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A-level Practical Chemistry experiment - Estimation of available chlorine in a commercial bleaching solution.

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Tang King Po School A-level Practical Chemistry Date: 13-11-2003 Class: 6A Class Number:11 Name: Kong Siu Wai Mark: ___________ Title Estimation of available chlorine in commercial bleaching solution. Aim Understand and handle iodometry. Learn to write a full report. Introduction For indirect determination of oxidizing agents, iodometry is widely applied. Here, standard solutions of sodium thiosulphate are required as the titrating agent. The procedure to determine an oxidizing agent is as follows: An oxidizing agent to be analyzed is added to an approximate concentration, excess of potassium iodide solution. The iodine liberated is then titrated with a standard solution of sodium thiosulphate until the end point is reached. In general, the reactions may be represented as follows: Oxidizing agent ( to be analyzed ) + I- (excess ) --> I2(aq) + other products I2(aq) + 2S2O32-(aq) --> 2I-(aq) +S4O62-(aq) In this case, iodide ion acts as a moderately effective reducing agent. The quantity of iodine formed is chemically equivalent to the amount of oxidizing agent and thus serves as the basis for the analysis. Similarly, for this experiment, the active ingredient is chlorate (I) ion in a bleach which will undergo redox reaction with excess of potassium iodine solution in the presence of acid, liberating iodine, which is then titrated against standard sodium thiosulphate solution. ...read more.


Second, it is also not necessary to measure the accurate amount of ethanoic acid because: Ethanoic acid is needed to be excess for speeding up the completion of reaction (1) and (2). In reaction (1) and reaction (2), both need H+ ions for the completion of the redox reactions. Thus, ethanoic acid is needed to be speed up the two reactions or it will make the great errors for the experiment. Apart from it, chlorine gas evolved in reaction (1) is easy to escape from the solution in the conical flask, which needs a short time for reaction to minimize the escaping of chlorine gas. In this way, the acid in excess amount can achieve this. Furthermore, NO3- , which is a very weak oxidizing agent compare to chlorine gas, so that it will not make any inaccuracy for the experiment though in excess amount. (2) What is the function of starch solution? Why we should NOT add the starch solution at the beginning of the titration? The starch solution acts as indicator, whose end point is to indicate the completion of reaction. In fact, there is the color change from iodine to iodide. It, however, cannot be used to accurately detect the end point (the change in color 'brown-->yellow-->colorless' is very difficult to observe), thus, starch (preserved with salicylic acid) ...read more.


(6) Suggest another application of iodometry besides analysis of bleach. Besides the analysis of chlorine bleach, it also can be used to determine the copper content in an alloy-a physically mixture of different atoms at least one in which is metal. It can be used by this way: The certain amount of copper-containing alloy was dissolved by the sample in noitric acid. After the excess nitric acid is removed by boiling, an excess of potassium iodide crystals are added to the resultant copper(II) nitrate solution. The liberated I2 then is titrated with standard sodium thiosulphate solution until the end point. (i.e. Starch is used as indicator.) The equations for the reactions are: Cu--> Cu2+ + 2e ---(1) 2Cu2+(aq) +4I-(aq) --> 2CuI(s) +I2(aq)------(2) I2(aq) + 2S2O32-(aq) --> 2I-(aq) +S4O62-(aq)-(3) By comparing the coefficients of those reactant, the content can be found. Apart from it, it also can be used to determine the oxidizing agents(e.g. Fe3+). In this experiment, excess NaI(aq) is added to an oxidizing agent, Fe3+ without unknown molarity. The I2 will be generated quantitatively.* 2Fe3+(aq) + 2I-(aq) --> 2Fe2+(aq) + I2(aq) oxidizing agent (stronger than I2) The iodine liberated is then determined by the titration with standard thiosulphate solution to a starch end point. [i.e. I2(aq) + 2S2O32-(aq) --> 2I-(aq) +S4O62-(aq)].The end point is indicated when the blue-black color disappears. Then use the data marked and then calculate the required molarity. By Kong Siu Wai Page 6 (c)All rights reserved. ...read more.

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