To estimate the strength of given Mohr's salt solution by titrating against Potassium dichromate solution. Standardize the Potassium dichromate solution with (approx. 0.025N) standard Mohr's salt solution prepared by you.

Chemicals

1. Mohr's Salt
2. Potassium dichromate
3. Sulphuric acid
4. Diphenylamine
5. Phosphoric acid

Glassware/Plastic ware

All glassware and plastic ware used should be sterilized:

1. Conical flasks 100 ml
2. Weighing bottle
3. Spatula
4. Micro tips
5. Pipettes
6. Funnel
7. Burette

Instruments/ equipment

1. Weighing balance

Redox-Titration

Redox stands for Reduction - Oxidation
Redox titrations involve the titration of an oxidizing agent (or oxidant) with a reducing agent (or reductant) or vice versa. There must be a sufficiently large difference between the oxidizing and reducing capabilities of theses agents for the reaction to undergo completion with a sharp end point.
Oxidation process involves loss of electrons while reduction process involves gain of electrons.
Thus an oxidizing agent is one which accepts electrons while a reducing agent is one which loses the electrons.
An oxidizing agent oxidizes the other substance by stripping off electrons from it. A reducing agent reduces the other substance by donating electrons to it. Oxidation and reduction reactions always occur simultaneously. One can not take place in isolation from the other. During a redox reaction the oxidizing agent itself undergoes reduction while the reducing agent undergoes its oxidation.

1. pH dependence of oxidizing behaviour

   It is important to note that for many oxidants the pH of the medium is of great importance and hence their oxidizing strength may vary depending on the medium in which its reaction is studied. For example potassium permanganate is oxidizing agent in all three mediums, acid, alkaline and neutral. However it is strongest in acidic medium.

2. Redox Indicators

   A redox indicator should be capable of producing a sudden potential change in the vicinity of the equivalence point during a redox titration. For this the the indicator itself should be redox active i.e., capable of undergoing a reversible oxidation or reduction process. The oxidized and reduced form of the indicator should have a contrast difference in the colours.
   ln_oxd + ne = ln_red
   At potential E, the ratio of the concentration of oxidized and reduced forms is given by the Nernst equation
   E= E⁰ + RT/nF ln [ln_oxd]/[ln_red]

3. Type of Redox Indicators

   1. Self indicator
   2. Internal indicator
   3. External indicator
   4. Potentiometric methods
   
   

Titration of Potassium dichromate with Mohr's salt.

Potassium dichromate is also a very strong oxidizing agent

(E0red=+1.33V)

However it is not as strong oxidizing agent as permanganate is

(E0red=+1.51V).

Still it is widely used in redox titrations because of several advantages over permanganate. Unlike potassium permanganate, potassium dichromate is available in high purity and is highly stable up to its melting point. Its aqueous solutions are not attacked by oxidisable impurities like rubber or any other organic matter and thus composition of aqueous solution does not change on keeping. The aqueous solutions are quite stable towards light. It is thus an excellent primary standard and its standard solutions can be prepared by direct weighing of an amount of it and dissolving in a known volume of distilled water.

1. Potassium dichromate acts as oxidizing agent in acidic medium only:

   The neutral aqueous solution of Potassium dichromate is 1:1 equilibrium mixture of dichromate and chromate, a consequence of hydrolysis of dichromate ions.
   
   Chromate ions are weaker oxidizing agent than dichromate. Thus oxidizing strength of dichromate is reduced in neutral solution. The above hydrolysis reaction however can be reversed by adding acid to the solution and this explains the necessity of acidic medium for the reaction.
   
   Also the reduction reaction of dichromate can be represented as:
   
   This reaction clearly shows the involvement of

   H+ in the reduction half reaction.

   
   The medium is generally acidified with dil. H₂SO₄. Unlike permanganate case, cold HCl can be used here for acidifying the reaction mixture provided the acid concentration does not increase beyond 1-2 M.
   Though the dichromate solutions are intensely orange coloured solutions and a single drop of it imparts yellow colour to a colourless solution, it can't be used as a self indicator like KMnO₄. This is because its reduction product (Cr3⁺) is green which hinders the visual detection of end point by observing dichromate colour. Thus an indicator is must required in this titration. The indicator should be redox active and must be properly chosen keeping in mind the electrode potential values of the reducing agent being titrated with dichromate. Suitable indicators for dichromate titrations are Diphenylamine (specifically sodium diphenylamine sulphonate)(0.2%aq. Soln) in presence of orthophosphoric acid, and N- phenylanthranilic acid (0.1% soln. in 0.005 M NaOH).

2. Mohr's Salt

   The reducing agent used in this titration is Mohr's salt which is a double salt. Its composition is

   FeSO4.(NH4)2SO4.6H2O.

