Download ST. PAUL`S CONVENT SCHOOL METALLIC RAINBOW 金屬彩虹

ST. PAUL’S CONVENT SCHOOL
METALLIC RAINBOW 金屬彩虹
Introduction
Water resources are seriously contaminated with heavy metal ions, yet they are difficult to be observed with naked
eyes. We can now rely on the formation of metal complexes to show the presence of metal ions in water.
Objectives
In part I, we selected ligands by comparing the colour intensity and specificity of their complexes formed with metal
ions. In part II, we constructed calibration curves and selected the ‘best’ ligands by comparing their absorbance
readings, specificity, chemical hazards, and environmental impacts. A handy heavy metal ion test kit was constructed
using filter paper and a colour scale in part IIIA, while a colour chart was developed using both laboratory and
household chemicals in part IIIB. In part IV, we investigated the precipitates formed by colourless metal ions with
common laboratory and household chemicals.
Chemical Principles
Part I – selecting ligands that form metal complexes with high colour intensities
 Metal complex formation
A complex is formed when a central metal ion/atom is datively bonded to ligands having lone pair of
electrons. Cu2+, Ni2+, Co2+, Cr3+ and Fe3+ are chosen for investigation because of their small sizes, high charges
and availability of vacant low-lying d-orbitals. When ligands bond to a transition metal ion, electrons in the
ligands repel electrons in the five d orbitals of the metal ion, splitting them into high and low energy orbitals.
When white light is passed through a complex, a specific frequency of light is absorbed to promote an electron
from a lower energy orbital to an upper one. As a consequence the other colours are transmitted. Different
complex ions absorb different frequencies of visible light, giving rise to different colours.
 Examples
Cu2+(aq) + 4NH3(aq)  [Cu(NH3)4]2+(aq)
Fe3+(aq) + SCN(aq)  [Fe(SCN)]2+(aq)
Part II – conducting colorimetry
Colorimetry is an analytical method that determines the concentration of a
coloured chemical solution by measuring the amount of light it absorbs. The
higher the concentration the metal complex ions, the greater the absorbance
/ colour intensity. Calibration curves are then constructed.
Part IIIA – constructing filter paper test kit
Complexes of eight concentrations are dripped into filter papers, forming a colour scale. For
example, when glycine reacts with Cu2+ ions, the colour scale would be eight different
shades of blue. A water sample containing an unknown metal ion will be dripped into those
filter papers soaked in ligands, and the colour that appeared can be matched with the colour
scale, thus identifying the type and rough concentration of the metal ion in the sample.
Laboratory
Chemicals
Part IIIB – developing colour chart using lab reagents and household chemicals
Ten environmentally friendly ligands, including laboratory and household chemicals, are selected, as they have similar
functional groups as the ligands chosen in parts I and II.
1. Potassium sodium 2. Tartaric acid
3. Citric acid
4. Glycine
5. Leucine
tartrate
7. Baking powder
8. MSG
9. Aspartame
10. Gelatine
Household
Chemicals
6. Cream of tartar
Part IV – testing for colourless metal ions through precipitation
Some colourless heavy metal may form insoluble salts upon precipitation with suitable reagents. For example, when
HgCl2 and AgNO3 solutions are added to KI solution separately, a red-orange and yellow precipitate will be formed
respectively.
Hg2+ (aq) + 2I (aq)  HgI2(s) red-orange
Ag+(aq) + I(aq)  AgI(s)
yellow
1
Experiments
Part I – selecting ligands that form metal complexes with high colour intensities
0.5M, 0.1M, 0.05M, 0.01M, 0.005M and 0.001M of CuSO4, NiCl2, CoCl2, CrCl3 and Fe2(SO4)3
solutions were prepared. 0.5 cm3 of metal ion solution was added into a 6-hole well plate according to
the sequence of concentrations. 16 different ligands were added into the well plates using droppers.
