Chemistry Practical for B. Sc. Part II) - Dr Deepak Pant (read e book txt) 📗

molecule becomes trapped in the precipitate. This may be due to inclusion or occlusion. Inclusion is the term used of a single subsitution in the crystal lattice by an ion of similar size. Occlusion refers to the physical trapping of a large pocket of impurities within the crystal. One technique for minimizing these problems is to remove the mother liquor, re-dissolve the precipitate and then re-precipitate. The second time the mother liquor will contain fewer unwanted ions capable of coprecipitation.
Sulfate Determination
The method of choice for sulfate in waters and wastewaters is the precipitative gravimetric procedure using barium. If Ba(+II) is added in excess under acidic conditions, BaSO4 is precipitated quantitatively. The reaction is allowed to continue for 2 hours or more at 80-90oC. This is to encourage the formation of BaSO4 crystals (non-filtrable) from the initially formed colloidal precipitate (partially filtrable). The precipitate is washed, and then dried at 800`C for 1 hour. Low pH is needed to avoid the precipitation of BaCO3 and Ba3(PO4)
Ba+2 + SO4-2 = BaSO4(ppt.)
Chloride Determination
Chloride may be determined by precipitation with silver. Interfering ions likely to form insoluble silver salts are the other halogens (bromide, iodide), cyanide, and reduced sulfur species (sulfite, sulfide, and thiosulfate). Fortunately, the reduced sulfur compounds can be pre-oxidized with hydrogen peroxide, and the others are rarely present at high concentrations. Although AgCl can be determined gravimetrically, the recommended procedure for water and wastewater is to use a volumetric procedure with chromate as an indicator.

3. Inorganic Synthesis
(i) Synthesis of Cuprous Chloride (work in a fume hood)
(a) Cu2+ + Cu + 2 Cl- → 2 CuCl
Material: CuSO4.5H2O, Cu, NaCl, HCl, Na2SO3, CH3COOH
Procedure: Prepare a solution of 10 g of powdered CuSO4.5H2O and 15 ml of
concentrated HCl in a 250-ml round-bottom flask, add 4 g of NaCl and heat to boiling.
Cover the flask with a little funnel. Add copper to hot solution in small portions. The
green colour of the solution will turn to yellow. Filter off the remaining Cu. Pour the
solution to one litre of cold water with 2 g of Na2SO3.
Wash the precipitated cuprous chloride 2 – 3 times with a solution of 1.5 g of
Na2SO3, 6 ml of HCl and 300 ml of H2O by decantation. Filter out the precipitate and wash it with concentrated acetic acid. Dry it in a drying oven at 100 oC.
( b) 2 Cu2+ + SO2 + 2 Cl- + 2 H2O → 2 CuCl + SO42- + 4 H+

Material: CuSO4.5H2O, NaCl, HCl, SO2, CH3COOH
Procedure: Add 5 g of NaCl to the warm solution (70 oC) of 10 g CuSO4.5H2O and
bubble SO2 through the mixture. CuCl precipitates. Filter out the precipitate and wash
it with a solution of SO2 in water and then with concentrated acetic acid.
CuCl – white crystals, insoluble in water, on air turns to green alkali copper chloride


