Chemistry (Redox)

Experiment 1 : Redox Titration Using Sodium Thiosulphate Abstract This experiment is to determine the concentration of oxidizing solution using the iodine/ thiosulphate titration where the reducing solution is potassium iodate solution and the oxidizing solution is sodium thiosulphate solution. Potassium iodate solution which is an oxidizing agent is added into an excess solution of acidified potassium iodide. This reaction will release iodine. Potassium iodide is acidified with sulphuric acid and the iodine released quickly titrated with sodium thiosulphate until it become light yellow.The iodine then detected with starch solution and it turn into dark blue solution and titrated again with sodium thiosulphate until colourless. From the reaction occur, the amount of the iodine can be determined and based on this amount, the concentration of oxidizing agent which released iodine can be determined. Introduction Redox titration using sodium thiosulphate as a reducing agent is also known as iodometric titration. The reaction is:- I2(aq) + 2NA2S2O3(aq) 2Nal(aq) + 2Na2S4O6(aq) I2(aq) + 2S2O32-(aq)2I-(aq) + S4O62-(aq) In this equation, I2 has been reduced to I-: S2O32-(aq)S4O62-(aq) + 2e- I2(aq) + 2e- 2I-(aq) The iodine/ thiosulphate titration is a general method for determining the concentration of oxidizing solution. A known volume of an oxidizing agent is added into an excess solution of acidified potassium iodide. The reaction will release iodine:- Example: a. With KMnO4 2MnO4-(aq) + 16H+ (aq)+ 10I-(aq) 2Mn2+(aq) + 5I2(aq) + 8H2O(I) b. With KIO3 O3-(aq) + 5I-(aq) + 6H (aq) 3I2(aq) + 3H2O(aq) The iodine that is released is titrated with a standard thiosulphate solution.From the stoichiometry of the reaction, the amount of iodine can be determined and from this the concentration of the oxidizing agent, which released the iodine, can be calculated. Aims a. To prepare a standard solution of potassium iodate use in determining the concentration of a sodium thiosulphate solution accurately. b. To learn the proper technique for titration. Theory In an iodometric titration, starch solution is used as an indicator as it absorbs the iodine that is released. This absorption will cause the solution to change to a dark blue colour.When this dark blue solution is titrated with the standard thiosulphate, iodine will react with the thiosulphate solution, the dark blue will disappear. So the end point of the titration is when the dark blue solour disappear. It is difficult for iodine to dissolve in water. Iodine is usually dissolves in water by adding an excess of KI so that KI3, which has similar properties to iodine, is formed. I2(aq) + KI(aq)KI3 .I3-(aq) + 2e-3I-(aq) APPARATUS AND CHEMICALS 50-mL beaker250-mL volumetric flask Glass rodBurette 25-mL pipettePipette filler 3 ? 250-mL conical flaskMagnetic fleaPotassium iodate crystals (KIO3)Potassium iodide (KI) 1M H2SO4Starch solution ca. 0. 1M sodium thiosulphate solution Level of miniscus Pull the stopcoc k in against the taper each time you turn it. A sheet of white paper or towel below the flask will help in recognizing the colour change at the end point. Swirl the flask continuosly until one drop of titrant causes a colour change throughout the entire solution. Swirl the flask continuously until the drop of titrant causes a colour change throughout the entire solution. METHODOLOGY : A. Preparation of potassium iodate solution . 0. 75 g of potassium iodate crystals is weighed accurately in 50-mL beaker. 2. 25 mL distilled water are added into beaker and stirred with glass rod to dissolve all the potassium iodate. 3. The potassium iodate solution is poured into a 250-mL volumetric flask. The beaker is rinsed with distilled water and the washings are poured into the volumetric flask. 4. Distilled water is added to the volumetric flask up to the graduated level. The volumetric flask is stoppered and shook well in order to get a homogeneous solution. B. Standardization of 0. M sodium t hiosulphate solution 1. A burette is filled with the 0. 1M sodium thiosulphate solution to be standardized. There are no air bubbles in the burette. 2. The initial reading of the burette is recorded in table 1. 1. 3. 25-mL of standard potassium iodate solution prepared in part A are pipetted into a 250-mL conical flask and a magnetic flea is added into conical flask. 4. 