ETERMINATION OF TOTAL H ARDNESS IN QUANTITATIVE DETERMINATION DRINKING W ATER BY COMPLEXOMETRIC EDTA TITRATION
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1. What is the significance of determining the hardness of water? Water that we use may contain metal cations. Due to the presence of the cations, it could affect the quality of water. The cations present such as calcium and magnesium may react with its surrounding, thus forming precipitates which may affect objects and equipment wherein the water runs through. Also, hard water may have may affect industrial functions, such as for food production. The harder the water is the more it is possible that precipitates would form. To avoid this happening either in the household or in an industrial use it is better to check the water hardness and adjust according to findings.[1]
titrations. A factor behind this is because EDTA is hexadentate, there are relatively many complexing sites, thus resulting to a cage-like structure. This structure helps cations to be separated from solvent molecules deeming it stable. EDTA as titrant react with metal ions in a 1:1 ratio. The charge does not matter. Due to this EDTA as titrant makes titrant calculations more convenient. [1]
2. Why do we express water in hardness as ppm CaCO3? The hardness water is due to the cations present. It is measured in terms of the concentration of CaCO 3, particularly in ‘”ppm” but it does not mean that Ca + is the only ion present, other cations are already accounted when we say ppm CaCO 3.
3. Discuss the use of EDTA as a complexing agent and titrant. EDTA as a complexing agent forms chelates with all cations except akali metal ions. Also, the chelates formed are stable enough for
4. Discuss the use of EBT as indicator. Why do we add MgCl2 6H2O crystals to the titrant? Eriochrome Black T (EBT) a typial metal ion indicator, also it can also be used as acid/base indicator. In the experiment it is used as a metal ion indicator. Ca-EBT complexes are not stable, thus in the experiment Mg-EBT complex, denoted as MgIn- (wine-red), is present. When titrated with EDTA, the Mg-EBT complex breaks up and to form the Mg-EDTA complex. The EBT then becomes, HIn 2(blue)-. The titration depends on the color change of the indicator. MgCl2 crystals are added to the EDTA titrantso that Mg + ions are introduced because CaIn - is less stable that MgIn-. With that the Ca + becomes completely free, and the analyte turns int o wine red. So Basically, Mg was added because Ca-EBT is not a good indicator.
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5. Why do we add NaOH to the EDTA solution if the disodium EDTA crystals do not dissolve? EDTA crystals do not dissolve easily because it tetraprotic. In order for the EDTA to dissolve further the pH must be high so that the protons will neutralize. When the pH becomes more basic, the EDTA becomes more unprotonated, thus more soluble.
monoprotic form of it dominates. When pH is greater than 10 the unprotonized form is dominant. The triprotic form is not usually there because the formation of it is less probable. With this it can be said that cations with charge 2-3+ are titrated better at pH 3-10. For 4+ it is at pH greater than 10, lastly for 1+ cations EDTA titration, it is more difficult. [1]
6. Outline and explain the pertinent chemical equations involved during titrations.
8. Enumerate some other applications of EDTA complexation(besides complexometric titration). Other than doing volumetric analysis on an EDTA complexation other applications are Potentiometric and Spectrophotometric methods. In Potentiometric method, potential are used to determine EDTA titration endpoints. In spectrophotometric methods, UV/visible light absorption is measure to determine the endpoint of titration.
The important chemical equations in the experiment are the ff:
[1] 2H2Y2-+Ca2++Mg2+ CaY2-+MgY2-+4H[2] H2Y2- + MgIn- HIn2- + MgY2- + H+ The first equation shows that when the EDTA titrant is added the free Ca2+ and Mg2+ reacts with the EDTA for the respective EDTA complex. When the free cations are already tirated. The MgIn - complex will then react with the EDTA titrant. Due to the Mg-EDTA being more stable than the Mg-EBT, the formation of the EDTA complex is more preferred. Thus when all the Mg-EBT complex is titrated the EBT becomes HIn 2-, which makes the analyte change color from wine red to blue.
