Removal of Hardness of Water Using Precipitation and Complexation Methods

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  • Topic: Water, Hard water, Carbonate
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KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY

COLLEGE OF

TITLE: REMOVAL OF HARDNESS OF WATER USING PRECIPITATION AND COMPLEXATION METHODS.

NAME: KWARTENG YAW PRINCE
COURSE: BSC. ENVIRONMENTAL SCIENCE
YEAR: FIRST YEAR
EXPERIMENT NO. : A.1.1.3.
T.A.: BRIGHT KOFI LEONARD
DATE: 7TH NOVEMBER, 2007.

Aims and Objectives:
1. To describe water hardness.
2. To soften hard water in terms of the species involved and the reactions they undergo. 3. To test the effectiveness of the methods.

INTRODUCTION
The experiment seeks to test the effectiveness of the two methods (precipitation and complexation) of separating hard water. Water containing Ca2+ and/or Mg2+ ions is called hard water, and water that is mostly free of these ions is called soft water. These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits. These deposits can make hard water unsuitable for many uses, and so a variety of means have been developed to "soften" hard water; i.e., remove the calcium and magnesium ions. Water can be softened in a number of ways. An automatic water softener connected to water supply pipes removes magnesium and calcium from water and replaces them with sodium. Sodium does not react with soap or detergents. If you don't have an automatic water softener, you can still soften laundry water by adding softeners directly to the wash water. These softeners combine with calcium and magnesium, preventing the minerals from forming a soap scum. Precipitation: One common type of reaction of reaction that occurs in aqueous solution is the precipitation reaction, which results in the formation of an insoluble product, or precipitate. A precipitate is an insoluble solid that separates from the solution. Precipitation reaction usually involves ionic compounds. Hard water cations Ca2+, Mg2+ and Fe3+ which precipitates soap from aqueous solution and cannot be removed by heating if bicarbonate is absent. Such solutions which cannot be softened by boiling are referred to as permanent hard water. Permanent hard water can be softened only by removing the offending cations. For example:

2CH3 (CH2) 16COONa(s) + Ca2+ → [CH3 (CH2) 16COO] 2Ca(s) + 2Na+ Soap curd/scum

For large-scale municipal operations, a process known as the "lime-soda process" or precipitation is used to remove Ca2+ and Mg2+ from the water supply. Ion-exchange reactions, similar to those you performed in this experiment, which result in the formation of an insoluble precipitate, are the basis of this process. The water is treated with a combination of slaked lime, Ca(OH)2, and soda ash, Na2CO3. Calcium precipitates as CaCO3, and magnesium precipitates as Mg(OH)2. These solids can be collected, thus removing the scale-forming cations from the water supply. In this experiment, calcium carbonate can be precipitated from a hard water sample containing Ca2+ simply by adding a soluble carbonate salt such as Na2CO3.

2Na (aq) + CO2-(aq) + Ca2+ → CaCO3(s) + 2Na+(aq)

A significant disadvantage of sodium carbonate as water softener is that it leaves a suspension of CaCO3(s) which dulls the colours of many laundered fabrics.

Complexation: It is the .A complex ion is an ion containing a central metal cation bounded to one or more molecules or ions. EDTA is a complex and as such has the ability to hold metal ions/atoms (both Ca2+ and Mg2+) like a claw, thus complexing the cations. In the second phase of the experiment, a small amount of EDTA will be added to the water after which some soap solution will be added to the solution and then shaken.

Calculations for water hardness
The concentration of EDTA solution is found by:
M EDTA = (wt. of CaCO3)(V10/100 mL)
((Mol wt CaCO3))(mL EDTA/1000)
where V10 is the calibrated volume of...
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