Alkaline Earth Metals: Case studies

Case studies

1. Magnesium hydroxide suspension

Magnesium hydroxide mixture is an aqueous oral suspension containing hydrated magnesium oxide. It is indicated for use in constipation in adults and children. Typical analysis methods used for quality purposes are based on titration reactions. A certain volume of the suspension containing hydrated magnesium oxide [Mg(OH)₂] is typically reacted with a known amount of sulfuric acid (H₂SO₄). The excess acid is then titrated with sodium hydroxide (NaOH) and methyl orange as an indicator .

(a)    Research the type of titration described. Describe the chemical structure and mode of action of the indicator.

(b)   Formulate the relevant chemical equations.

(c)    For the analysis, 10 g of the suspension was reacted with 50 ml of 0.5 M H₂SO₄. The excess H₂SO₄ was titrated with 1 M NaOH using methyl orange as indicator. For each titration, the following volume of NaOH has been used:

11.0 ml 11.2 ml 10.9 ml

Calculate the amount of Mg(OH)₂ present in your sample. Express your answer in grams and moles.

2. Calcium carbonate tablets

Your pharmaceutical analysis company has been contacted by an important client and asked to analyse a batch of injections containing calcium carbonate (CaCO₃). The description of your brief states that you are supposed to analyse the active pharmaceutical ingredient (API) in these tablets following standard quality assurance guidelines.

Typical analysis methods used for quality purposes are based on titration reactions. A certain amount of the tablet powder is dissolved in water and hydrochloride acid (HCl). A known amount of disodium edetate is added. After adjustment of the pH, the excess disodium edetate is titrated with zinc chloride (ZnCl) using morbant black II solution as indicator .

(a)    Research the type of titration described. Describe the chemical structure and mode of action of the indicator. You may want to familiarise yourself with chelation.

(b)   Formulate the relevant chemical equations.

(c)    The package states that each tablet contains 1.5 g of CaCO₃. For the experiment, 20 tablets are weighed (total weight 42.6 g) and powdered. An amount of powder containing 50 mg of Ca²⁺ is dissolved in water and HCl and reacted with 50 ml of 0.05 M disodium edetate. After adjusting the pH to 10.9, the excess disodium edetate is titrated with 0.05 M ZnCl₂ solution. For each titration, the following volume of ZnCl₂ has been used:

25.0 ml 24.8 ml 25.3 ml

Calculate the amount of CaCO₃ present in your sample. Express your answer in grams and moles.

(d)   Critically discuss your result in context with the stated value for the API.

(e)    Research the typically accepted error margins.