Notes about Physiology

American University of Science & Technology Department of Laboratory Sciences and Technology CHE: 205L: CHEMICAL ANALYSIS LABORATORY Experiment IV Redox Titration Name: Helena Al Jawhary / Partner’s Name: Eyad Aridi Instructor’s Name: Dr Juliana El Khoury Date of experiment: 10/3/2013 Redox Titration Objective: • To learn some technique in volumetric analysis: Redox titration. • To review the stoichiometry of an oxidation-reduction reaction. • To practice the titration technique.

• To determine the concentration of an unknown sodium oxalate (Na2C2O4) solution by titrating it against standardized potassium permanganate solution (KMnO4).Principle: Redox reaction is just like an acid-base reaction. An acid can show its acidic properties in the presence of base only. Like acid-base reaction, redox reactions are concerned with the transfer of electrons between species. One compound has to give electron and oxidizes and another compound has to accept electron and reduces.

Hence redox reaction is a combination of oxidation-reduction reaction. Each reaction by itself is known a "half-reaction" and whole reaction is called as redox reaction. For that, the half reactions (oxidation half reaction and reduction half reaction) must be balanced in order to get the fully balanced redox eaction. Titration is a laboratory method of quantitative analysis to determine the concentration of a given sample. Titration is used to determine the concentration of an unknown substance by using standard compound. This method is based on either acid-base reaction or oxidation-reduction (Redox) reaction.

Usually potassium permanganate solution is used as a standard solution in such redox reactions. Since potassium permanganate ion is a strong oxidizing agent , hence can be used to analyse the solutions.The main advantage of permanganate solution in the titration of colorless unknown solutions is that it acts as self-indicating. The color of MnO4- quickly disappears as it is reduced to Mn2+ in the presence of the reducing agent. Finally at the endpoint, all the reducing agent has been used up and next drop of MnO4- solution detected color change where a persistent pink color appears for almost 30 sec.

By using the used concentration of the oxidizing agent added, we can figure out the concentration of reducing agent present in the unknown sample using a known concentration of standard compound.Materials and methods: In order to get the titration apparatus complete, we must prepare the solution, and thus we need the following equipment to be ready: -Equipment used: 250 ml volumetric flask – beakers – 25 ml pipet with pipet filler – titration stand – buret clamp –100 ml graduated cylinder – 250 ml Erlenmeyer flask- electric heater – funnel - thermometer -Key reagents used: Na2C2O4 solid - oxalate solution – potassium permanganate – H2SO4 solution.Procedure: Part 1: 1. Preparation of a primary standard Na2C2O4 solution: a. weigh the mass of Na2C2O4 using analytical single-pan balance. b.

transfer them into 250 ml volumetric flask. c. add distilled water slowly while homogenizing the solution until reaching line mark. And now we have the required Na2C2O4 solution needed for titration. Part 2: Standardization of KMnO4: a.

Pipet 25. 00 ml Na2C2O4 into a 250 ml Erlenmeyer flask using 25 ml pipet with pipet filler. b. Add 75 ml of 0.

75 M H2SO4 using graduated cylinder. c. Heat the oxalate solution to 80° - 90°C and allow it to stir for 10 minutes. d. inse the graduated buret with few ml of distilled water. e.

rinse the graduated buret with few ml potassium permanganate solution to prevent contamination with distilled water. f. fill the graduated buret with potassium permanganate solution until 0. 00 g. open the tap and start the stirrer then start the titration until a persistent pink color occurs. h.

Repeat the same procedure twice in order to reduce the percent error. Part 3 : Analysis of an unknown oxalate: a. obtain the unknown from the stock room prepared by the assistant; and record its number b. we Dissolve it in 10 mL of water in a 250 mL Erlenmeyer flask.

c.Repeat same procedure as in part 2. 5. Results: Part 1: Table 1: Preparation of standard H2C2O4 Solution. Weight of Na2C2O4| 1.

679 g| Volume of solution| 250 ml| Molarity of (Na2C2O4) = 0. 04 mol/l Part 2: Table 2: titration of oxalate with unknown KMnO4 solution Trial| Buret upper reading| Buret lower reading| Volume of KMnO4| 1| 0. 00| 24. 2| 24.

2 ml| 2| 24. 2| 49. 3| 25. 1 ml| Average volume of KMnO4 = 24.

6 ml Molarity of KMnO4= 0. 01 mol/l Ionic equation: 2MnO4- + 5 C2O4 2- + 16 H+ > 2 Mn2+ + 10 CO2 + 8 H2O Part 3: Table 3: analysis of unknown oxalate: Trial| Buret upper reading | Buret lower reading| Volume of KMnO4| 1| 0. 0| 10. 4 | 10. 4 ml| The mass of Na2C2O4 = 0.

34 g 6. Calculations: Part 1: preparation of primary standard Na2C2O4 solution: M Na2C2O4 = number of molesvolume of solution But we are already given that N (normality)= 0. 1 N And N= C ? x = C ? 2 This gives C= 0. 1/2= 0.

05 mol/l And thus n= C/V = 0. 01 moles And thus m=MM ? n = 1. 675 g (we have Molar Mass of Na2C2O4 = 134. 00g/mol) Using analytical single-pan balance we got m=1.

