EXPT. 10 DETERMINATION OF ZINC BY PRECIPITATION WITH POTASSIUM FERROCYANIDE USING INTERNAL INDICATOR

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1 EXPT. 10 DETERINATION OF ZINC BY PRECIPITATION WITH POTASSIU FERROCYANIDE USING INTERNAL INDICATOR Structure 10.1 Introduction Objectives 10. Principle 10.3 Requirements 10. Solutions Provided 10.5 Procedure 10. Observations and Calculations 10.7 Result 10.1 INTRODUCTION In the previous experiment you learnt and performed precipitation titrations called argentometric titrations. These involved the determination of chloride ions in a given solution by precipitating them as silver chloride using silver nitrate as the titrant. The determination was done by two different methods, based on the type of indicator used for the determination of the end point. While the ohr s method used a precipitation indicator, the Fajan s method employed an adsorption indicator. In this last experiment of this course you will learn and perform the determination of zinc ions in a given solution, by precipitating the ions with potassium ferrocyanide. Interestingly, you would be using a redox indicator for the determination of the end point. Objectives After studying and performing this experiment, you should be able to: state the principle of the determination of zinc ions in a given solution by precipitating the ions with potassium ferrocyanide, explain various basis of end point determination in the precipitation titrations, prepare a standard solution of zinc sulphate and use it for the standradisation of potassium ferrocyanide, titrate the given solution of zinc ions with potassium ferrocyanide by using diphenylamine as internal indicator, and calculate the amount of zinc ions in the given aqueous solution. 10. PRINCIPLE The determination of Zn ++ ions in neutral or weakly acidic solution by precipitation titration using standard K [Fe(CN) ] solution is based on the following reaction: Zn3 + K In the titration of zinc ions with potassium ferrocyanide, the continuous addition of the titrant precipitates the analyte ions and immediately after the equivalence point there would be an excess of the titrant. This stage could be ascertained with the help of a suitable indicator. As mentioned above, in this titration you are going to use a redox indicator i.e., diphenylamine (transition range V) for the purpose. 7

2 In order to use the redox indicator we need to create a redox system. You know that potassium ferrocyanide K [Fe(CN) ] is a complex salt of ferrous ions and like other ferrous compounds, it has reducing properties. Therefore, on oxidation, K [Fe(CN) ] forms the corresponding ferric salt; that is potassium ferricyanide, K 3 [Fe(CN) ]. The presence of these two moieties in solution constitutes a redox system whose potential is given by the following equation: [ Fe( CN) ] E = 0.3+ log 1 [ Fe( CN) ] In order to create the above mentioned redox couple we need to add a little of potassium ferricyanide in the analyte solution. Therefore, in one of the ways of performing this determination, a small volume of the solutions of, potassium ferricyanide, K 3 [Fe(CN) ] is added to the titrand ( zinc ion solution) along with diphenylamine indicator. This imparts a blue-violet colour to the solution. This can then be titrated with a solution of potassium ferrocyanide ions that precipitate the zinc ions as per the equation Zn3 + K Since the added titrant is used up in precipitating the zinc ions, there are no free [ Fe( CN) ] ions, the potential of the solution does not change and the colour of the solution remains the same till all the zinc ions are precipitated out. Thereafter, first drop of K [Fe(CN) ] in excess lowers the oxidation potential of the solution to an extent that the indicator becomes colourless. This marks the end point of the titration. The precipitation of zinc with ferrocyanide is a little slow therefore the titration should not be performed rapidly. Earlier, the equivalence point in this titration was determined generally by means of an external indicator, uranyl nitrate, which forms a brick-red precipitate of a uranyl salt with K [Fe(CN) ] REQUIREENTS Apparatus Volumetric flask (100 cm 3 ) 1 Burette (50 cm 3 ) 1 Pipettes (10 cm 3 ) 1 Weighing bottle 1 Burette stand with clamp 1 Conical flasks (100 cm 3 ) Funnel 1 Beakers (50 cm 3 ) Chemicals heptahydrate Potassium ferricyanide Diphenylamine Ammonium sulphate Sulphuric acid 10. LUTIONS PROVIDED potassium ferrocyanide, K [Fe(CN) ]: It is prepared by dissolving about 1.85g of potassium ferrocyanide in about 0 cm 3 of distilled water taken in a 100 cm 3 volumetric flask and making up the volume with more distilled water.. Diphenylamine indicator: It is prepared by dissolving about 1.0 g of diphenylamine in 100 cm 3 of concentrated sulphuric acid. 3. Potassium ferricyanide, K 3 [Fe(CN) ]: It is prepared by dissolving about 1.5 g of potassium ferrocyanide in about 0 cm 3 of distilled water taken in a 100 cm 3 volumetric flask and making up the volume with more distilled water. 75

