EDTA Complexometric Titrations - Assignment Example

Introduction Titration will usually involve dissolving accurately measured mass of sample in aqueous solution with some additional chemical treatment such as acid digestion of solids diluted with water which is highly purified and measured accurately. An accurate volume of a sample solution which is referred to as aliquot is pipette in a vessel (titration vessel) and then there is titration of an analyte with a standardized solution that is appropriate to the titrant to the equivalent point (end point) of titration. It is important that the end point be known for this procedure to be successful. An indicator is used to accomplish this where there is change of colour at at the end point. Having the volumes and molarity it is possible to determine the number of moles of the titrant that will have been used. Using the known stoichiometry of the reaction between the analyte and the titrant, it is possible to calculate the number of moles of analyte and consequently the mass and molarity of the analyte. Through calculation there is determination analyte’s concentration and total mass in the sample to bring the process to completion. Preparation of Samples He experiment involves use of antacid powder which contains calcium carbonate, aluminium hydroxide and magnesium carbonate. Approximately o.65g of the sample was weighed in a conical flask of 250ml capacity and this was recorded. There was then addition of 5ml of water followed by addition of 10ml 3mol HCL. Using a microwave heating was done to boiling and the mixture shaken to dissolve. Next was addition of 100ml of water and then the solution was quantitatively transferred to 250ml volumetric flask ten diluted to volume. Procedure Assay for Al3+ Out of the prepared sample 25.00ml was transferred to a 250ml conical flask using pipette and then 20L of water was added. While swirling 25.ooml of 0.01 mol per liter of EDTA was added to the conical flask. While continuing string there was addition of 20ml of Ph 4.5 buffer with emphasis being put on the order in which the addition was being done. The solution was heated with a microwave foe 2 minutes,and after being cooled there was addition of 0ml ethanol followed by addition of 2ml dithizone indicator with use of undissolved dithiozone being avoided completely. There was then titration using 0.01mol/L zinc sulfate standard to a point where there was change in colour from green-violet to rose pink with the titre being approximated to 2 decimal points. A blank was then performed using a 25ml with the prepared sample being replaced by deionized water. The process was then repeated in order to obtain three concordant values. Assay for Mg2+ + Ca2+ Using a pipette 10.00ml of the prepared sample was transferred into 250ml conical flask with 34ml of aqueous triethanolamine solution being added next and mixed. The next step was the addition of 2oml buffer with a Ph of 10 and then 2 drops of an aqueous triethanolamine solution was then added and the contents mixed. The solution was then cooled to about 3-4 °C in an ice bath with care being taken to avoid the solution getting in contact with the solution as this result in loss of some solution on removal of the thermometer. The next step was titration with a standard 0.01 mol/L EDTA up the point Assay for Ca2+ Using a pipette 10.00ml of the prepared sample was transferred into 250ml conical flask. Next was the addition of 70Ml water, followed by addition of 50% NaOH solution after which there was addition of 10mg of hydroxynapthol blue. The next step was titration with 0.01mol/L of EDTA up to the point where there was distinct blue colour. A blank was then performed. The titration was repeated for three concordant values to be obtained. The results obtained in the three assays were then recorded in excel for the entire class with each students performing their independent analysis. Results sample Sample mass 1st / mL 2nd / mL 3rd / mL Blank/mL M SD RSD Al 0.6507 6.95 7 7.05 1 7 0.05 0.0071 Mg 0.6520 6.74 6.68 6.78 0.00 6.733 0.05 0.0074 Ca 0.6499 6.90 7.10 7.20 0.40 7.06 0.15 0.021 Calculations MCa= Mass of Al (mg) =  The original sample was 250ml and thus sample used is 10times  Actual mass Al= 0.175mg Mass of Mg (mg) =  The original sample was 250ml and thus sample used is 25times  Actual mass Mg = 0.0158gx25=0.395mg Mass of Ca (mg) = The original sample was 250ml and thus sample used is 25times  Actual mass Ca= 0.02606x10mg=0.2606mg Calculation of the 95% confidence intervals for the mean titres Al3+, Ca2+ and Mg2+ of future antacid batches. Confidence interval Al3+=  Confidence interval Mg2+ =  Confidence interval Ca2+ =  Calculation of ions Ions mass % Al3+ 0.175 0.67 Mg2+ 0.395 1.51 Ca2+ 0.2606 1.00% Conclusion In this experiment we have seen how various techniques are used in determination of quantities of various metal ions. In performing titration it is very important to observe very high level of accuracy in order to get the correct outcome. Using the right apparatus and following the right sequence when mixing the chemicals has also been seen to be some of the factors which determine the success of experiment. Additional exercise 1. In the determination of Al3+ back titration was used. This technique is used where there is no suitable indicator, suppose there is precipitation of analyte in absence of EDTA or if the reaction of the analytes takes place at a very slow pace. The metal ion that is used in back titration is not supposed to displace the analyte metal ion from the EDTA complex. 2. This involves ions which forms precipitates with some metal ions whose analysis with EDTA can be accomplished by indirect titration. A good example being the case of precipitation of sulfate by excess Ba2+ at a pH of 1. The precipitates of BaSO4 are filtered then washed followed by boiling the precipitate in excess EDTA at a Ph of 10 and thus bringing the Ba2+into solution in form of Ba(EDTA)2-. Then there is back titration of the excess Mg2+ with excess EDTA. References D.C.Harris, Exploring Chemical Analysis, Chapter 15 (1997). D.C.Harris, Quantitative Chemical Analysis, 5th edition, Chapter 13 (2003). D.A.Skoog, D.M.West, F.J.Holler, S.R.Crouch Fundamentals of Analytical Chemistry, 9th Edition, Chapter 17 (2014). A.I.Vogel, A Textbook of Quantitative Inorganic Analysis. (1962). Read More