Alkali Metals: Case studies

Case studies

1. Lithium carbonate (Li₂CO₃) tablets

Your pharmaceutical analysis company has been contacted by an important client and asked to analyse a batch of formulated Li₂CO₃ tablets. The description of your brief states that you are supposed to analyse the 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 powdered Li₂CO₃ tablets is dissolved in water, and a known amount of HCl is added. The solution is boiled to remove any CO₂. The excess acid is then titrated with NaOH using 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)    The package states that each tablet contains 250 mg Li₂CO₃. For the experiment, 20 tablets are weighed and powdered (total weight 9.7 g). Powder containing 1 g of Li₂CO₃ is dissolved in 100 ml water, and 50 ml of 1 M HCl is added. After boiling, the solution is titrated against 1 M NaOH using methyl orange as the indicator. For each titration, the following volume of NaOH was used:

35.0 ml 35.5 ml 34.5 ml

Calculate the amount of Li₂CO₃ 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 and critically discuss the typically accepted error margins.

2. Sodium chloride eye drops

Your pharmaceutical analysis company has been contacted by an important client and asked to analyse a batch of eye drops containing a NaCl solution. The description of your brief states that you are supposed to analyse the API in these tablets following standard quality assurance guidelines.

Typical analysis methods used for quality purposes are based on titration reactions. A certain volume of NaCl solution is titrated with silver nitrate (AgNO₃). Potassium chromate is used as the appropriate 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)    The package states that the eye drops are a 0.9% w/v aqueous solution of NaCl. For the experiment, a volume containing 0.1 g of NaCl is titrated with a 0.1 M AgNO₃ solution. For each titration, the following volume of AgNO₃ is used:

16.9 ml 17.0 ml 17.4 ml

Calculate the amount of NaCl 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.