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IIIB Se.VI SEM Pap-VII Lab Manual Maharani’s science college for women, Bangalore., , Experiment No -1, Estimation of Zn™* using EDTA and standard ZnSO, solution / 47, ‘Aim: To estimate the amount of Zn™ present in 250 cm’ of the given solution using standard EDT solution., Principle: Ethylenediaminetetracetic acid (EDTA) forms colourless, stable, water soluble 1:1 complexes, with a large number of polyvalent cations such as Ca°*, Mg", Zn, Ni? ete. Hence itis used in volumetric, estimations of various cations in solution by formation of complexes, hence called as complexometric, titrations. EDTA is sparingly soluble, hence the more soluble disodium salt of EDTA (NasEDTA) is used to, prepare a standard solution. The structure of Na; EDTA is as shown below,, CH:COONa, sen C, "HCOOH, ‘The disodium salt of EDTA has two replaceable hydrogen ions and may be represented as NasH.Y. It reacts,, with M?* in basic medium as follows,, M+ Hay? MY? + 2H", ‘The dissociation of the complexMY" is governed by the p! of the solution and is stable in slightly acidic or, in basic medium. The medium is maintained basic at p"" 10 using NH,OH-NH,C! buffer in estimation of zinc., , , , , , , , Zinc ions in solution forms wine red coloured complex with the EBT indicator and colorless complex with, EDTA. Zinc-EBT indicator complex is less stable than Zn-EDTA complex. At the end point when all the, free Zinc ions in solution have been complexed with EDTA, the added EDTA removes the meta ion from, the less stable metalindicator complex, releasing the free indicator which has a blue colour at pH-10, hence, the end point is wine red to blue., , ZnIn +EDTA ——» Zn-EDTA+In, , Wine Be, Procedure:, Part I: Preparation of standard ZnSO, 7H0 solution, Weigh accurately about 1.8g of ZnSO, 7H,O crystals and transfer into 250 cm’ standard flask. Dissolve the, crystals in distilled water and add a drop of conc. H:SOs. Make up the solution upto the mark using distilled, water. Shake well for uniform concentration. Calculate the molarity of ZnSO, 7H;0 solution, Part II: Standardization of EDTA solution, Pipette out 25 cm’ of the above solution into a clean conical flask. Two Test tubes of distilled water, Lem’ of, NHLOH-NH.CI buffer and a pinch of eriochrome black-T indicator are added. The solution turns to wine red, color. Titrate this solution against EDTA solution taken in the burette till the colour of the solution changes, to blue. Repeat the titration to get concordant values. The molarity of EDTA solution is calculated,, Part III: Estimation of Zn” using standardized EDTA solution, Make up the given solution in the standard flask upto the mark using distilled water and shake well, Pipette, out 25 cm’ of this solution into a clean conical flask. Two Test tubes of distilled water, Lem’ of, NHLOH-NH,CI buffer and a pinch of eriochrome black: indicator are added. The solution turns to wine red, color. Titrate this solution against EDTA solution taken in the burette till the colour of the solution changes, to blue. Repeat the titration to get concordant values. The molarity ofthe given zinc sulphate solution and the, weight of zinc is calculated., Result: Amount of Zn” present in the given 250 em solution is