22-02-2013, 02:24 PM
DETERMINATION OF ZINC IN BRASS
Theory :
A known weight of the brass sample is heated with conc.HNO3.
Copper, zinc, lead and iron pass into solution while tin gets
precipitated as metastannic acid, H2SnO3. This is filtered, dried,
ignited and weighed to get the percentage of tin in the alloy. The
filtrate is evaporated to dryness with conc. H2SO4 which expels
off nitrogen oxides. The dry mass is then treated with distilled
water. White precipitate of PbSO4 appears which is filtered,
washed with cold water, dried, ignited and weighed to get the
percentage of lead. The filtrate contains CuSO4, ZnSO4 and
FeSO4. This solution is then adjusted to pH = 3 by adding dil.
HCl, warmed and treated with saturated HS so as to precipitate
copper as black CuS. Copper sulphide is filtered off, washed with
cold water and filtrate is taken for the determination of zinc. First
H2S is removed completely by boiling. A known aliquot of the
solution is then adjusted to pH = 10 by using the (NH4Cl +
NH4OH) buffer. This is than titrated against a saturated EDTA
solution. This gives the amount of zinc in brass sample can be
calculated.
Procedure :
I. Preparation of the solution of the sample
1. Weight accurately of 1.0 gm of brass turnings (freshly drilled)
on a porcelain dish.
2. Treat it with 30 ml conc.HNO3 with stirring till the turnings
dissolve completely. Heat in a water bath (90 to 100° C) for
about 10 minutes.
3. White precipitate of H2SnO3 appears. Cool and filter through
Whatmann No. 41. Wash with hot water. Take the residue for
tin.
4. Transfer the filtrate and washings into another porcelain dish,
add 50 ml of conc. H2SO4 and evaporate the solution to
dryness.
5. Cool and treat the dry mass with 100 ml distilled water. Boil
the contents fir 5 minutes. Lead (if present) appears as a
white precipitate.
6. Filter and wash the residue with cold water. Take the residue
for lead. Collect the filtrate and the washings together in a
500 ml beaker.
7. Add about 100 ml 2N HCl, heat gently and add saturated H2S
with constant stirring (till the smell of H2S comes out from the
beaker).
8. Filter off the precipitate and wash with water. Test the filtrate
for copper by adding saturated H2S to a small portion. The
copper-free filtrate is boiled vigorously to expel off H2S (test
with lead acetate paper) and transfer the solution into a 250
ml volumetric flask. This filtrate is taken for the estimation of
zinc
Result : In the given sample of brass, the amount of zinc is = ______
Theory :
A known weight of the brass sample is heated with conc.HNO3.
Copper, zinc, lead and iron pass into solution while tin gets
precipitated as metastannic acid, H2SnO3. This is filtered, dried,
ignited and weighed to get the percentage of tin in the alloy. The
filtrate is evaporated to dryness with conc. H2SO4 which expels
off nitrogen oxides. The dry mass is then treated with distilled
water. White precipitate of PbSO4 appears which is filtered,
washed with cold water, dried, ignited and weighed to get the
percentage of lead. The filtrate contains CuSO4, ZnSO4 and
FeSO4. This solution is then adjusted to pH = 3 by adding dil.
HCl, warmed and treated with saturated HS so as to precipitate
copper as black CuS. Copper sulphide is filtered off, washed with
cold water and filtrate is taken for the determination of zinc. First
H2S is removed completely by boiling. A known aliquot of the
solution is then adjusted to pH = 10 by using the (NH4Cl +
NH4OH) buffer. This is than titrated against a saturated EDTA
solution. This gives the amount of zinc in brass sample can be
calculated.
Procedure :
I. Preparation of the solution of the sample
1. Weight accurately of 1.0 gm of brass turnings (freshly drilled)
on a porcelain dish.
2. Treat it with 30 ml conc.HNO3 with stirring till the turnings
dissolve completely. Heat in a water bath (90 to 100° C) for
about 10 minutes.
3. White precipitate of H2SnO3 appears. Cool and filter through
Whatmann No. 41. Wash with hot water. Take the residue for
tin.
4. Transfer the filtrate and washings into another porcelain dish,
add 50 ml of conc. H2SO4 and evaporate the solution to
dryness.
5. Cool and treat the dry mass with 100 ml distilled water. Boil
the contents fir 5 minutes. Lead (if present) appears as a
white precipitate.
6. Filter and wash the residue with cold water. Take the residue
for lead. Collect the filtrate and the washings together in a
500 ml beaker.
7. Add about 100 ml 2N HCl, heat gently and add saturated H2S
with constant stirring (till the smell of H2S comes out from the
beaker).
8. Filter off the precipitate and wash with water. Test the filtrate
for copper by adding saturated H2S to a small portion. The
copper-free filtrate is boiled vigorously to expel off H2S (test
with lead acetate paper) and transfer the solution into a 250
ml volumetric flask. This filtrate is taken for the estimation of
zinc
Result : In the given sample of brass, the amount of zinc is = ______