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Determination of Total Hardness in Water by EDTA Titration

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Introduction

Principles of Environmental Chemistry Experiment 3: Determination of Total Hardness in Water by EDTA Titration Introduction Hardness of water is caused by divalent and multivalent metal that mainly came from contacting with the soil and rock formations. Hardness values are normally expressed in an equivalent amount of CaCO3 in mgl-1. If the hardness of water is smaller than 75mg/l, then it is described as soft water. EDTA can form very stable complex with metal ions (e.g. Mg2+ & Ca2+). The equation: M2+ + Na2H2EDTA � MEDTA2- + 2H+ + 2Na+ In this practical, EDTA (i.e. ethylene diamine tetraacetic acid) is used to determine the hardness of a water sample. EDTA can form very stable complex with metal ions (e.g. Mg2+ & Ca2+) i.e. M2+ + Na2H2EDTA � MEDTA2- + 2H+ + 2Na+ EDTA is added from a burette to the hard water sample and form (MEBT)- complex. At the equivalence point, EDTA removes the metal ion from the (MEBT)- by forming the more stable(MEBTA)-2 complex and free EBT- molecule that is blue in color. ...read more.

Middle

/ 3 = 10.40ml Determination of hardness of an unknown water sample (25-ml was used) Trail 1 2 3 Initial Reading 0.00 19.50 10.60 Final Reading 19.50 39.00 30.50 Volume of EDTA used (ml) 19.50 19.5 19.90 Average volume of EDTA used = (19.5+19.5+19.9) / 3 = 19.63ml Data treatment and Questions: Standardization of EDTA Solution Ca2+(aq) + EDTA4-(aq) ? [CaEDTA]2-(aq) Molarity of standard calcium chloride = 0.008 M Volume of standard calcium chloride used = 25 ml No. of moles of standard calcium chloride in 25 ml = 0.008 X 0.025 = 2 x 10-4 mole From the equation above, mole ratio of EDTA: calcium chloride = 1: 1 No. of moles of EDTA used = 2 x 10-4 moles Volume of EDTA solution used= 39.07/ 1000 = 0.03907 dm3 The molarity of EDTA solution = 2 x 10-4 / 0.03907 dm3 = Determination of hardness of a tap water sample: Molarity of EDTA solution = 0.00512M Volume of EDTA solution used= 10.40ml = 10.40/1000 dm3 = 0.0104 dm3 No. ...read more.

Conclusion

It is because EDTA solution made by the latter method is not stable, it is hygroscopic hence it cannot be a primary standard solution. We have to standardize it by calcium chloride solution. On the other hand, calcium chloride solution is a bit unstable indeed, hence the way to produce calcium chloride solution would be from calcium carbonate, which is the "real" primary standard as it is stable enough, in order to produce calcium chloride by reaction. At the second titration, which is EDTA being titrated over tap water, the titrant volume of EDTA is recommended to over 10ml at least. The larger the volume used, the more accurate the result. As error would be minimized due to the following equation: Use the burette with a narrower meniscus to decrease the observation error. The larger the sample size, the smaller the relative error. Precaution: 1. While adding indicator, do not put too much amount of indicator into the conical flask, otherwise the too deep colour affects the judgement of end-points of the titration. ...read more.

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