To determine the amount of magnesium in a solution, a sample of the liquid was first acidified with — Analytical Chemistry Chemistry Question
Magnesium determination
To determine the amount of magnesium in a solution, a sample of the liquid was first acidified with HCl, then made slightly alkaline by addition of NH3 and then combined with an excess (NH4)2HPO4 in water. The precipitate of MgNH4PO4 formed was filtered off, washed with diluted aqueous NH3, annealed at 1000 °C to constant mass and weighed. Answer the following questions using numerical data given in the end of the text whenever necessary.
Reference data:
H3PO4 acidity constants: Ka1 = 7.1×10–3, Ka2 = 6.2×10–8, Ka3 = 5.0×10–13
NH3 basicity constant: Kb = 1.8×10–5
Mg(OH)2 solubility product: Ksp = 6.0×10–10
H2O ionic product: Kw = 1.0×10–14
Write down the net ionic equation for the precipitation reaction taking place in course of the analysis.
Model Answer
Mg2+ + HPO42– + NH3 → MgNH4PO4 (s)
Write down the equation for the reaction taking place in the course of annealing.
Model Answer
2 MgNH4PO4 → Mg2P2O7 + 2 NH3 + H2O
When determining the content of magnesium in a granulated medicine preparation calmagin 0.1532 g of the annealed precipitate were obtained from a 1.8005 g sample of calmagin. Calculate the mass percent of MgO in the preparation.
Model Answer
Mr(MgO) = 24.31 + 16.00 = 40.31;
Mr(Mg2P2O7) = 2 × 24.31 + 2 × 30.97 + 7 × 16.00 = 222.56;
w(MgO) = (2 × 40.31 × 0.1532) / (222.56 × 1.8005) × 100 % = 3.08 %
During the precipitation of MgNH4PO4 some impurities may coprecipitate such as MgHPO4, Mg(NH4)4(PO4)2, Mg3(PO4)2, Mg(OH)2, (NH4)2HPO4 and NH4Cl. Some of these substances can undergo thermal decomposition at annealing. Write down the equations of the corresponding reactions.
Model Answer
2 MgHPO4 → Mg2P2O7 + H2O
Mg(NH4)4(PO4)2 → Mg(PO3)2 + 4 NH3 + 2 H2O
(Mg3(PO4)2 → no changes)
Mg(OH)2 → MgO + H2O
(NH4)2HPO4 → HPO3 + 2 NH3 + H2O
NH4Cl → NH3 + HCl
Indicate if the presence of the impurities listed in Table below can lead to an error in the magnesium content as determined by the method described above. Put 0 in the Table if no error is expected, plus or minus sign if the error will be positive or negative, respectively.
[VISUAL]
Model Answer
Impurity Error
MgHPO4 0
Mg(NH4)4(PO4)2 +
Mg3(PO4)2 –
Mg(OH)2 –
(NH4)2HPO4 +
NH4Cl 0
Explanation: The error is positive if the percentage (by mass) of magnesium in the annealing product is lower than that in Mg2P2O7, negative if higher and equal to zero if the same or if the impurity completely volatilizes during annealing.
At what maximum pH value the precipitation of MgNH4PO4 may be carried out to avoid simultaneous precipitation of Mg(OH)2? Assume that the volume of the original sample was 200 cm3 and the content of magnesium in it was 0.10 g.
Model Answer
pH = – log [H+] = – log Kw + log [OH–]
[OH–] = sqrt(Ksp(Mg(OH)2) / [Mg2+])
[Mg2+] = 0.10 g / (0.200 dm3 × 24.31 g mol-1) ≈ 2.1 × 10–2 mol dm-3
[OH–] = sqrt(6.0 × 10–10 / (2.1 × 10–2)) = 1.7 × 10–4 mol dm-3; pOH = 3.8; pH = 14.00 – 3.8 = 10.2
To determine the solubility product (Ksp) of MgNH4PO4 a NaOH solution was added dropwise until the beginning of precipitation to a 100 cm3 of a solution containing MgCl2, NH4Cl and NaH2PO4 with a concentration of 0.010 mol dm-3 each. The precipitation started at pH 6.48. Calculate Ksp. Neglect the volume change during the experiment.
Model Answer
At pH = 6.48, [H+] = 3.31 × 10–7 mol dm-3
[PO43–] = c(PO4) × (Ka1 × Ka2 × Ka3) / ([H+]3 + Ka1 × [H+]2 + Ka1 × Ka2 × [H+] + Ka1 × Ka2 × Ka3) = 2.4 × 10–9 mol dm-3
[NH4+] ≈ c(NH4+) = 0.010 mol dm–3 (since pH << pKa(NH3) = pKw - pKb(NH3) = 9.25)
[Mg2+] = 0.010 mol dm-3
Ksp = [Mg2+][NH4+][PO43–] = 2.4 × 10–13