Question

outline a titration method to quantify calcium and magnesium in a pharmaceutical sample, using EDTA.

enough detail in answer please

Answer 1

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 acid with 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.

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 Eriochrome Black T indicator solution.
3. Titrate the EDTA 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.

Titration Method for pharmaceutical Samples
1. Pipette 10 mL of the sample solution into a conical flask.
2. Add 20 mL of 0.05 mol L−1 EDTA solution.
3. Add 10 mL of ammonia buffer, 50 mL of distilled water and 1 mL of Eriochrome Black T indicator solution.
4. Titrate the sample with the standard 0.025 molL−1 magnesium chloride solution until a permanent pink colour appears.

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.