EXPT. 8 IODOMETRIC DETERMINATION OF AVAILABLE CHLORINE IN A SAMPLE OF BLEACHING POWDER Structure 8.1 Introduction Objectives 8. Principle 8.3 Requirements 8.4 Solutions Provided 8.5 Procedure 8.6 Observations and Calculations 8.7 Result 8.1 INTRODUCTION In the previous experiment you learnt about and performed iodimetric determination of ascorbic acid in a tablet of Vitamin C. In this process we recalled from Unit 10 of the Basic Analytical Chemistry course (Section 10.7.1) that the I /I couple is of medium oxidising power.molecular iodine is a weak oxidant, whereas the iodide ions are relatively weak reductant. Accordingly, the I /I redox couple can be used for the determination of reductants as well as the oxidants. In the previous experiment you used I (generated in situ from acidification of KIO 3 ) as the oxidizing agent to determine the reductant, ascorbic acid. In the present experiment you will be using iodide ions (from KI) to determine available chlorine in bleaching powder. Such a determination wherein iodide ions are used as a reducing agent is termed as iodometric determination. Bleaching powder, also known as chlorinated lime, is a yellowish-white powder having a smell of chlorine and is readily soluble in water. It is prepared by passing chlorine gas over slaked lime at a temperature of 35-45 0 and consists of a mixture of calcium hypochlorite Ca(OCl) and calcium chloride CaCl ; in addition some amount of free slaked lime i.e. Ca(OH).H O is generally present. Of these, Ca(OCl ) is responsible for the bleaching action of bleaching powder. On treatment with glacial acetic acid, it liberates chlorine gas (Cl ) as per the following reaction. ( OCI ) CH3COOH Ca( CH3COO) + HO CI Ca + +.(8.1) The amount of chlorine liberated by the action of an acid on bleaching powder (CaOCl ) is termed as available chlorine. The chlorine content of bleaching powder varies from 35 40%. Besides bleaching action it has got strong germicidal and disinfectant properties also. Accordingly, it finds application as a disinfectant for drinking water or swimming pool water. Industrially, the bleaching powder finds major use in chemical, paper, textile and oil industries. The bleaching, oxidizing or disinfecting potential of a sample of bleaching powder depends on the percentage of chlorine liberated on action of acid. We may define available chlorine to be the grams of chlorine liberated from 100 g of the bleaching powder on treatment with dilute acid. Due to its hygroscopic nature, bleaching powder absorbs moisture from atmosphere and evolves chlorine as per the following reaction ( OH) CI - + OCl + Cl + Ca + HO Ca +.(8.) Due to this deterioration, a sample of bleaching powder may always contain lesser amount of chlorine than expected and therefore a sample of bleaching powder needs to be analysed for its effective or available chlorine. In the next experiment you would 58
learn about precipitation titrations and perform a precipitation titration for the determination of chloride ions in a solution. 8. OBJECTIVES After studying and performing the experiment, you should be able to: define available chlorine, define iodometric titrations, state and explain the principle of iodometric titrations with reference to the determination of available chlorine in a sample of bleaching powder, state the reasons for using acetic acid to liberate chlorine from bleaching powder, prepare a standard solution of potassium dichromate and use it to standradise a solution of sodium thiosulphate, write the chemical equations involved in the titration of a solution of bleaching powder solution with sodium thiosulphate, prepare a solution of bleaching powder from the given sample, perform the determination of available chlorine in the solution of bleaching powder, and calculate the amount of available chlorine in the solution of bleaching powder. 8.3 PRINCIPLE As mentioned in the introduction, a sample of bleaching powder liberates chlorine gas (Cl ) on treatment with glacial acetic acid, as per the following reaction. ( OCI ) CH3COOH Ca( CH3COO) + HO CI Ca + + (8.1) The amount of chlorine so liberated is termed as available chlorine. The liberated chlorine can be used to oxidize KI (taken in excess) in presence of acid and liberate out an equivalent amount of iodine as per the following equation: Cl + KI KCl+ (8.3) I This iodine can then be determined by titrating against a standardised solution of sodium thiosulphate using freshly prepared starch solution as an indicator. The chemical reactions involved can be given as follows Na SO3 + I Na S4O6 + NaI (8.4) The overall reaction between the chlorine liberated from the bleaching powder and sodium thiosulphate mediated by potassium iodide can be obtained by adding eq. 8.3 and eq. 8.4 can be written as follows, Cl + KI KCl + (8.3) I Sodium thiosulphate, Na S 0 3.5H 0, can be obtained chemically pure. However, a standard solution of thiosulphate cannot be made by exact weighing as it reacts with atmospheric O and also the CO dissolved in water. More so, even some microorganisms can decompose thiosulphate Na SO3 + I Na S4O6 + NaI (8.4) Na SO3 + Cl + KI Na S4O6 + NaI + KCl (8.5) The sodium thiosulphate can be standardized by titrating against a primary standard solution of potassium dichromate. (alternatively, you can use potassium iodate for the purpose as described in the previous experiment) 59
