Quimica Organica

Determination of Total Calcium and Magnesium Ion Concentration
Introduction
This method, called a complexometric titration, is used to find the total calcium and magnesium content of milk, sea water and various solid materials. It can also be used to determine the total hardness of fresh water provided the solutions used are diluted. The combined concentration of calcium and magnesium ions isconsidered to be the measure of water hardness. The method uses a very large molecule called EDTA which forms a complex with calcium and magnesium ions. EDTA is short for ethylenediaminetetraacetic acid. A blue dye called Eriochrome Black T (ErioT) is used as the indicator. This blue dye also forms a complex with the calcium and magnesium ions, changing colour from blue to pink in the process. Thedye–metal ion complex is less stable than the EDTA–metal ion complex. For the titration, the sample solution containing the calcium and magnesium ions is reacted with an excess of EDTA. The indicator is added and remains blue as all the 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 theexcess 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:

Solutions Needed
EDTA: (ethylenediaminetetraaceticacid) 500 mL of a 0.05 molL-1 solution. Weigh 9.31 g of the EDTA salt and dissolveit in 500 mL of distilled water in a volumetric flask. Buffer: Dissolve 7.0 g of ammonium chloride in 57 mL concentrated ammonia (see safety notes). Dilute to 100 mL with distilled water in a volumetric flask. The pH should be 10.5. MgCl2.6H2O: 0.025 molL-1 solution. Weigh 2.54 g of magnesium chloride hexahydrate and dilute to 500 mL with distilled water in a volumetric flask. ErioT indicator:Dissolve 0.2 g of Eriochrome Black T indicator in 15 mL of concentrated ammonia solution (or 15 mL of triethanolamine) (See safety notes) and 5 mL absolute ethanol. Do not store more than one to two days before use. You may be able to get the ErioT indicator from the University of Canterbury, see the contact details at the end.

Method
Sample Preparation
For samples that are already in solution,such as freshwater, seawater and milk, no further preparation is needed. For solid samples such as eggshells and limestone, the samples must first be dissolved in acid. Accurately weigh about 0.5 g of the solid into a small beaker or conical flask, add about 20 mL dilute hydrochloric acid and allow the solid to completely dissolve (this may take several minutes). Neutralise the unreacted acidwith dilute sodium hydroxide solution until the pH of the solution is almost 7 (according to pH indicator paper). For eggshells, the inner membrane will remain undissolved and may be carefully removed from the solution. Transfer the solution to a 100 mL volumetric flask and make up to the mark with distilled water.

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

Equipment Needed
Burette 20 mL pipette 250 mL conical flasks 100 mL volumetric cylinder

Standardisation of the EDTA Solution
1. Pipette a 10 mL sample of the EDTA solution into a conical flask. 2. Add 10 mL of ammonia buffer solution and 1 mL of

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Eriochrome Black T indicator solution. 3. Titrate theEDTA with the magnesium chloride solution until the endpoint is reached – a permanent colour change from blue to pink. 4. Having determined the average titre of the magnesium chloride solution, determine the number of moles used. 5. Given the Mg2+ : EDTA ratio of 1 : 1, calculate the concentration of your EDTA solution.

4. Prepare a 0.0025 mol L−1 magnesium chloride solution by diluting the...