   The redox active species in this compound is Fe²⁺ whose oxidation can be represented as:
   Fe²⁺ --> Fe³⁺ + e^-
   
   Mohr's salt is a primary standard and therefore its standard solution can be easily prepared by direct weighing a known quantity of it and dissolving in a known volume of distilled water. However The Ferrous ions in aqueous solutions are highly susceptible towards hydrolysis resulting in precipitation of ferrous hydroxide.
   
   The reaction sequence however can be reversed by adding acid. Hence while preparing standard solution of Mohr's salt it is necessary to add about 1 ml of dil. H2SO4 to the measuring flask. Simple Ferrous sulphate is not a primary standard and hence can never be used for preparing the standard solution. The solid sample of ferrous sulphate contains a high percentage of ferric ions produced by aerial oxidation of the ferrous ions. Thus composition is not accurately known.
   The reaction between Potassium dichromate and Mohr's salt can be represented as:
   
   The end point of the titration as indicated earlier has to be defined with the help of an indicator. Diphenylamine is one such indicator (internal indicator as it is added to the reaction mixture). The end point is marked with an intense blue violet colouration. The reduction potential value of this system is E⁰_red = +0.76V which is very near to that of ferrous- ferric system (E⁰_red = + 0.77V). Thus phosphoric acid is required in this titration as it reacts with the product ( yellow ) Fe3⁺ ion to form the complex ion [Fe(HPO₄)]⁺ , thus lowering the formal potential of the Fe(III)/Fe(II) system thereby increasing its reducing power. Thus the end point is sharper.
   The diphenyl amine (I) undergoes oxidation first into a colourless diphenylbenzidine (II) which is the real indicator and is reversibly further oxidized to diphenylbenzidine violet (III). The titrations done here are double titrations. This means in the first titration K₂Cr₂O₇ is standardized by titrating with a standard solution of Mohr's salt and in the second titration this standardized K₂Cr₂O₇ is titrated against a given oxalate ion solution of unknown strength.

1. Preparation of standard solution of Mohr's salt (roughly N/40)

   Take a clean dry weighing bottle. Weigh it when empty without the bottle lid. (as demonstrated in lab session) This is weight of empty weighing bottle (w1). Now put an approximate quantity of Mohr's salt to be weighed in the weighing bottle. Weigh again. In case the added amount is too much than required , take out some with the help of a clean spatula and weigh the weighing bottle. In case the added amount is less than required, add some more into the weighing bottle. (Never add compound to the weighing bottle, when it is still placed on the pan of the weighing balance. Always take it out, and then add or remove the compound). Note down the weighing readings as weight of weighing bottle with the compound (w2) Transfer the compound into 100 mL measuring flask with the help of a funnel and keep the weighing bottle back on the pan of weighing balance. Note down the weight as weight of weighing bottle after transference of compound (w3). The difference of (w3) and (w2) gives the amount of compound actually transferred to the measuring flask. Dissolve the solid in minimum amount of water. Add a few drops of conc. H2SO4 acid. Shake and make up the volume till the 100 ml mark on the flask. Put the stopper on the flask and shake a number of times, vigorously to homogenize the solution



2. Standardisation of K2Cr2O7 soln. by titrating with standard Mohr's salt solution

   Using Internal Indicator Diphenylamine

   • Pipette out 10 mL of Mohr's salt solution in the conical flask .
   • Add approximately 10 mL of dil. H2SO4 to the same flask.
   • Add 2 drops of the indicator diphenylamine and 2 mL of 1:1 ortho H3PO4.
   • Titrate the reaction mixture with potassium dichromate solution taken in burette till a colour change from light green to blue- violet is obtained.
   • Repeat the titration for three concordant readings.

   Using External Indicator Potassium Ferricyanide

   • Take a clean grooved tile. Add 2 drops of the indicator-Potassium Ferricyanide (2% aq. Soln.) in each groove.
   • Pipette out 10 mL of Mohr's salt solution in the conical flask .
   • Add approximately 10 mL of dil. H2SO4 to the same flask.
   • Titrate the reaction mixture with potassium dichromate solution taken in burette. Stop at the point when some green colour in the solution starts appearing.
   • Dip a clean glass rod in the reaction mixture and then in the indicator taken on the grooved tile.
   • Notice the colour. A prussian blue (Turnbull's blue) colouration or precipitate indicates the presence of reactant Fe2+ in the solution.
   • Continue the titration again with the same reaction mixture. Again after adding about 1 mL, repeat the indicator step and observe.
   • Repeat above two steps till no prussian blue colouration is observed when reaction mixture is added to the indicator.
   • Repeat the titration again, now checking the reaction mixture near end point after interval of 0.1 mL addition of Dichromate each time.
   • Repeat the titration for three concordant readings.



3. Titration of Standardised K2Cr2O7 soln. with Mohr's salt solution of unknown strength.

   Repeat the above titration, now taking Mohr's salt solution (whose concentration is to be determined) in place of standard Mohr's salt solution.

The strength of given Mohr's salt solution was found to be....................g/L

1. Vogel's Textbook of Quantitative Chemical Analysis
2. Analytical Chemistry by G.D.Christian