The standradisation of sodium thiosulphate by using potassium dichromate is also based on iodimetric titrations. The reaction between potassium dichromate and sodium thiosulphate is mediated through the participation of iodide ions provided by potassium iodide as explained below. In acidic medium, Cr O 7 - ions gets reduced to Cr (III) ions as per the following equation + - 3+ Cr O7 + 14H + 6e Cr + 7HO The iodide ions from KI on the other hand can get oxidised to I as follows: (8.6) I I + e (8.7) In order to maintain the electron balance, we can multiply equation 8.7 by 3 to get the following, 6I 3I + 6e (8.8) Adding equations 8.6 and 8.8, we get the overall ionic equation for the reaction between potassium dichromate and potassium iodide as : + 3+ Cr O7 + 14H + 6I Cr + 3I + 7HO (8.9) Thus, from Eq. 8.9, one mole of potassium dichromate reacts with 6 moles of potassium iodide and liberates 3 moles of iodine in the process. The liberated iodine, in turn, reacts with sodium thiosulphate solution as per the following equation 3I (8.10 ) + 6SO3 = 6I + 3S4O6 The iodide ions are regenerated back. The net chemical reaction involving a titration of potassium dichromate and sodium thiosulphate in the presence of excess potassium iodide can be written by combining Eq. 8.8 and Eq. 8.9, as shown below, + 3+ Cr O7 + 14H + 6I Cr + 3I + 7HO + 6SO3 = 6I + 3S4O6... (8.9) 3I... (8.10) + 3+ Cr O7 + 14H + 6SO3 Cr + 3S4O6 + 7HO (8.11) We see from Eq. 8.11 that one mole of potassium dichromate is equivalent to 6 moles of sodium thiosulphate. Therefore, the molarities are related by the following relationship. 6 M Dichromate V Dichromate = M Thiosulphate V Thiosulphate (8.1) You would be using this equation to compute the molarity of the given solution of sodium thiosulphate. 8.4 REQUIREMENTS Apparatus Chemicals 60
Volumetric flask (100 cm 3 ) 1 Burette (50 cm 3 ) 1 Pipette (10 cm 3 ) 1 Weighing bottle 1 Burette stand with clamp 1 Conical flasks (100 cm 3 ) Funnel 1 Beakers (50 cm 3 ) Bleaching powder Potassium dichromate Potassium iodide Sodium thiosulphate Sodium carbonate Sulphuric acid Sodium bicarbonate Acetic acid Starch 8.5 SOLUTIONS PROVIDED 1. ~0.05 M Sodiun thiosulphate: It is prepared by dissolving about 7.9 g of sodium thiosulfate pentahydrate in about 00 cm 3 of distilled water taken in a 1.0 dm 3 volumetric flask and adding about 0.1 g of sodium carbonate to it. The solution is then diluted to the mark with distilled water.. 0.5% Starch indicator solution: It is prepared by mixing 0.5g of soluble starch with 50 cm 3 of distilled water taken in a 100 cm 3 conical flask or beaker and heating it with stirring at about 80 o C for about 5 minutes. The solution is then allowed to cool to room temperature. Sodium carbonate is added to adjust the ph to approximately 9.3 so as to inhibit the formation of thiosulfonic acid. 3. 10% Potassium iodide solution: It is prepared by dissolving 100g of KI in about 00 cm 3 of distilled water taken in a 1 dm 3 beaker or conical flask and stirring well to dissolve it. It is followed by making up the volume to 1 dm 3 by adding more distilled water. 8.6 PROCEDURE The determination of available chlorine in a sample of bleaching powder using iodometric titration consists of the following steps: a) Preparation of potassium dichromate primary standard b) Standradisation of sodium thiosulphate solution c) Preparation of the solution of the bleaching powder sample d) Determination of available chlorine in the above solution by iodimetric titration Follow the instructions given below in sequential manner a) Preparation of potassium dichromate primary standard Accurately weigh about 0.3g of potassium dichromate in a clean dry weighing bottle, and transfer the same to a clean conical flask of 100 cm 3 capacity through a glass funnel. Add about 0 cm 3 of distilled water and swirl the contents of the flask until all the potassium dichromate is dissolved. Make the volume upto the mark by adding more distilled water. b) Standradisation of sodium thiosulphate solution 61
