Iodometry

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Iodometry

  1. 1. KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY DEPARTMENT OF CHEMISTRY YEAR TWO (CHEM 270) TITLE: IODOMETRY NAME: OPOKU ERNEST DATE: 28TH JANUARY, 2014 Page 1 of 10
  2. 2. TITLE: IODOMETRY AIMS AND OBJECTIVES: 1. To determine the amount of the active ingredient, hypochlorite in two household bleaches, by titration of iodine released by reaction of hypochlorite with iodide. 2. To determine which bleach is the best to buy. INTRODUCTION Iodometry is the quantitative analysis of a solution of an oxidizing agent by adding an iodide which reacts to form iodine, which is then titrated. Iodometric titration, is a method of volumetric analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point. Iodometry involves indirect titration of iodine liberated by reaction with the analyte, whereas iodimetry involves direct titration using iodine as the titrant. Iodine can be used as an oxidizing agent in many oxidation-reduction titrations and iodide can be used as a reducing agent in other oxidation-reduction titrations: I2 + 2 e– = 2 I– (1) If a standard iodine solution is used as a titrant for an oxidizable analyte, the technique is iodimetry. If an excess of iodide is used to quantitatively reduce a chemical species while simultaneously forming iodine, and if the iodine is subsequently titrated with thiosulfate, the technique is iodometry. Iodometry is an example of an indirect determination since a product of a preliminary reaction is titrated. The use of iodine as a titrant suffers from two major disadvantages. First, iodine is not particularly soluble in water, and second, iodine is somewhat volatile. Consequently, there is an escape of significant amounts of dissolved iodine from the solution. Both of these disadvantages are overcome by adding iodide (I–) to iodine (I ) solutions. In the 2 presence of iodide, iodine reacts to form triiodide (I3) which is highly soluble and not volatile. I2 + I– = I3 (2) The major chemical species present in these solutions is triiodide. The reduction of triiodide to iodide is analogous to the reduction of iodine. (3) I3 + 2 e = 3 ITriiodide reacts with thiosulfate to yield iodide and tetrathionate. I2 + 2S2O2-3 2I- + S4O2-6 (4) Page 2 of 10
  3. 3. Dilute triiodide solutions are yellow, more concentrated solutions are brown, and even more concentrated solutions are violet. Iodide solutions are colorless. If all of the other solution components are colorless, it is possible to detect the endpoint of titrations involving triiodide without the use of an indicator. Endpoint detection is considerably easier, however, with an indicator. The indicator that is usually chosen for titrations involving iodine (triiodide) is starch. Starch forms a dark blue complex with iodine. The end point in iodimetry corresponds to a sudden color change to blue. Likewise the end point in iodometry corresponds to a sudden loss of blue color due to the complex. Potato starch, rather than corn starch, is preferred for making the indicator solution since the color change due to the starch complex at the end point is sharper. In iodometry the starch is added only after the color due to triiodide has begun to fade, i.e., near the endpoint, because starch can be destroyed in the presence of excess triiodide. In the first portion of this experiment a sodium thiosulfate solution is prepared and standardized with the primary standard potassium iodate. Iodate (IO3) reacts with an excess of iodide in acid solution to yield triiodide, which is subsequently titrated with the standardized thiosulfate solution. Iodometric Determination of Hypochlorite in Commercial Bleach Product In most liquid laundry bleaches and in some solid bleaches, the active ingredient is hypochlorite (OCl–). Liquid bleaches usually contain sodium hypochlorite and the bleaching action is caused by the strong oxidizing properties of OCl–, which also are exploited in this iodometric determination of hypochlorite. In the experiment, iodide is used to reduce the hypochlorite in bleach. The reaction in acid solution yields chloride and triiodide. OCl– + 3 I– + 2 H+ = Cl– + I3 + H2O (5) The triiodide formed in this reaction is titrated with the standard thiosulfate solution that was prepared earlier. (6) + 3 I2 S2O32– + I3- = S4O6 Thiosulfate ↑ ↑ Tetrathionate The endpoint of the titration is located with the starch indicator solution that was also prepared earlier. CHEMICALS AND EQUIPMENT 1. 2. 3. 4. 5. 6. 0.1N Sodium thiosulphate Potassium iodide 2M sulphuric acid 0.1M KMnO4 Starch indicator White magic bleaching solution ( GH¢3.50) Page 3 of 10
