Water hardness is described as the amount of dissolved magnesium and calcium ions that may result from the presence of the two minerals’ carbonates. The key sources of hardness have been identified as seepage from sedimentary rocks and runoffs. Magnesium and calcium ions are essential in biological processes (Harris 246). Calcium is needed for the development of coral reefs, and the ion can be used as an important nutritional ingredient of most animal diets. Magnesium is needed in biochemical processes such as chlorophyll formation. Even though there are no hazards associated with mineral content in water it can be noted that water hardness causes some effects such as the formation of scales and reduces lathering in soaps.
Determination of water hardness is done mainly to quantify the amount of calcium and magnesium in a particular water body. It is necessary to determine water hardness so as to assess the suitability of the water for use in washing, in fish tanks and water heating pipes. The essence of determining the water hardness is noted in the environmental significance of the phenomenon. Hard water (high amount of dissolved Ca2+ and Mg+) is not suitable for washing due to excessive use of soap. Formation of scales in kitchen heating appliances is not favorable as the scales can cause destruction as well as blockage of pipes.
The titration technique is normally used to determine an unknown concentration of a solution through the use of a known concentration of a particular solution. Complexometric titration with EDTA is the main way of determining the hardness in water (Harris n.p). EDTA (Ethylenediaminetetraacetic acid) is suitable for the determination of the water hardness due to its ability to react directly with many metal cations to form a chelated complex. Complexometric titration with EDTA is used due to the fact that EDTA uses its six bonds to form a single metal complex. Since the equivalence point is obtained at the end-point, the unknown concentration of calcium ions can be determined based on the number of moles of EDTA (titer volume) that has been used to react with the complex ions to completion.
At around pH 10, EDTA reacts with the Ca2+ and Mg+ to form complexes. The calcium complex forms first due to its higher stability as compared to the magnesium complex. In testing the water sample for hardness, a sample of water is buffered to pH of 10.1 and a suitable indicator added. The main use of buffering of the test sample can be seen to assist in maintaining the pH at 10.1 ± 1. The addition of the buffer enables removal of the hydrogen ions that are present in the solution and hence enabling the complexing reactions to take place. The addition of EDTA can be seen to significantly increase the presence of hydrogen ions in the solution. A buffer solution can also be seen to be resistant to pH change and hence also assist in stabilizing the condition caused due to water hardness.
In the event of the addition of the Eriochrome Black T (EBT) indicator to a sample containing the Ca2+ and Mg2+ the red wine color of the indicator is observed. The titration with EDTA, however, leads to a color change to blue indicating that all the metal ions have been chelated with the EDTA. The color change is essential as it enables the determination of end-point. The addition of murexide (ammonium purpurate) indicator leads to complexing with Ca2+ ions leading to the formation of a pink a color (Harris 250). The pink color turns to purple upon the reaction with EDTA as the Ca2+ ions react with the EDTA to form the complex.
The main reactions in the test involved the EDTA reacting with each of the metal ions to form complexes.
Ca2+ + EDTA4- → CaEDTA2-
Mg2+ + EDTA4- → MgEDTA2-
Detection of the End-Point
End-point detection is essential in all titration exercises. In titration with EDTA to determine water hardness, the end-point is achieved by the use of the EBT indicator. Since calcium is complexed first, the indicator is used to show the completion of the reaction with the magnesium ions (Harris n.p). In the cases where there is a lack of magnesium ions in the water, magnesium complex MgEDTA2+ is added to help with the detection of the end-point. However, it can be observed that is very rare to find the instances in which there is a lack of magnesium ions being present in the water.
Water Hardness and Marine Life
The present experiment aims to determine the levels of calcium and magnesium within the laboratory aquarium so as to the suitability of the water to support the fish since some fish can only survive in particular water hardness levels (Standard Methods for the Examination of Water and Wastewater 2-35). The hardness of water also affects the various factors such as the pH of water. The aquarium water is always de-ionized to remove the excess mineral ions to enable easier control of pH. The expected result of the experiment is that the water will have significantly low calcium and magnesium levels.
Harris, Daniel C. Quantitative chemical analysis. Macmillan, 2010.
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Standard Methods for the Examination of Water and Wastewater. New York: American Public Health Association, 1995. Print.
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