Pipette out 10 cm 3 of potassium dichromate solution in a 100 cm 3 conical flask, add 10 cm 3 of dilute sulphuric acid and 1 g sodium hydrogen carbonate with gentle swirling to liberate carbon dioxide. Add 10 cm 3 of 10% KI solution, swirl, cover the flask with watch glass and allow the solution to stand for about 5 minutes in a dark place. Titrate the liberated iodine against the sodium thiosulphate solution taken in the burette until the solution acquires a light pale yellow colour. Sodiu carbon an atm CO i which air an oxidat from a Add about cm 3 of starch solution and continue adding sodium thiosulphate dropwise until the violet colour of the starch iodine complex just disappears. Repeat the standardization procedure at least three times and record your observations in Observation Table 8.1. c) Preparation of the solution of the bleaching powder sample Accurately weigh about 3-4 g of the bleaching powder and put it into a clean glass mortar. Add a little water, and rub the mixture to a smooth paste. Add a little more water, triturate with the pestle and allow the mixture to settle. Pour off the milky liquid into a 500-cm 3 volumetric flask. Grind the residue with a little more water, and repeat the operation until the whole of the sample has been transferred to the flask either in solution or in a state of very fine suspension, and the mortar washed quite clean. Make the volume upto the mark by adding more distilled water. d) Determination of available chlorine in the above solution by iodometric titration Wash the burette with distilled water and rinse with standard solution of sodium thiosulphate and then fill the burette with the same. Carefully pipette out 10 cm 3 of homogenous solution of bleaching powder and transfer into a 100 cm 3 conical flask. Add about 10 cm 3 of 10% potassium iodide (KI) solution and about half test tube of glacial acetic acid to the flask. Keep the flask in a dark place for about 5 minutes Titrate the liberated iodine against the sodium thiosulphate solution taken in the burette until the solution acquires a light pale yellow colour. Add about cm 3 of starch solution and continue adding sodium thiosulphate dropwise until the violet colour of the starch iodine complex just disappears. Repeat the standardization procedure at least three times and record your observations in Observation Table 8.. 8.7 OBSERVATIONS AND CALCULATIONS 6
a) Preparation of standard solution of potassium dichromate Mass of weighing bottle + potassium dichromate = m 1 g =...g Mass of weighing bottle (after transferring potassium dichromate) = m g =... g Amount of potassium dichromate transferred = m 1 m = m g =... g Molar mass (M m ) of potassium dichromate = 94.18 g mol 1 Volume of potassium dichromate prepared = 100 cm 3 Molarity of standard potassium dichromate solution = M K Cr O 7 m 1000 10m = = =... M 100 94. 94. b) Standardisation of sodium thiosulphate solution Volume of standard K Cr O 7 solution taken in conical flask, V dichromate = cm 3 Volume of 10% KI added : 10 cm 3 Volume of dilute sulphuric acid added : 10 cm 3 Solution in the burette: Sodium thiosulphate Indicator used: Starch Observation Table 8.1: Standardisation of sodium thiosulphate S.No. Volume of potassium dichromate (in cm 3 ) 1 3 Burette reading Initial Final Concordant reading Titre value (in cm 3 ) (Final-initial reading) The concentration of the given sodium thiosulphate solution can be determined as follows. The reactions involved: + 3+ Cr O7 + 14H + 6SO3 Cr + 3S4O6 + 7HO Molarity equation: 6M Dichromate V Dichromate = M Thiosulphate V Thiosulphate M Thiosulphate = 6M Dichromate V Dichromate V Thiosulphate Substituting the values, the molarity of thiosulphate = The molarity of given thiosulphate solution is =.M c) Preparation of the solution of the bleaching powder sample 63
Mass of bleaching powder taken = d) Determination of available chlorine in the given sample of bleaching powder Volume of bleaching powder solution taken in conical flask, = 10 cm 3 Volume of acetic acid added : 10 cm 3 Volume of 10% KI added : 10 cm 3 Solution in the burette: Sodium thiosulphate Indicator used: Starch Observation Table 8.: Determination of the amount of available chlorine in the solution of given bleaching powder S.No. Volume of bleaching powder solution (in cm 3 ) 1 3 Burette reading Initial Final Concordant reading Titre value (in cm 3 ) (Final-initial reading) The molarity of the iodine liberated from KI solution (which in turn is equal to the amount of chlorine liberated from the bleaching powder on the action of acetic acid) can be determined as follows. The reaction involved: Na SO3 + Cl + KI Na S4O6 + NaI + Cl Molarity equation: M Chlorine V Chlorine = M Thiosulphate V Thiosulphate M V Thiosulphate Thiosulphate M = Chlorine V Chlorine Substituting the values, of the molarity and the volume of thiosulphate used, the molarity of chlorine is found to be: =...M The mass of chlorine liberated = molarity molar mass =.M 70.90 g mol 1 =. 70.90 g = P g dm -3 The mass of bleaching powder dissolved per liter = w g (the solution was w g/ 500 cm 3 ) Thus, the amount of available chlorine in w g of bleaching powder = P g The amount of available chlorine in 100.0 g of bleaching powder = 50 P / w g The available chlorine (the grams of chlorine liberated from 100 grams of the bleaching powder on treatment with dilute acid) = 50 P/w g 8.8 RESULTS The available chlorine (the grams of chlorine liberated from 100 g of the bleaching powder on treatment with dilute acid) =.. g 64