Part II – conducting colorimetry
3.0 cm3 of each concentration of each metal ion solution was pipetted into a test tube. 3.0 cm3 of
a ligand reagent was pipetted into the same test tube. The absorbance of each solution mixture
was measured by a calibrated colorimeter. Best two to three complex ions for each metal ion
were chosen.
Part IIIA – constructing filter paper test kit
A few drops of each concentration of a particular complex were added to the pieces of
filter paper. The filter papers were allowed to dry. Pieces of filter paper were dipped
into 0.1 M ethane-1,2-diamine for 10 minutes. They were then dried in the oven.
Part IIIB – developing colour chart using lab reagents and household chemicals
1 spoonful of each household chemical was added into a microscale 8-well reaction strips using a
lab micro spatula. 3 drops of metal ion solution were added.
Part IV – testing for colourless metal ions through precipitation
2 cm3 of the 11 chosen solutions and a control were added into the 12 wells of 2 well plates.
2 cm3 of each colourless metal ion solution was then added.
Results
Part I – selecting ligands that form metal complexes with high colour intensities
2
Part II – conducting colorimetry
Part IIIA – constructing filter paper test kit
Cu2+
Ni2+
Co2+
Cr3+
Fe3+
Ethane-1,2-diamine
Sodium oxalate
Ethane-1,2-diamine
EDTA
Potassium thiocyanate
Glycine
Potassium sodium
tartrate
Potassium sodium
tartrate
Leucine
Sodium citrate
Glycine
Limitations of the filter paper test kit
The colours of complexes shown on filter papers are not easily observed at low
concentrations of metal ions. Thus we devise another test kit using reaction well strips
for testing the concentration of metal ions conveniently.
3
Part IIIB – developing colour chart using lab reagents and household chemicals
Cu2+
Ni2+
Co2+
Cr3+
Fe3+
Tartrate
Tartrate
Tartrate
Tartrate
Citric acid
Glycine
Glycine
Glycine
Glycine
Glycine
Leucine
Cream of tartar
MSG
Baking powder
Gelatine
MSG
Cream of tartar
Baking powder
MSG
MSG
Baking powder
Tartaric acid
MSG
Gelatine
Gelatine
Part IV – testing for colourless metal ions through precipitation
Zn2+
Pb2+
Ag+
Hg2+
Pb2+/ Ag+/ Hg2+
Discussion
 In part I, NH3, en, citrate and glycine were chosen for Cu2+ for the conduction of colorimetry; EDTA, oxalate
and tartrate for Ni2+; conc. HCl, en and tartrate for Co2+; NaBr, EDTA and leucine for Cr3+; KSCN, rust
indicator, en, citrate and glycine for Fe3+.
 In part II, en and glycine were selected constructing the test kit for Cu2+; oxalate and tartrate for Ni2+; en and
tartrate for Co2+; EDTA and leucine for Cr3+; KSCN, citrate and glycine for Fe3+
 In part IIIA, glycine was chosen for the construction of test kit which tests for Cu2; tartrate for both Ni2+ and
Co2+; leucine for Cr3+; citrate and glycine for Fe3+. However, the colours of the complexes formed are rather
pale on the filter papers.
 In part IIIB, glycine and MSG were chosen to construct the final test kit.
 In part IV, KI and I2 tincture could be used to test for the presence of colourless Pb2+, Ag+ and Hg2+ ions in
polluted water.
 We can easily deduce which metal ions and its concentration in the unknown samples simply by referring to
the colour chart made based on the reaction between MSG/glycine and metal ions of different concentrations.
 There are currently some existing testing instruments
like spectrophotometers, which are very costly, while
some commercial test kits contain poisonous
substances. When compared to them, our test kit is
safer, more handy, and most importantly, more
economic, as it only costs 0.133 cents per spoonful.
Conclusion
Glycine and MSG were chosen to make the final test kit. There were
significant colour changes even when very low concentrations of metal ions
were used. In particular, MSG is cheap and easily accessible. People can
easily identify metal ions in water with a palm-sized test kit.
4