  4. 4. 7. Janola bleaching solution ( GH¢ 5.0) 8. 10ml pipette 9. 3 Conical flask 10. Electronic balance 11. Burette 12. Funnel PROCEDURE 1. Saturation of sodium thiosulphate Experiment 2g of KI + 10ml of KMnO4 2g of KI + 10ml of KMnO4 + 5ml of 2M H2SO4, follow by titration with the sodium thiosulphate. Observation Purple colour decolourised The decolouration persist, Colour changes were observed until pale yellow colouration occurs, the colour change was left for a while. 1ml of freshly prepared iodine was added to A blue black solution was formed indicating the pale yellow solution the presence of iodine. The blue solution was titrated against a drop The blue black colour disappeared to form a of sodium thiosulphate. colourless solution. The procedure was repeated for another two experiments. 2. Analysis of bleaching solution. JANOLA Experiment Observation 1g of KI was measured into a conical flask and then 20ml of distilled water added to it. 1ml of the bleach (janola) + 10ml of 2M Orange solution was observed H2SO4 was added to the prepared solution. The liberated iodine was then titrated against There was a colour change from orange to the thiosulphate solution. pale yellow 1ml of starch solution was added to the pale A blue black solution form indicating the yellow solution presence of iodine. The blue solution was titrated against a drop The blue black colour disappeared to form a of sodium thiosulphate. colourless solution The procedure was repeated for two more experiment. Page 4 of 10
  5. 5. WHITE MAGIC Experiment Observation 1g of KI was measured into a conical flask and then 20ml of distilled water added to it. 1ml of the bleach solution (white magic) was added to the prepared solution. The liberated iodine was titrated against the The colour change from orange to pale thiosulphate solution. yellow. 1ml of starch solution was added to the pale A blue black solution form indicating the yellow solution presence of iodine. The blue solution was titrated against a drop The blue black colour disappeared to form a of sodium thiosulphate colourless solution The procedure was repeated for two more experiment. TABLE OF RESULTS STANDARDIZATION OF SODIUM THIOSULPHATE Titration Final volume/ml Initial volume/ml Titre value/ml 1 10.80 0.00 10.80 2 10.85 0.00 10.85 3 10.80 0.00 10.80 Average titre = (10.80 + 10.85 +10.80) = 10.82ml 3 JANOLA Titration 1 Final volume/ml 1.50 Initial volume/ml 0.00 Titre value/ml 1.50 Average titre= (1.30 + 1.30)= 1.30ml 2 2 2.80 1.50 1.30 3 4.10 2.80 1.30 2 6.65 6.45 0.20 3 6.89 6.65 0.24 WHITE MAGIC Titration Final volume/ml Initial volume/ml Titre value/ml 1 6.45 6.10 0.35 Average titre= (0.20 + 0.24) = 0.22ml 2 Page 5 of 10
  6. 6. CALCULATIONS AND EVALUATION OF DATA From the reaction equation 2MnO-4 + 16H+ + 10I- 2Mn2+ + 5I2 + 8H2O n(MnO-4) = 2 n(I2) 5 n(I2)= 5 × n(MnO2 n(MnO-4) = concentration x volume n(MnO-4)= 0.1mol/L × 0.01L = 0.001mol n(I2) = 5 × 0.001mol= 2.5×10-3mol 2 from the equation I2 + 2S2O42- 2I- + S4O-26 n(S2O-23) = 2 × 2.5×10-3mol = 0.005mol from the titration average titre = 10.82ml = 0.01082L concentration = mole volume [S2O-23] = 0.005mol 0.01082L = 0.462M JANOLA From I2 + 2S2O2-3 2I- + S4O-26 n(S2O2-3) that reacted with the liberated I2 = conc. x average titre volume n(S2O2-3) that reacted with the liberated I2 = 0.462 M × 0.00130L = 6.006 × 10-4mol Page 6 of 10
  7. 7. but n(I2) = ½ × n(S2O-23) = ½ × 6.006 × 10-4= 3.003 ×10-4mol hence n(I2) liberated= 3.003 ×10-4mol from the equation ClO- + 2I- + 2H+ Cl- + I2 + H2O n(I2) liberated = n(ClO-) hence n(ClO-)= 3.003 ×10-4mol since the volume of bleach taken = 1ml =0.001L then [ClO-]= (3.003×10-4) = 0.3003M 0.001L Mass(ClO-) = mole (ClO-) x molar mass of (ClO-) Molar mass(ClO-) = (35.5 + 16) = 51.5g/mol m (ClO-) in Janola= 3.003 ×10-4 × 51.5 = 0.0155g WHITE MAGIC n(S2O2-3) that reacted with the liberated I2 = 0.462×0.22×10-3 = 1.0164 ×10-4mol but n(I2) = ½ × n(S2O2-3)= ½ ×1.10164×10-4 = 5.082 × 10-5mol since n(I2)= n(ClO-), n(ClO-)= 5.082 ×10-5mol since the volume of bleach used = 1ml = 0.001M thus [ClO-]= (5.082 × 10-5= 0.05082M 0.001L Mass(ClO-) = mole (ClO-) x molar mass of (ClO-) Page 7 of 10
