[Harry]

Hi there, a few weeks ago I did a chemistry experiment on back titration. However, I misplaced the worksheet given to me with questions on the practical work we did, hence left it til the last minute. Now I've found it and I'm having trouble with some of the questions. On top of that my chemistry teacher says he will fail me if I don't hand this by next lesson. Below are the intructions to the experiment that we did and the relevant questions given. I think it's a pretty common experiment so many people here would probably be familiar with it (or have done a similar experiment). If anyone could help me out just this once that would be appreciated. And I'm not asking for solutions here but just a few hints for each of the questions below. I'll ask questions accordingly and make my own deductions on how to answer the questions. I am also interested to know this experiment for why each step must be carried out as I've been told that I will be doing it again next year. So, if anyone knows of any sites they could direct me that would be helpful.


Purpose
To use a back titration to determine the alcohol content of wine.

Materials

• 10 mL white wine
• 60 mL standard potassium dichromate solution (about 0.04 M)
• 100 mL standard sodium thiosulfate solution (about 0.1 M)
• 30 mL 40% sulfuric acid
• 6 g potassium iodide
• 250 mL distilled water
• starch indicator
• 10 mL pipette
• 2 X 20 mL pipettes
• pipette filler
• 250 mL volumetric flask
• 3 X 250 mL conical flasks and stoppers
• 10 mL measuring cylinder
• small funnel
• burette and stand
• white tile
• hot-water bath
• thermometer, -10°C - 110°C
• spatula
• disposable gloves
• electronic balance
• safety glasses

Procedure
Part A - Oxidation of alcohol

1) Record the brand of wine to be analysed and the manufacturer's specification of the alcohol content.
2) Use a pipette to transfer 10.00 mL wine to a 250 mL volumetric flask. Make up the volume to 150.0 mL using distilled water and mix throughly.
3) Place 20.00 mL aliquots of the diluted wine into each of three 250 mL conical flasks.
4) Record the concentration of the standard potassium dichromate solution supplied. Place a 10.00 mL aliquot of this solution in each flask. Wearing gloves and using a measuring cylinder, slowly add approximately 10 mL 40% sulfuric acid solution to each flask.
5) Stopper each flask loosely and heat for 10 minutes in a water bath at 45 - 50° C. (Do not allow the temperature to exceed 50° C.)

Part B - Determination of excess dichromate

1) After heating for 10 minutes, add about 2 g of potassium iodide to each flask.
2) Fill a burette with standard sodium thiosulfate solution. Record the initial volume of solution in the burette and the concentration of the solution.
3) Titrate the contents of each cnoical flask with the sodium thiosulfate solution, adding 1 - 2 mL starch indicator when the brown colour of the solution develops a green tinge. The equivalence point of each titration is reached when the blue colour of the starch-iodine complex just disappears, leaving a clear, green colour. Record the final volume of liquid in the burette.

Relevant equations:

3C₂H₅OH(aq) + Cr₂O₇^2-(aq) + 8H⁺ (aq) ---> 3CH₃CHO(aq) + 2Cr³⁺(aq) + 7H₂O(l)

Cr₂O₇^2-(aq) + 14H⁺(aq) + 6I^-(aq) ---> 2Cr²⁺(aq) + 7H₂O(l) + 3I₂(aq)

Next the iodine is titrated with standard sodium thiosulfate solution. The equation for this reaction is:

I₂(aq) + 2S₂O₃^2-(aq) ---> 2I^-(aq) + S₄O₆^2-(aq)


Questions
1) Calculate the amount of Cr₂O₇^2- ions, in mol, in each 20.00 mL aliquot of potasium dichromate solution.
2) Calculate the average  titre of standard sodium thiosulfate solution and the amount of thiosulfate ions (S₂O₃^2-) in this titre.
3) Deduce the amount of iodine (I₂) formed during the reaction with excess dichromate.
4) Use the equation for the reaction between Cr₂O₇^2- ions and I^- ions to deduce the amount of Cr₂O₇^2- ions in excess after oxidation of the ethanol.
5) By subtracting the amount of excess Cr₂O₇^2- ions from the original amount of Cr₂O₇^2- ions added to the diluted wine (from question 1), calculate the amount of Cr₂O₇^2- ions that reacted with the ethanol.
6) Using the equation for the reaction between ethanol and Cr₂O₇^2- ions, find the amount of ethanol in each 20.00 mL aliquot of diluted wine.
7) Calculate the amount of ethanol in the 250 mL volumetric flask.


Feel free to ask me about any part you don't understand or find confusing.

[Borek]

Start following the directions - they look quite simple.

http://www.titrations.info/titration-calculation

http://www.titrations.info/back-titration