POTENTIOMETRIC ESTIMATION OF CHROMIUM AND VANADIUM

Potentiometric titration is a technique of one kind of Redox Reaction. For this titration two electrodes are used, one is indicator electrode, indicating the Redox Reaction change in potential(pt, electrode) and another is reference electrode. Reference electrodes used are calomel electrode. The indicator electrode forms an electrochemical half cell, Emf of the Redox half cell is given by the Nernst equation E=E⁰+(0.059/n)log[Ox]/[Red] at 25°C for any Redox system, where Oxidant+ne= Reductant where n is integer indicating no of electron transfer with the interested ions in the test solution. The reference electrode forms the other half cell.

The overall electric potential is calculated as Ecell = Eind – Eref + Esol. Esol is the potential drop over the test solution between the two electrodes. Ecell is recorded at intervals as the titrant is added. A graph of potential against volume added can be drawn and the end point of the reaction is half way between the jumps in voltage. Ecell depends on the concentration of the interested ions with which the indicator electrode is in contact.

In this method, we are presenting the results of our investigations on the titration of Chromium and Vanadium in their ore and Metal Form.

The Method here utilises two Redox Reactions, for estimation of Cr as well as V.  In both the cases reducing agent is common i.e. Ferrous Ammonium Sulphate(FAS). 

For Chromium:

Cr is oxidized to Cr(VI) and Cr (VI) is titrated with Standard Ferrous Ammonium Sulphate. End Point being detected by Sharp Change in Emf recorded in a Potentiometer.

For Vanadium:

Vanadium is oxidized to V (V) and this pentavalent Vanadium is titrated with Std FAS.

Instrument Used:

Potentiometer 

Reagent Used:
For Chromium
  1. Sodium Peroxide (Na2O2)
  2. Sodium hydroxide (NaOH)
  3. Hydrochloric Acid (HCl)
  4. 1:1 Sulphuric Acid (H2SO4)
  5. Ferrous Ammonium Sulphate
For Vanadium
  1. Sodium Peroxide (Na2O2)
  2. Sodium hydroxide (NaOH)
  3. Hydrochloric Acid (HCl)
  4. 1:1 Sulphuric Acid (H2SO4)
  5. Mohr salt (FeSO4. (NH4 )2 SO4 .6H2 O )
  6. Potassium Permanganate (KMnO4)
  7. Sodium Nitrite (Na NO2)
  8. Urea
Procedure:

The process followed is fusion of a test portion with Sodium Peroxide, followed by dissolution of the melt in water and acidification of aqueous solution with sulphuric acid. Alternatively, for some Ferroalloys, dissolution of a test portion by direct acid attack. Sum of Chromium and Vanadium is first determined by Potentiometric titration with Ammonium Iron(II) Sulphate. Then Vanadium is re-oxidized by Potassium permanganate. Excess of permanganate is destroyed by nitrite followed by destruction of the excess of nitrite by urea. Determination of vanadium by Potentiometric titration with Ferrous Ammonium Sulphate and determination of the chromium content by difference.

The reactions are the following:

Cr2O72-+6Fe2++14H→2Cr3++6Fe3++7H2O

VO2++Fe2++2H+→VO2++Fe3++H2O

5VO2++MnO4+H2O→5VO2++Mn2++2H+

5NO2+2MnO4+6H+→5NO3+2Mn2++3H2O

Graph for Chromium Titration

crv1

crv2

Where,
dV = Electromotive Force in milivolt. 
 V  = Volume of Ferrous Ammonium Sulphate consumed in ml.

crv3

crv4

Where,
dV = Electromotive Force in milivolt. 
 V  = Volume of Ferrous Ammonium Sulphate consumed in ml.

Graph For Vanadium Titration

crv5

crv6

 

Where,
dV = Electromotive Force in milivolt. 
 V  = Volume of Ferrous Ammonium Sulphate consumed in ml.

Recovery Data:

crv7

 

Conclusion:

Common volumetric titration method of Chromium and Vanadium is quite tedious and time consuming and therefore there is a need for simpler and more rapid method. This Potentiometric method is very rapid and cost effective. In this method, no indicator is used and end point detection is also very easy. Chromium is titrated with Ferrous Ammonium Sulphate instead of Potassium Dichromate so that no back titration is required.  Therefore, this method is very useful for regular analysis of Chrome Ore, Ferrochrome and Ferrovanadium.

Contributed by:  Ms. Alakta Saha under the guidance of Prof. Barun Gupta

DETERMINATION OF TUNGSTEN IN FERRO TUNGSTEN BY GRAVIMETRIC METHOD
CENOSPHERES – AN ANALYTICAL STUDY

Leave a Reply

Your email address will not be published. Required fields are marked *

RECENT POSTS