ammonium zinc phosphate, gravimetry, zinc pyrophosphate, zn2p2o7

1. SHREE MALLIKARJUN COLLEGE SYBSC
SAFETY: Refer to MSDS of zinc nitrate, zinc sulphate, diammonium hydrogen phosphate, H2S gas, HCl.
-Dr. Mithil S. Fal Desai
Aim: To estimate the amount of Zinc as Zn2P2O7 in the given solution of zinc sulphate containing copper
sulphate and free H2SO4.
Chemicals:Zinc salt (zinc nitrate, zinc sulphate), diammonium hydrogen phosphate, H2Sgas, HCl, H2SO4.
Apparatus: Beaker, conical flask, funnel, Whatman 41, silica crucible, glass rod with policemen, water
bath, oven.
Theory: Zn(II) reacts with diammonium hydrogen phosphate in a mild basic medium to form a precipitate
of ammonium zinc phosphate. Igniting this precipitate results in the formation of zinc pyrophosphate
(Zn2P2O7). In a highly basic (pH<10) and acidic medium, zinc forms soluble tetraamminezinc(II)
[Zn(NH3)4]2+
and tetrachlorozincate(II) [Zn(Cl)4]2-
complex, respectively. The addition of methyl orange
indicator is desirable to observe the mild basic condition of solution. To determine Zn from a solution of
copper and zinc we need to separate copper by selective precipitation. The Cu2+
precipitates in an acidic
solution as a copper sulfide while Zn remains in the solution.
Reaction:
Cu2+
(aq) + H2S(g)  CuS(s) ↓ + 2H+
(aq)
Zn2+
(aq) + (NH4)2HPO4(aq)  NH4ZnPO4(s)↓+ NH4
+
(aq) + H+
(aq)
2NH4ZnPO4(s)+ heat  Zn2P2O7(s) + 2NH3 + H2O
Procedure:
1. Pipette out 25 mL of the aqueous mixture of Cu and Zn ions in a 250 mL beaker,add 1 mL of concentrated
HCl and warm the solution.
2. Pass H2S gas through the solution and allow the solution to settle. (Check for complete precipitation).
3. Filter the solution and collect the Zn ions quantitatively in a 250 mL beaker.
4. Boil the solution free from copper ions to expel dissolved H2S gas. (Test with lead acetate paper).
5. To this hot solution add 2 g of NH4Cl and few drops of methyl red.
6. Add 1:1 NH3 dropwise till the solution becomes yellow and heat it nearly to boiling.
7. Add 25% diammonium hydrogen phosphate till the precipitation of Zn is complete. (Check for complete
precipitation)
8. Filter the precipitate through a Whatman filter paper No. 41, wash with cold water and ignite it in a
previously weighed silica crucible.
9. Cool the crucible and weigh. The weight of residue corresponds to Zn2P2O7.
Observation
Table 1.
The constant
weight of silica
crucible without
precipitate
“W1’ (g)
The constant
weight of silica
crucible with
precipitate
“W2’ (g)
Weight of
Zn2P2O7
‘A’=W2 – W1
(g)
Molecular weight
of Zn2P2O7
‘B’ (g)
Atomic weight of
Zn
‘C’(g)
Amount of Zn in
25 mL
‘x' = A*C*2/B
(g)
Result: Amount of Zn in 500 mL is ______g.