Question

Prepare 250 mL of-0.02 M K2Cr2O7 by transferring an accurately weighed amount of the dried dichromate into a 250 mL volumetric flask and diluting to the mark. Since this is a volumetric preparation, the exact concentration of the titrant will be based on the analytical weight of the potassium dichromate used to prepare the solution. . Analysis of the Unknown FeO Sample Accurately weigh four (4)-0.3 g samples of the dried unknown into four 400 or 600 mL beakers. Add 50 mL of 6 M HCI to each sample beaker and heat in the fume hood until the solutions boil for 30 seconds or until the sample dissolves. (Do not perform a rough titration). NOTE: Treat each sample individually from this point on (see question 4). Note: Perform this step in the fume hood -gloves and safety glasses required. Obtain approximately 20 mL of stannous chloride solution (SnCl2) from the reagent hood. Add V SnCb to the hot unknown solution drop-wise with a disposable plastic pipet until the solution changes from yellow to light green. Add three drops of SnCl2 solution in excess. emove the sample from the fume hood and cool the solution to room temperature (a /room temperature tap water bath is OK but no ice). After cooling, rapidly add 10 mL of the saturated HgClh solution (obtained from the reagent hood). Allow the sample solution to stand for 3 minutes - a precipitate should form (what is this?) To the sample solution add 60 mL of 3 M H2S04, 15 mL of 85% H3PO4 and 100 mL of D.I. water. Allow the solution cool briefly (two minutes), then add 8 drops of diphenylamine sulfonate indicator. Titrate immediately with the dichromate solution to violet-blue endpoint. Repeat for the remaining three samples. F. Questions 1. What is the purpose of the phosphoric acid? Be specific. 2. How does the indicator work? What chemical reaction causes the color change? How is an indicator chosen in a redox reaction? 3. Prepare a complete analysis procedure using KMnO4 as the oxidant instead of K2Cr20r. Include sample and solution preparation, approximate weights of samples and reagents, procedure and chemical reactions. In particular, what is the purpose of the Zimmermann-Reinhardt reagent?

Answer 1

1) Unlike permangante , dichromate solution treatment requires indicator. According to Kolthoff and Sandell , the reduction potential of the indicator must lie within the range of the reduction potentials of the two reactions . The indicator reduction potential doesn't lie within the range of reduction potential of Fe 3+ to Fe 2+ and Cr₂O₇^2- to Cr ³⁺ . Hence we use phosphoric acid which bonds with the Fe2+ ion and forms complex and the reduction potential of Fe3+ to Fe2+ becomes higher than standard potential. Now , the indicator reduction potential comes within the range of the reduction potential of the two reduction reactions.

2) The redox indicator has a definite color at a definite potential. In the reaction , the equivalence potential must also be more or less same as the indicator range. At the equivalence potential , when the metal center changes it's oxidation state , it also gets oxidized or reduced forming another color and this sharp color change provides us the point of equivalence.

Oxidation or reduction causes color change of  the species.

In a redox reaction an indicator is chosen such that it's reduction potential resides within the range of the reduction potential of the reaction.

3) The procedure is -

a) Prepare a known concentration of permanganate solution and take it to the burette ( 0.2M)

b) Reduce the Fe3+ solution using SnCl2 solution

c) Add Reinhardt-Zimmerman solution (25 mL + 400 mL water) to the reaction mixture so that permanganate can not oxidize chloride to chlorine

The reduction potential of chloride to chlorine resides within the range of manganate to Mn2+ . Hence there is a chance of chloride to get oxidized to chlorine. So, adding Mn2+ and H2SO4 solution we bring down the reduction potential of manganate below standard so that chlorine can not be oxidized.