Experiment # 4a
Title: Determination of the Hardness of Water from a Waterfall Aim: To determine the molarity of EDTA and to determine the hardness of water by measuring the concentrations of calcium and magnesium in a water sample by titration. Abstract: The aim of this experiment was to determine the molarity of a sample of EDTA and then to use this sample to determine the hardness of a sample of water. This was done using a titrimetric method. This was standardized using calcium chloride. The calcium chloride was weighed, dissolved and diluted to a known volume. A buffer and indicator were added to the solution which was then titrated with the EDTA. The water sample was then used as the titrating solution and the procedure repeated. The hardness of water was determined and was found to be 9.25 x 104 ppm. Introduction: The main purpose of this experiment was to determine the hardness of water. This is the expression of the total concentration of magnesium and calcium ions in a water sample. This was used because of the fact that these ions combine with soap molecules in water making it ‘hard’ to get sudsy; consequently the soap will not clean effectively. When water containing calcium and bicarbonate ions is heated, some carbon dioxide is given off. As a result, the solution becomes less acidic and insoluble calcium carbonate is formed: Ca2+ (aq) + 2HCO3- (aq) → CaCO3 (s) + H2O (l) +CO2 (g)
Hardness of water is normally expressed as parts per million (ppm) CaCO3 or mg CaCO3/L solution. According to the American Water Works Association, ideal water quality has no more than 80 mg CaCO3/L solution. The ions involved can be determined by method of titration with EDTA. EDTA (ethylenediaminetetraacetic acid), is a weak acid which can form soluble complexes with magnesium and calcium in its ionized form (so neither of them have free ions in solution). The endpoint of the titration can be determined using Eriochrome Black T which initially forms a complex with the magnesium and calcium ions. This complex has a wine-red colour. As the EDTA is added, it replaces the Eriochrome Black T and forms more stable complexes with calcium and magnesium. A buffer is a solution that resists changes in pH with the addition of a small amount of acid or base. The formation of a metal ion and EDTA is depended on the pH which must be maintained at about pH 10 so the complex formation may be quantitative, hence the use of a buffer solution in the experiment. The indicator is subsequently released giving a blue colour as the EDTA is added. This means that there are no more magnesium or calcium ions which are not combined with the EDTA. The endpoint of the titration can then be determined by a change from wine-red to purple then to blue with no traces of red.
Procedure: 0.4 g of primary standard calcium carbonate was weighed. The solid was transferred using a funnel to a 500mL volumetric flask and diluted with distilled water to the mark. 1:1 HCl was added drop wise until the effervescence stopped and the solution was clear. The solution was mixed thoroughly. A 50 mL portion of the solution was pipetted into a 250 mL Erlenmeyer flask and 10 mL of ammonia/ammonium chloride buffer solution was added. 5 drops of Eriochrome Black T indicator was added. This was titrated with the EDTA solution until a colour change from wine-red to pure blue was observed. 50 mL of the water sample was pipetted into a 250 Erlenmeyer flask. 10 mL of the buffer solution was added along with 5 drops of the Eriochrome Black T. This solution was titrated with the EDTA solution until a colour change was observed. This was repeated for two more readings. Results:
Table 1 Showing the Standardization of EDTA
Volume of CaCl (mL)
| Trial 1
| Trial 2
| Trial 3
Final Volume (mL)
Initial Volume (mL)
Volume Used (mL)
Eriochrome Black T indicator changed from red to blue
Table 2 Showing...
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