(ii) Preparation of chrome alum - KCr(SO4)2.12H2O
a) K2Cr2O7 + 3 SO2 + H2SO4 → KCr(SO4)2 + H2O
Material: K2Cr2O7, SO2, H2SO4
Procedure: Blow the SO2 gas through a gas washing bottle with K2Cr2O7 solution acidified with H2SO4. Do not permit the temperature to rise above 60 oC. Above this temperature complexes of chromium (III) sulphate are formed. These complexes contain sulphate in a non-ionisable form and are difficult to crystallise.
Bubble the unreacted gas through a washing bottle with 10 % NaOH solution. Test for the end of the reaction – to the sample of reduced solution in a test tube add small amount of Na2CO3 crystals and heat the mixture just below the boiling point. Let the precipitate settle, the solution over the precipitate has to be colourless. Set the solution aside to crystallise after the end of reduction.When the crystallization is complete, filter off the crystals and wash them with a small amount of water.
Transfer the product to a dry filter paper and let them dry in air.
b) K2Cr2O7 + 3 C2H5OH + 4 H2SO4 → KCr(SO4)2 + 3 CH3CHO + 7 H2O
Material: K2Cr2O7, C2H5OH, H2SO4
Procedure: Dissolve crushed K2Cr2O7 in diluted H2SO4 (1:3) and add, in small portions with stirring, calculated volume (+ 10 % excess) of C2H5OH. Do not permit the temperature to rise above 60°C. Continue like in procedure a).
KCr(SO4)2.12H2O – dark violet crystals, crystallize in regular octahedra, soluble in
water.
(ii) Preparation of potassium tris(oxalate)ferrate(III) trihydrate
Fe(OH)3 + 3 KHC2O4 → K3[Fe(C2O4)3] + 3 H2O
Material: FeSO4.7H2O or (NH4)2Fe(SO4)2.4H2O, K2C2O4, H2C2O4, HNO3, ethanol
Procedure: Dissolve 35 g of FeSO4.7H2O in 100 ml of warm water and add slowly
diluted HNO3 (1:1) to oxidize Fe2+. Add NH3(aq) to the solution until the precipitation
of Fe(OH)3 is completed. Let the precipitate settle and decant the liquid. Filter out the
precipitate and wash it with hot water. Prepare a hot solution of 44 g of KHC2O4
(calculate it as a mixture of K2C2O4 and H2C2O4) in 100 ml H2O. Add precipitate of
Fe(OH)3 in small portions to this solution. Filter the resulting solution and evaporate it
on a steam bath to crystallization. Filter out and wash the crystals on the Buchner
funnel with ethanol/water 1:1 and finally with acetone. Transfer the product to a dry
filter paper and let it dry in air.
K3[Fe(C2O4)3].3H2O – green crystals, photosensitive and decomposes due to
influence of light:
2 K3[Fe(C2O4)3] → K2[Fe(C2O4)2] + K2C2O4 + 2 CO2


(iv) Preparation of iron alum (Ferrous ammoninium sulphate)
2 FeSO4 + H2O2 +H2SO4 → Fe2(SO4)3
Fe2(SO4)3 + (NH4)2SO4 → 2 NH4Fe(SO4)2
Material: FeSO4.7H2O, H2O2, H2SO4, (NH4)2SO4
Procedure: Dissolve FeSO4.7H2O in water to a solution with w(FeSO4) = 0.15. Filter
this solution if necessary and carefully add concentrated H2SO4 in 10 % excess to the
stoichiometry. Then slowly add H2O2 (double amount compared to stoichiometry),
while stirring the mixture continuously. Heat the mixture to the boiling. Make sure that
Fe2+ was oxidized to Fe3+ by a reaction of sample with K3[Fe(CN)6]. Add further H2O2
if necessary.
Evaporate the solution on a steam bath to half the volume and add warm saturated
(by 60 oC) solution of calculated amount of (NH4)2SO4. Let the solution crystallize.
Put the crystals on a dry filter paper and let them dry in air.
NH4Fe(SO4)2.12H2O – colourless or light violet crystals, turn brown on air.


(v) Preparation of lead carbonate
Pb(CH3COO)2 + (NH4)2CO3 → PbCO3 + 2 CH3COONH4
Pb(NO3)2 + 2 NaHCO3 → PbCO3 + 2 NaNO3 + CO2 + H2O
Material: Pb(CH3COO)2 or Pb(NO3)2, (NH4)2CO3 or NaHCO3
Procedure: Add saturated (NH4)2CO3 or NaHCO3 solution (10 % excess to stoichiometry) to saturated Pb2+ salt solution while continuously stirring. Decant the precipitated product with water, filter it and dry at laboratory temperature.
PbCO3 – white powder, insoluble in water, easy soluble in acids and hydroxides,
decomposes by on heating. If Na2CO3 is used for precipitation, the alkali carbonate
2PbCO3.Pb(OH)2 creates.