1 g of potassium iodide, KI, is weighed approximately and it is placed in a 50-mL beaker. 5. 10. 0 mL of 1. 0M sulphuric acid solution are added and are swirled until all the KI dissolved. . This solution is added to the conical flask containing the potassium iodate solution and it is immediately titrated with the sodium thiosulphate solution, while it is stirred using a stirrer hot plate, until a ligth yellow solution is obtained. Then, the solution is diluted with distilled water until the total volume is about 100 mL. Then, 1. 0 mL of starch solution is added and the titration is continued until the blue colour di sappears and the solution become colourless. . The final reading of the burette is recorded in the table 1. 1 . The procedures are repeated twice as the results is more accurate. CALCULATIONS : 1. Complete chemical reaction equation for reaction between ; i. iodate and iodide ions, IO3- + 5I-+ 6H+ >3I2 + 3H2O ii. iodine and thiosulphate ions, [ I2 + S2 O32- > 2I- + S4O62-]? 3 > 3I2 + 6S2 O32- > 6I- + 3S4O62- 2. Calculate the molarity of the KIO3 solution. Mass of KIO3 = 0. 75 g Volume of KIO3 = 250 mL Relative molecular mass of KIO3, = 39. 0983+126. 9045+3(15. 994) = 214. 001 g mol-1 Number of mole KIO3, = 0. 75 g ? 214. 001 g mol-1 = 0. 0035 mol Thus, Molarity of KIO3, = 0. 0035 mol ? 0. 25 L = 0. 014 mol L-1 3. The mole ratio between the iodate and thiosulphate ions is determine from the equation given are IO3- + 5I-+ 6H+ >3I2 + 3H2O [ I2 + S2 O32- > 2I- + S4O62-]? 3 3I2 + 6S2O32-> 6I- + 3S4O62- IO- : 6S4O62- 1:6 4. The molarity of the sodium thiosulphate bM1V1 = aM2V2 6(0. 014)(0 . 50) = 1(M2)(0. 021) M2= 2 M2= 0. 0210. 0211 = 0. 995 mol 5. The molarity of potassium iodide solution IO3- + 5I-+ 6H+ >3I2 + 3H2O Moles of KI- = 1g ? 1 mole of KI39+127 Molarity of KI3 = aM2V2bV2 = 1(21. 14100L)(0. 1008M)6101000L =0. 0355 M KIO3 Moles of KI3 = MV1000 =0. 003? 101000 =0. 00036 mol KIO3 Excess mole of I- = 0. 0060 – 0. 0018 =0. 0042 mol I- 0. 0060 mol0. 0042 mol = 250 mlx X =17. 5 mL 3I2 + 6S2O32-> 6I- + 3S4O62- Ratio of iodide to thiosulphate 5 : 6 bM1V1 = aM2V2 Molarity of KI = 50. 1008g(21. 141000l)6(17. 51000l) =0. 1014 M KI DISCUSSIONRedox titration using sodium thiosulphate is also known as iodometric titration. Sodium thiosulphate acts as reducing agent an at the same time it undergoes oxidation and release electron. In this titration, potassium iodate, KIO3, is used as an oxidizing agent. Potassium iodate solution is added into an excess solution of acidified potassium. This reaction release iodine. Iodine reacts with sodium thiosulphate. The reaction is: I2 (aq) + 2Na2S2O3 (aq) 2NaI (aq) +Na2S4O6 (aq) I2 (aq) + 2S2O32- (aq) 2I- (aq) + N4O62- (aq)In this equation I2 has been reduced to I- 2S2O32- (aq) S4O62- (aq) + 2e- I2 (aq) + 2e- 2I- (aq) In this iodometric titration, we use starch solution as an indicator as it absorbs the iodine that is released. The absorption causes the solution to change to dark blue colour. When the dark blue solution is titrated with the standard thiosulphate, iodine react with the thiosulphate solution. When all the iodine has reacted with the thiosulphate solution, the dark blue colour disappear. So, it is the end point of the titration when the dark blue colour disappear.However, there are a few aspects that need to be considered during the experiment. Firstly, the indicator should only be added after the titration has begun. This is because it is difficult to release the iodine while titrating with the thiosulphate. This will affect the end point. Hence, the addition of the starch should only be do ne when the colour of the solution is light yellow. Secondly, to avoid the iodine from evaporating, the solution must be titrated immediately with the sodium thiosulphate after the solution is mixed with potassium iodide,KI. The precaution taken: Wear safety goggles in the laboratory during the experiment. * Washed the effected skin immediately when in contact with chemicals * Wear gloves when taken the chemical substances. CONCLUSION The experiment was performed in order to determine the concentration of oxidizing solution. Potassium iodate, KIO3, as an oxidizing agent is added into an excess solution of acidified potassium iodide and from the reaction, iodine is released. The iodine that is released is titrated with a standard sodium thiosulphate. From the calculation, the molarity of potassium iodate is 0. 014g/cm3.