CO² + Ca² → CaCO
In this experiment the hardness of an unknown water sample will be determined by titration against EDTA, the anions H EDTA² react with metal ions in aqueous solution on a l:1 ratio. Because of the equilibrium constraints for the reactions are different concentrations of both Ca² and Mg² can be determined individually. At ph10, both metal are complexed by EDTA, but at ph12 only Ca are complexed, Mg² precipitate as Mg(OH) . Therefore its concentration can be calculated:
[Mg² ] = [M² ] – [Ca² ]
The EDTA (Ethylene diammine tetra-acetic acid) is usually used in its disodium salt form, the large number of oxygen and nitrogen atoms in its structure (below) allow it to bind to magnesium and calcium ions easily. The free electrons (binding pairs) of the EDTA attract the positive charge of the metal ions, and form a complex called a chelate.
Metal chelates are very important compounds in nature, and are common as a means of transporting metals in the environment. The indicators that are added in this experiment (Erichrome Black T and Hydroxy Napthol Blue) also form complexes with the metal ions. These complexes are weaker than the ones formed with EDTA. As the EDTA is added, the indicator begins to complex the metal ions. The end point is reached when the EDTA begins to take the metal ions from the weak indicator metal ion complex. Causing a colour change from red to blue. Beautiful
Titration at PH 10
Average titre= 19cm³
Titration at PH 12
Average titre = 6cm³
Total concentration of metal ions from titration at ph10
Concentration of EDTA = 0.01moldm ³
Number of moles = concentration x volume (average titre)
= 0.01 x 0.019
= 1.9x10 moles
As the EDTA reacts with the metal ions on a 1:1 ratio, there was am equal number of metal ions used.
Concentration of metal ions = Number or moles
Volume
= 1.9x 10
0.05
= 3.8x 10 ³ mol dm ³
Concentration of Ca² ions in water – from titration at ph12
Average titre of EDTA = 6cm³
Number of moles = 0.01 x 6.0x 10 ³
= 6 x 10 moles
Therefore there was also 6x10 moles of Ca² ions in the reaction
Concentration of Ca² ions = Number of moles _ = 6x10 = 1.2x10 ³ mol dm ³
Volume 0.05
Therefore number of Mg² ions in the water will be the remaining ions after deducting the Ca² concentration from the total ion concentration.
[Mg²] = [M² ] – [Ca² ]
= (3.8x10 ³) – (1.2x10 ³)
= 2.6x10 ³ mol dm ³
Concentration of Ca² = 1.2x10 ³ mol dm ³
Mass of Ca² = 1.2x10 ³ x molar mass
= 1.2x10 ³ x 40.078
= 0.0481g (4d.p) x 1000
= 48.1mgdm ³
Concentration of Mg² = 2.6x10 ³ mol dm ³
Mass of Mg² = 2.6x10 ³ x 24.305
= 0.0632 (4 d.p) x 1000
= 63.2 mg dm ³
Flame photometry and atomic absorption spectroscopy
Ca²
Unknown absorbance = 55 therefore concentration = 53
Mg²
Absorbance of unknown = 0.045 so therefore concentration = 1.25
Discussion
- I prefer the ASS and FP technique due to it reducing human error and giving more accurate results.
- The purpose of the filter is so you can filter out the substance you want and means less impurities
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Mg² can not be determined by titration at ph12 because the mg2+ reacts to form mg(oh)2 and must therefore be neutralised to stop this as the reaction obscures end point. Mg can’t be determined by flame photometry as it’s a low temperature flame so does not give off ionisation of Mg ions.
- AAS differs from FP in the following 3 ways:
- Flame photometer is free from ionisation interference because the degree of ionisation increases for the alkali metals with increased temperature
- AAS uses a plasma instead of a flame
- The flame photometer is free from spectral interferences - low flame temperature means only a few emission lines are obtained from flame.