  8. 8. Molar mass(ClO-) = (35.5 + 16) = 51.5g/mol m (ClO-) in white magic = 5.082×10-5 × 51.5= 2.61723 × 10-3g 1. Janola If the cost is 5.00 Ghana cedis A unit in cedis per gram = GH¢ 5.00 = GH¢ 322.58p/g 0.0155g = GH¢ 322.58p/g ii. white magic If the cost is GH¢ 3.50p A unit in cedis per gram = GH¢ 3.50p = GH¢ 1337.29p/g -3 2.61723 × 10 g = GH¢ 1337.29p/g Based on my results, Janola is the best to buy amongst the two. DISCUSSION From the experiment performed above, the reaction between the measured gram of potassium iodide and 10ml of potassium permanganate, the purple colour of the permanganate was decolourised by iodide ion. 5ml of sulphuric acid was added to the permanganate- iodide solution to prevent the decomposition of the ions in the solution and also to create medium for the solution to proceed. 2MnO-4 + 16H+ + 10I- 2Mn2+ + 5I2 + 8H2O The solution was then titrated against a standard sodium thiosulphate, the titration proceeds with colour changes till pale yellow colour was observe, showing the presence of iodine in solution. The solution was allow for two minutes, this was done to see whether the pale yellow colour will stand, 1ml of starch solution was added to the pale yellow solution and the colour changes to blue black, confirming the presence of the iodine. A few drop of the thiosulphate turn the solution colourless. The volume of the titration was recorded and addition two experiments were conducted. Average titre value was determined for the three experiments. The iodide content of two different bleaches (Janola and White magic) were given, 1ml of each sample was taken into different conical flask, a measured amount of potassium iodide was dissolves into a certain volume of water. The measured samples of the bleach was transferred into the iodide solution which gives yellow solution. This was also titrated against sodium thiosulphate till a pale yellow solution was obtain, 1ml of starch solution was Page 8 of 10
  9. 9. added to the pale yellow solution which turns blue black. A drop or two of thiosulphate changes the solution to colourless. It was observed that the Janola has higher titre values whilst white magic has less titre values. I2 + 2S2O32- 2I- + S4O62- In the calculation, the mole ratio of permanganate and iodine was used together with their stochiometric ratio was used to find mole of the iodine, by so doing concentration and volume of the permanganate was used. Knowing the mole of iodine, which is in ratio one is to two (1:2) with the thiosulphate, mole of thiosulphate is equal two times the mole of iodine. So knowing the mole of thiosulphate, the concentration of the thiosulphate was calculated using the volume and the mole. With the mole of iodine, which is in ratio of (1:1) with perchlorate? The mole and concentration of hypochlorate was calculated, masses of the hypochlorates in both janola and white magic are calculated. These masses are used to determine the price of each of the bleach. ClO- + 2I- + 2H+ Cl- + I2 + H2O. CONCLUSION From the experiment and the calculation above, we can conclude that the active ingredients of the two bleaches are the hypochlorite. The respective masses of hypochlorite present in Janola and White magic were found to be 15.5mg and 2.86172mg. Their price tags were found to be right because of the quantity difference. However it is recommended that Janola is the best to buy. PRECAUTIONS 1. The gases produced during the dissolution of the brass sample are toxic. Therefore all reactions that involved the evolution of a gas were done in fumehood. 2. Goggles were worn to prevent any of the solution from getting into contact with the eye during boiling. 3. Gloves were worn to prevent any of the solution from getting into contact with the body. REFERENCES 1. Modern Inorganic Chemistry, Second Edition by William L. Jolly, 2010, pages 46 to 49. 2. Journal of Solid State Chemistry, 2012, pages 4 to 7, retrieved from http://www.sciencedirect.com/science. Page 9 of 10
  10. 10. 3. Concise Inorganic Chemistry, Fifth Edition by J.D Lee, 2007, pages 202-204 and 951. 4. KNUST Chemistry laboratory manual for second year, CHEM 269 & 270, page 32- 34. Page 10 of 10

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