(vi) Preparation of lead dioxide
a) Pb(NO3)2 + 2 NaOH → Pb(OH)2 + 2 NaNO3
Pb(OH)2 + 2 NaOH → Na2[Pb(OH)4]
Na2[Pb(OH)4] + CaOCl2 → PbO2 + CaCl2 + 2 NaOH + H2O
Material: Pb(NO3)2 or Pb(CH3COO)2, NaOH, CaOCl2 or NaClO, HNO3
Procedure: Dissolve 0.1 mol of Pb2+ salt in 300 ml of hot water and cool the solution.
If a small amount of substance remains undissolved, it will not affect the result. Add a solution of 20 g NaOH in 180 ml of water. First white precipitate of Pb(OH)2 appears, which dissolves in excess of NaOH to Na2[Pb(OH)4].
Mix 40 g of CaOCl2 with 50 ml of water and the necessary amount of Na2CO3 for
reaction with Ca2+. Add 200 ml of water to the mixture and filter it.
Add the filtrate to the boiling solution of Na2[Pb(OH)4] until the test for presence of
Pb2+ in the final solution is negative.
Test for presence of Pb2+: add one drop of Na2S solution to one drop of the
reaction mixture on filter paper. Black precipitate (PbS) indicates presence of Pb2+.
Pour the mixture into 500 ml of water and decant it. Add 100 ml of diluted HNO3
(1:3) to the precipitate, stir and decant it until pH is neutral. Filter off the lead dioxide,
wash it with boiling water and dry.
b) Pb(NO3)2 + 2NaOH → Pb(OH)2 + 2NaNO3
Pb(OH)2 + 2NaOH + Cl2 → PbO2 + 2NaCl + 2H2O
Material: Pb(NO3)2 or Pb(CH3COO)2, NaOH, Cl2
Procedure: Dissolve 10.0 g of Pb(NO3)2 in 80 ml of water acidified with a drop of
concentrated nitric acid and add solution of 2.4 g of sodium hydroxide dissolved in
50 ml of water slightly with constant stirring. Heat the precipitated lead(II) hydroxide
suspension to 70 - 80 oC and bubble chlorine through it at the same time (do not heat
it over 80 oC or PbO will be obtained!). Decant the precipitated brown-black product
with diluted nitric acid (1:3) and then with water. Dry it at 100 oC.
PbO2 – brown powder, insoluble in water, decomposes on heating to Pb3O4 or PbO and oxygen. Good oxidizer.


Preparation of Potash alum K2(SO4).Al(SO4)3•12H2O)
It is white crystalline solid, soluble in water, used for the purification of water, leather industry paper industry and as fire extinguisher.
Melting point is 92oC
Potash alum is commonly known as "PHITKARI"
Potash alum is prepared by mixing equi-molecular masses of potassium sulphate and aluminum sulphate in water followed by evaporation
K2SO4 + Al2(SO4)3 + 24H2O-- K2SO4.Al2(SO4)3.24H2O


(vii) Preparation of copper ammine sulphate - perform this experiment in a fume hood
CuSO4 + 4 NH3(aq) + H2O → [Cu(NH3)4]SO4.H2O
Material: CuSO4.5H2O, NH3(aq), C2H5OH
Procedure: Place 5 g of finely powdered copper sulphate, CuSO4.5H2O, in a small
beaker, pour upon it 7.5 ml of concentrated ammonia and 3 ml of water. Shake it for
about 1 minute and then heat it gently until all the solid dissolves. Add about 10 ml of
ethanol to the solution, let it stand for about one hour and filter off the crystals. Wash
them with a mixture of 5 ml of concentrated ammonia and 5 ml of ethanol. Dry them
on air in the hood.
[Cu(NH3)4]SO4.H2O – dark blue crystals, soluble in water (18 g in 100 ml of water at
21.5 oC), stable on air.


(viii) Preparation of cuprous chloride – work in a fume hood
a) Cu2+ + Cu + 2 Cl- → 2 CuCl
Material: CuSO4.5H2O, Cu, NaCl, HCl, Na2SO3, CH3COOH
Procedure: Prepare a solution of 10 g of powdered CuSO4.5H2O and 15 ml of concentrated HCl in a 250-ml round-bottom flask, add 4 g of NaCl and heat to boiling.
Cover the flask with a little funnel. Add copper to hot solution in small portions. The green colour of the solution will turn to yellow. Filter off the remaining Cu. Pour the solution to one litre of cold water with 2 g of Na2SO3.
Wash the precipitated cuprous chloride 2 – 3 times with a solution of 1.5 g of Na2SO3, 6 ml of HCl and 300 ml of H2O by decantation. Filter out the precipitate and wash it with concentrated acetic acid. Dry it in a drying oven at 100 oC.
b) 2 Cu2+ + SO2 + 2 Cl- + 2 H2O → 2 CuCl + SO4
2- + 4 H+
Material: CuSO4.5H2O, NaCl, HCl, SO2, CH3COOH
Procedure: Add 5 g of NaCl to the warm solution (70 oC) of 10 g CuSO4.5H2O and bubble SO2 through the mixture. CuCl precipitates. Filter out the precipitate and wash
it with a solution of SO2 in water and then with concentrated acetic acid.
CuCl – white crystals, insoluble in water, on air turns to green alkali copper chloride