s just play with the simple observable theory we have for it,

Say the ratio of  [Fe3+]/[Fe2+] (just the ratio here onwards) after adding some volume of oxidizing agent is 100/900 = .11

Now, Let’s add 20 more molecules of K2Cr2O7 so, if we have 1 molecule of K2Cr2O7 oxidizing 6 molecule of Fe3+ into Fe2+,

Then we have the ratio changing to,

100 + (20*6)/ 900 – (20*6)  = 220/780 = 0.2820

Similarly adding another 20 molecules will change the ratio as,

220 + (20*6)/ 780 – (20*6) = 340/660 = 0.5151

and again and again till,

340+ (20*6) /660 – (20*6) = 460/540 = 0.8518

460+ (20*6) /540 – (20*6) = 580/420 = 1.3809

580+ (20*6) /420 – (20*6) = 700/300 = 2.3333

700+ (20*6) /300 – (20*6) = 820/180 = 4.5555

820+ (20*6) /180 – (20*6) = 940/60   = 15.666

Now if we look at the ratio, there is a drastic change in the ratio.

Before any more adding of oxidizing agent observe that there are not enough molecules to be oxidized, so what do you think that will happen?

Actually say if you are in a room, full of two balls one steel ball say blue in colour and other balloon of say red in colour

Now the oxidizing agent is like an arrow. See the figure and observe what is happening,

So when the amount of targets is decreased, number of shots, also decrease.

Thus the next addition( 120 arrows) will not get all the balloons yes but they will get say 30-40 say but not all.

So, the ratio will be,

(940 + 50)/ (60-50) = 990/10 = 99.00

Similarly next will take another   i.e.  More in next 120 arrows, so the ratio will give,

(990+2)/ (10-2) = 124.00

(992+1)/ (8-1)   = 141.85

Now we don’t have other parameter so we can’t predict the E0 Values but what we can is the change in E0,

And from the above data we can see that it increase more and more when come close to the equivalence point, i.e. where all the ions are converted.

The graph is expected as follows-

graph

  1. From the volume of Potassium dichromate at equivalence point, calculate normality of K2Cr2O7 and the weight of FAS in the given solution.

        

 

   Experimental points to remember –

Now there are some points we need to understand from the process we observed happening in beaker.

  1. The reaction should be done fast, as we know that if we leave the beaker the arrows will get to the rest of the balloons, and eventually that is why in the experiment we observe that the value keep on increasing slightly, if we allow it to, So Don’t let it settle.
  2. To make a quick mix of the oxidiser, we use to bubble it with the straw, it is preferred to leave the straw with the set up.
  3. Don’t move the electrodes, it will disturb the reading of the potentiometer, by all mixing and settling thing.

So, this is how we use the electrochemical cell, to calculate the concentration of the metal in a solution. We will see a similar experiment with different indicator electrode to calculate the H+ ion concentration.

Abhishek Kumar jha

(Chemistry  at Utkarshini)

 

 

 

 

Note copy for download//potentiometric-estimation

 

 

 

 

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