Carbon containing compounds can be quantified using a redox titration procedure.
ID: 995090 • Letter: C
Question
Carbon containing compounds can be quantified using a redox titration procedure. Carbon containing compounds are oxidized to CO_2 and water quantitatively using an excess of a standard dichromate solution in acidic media (step 1); followed by a back titration of unreacted dichromate using a standard Fe^+2 solution (step 2); with the data from this procedure ethanol in a sample is quantified as follows. A 50.00 mL sample of ethanol was mixed with 10.00mL of 0.200M K_2Cr_2O_7 solution and the resulting solution acidified sufficiently. Assume a constant pH of 0 throughout the procedure. The reaction was allowed to take place and the resulting solution was titrated vs. standard Fe^+2 solution (0.0300M) and it required 24.65mL of the ferrous solution to react with unreacted dichromate. Write the three half reactions (involving ethanol, ferrous and dichromate) involved in the above procedure. Write the two complete (net ionic) reactions (step 1 and 2). How many moles of dichromate were left over after the reaction in step 1? Calculate the concentration of ethanol in the sample used. How would you detect the end point of step 2?Explanation / Answer
1. Titration and estimation
a. Half reactions,
C2H6O ----> C2H4O + 2H+ + 2e-
Fe2+ ---> Fe3+ + e-
Cr2O7^2- + 14H+ + 6e- ---> 2Cr3+ + 7H2O
b. Net ionic reactions,
3C2H6O + Cr2O7^2- + 8H+ -----> 3C2H4O + 2Cr3+ + 7H2O
6Fe2+ + Cr2O7^2- + 14H+ ----> 6Fe3+ + 2Cr3+ + 7H2O
c. moles of dichromate used for step 1 = 0.2 M x 10 ml - 0.03 M x 24.65 ml = 1.2605 mmol
d. Concentration of ethanol in sample = 3 x 1.2605 mmol/50 ml = 0.07563 M
e. The end point is identified by green solor of solution for Cr3+ from purple of Cr2O7^2-.