679 g Then n=m/MM = 1. 679/134. 00 = 0. 01 and thus molarity (M) = 0.

01/0. 250 = 0. 04 mol/l Part 2: standardization of unknown potassium permanganate uring titration the equation of the reaction taking place is the following: 2MnO4- + 5 C2O4 2- + 16 H+ > 2 Mn2+ + 10 CO2 + 8 H2O In the standardization step of KMnO4, at the equivalence point: Number of moles added from buret = number of moles present in beaker n KMnO4 = n Na2C2O4 And according to stoic. Ratios : / 2 = C2O4-/5 Thus 5 MnO4- = 2 C2O4- 5 M? V(MnO4- )= 2 M? V(C2O4- ) 5? M? 24. 6? 10-3 = 2? 0.

04? 0. 025 then M (KMnO4) = 0. 01 mol/l Part 3: Standardization of unknown oxalate In the standardization of unknown oxalate we have same reaction: 2MnO4- + 5 C2O4 2- + 16 H+ > 2 Mn2+ + 10 CO2 + 8 H2OAccording to stoichiometric ratios: 5 n MnO4- = 2 n C2O4 2- 5 ( M? V) MnO4- = 2 n C2O42- But we already calculated the molarity of potassium permanganate solution in the first part of the experiment which is M= 0. 01 mol/l Thus we can use this value in the following calculations: According to the titration technique, the volume of potassium permanganate needed to standarize the oxalate solution is 10. 4 ml, then: 5 (0. 01 ? 10.

4 ? 10-3 ml ) = 2n Thus n (C2O42-) = 2. 6 ? 10-4 moles And we are already given the molar mass to be 134. 00 And n = m/MM Hence, the mass of unknown oxalate given by the instructor is: m = 26? 0-4 ? 134. 00 = 0. 34 g 7. Discussion: In this lab we are trying to determine the molarity of unknown potassium permanganate and then the mass of unknown oxalate by performing redox titration using potassium permanganate as our titrant.

In the first part of the experiment, which was the easiest part since the same technique was followed in the acid-base titration done in the previous session, we were able to prepare the sodium oxalate solution needed by calculating the theoretical mass needed and obtaining it using analytical single-pan balance with a slight difference of 0. 02 between them. And thus we prepared the required solution. The second part required more accuracy in following the titration technique. Potassium permanganate is the most common oxidizing agent for redox titrations because the purple colored permanganate ion acts as its own indicator.

In the titration with permanganate the solution remains colorless until the oxalate is completely consumed, the end-point is reached. Then, the addition of one drop of permanganate solution gives a pink color..This titration had to be run slowly rather than the acid-base titration because we were dealing with potassium permanganate as an indicator. Thus if this titration is carried quickly the acid used ( H2SO4) would be exhausted, and if not enough acid is present a brown precipitate will form (MnO2 ) instead of Mn2+ according to the following reaction: 4 MnO4- + H2O > 4 MnO2 + 4 OH- And the formation of brown precipitate will mask the endpoint that is revealed by the persistent pink color of Mn2+.

This was obviously experienced with some groups in the lab session where they witnessed the brown precipitate instead of pink color. But me and my partner took our time in running the titration drop by drop and so we were able to achieve the goal of this titration that is to calculate the molarity of unknown potassium permanganate by not encountering such problems in both trials. We were able to detect the endpoint when our solution turned into light persistent pink due to the formation of manganese ions. And then we calculated the molarity of unknown permanganate to be 0. 1 mol/l Our percentage error was relatively low; therefore we can say that our experiment was somehow reliable, but still there is a percentage of error and an inefficiency in the designed experiment and that is due to weighing errors and buret reading errors. The third part of the experiment resembled the second one but with a different purpose.

The instructor’s objective was to calculate the mass of oxalate by following same titration but with known potassium permanganate and unknown oxalate solution through calculating the volume of potassium permanganate needed and following stoichiometric calculations to reach the mass of oxalate used.And thus we were able to figure out the mass of oxalate given by the instructor. 8. Questions: 1. a- The equation of the reaction that takes place between potassium permanganate and iron II sulfate: MnO4- + 8H+ + 5e- Mn2+ + 4H2O Fe2+ Fe3+ + 1e- Adding both equation we get overall equation: MnO2- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + H2O b- according to stoic.

Ratios: n KMnO4 /1 = n FeSO4 /5 THEN 5 MnO4- = Fe2+ 5 ( MV ) MnO4- = MV Fe2+ 5 ( 16. 42 ? 0. 1327 ) = ( M ? 20. 00) Which gives M (ferrous sulfate) = 0.

544 mol/l 2.Compound| Element 1/ oxidation num| Element 2 / oxidation num| Mg3N2: | Mg : 2+| N: 3-| CsO2:| Cs : 4+| O : 2-| CO32- :| C : 4+| O: 2-| C2O42- :| C : 3+| O : 2-| PtCl62- :| Pt : 4+| Cl : -1| Al2O3: | Al : 3+| O : 2-| 9. Conclusions: Eventually, redox reactions are very important reactions in the field of chemistry. In addition to the role of redox titrations in giving an estimation of the molarity of unknown compound, they also play a role in biochemical reactions. The electron transfer system in cells and oxidation of glucose give a great evidence of redox reactions.