Determination of Hardness of Water

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Abstract

Water “hardness” was analyzed in this experiment, through the determination of CaCO3 concentration. This was achieved by the titration of an unknown solution using a standardized 0.1M EDTA, and addition of Eriochrome Black T to the unknown, to indicate the endpoint of the titration. The average concentration of CaCO3 obtained was 1034 ppm, with a standard deviation of 2.4495. The results indicate that the unknown solution can be considered as hard water.

Introduction

The hardness of water is defined in terms of its cation content, which includes calcium, magnesium, iron, zinc, and other polyvalent metal ions. These metal ions interfere with the use of the water for many applications. For example, these ions diminish the effectiveness of soap and detergent for cleansing operations; they diminish the drinking quality of water, and they contribute to the accumulation of insoluble salt deposits in storage vessels or plumbing1.

Water hardness can be easily determined by titration with the chelating agent EDTA (ethylenediaminetetraacetic acid), where its completely deprotonated anion forms a 1:1 complex with metal ions such as Ca2+ and Mg2+. In a titration, to establish the concentration of a metal ion, the EDTA that is added combines quantitatively with the cation to form the complex. The end point occurs when essentially all of the cation has reacted. The equation is shown below.

M + EDTA -----> M(EDTA)-complex

Both calcium and EDTA solutions are colorless, so it is necessary to add a metallochromic indicator to determine the endpoint of the titration, such as Eriochrome Black T which forms a stable wine-red complex ion (1). As EDTA is added, it binds calcium ion more strongly than Eriochrome Black T allowing the indicator to return (through shades of violet) to a pure blue color establishing the endpoint of the titration (2). Mg2+ is added to the titration solution to enhance the sharpness of the titration endpoint,



References: 1 http://www.sjsu.edu/faculty/chem55/55mgca.htm 2 Briggs, J.C., and Ficke, J.F., 1977, Quality of Rivers of the United States, 1975 Water Year -- National Stream Quality Accounting Network (NASQAN): U.S. Geological Survey Open-File Report 78-200, 436 p.) 3 Quantitative Chemical Analysis, 7th Edition, Daniel C. Harris. 2007 W.H freeman and company, USA 4 Laboratory Manual Experiment 1: Hardness of Water by EDTA Titration, CSULA Chemistry Department.

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