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Compleximetric Determination of Water Hardness

Caindec, Patricia Ysabel B.

Water hardness is a measure of the amount of calcium and magnesium carbonate dissolved as Ca 2+ and Mg 2+ in water. There are no health hazards associated with water hardness, however, it causes scaling, as well as forming of soap suds. Compleximetric titration is one of the best ways of measuring total water hardness using a standard ethylenediaminetetraacetic acid (EDTA) solution. EDTA solution is used as it has the ability to easily bind with calcium and magnesium ions. EDTA and Ca2+ react at the ratio of 1:1. The purpose of this experiment is to determine the hardness of water by measuring the concentrations of calcium and magnesium in water samples by titration. The total hardness is expressed in terms of the equivalent concentration of calcium carbonate in parts per million. The amount of CaCO3, in this experiment, is determined by titrating calcium ions from a sample tap water (from Baguio).

Water hardness is a commonly reported aspect of water quality. Hard water is water that contains high concentrations of Ca2+ and Mg2+ ions. Even with this concentration it is safe to drink and is apt for cooking and cleaning and is not a health hazard. Calcium carbonate (CaCO3) is a common substance found in rocks. It is usually the principal cause of hard water. Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral.  Hardness is traditionally measured by chemical titration.  The hardness of a water sample is reported in milligrams per liter or parts per million. In a compleximetric titration, a solution containing the free metal ion of interest is titrated with a solution of chelating agent until all of the metal ions are completely complexed.  The endpoint is usually measured with an indicator ligand that forms a colored complex with the free metal ion. The most important chelating agent in analytical chemistry is ethelyenediaminetetraacetic acid (EDTA).  It is colorless, water soluble solid. It is widely used to dissolve scale, caused by minerals (like Ca2+) from water. In analytical chemistry, it is used as a ligand and chelating agent. EDTA is used in compleximetric titrations and analysis of water hardness because it has the ability to grab hold of metal ions such as Ca2+ and Mg2+. If drinking water is concerned it is not important to know the hardness apart from the quantity and kind of mineral contained. In other words, for potable purposes, the hardness of water has no consequence. The effect of the some minerals upon the health should be of principal concern, however, the matter is rather different. Water is used in a steam boiler. Often the tubes of the boiler become coated with the deposited mineral therefore making it impossible to generate steam, prefiguring the danger of an explosion. Knowing the hardness of water can help determine the length of a steam boiler’s life span, thus experts know when to clean/replace a steam boiler.

Experimental Section
For the preparation of (5 x 10-3M) EDTA (ethylenediaminetetraacetic acid) titrant (stock titrant), in a 250 ml of distilled water, approximately 0.47 grams of EDTA was dissolved. A pinch of approximately .05 grams of MgCl2•6H2O was added. A 10.00 ml aliquot of the prepared titrant was diluted to volume in a 100.0 ml volumetric flask. It was shaken and is called the diluted EDTA titrant. In the preparation of standard CaCO3, 0.10 – 0.15 grams of CaCO3 was weighed in a 250 ml beaker. Until all the CaCO3 is dissolved, concentrated HCl were added drop by drop. The beaker was covered with a watch glass and the HCl was evaporated. The residue was quantitatively transferred and diluted to volume to a 500.0 ml volumetric flask.’ For the determination of the exact concentration of EDTA (standardization), a 10.00 ml of the standard CaCO3 was placed in a titration vessel. 5 ml of an ammonia buffer with pH 10 and 5...
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