3 10.5 PROCEDURE The determination of zinc ions in a given solution using potassium ferrocyanide consists of the following steps: b) Standradisation of potassium ferrocyanide solution Follow the instructions given below in sequential manner to carry out the determination of zinc ions by using potassium ferrocyanide. Accurately weigh about.87 g of zinc sulphate heptahydrate in a clean dry weighing bottle, and transfer the same to a clean volumetric 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 zinc sulphate heptahydrate is dissolved ake the volume upto the mark by adding more distilled water b) Standradisation of potassium ferrocyanide solution Pipette 10 cm 3 of the standard zinc sulphate into a 100 cm 3 conical flask and add g of (NH ), about 0 cm 3 of 3 H solution, two or three drops of K 3 [Fe(CN) ] solution, and 3 drops of diphenylamine indicator to it. The solution acquires a blue colour, titrate the solution with potassium ferrocyanide solution taken in a burette. Continue the titration till the colour of the solution changes sharply (it becomes colourless) after addition of a single drop. Repeat the standardization procedure at least three times and record your observations in Observation Table Use the data so obtained to determine the molar concentration of the potassium ferrocyanide solution. Pipette 10 cm 3 of the given zinc sulphate solution into a 100 cm 3 conical flask and add g of (NH ), about 0 cm 3 of 3 H solution, two or three drops of K 3 [Fe(CN) ] solution, & 3 drops of diphenylamine indicator to it, The solution acquires a blue colour, titrate the solution with potassium ferrocyanide solution taken in a burette, Continue the titration till the solution colour changes sharply (it becomes colourless) after addition of a single drop. Repeat the procedure at least three times and record your observation in Observation Table 10.. Use the data so obtained to determine the molar concentration of the zinc ion solution 7

4 10. OBSERVATIONS AND CALCULATIONS ass of weighing bottle + zinc sulphate= m 1 g =...g ass of weighing bottle (after transferring zinc sulphate) = m g =... g Amount of zinc sulphate transferred = m 1 m = m g =... g olar mass ( m ) of zinc sulphate heptahydrate = g mol 1 Volume of zinc sulphate prepared = 100 cm 3 olarity of standard zinc sulphate solution = Zn m m = = = b) Standardisation of potassium ferrocyanide solution Volume of standard Zn solution taken in conical flask, V Zn = cm 3 Solution in the burette: Indicator used: Diphenylamine Observation Table 10.1: Standardisation of potassium ferrocyanide S.No. Volume of zinc sulphate (in cm 3 ) 1 3 Burette reading Initial Final Concordant reading Titre value (in cm 3 ) (Final-initial reading) The concentration of the given potassium ferrocyanide solution can be determined as follows. The reactions involved: Zn3 + K olarity equation: V = 3 V Zn Zn = 3V V Substituting the values, the molarity of potassium ferrocyanide = The molarity of given potassium ferrocyanide solution is =. Volume of standard Zn solution taken in conical flask, V Zn = cm 3 Solution in the burette: 77

5 Indicator used: Diphenylamine Observation Table 10.: Titration of given zinc sulphate solution with standradised solution of potassium ferrocyanide S.No. Volume of zinc sulphate (in cm 3 ) 1 3 Burette reading Initial Final Concordant reading Titre value (in cm 3 ) (Final-initial reading) The concentration of the given zinc sulphate solution can be determined as follows. The reactions involved: Zn3 + K olarity equation: V = 3 V Zn Zn 3 = V V Substituting the values, the molarity of given zinc sulphate = The molarity of given zinc sulphate solution is = RESULTS The molarity of zinc ions in the given solution is found to be =.. 78