I really need help with these calculations. Please help! Experiment 4: EDTA titr
ID: 1067763 • Letter: I
Question
I really need help with these calculations. Please help!Experiment 4: EDTA titration. EDTA is one of the great chelating agents of analytical chemistry. As a chelator, EDTA reacts 1:1 with several metal ions including both calcium and magnesium. In this experiment be using EDTA as the titrant to determine the amount ofcalcium and magnesium present in your unknown. You will need to calculate the concentration of each ion as well as the mass percent of each compound in the solid unknown. Your solid unknown is a mixture of calcium carbonate and magnesium carbonate. Detecting the endpoint of this titration is a bit more difficult than in acid-base titrations. The EBT that you will be using forms a red complex with metal ions but is blue by itself. The indicator competes with the EDTA for possession of the metal ions in the solution. When you first add the indicator, your solution should be red. As the titration proceeds, the EDTA starts to steal ions away from the indicator, thus it will turn blue (which looks purple because blue and red make purple). The endpoint of this titration occurs when the very last metal ion complexes with the EDTA- when the solution turns completely blue. Because the quantitative reaction in this experiment is a complexation reaction, the chemistry is much slower than in acid-base titrations. As you get closer to the endpoint you need to allow the solution to sit for 20-30 seconds to be sure the reaction is complete. It is also necessary to a the EDTA drop wise while swirling the flask to mix thoroughly the reactants. This is a two-part experiment. The first art is the titration to determine the total concentration of ions in the unknown solution. This part of the experiment requires the use ofEBT as an indicator. The second part is the titration of only the calcium in the solution. In the second part, you will effectively remove all of the magnesium from the solution by adding hydroxide and titrate the calcium that is left. You will be using a different indicator for this part of the experiment (hydroxynaphthol blue, a solid) Experimental Procedure Preparing the solutions. pH 10 buffer: 28.5 mL concentrated ammonia (ammonium hydroxide) 3.5 gammonium chloride diluted to 50 mL with DI water. Store this in a tightly capped plastic bottle. EDTA solution: Accurately wei out-0.6 g ofNa2H2EDTA-2H2O (FM 372.24 g/mol). Dissolve the EDTA in a 500 mL volumetric flask that is full of DI water. Add 3 pellets of solid NaOH to your EDTA solution. Swirl the solution to dissolve the EDTA. (Ifit doesn't dissolve immediately, prepare your unknown and come back to this). Make sure your EDTA is completely dissolved and fill the flask to the mark with DI water and invert to mix Unknown solution: Accurately weigh out 0.25-0.35 g of your unknown directly into a clean, dry 250 mL Erlenmeyer flask. Use a minimum amount of-20% HCI (20 mL conc, HCI 80 mL DI water to dissolve the unknown solid. Your unknown is limestone and contains a mixture of calcium and magnesium carbonates it will bubble so when the acid is added be careful and add the acid slowly. You will know that the solid is completely dissolved w adding acid no longer results in the formation of bubbles (or the pH ofyour solution falls below 4).
Explanation / Answer
ok your reading are good
use this method for calculation
Result Calculations 1. Calculate the total moles of EDTA added to the sample solution. 2. Calculate the moles of the magnesium chloride solution used in the back titration from your concordant results. From the equation of the titration below, the moles of Mg2+ will be equivalent to the moles of excess EDTA. EDTA4 + Mg2+ [Mg-EDTA]2 3. Given the ratio of Ca2+ + Mg2+ : EDTA = 1 : 1, calculate the moles of Ca2+ and Mg2+ that must have been complexed with EDTA by subtracting the excess EDTA from the total moles of EDTA added to the sample. This result is the moles of Ca2+ and Mg2+ in the sample solution. Additional Notes SafetyThe concentrated ammonia solution used in preparing the buffer and indicator solutions is highly corrosive and should be handled with care. Wear safety glasses and rubber gloves. Both the buffer and indicator (and thus also the titration solution) will liberate ammonia gas to some extent. This gas may be harmful if inhaled in large quantities. Work in a fumehood or well ventilated area. Additional Notes1. Ethylenediaminetetraacetic acid, EDTA is a large molecule which creates a complex with a metal ion, b 1. Ethylenediaminetetraacetic acid, EDTA is a large molecule which creates a complex with a metal ion, bonding through six coordination sites. Complex formed by EDTA and calcium ions 2. The ammonia buffer (pH ~ 10.5) used here is needed as Eriochrome Black T only changes colour in the pH range 7 – 11. nature and source of the sample it may be necessary to vary the concentration of the EDTA( if the titre volume is too low) or to dilute your solutions (if the titre volume is too high). The average titre volume should be in the range of 10 – 30 mL.
Ca2+ and Mg2+ ions present are complexed with the EDTA.
A back titration is carried out using a solution of magnesium chloride. This forms a complex with the excess EDTA molecules until the end-point, when all the excess EDTA has been complexed. The remaining magnesium ions of the magnesium chloride solution then start to complex with ErioT indicator, immediately changing its colour from blue to pink.
The main reaction is:
Ca2+ + EDTA4- --> [Ca-EDTA]2-
Back titration:
EDTA4- + Mg2+ --> [Mg-EDTA]2-
Indicator reaction: Note: ErioT is blue and ErioT-Mg is pink
ErioT + Mg2+ --> ErioT-Mg