Calcium Carbonate and Ca2+ Ions

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Experiment. EDTA Titration of Ca2+ in an unknown solution

Experiment. EDTA Titration of Ca2+ in an unknown water sample

Modified 9/2012

Objective: The most common multivalent metal ions in natural waters are Ca2+ and Mg2+. In this experiment, you will find the total concentration of calcium ions that can react with EDTA with the assumptions that EDTA reacts 1:1 with metal (Ca2+) ions.

Equipment 250-mL Erlenmeyer flask (3) 50-mL Buret Ring-stand and hardware Desiccator 400-mL Beaker 500-mL Vol. flask 250-mL Vol. flask 1.0-mL Vol Pipette 100-mL Grad cylinder Hot plate Safety and Waste Disposal

Chemicals

Buffer (pH 10): Add 142 mL of 28 wt % aqueous NH3 to 17.5 g of NH4Cl and dilute to 250 mL with water.

Eriochrome black T indicator: Dissolve 0.2 g of the solid indicator in 15 mL of triethanolamine plus 5 mL of absolute ethanol.

50 wt % NaOH: Dissolve 100 g of NaOH in 100 g of H2O in a 250-mL plastic bottle. Store tightly capped. When you remove solution with a pipet, try not to disturb the solid Na2CO3 precipitate.

Discussion: Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca2+, Mg2+, Fe3+, SO42-, and HCO3-. Generally hard water arises because rainwater moves through limestone, CaCO3 underground that occurs in our area to the aquifer. This is why we measure hardness in terms of CaCO3. The concentration of the Ca2+ ions is greater than the concentration of any other metal ion in our water.

The determination of water hardness is routinely used to measure the quality of water that the general public uses. Originally, water hardness was defined as the measure of the capacity of the water to precipitate soap. Hard water is not a health hazard since the main chemical in hard water is calcium. People regularly take calcium supplements. In fact, hard water can be a source of necessary minerals (calcium and magnesium) that is necessary for good health. Indeed, the National Academy of Science go so far as stating that that consuming extremely hard water could be a major contributor of calcium and magnesium to the diet.

The problem with hard water is that it cause soap scum, clog pipes and clog boilers. Soap scum is formed when the calcium ion binds with the soap. This causes an insoluble compound that precipitates to form the scum you see. Soap actually softens hard water by removing the Ca2+ ions from the water. When hard water is heated, CaCO3 precipitates out, which then clogs pipes and industrial boilers. This leads to malfunction or damage and is expensive to remove. There are two types of water hardness, temporary and permanent. Temporary hardness is due to the bicarbonate ion, HCO3-, being present in the water. This type of hardness can be removed by boiling the water to expel the CO2, as indicated by the following equation: HCO3- (aq)  H2O (l) + CO2 (g). Because bicarbonate can be removed it is classified as temporary hardness. Permanent hardness is due to the presence of the ions Ca2+, Mg+2, Fe3+ and SO4-2. Because boiling cannot eliminate this type of hardness, the water is said to be permanently hard.

The table below shows the degree of hardness of the water in terms of its calcium carbonate concentration in ppm and grains. Hardness rating Soft Medium Hard Hard Very Hard Concentration of Calcium Carbonate (mg/L or ppm) 0 < 75 75 to < 150 150 to < 300 300 and greater Concentration of Calcium Carbonate (grains/US gallon) 0 to < 5.2 5.2 to < 10.5 10.5 to < 21 21 and greater

Permanent hardness is usually determined by titrating it with a standard solution of ethylenediamminetetraacetic acid, EDTA. The EDTA is a complexing, or chelating agent used to capture the metal ions. This causes water to soften, but the metal ions however, are not removed from the water. EDTA simply binds the metal ions so that the ions do not precipitate to form soap scum. EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules...
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