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Determination of calcium and magnesium in water by EDTA

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Introduction

Determination of calcium and magnesium in water by EDTA Water hardness is a very important quality and is one of the most commonly determined water quality parameters. Metal ions present in water contribute to the hardness of water. Cations of Calcium and Magnesium are the main contributors, and to a lesser extent cations of barium and strontium. The Mg� and Ca� ions are therefore the measure of the hardness of water. These ions present in water are usually from water sources that originate from limestone or chalk geological formations. They often accompany the alkalinity ions: carbonate, bicarbonate and hydroxide. Thus the alkalinity of a water sample can be a reasonable guide to water hardness. Water hardness is an important quality used for municipal or industrial purposes. A certain degree is necessary in municipal water systems to prevent corrosion of pipes. Hard water causes curdy precipitate with soap, therefore reducing its ability to solubilize dirt. Ca� + H C-(CH ) -COO �[H C-(CH ) ...read more.

Middle

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 1st 2nd 18.8 19.2 Average titre= 19cm� Titration at PH 12 1st 2nd 5.8 6.2 Average titre = 6cm� Total concentration of metal ions from titration at ph10 Concentration of EDTA = 0.01moldm � 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. ...read more.

Conclusion

0 20 40 60 80 100 Concentration 0 21 42 63 82 100 Unknown absorbance = 55 therefore concentration = 53 Mg� Absorbance 0 0.5 1.0 1.5 2.0 Concentration 0 0.043 0.046 0.051 0.055 Absorbance of unknown = 0.045 so therefore concentration = 1.25 Discussion 1) I prefer the ASS and FP technique due to it reducing human error and giving more accurate results. 2) The purpose of the filter is so you can filter out the substance you want and means less impurities 3) Mg� 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. 4) 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. ...read more.

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