Case studies 1. Phosphate solution for rectal use 2. Forensic test for arsenic
1. Oral
phosphate preparations contain typically monobasic sodium phosphate, and/or
disodium phosphate. Determine the chemical formulae of these three compounds.
2. Draw
the Lewis structure of the following molecules and determine the oxidation
state of phosphorus:
(a)
H3PO4
(b)
PCl3
(c)
Na2HPO4
3. Determine the
oxidation state of arsenic in the following molecules:
(a)
As2O3
(b)
H3AsO4
(c)
AsF5
4. A
typical IV solution for the treatment of hypophosphataemia contains 100 mmol/l
phosphate, 19 mmol/l potassium ions and 162 mmol/l sodium ions. Calculate the
amount of phosphate, potassium and sodium ions present in 500 ml. Express your
answer in grams.
1. Phosphate
solution for rectal use
Your pharmaceutical analysis company has been contacted by an
important client and asked to analyse a batch of phosphate solutions for use in
enemas. The description of your brief states that you are supposed to analyse
the APIs in this solution following standard quality assurance guidelines.
Typical analysis methods used for quality purposes are based on
titration reactions. A certain amount of solution is diluted with water. A
known amount of this solution is then titrated with sodium hydroxide or
hydrochloric acid depending on the API. Phenolphthalein and methyl red
solutions are typically used as indicators.
(a)
Research the APIs typically present in a
solution used for enemas and describe their chemical structure. Describe the
type of titration suggested. Describe the chemical structure and mode of action
of the indicators.
(b)
Formulate the relevant chemical equations.
(c)
Phosphate enemas typically contain 12.8 g
sodium dihydrogen phosphate dihydrate and 10.24 g dis-odium phosphate
dodecahydrate and water made up to a 128 ml solution.
For the determination of the sodium dihydrogen
phosphate dehydrate content, 20 ml of this solution is diluted with 80 ml of
water and titrated with 0.5 M sodium hydroxide solution using phenolphthalein
as indicator. The following amounts of sodium hydroxide are used:
25.0 ml 25.6 ml 25.4 ml
For the determination of the disodium hydrogen
phosphate dodecahydrate content, 50 ml of this solu-tion is titrated with 0.5 M
hydrochloric acid using methyl red as indicator. The following amounts of
hydrochloric acid are used:
22.0 ml 22.3 ml 22.4 ml
Calculate the amount of sodium dihydrogen phosphate and disodium
hydrogen phosphate 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.
Arsenic compounds are known for their toxicity and are often
used in poisons. In order to obtain forensic evidence, specific tests were
developed for the qualitative analysis of arsenic-based compounds. The
so-called Marsh test was published in 1836 and is based on the reduction of As3+
in the presence of Zn.
(a)
Research the conditions needed for the Marsh
test.
(b)
Formulate the relevant reduction and oxidation
equations.
(c)
Is it possible to use the Marsh test as a
quantitative test?
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