7. Discuss the relationship of pH and feasibility of titration of cations using EDTA. EDTA basically is a tetraprotic acid. Due to this, EDTA at lower pH comes takes the fully protonated form. As the pH gets higher the EDTA slowly become unprotonated. Specifically at ph range 3-10 the diprotic and
9. Why do we perform the analysis of Ca and Mg at pH 10? The formation of Ca and MgEDTA occurs at high pH. Particularly, Ca-EDTA is at around pH=7.3, and on the other hand Mg-EDTA is at around pH=10. Therefore the minimum possible pH to achieve complexation is at pH=10. In addition to that EDTA as a tetraprotonic acid is amphoteric in its other forms, to avoid the tendency of EDTA to change properties drastically it better to maintain the pH high. [2]
10. What is the effect, if any, of using too much buffer in this analysis? The buffer’s purpose is to regulate the possibility of a sudden change in pH. The complex formation used in the experiment are mostly pH dependent. It is a right thing to use a
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buffer in this case. If too much buffer is used pH changes will be very much resistant. Due to this there is a possibility that pH would remain very much constant thus affecting other components in the experiment that may need lower pH to thrive. 11. What is the purpose of auxiliary complexing agents? What reagent, if any, served as auxiliary complexing agent in this titration? Discuss. An auxiliary complexing agent purpose is bind to metal strongly to avoid the formation of metal hydroxide especially in high pH. It will also be readily able to give away its metal when EDTA comes. In the experiment, the agent that acted as an auxiliary complexing agent is the EBT complex. It acted as the complexing agent because it reacted with Mg 2+, but when EDTA was started to be titrated, the formation of Mg-EDTA complex was preferred. [3]
12. Correlate the stability of the Mg-EDTA, Ca-EDTA, Mg-EBT, and Ca-EBT complexes with the values of the formation constant. Ca-EDTA Kf = 5.0 x 1010 Mg-EDTA Kf = 4.9 x 108 Ca-EBT Kf = 2.5 x 105 Mg-EBT Kf = 1.0 x 10 7
Base on the K f values above the extent of formation is ranked as CaEDTA > Mg-EDTA > Mg-EBT > Ca-EBT. This means that Ca-EDTA would be the most stable. Ca-EDTA is preferred than Mg-EDTA. When EDTA is introduced to a Mg-EBT complex, it become MgEDTA because this is more preferred. Lastly, Ca-EBT would be the most unstable become it will most likely not form among the four complexes.
13. Discuss the difference between the experimental value of water hardness and the stated value of water hardness on the label of your water sample. The water hardness on the label of the water sample is at 58.05 ppm CaCO3. On the other hand the experimental value was calculated as 41.43 ppm CaCO3. The experimental titrated cations is lower than what is stated in the water sample. This means that the cations present in the water sample is not in accordance to what is stated in the water sample, relative to what was measured in the experiment. The stated hardness by the company is assumedly measured with higher precision and accurancy. Therefore, the experiment that was done had many errors. 14. Enumerate and discuss some possible sources of error. Determine their effect on your calculated parameters. Besides the normal equipment and human error that occurs there are also other errors that may affect calculated parameters. An example of this error is the parallax error. Reading burettes graduation, might be a cause of error if not done right, specifically reading it not in an eye level. Another error is because of carbonate error. Due to present carbonates in solution, it may react with the cations present. Due to the carbonate present titration will be more than what is really needed. [4]
REFERENCES
[1] Skoog, D. A., West D. M. Holler F. J., Crouch S. R., Fundamentals of Analytical Chemistry 8th ed . Thomson Learning Asia. 3
Singapore, 2004. pg 338, 461, 476, 459 [2] Khopka, S.M. Basic Concepts of Analytical Chemistry. New Age Ineternationals, 2004. Pg 63-76. [3] Harris, D. Quantitative Chemical Analysis 8th edition. [4]“Determination of water total hardness by complexometric titration”. http://www.titrations.info/EDTA-titrationwater-hardness”. Web. (accessed 15 Mar, 2016)
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