ST. PAUL’S CONVENT SCHOOL METALLIC …20Rainbow%20abstract.pdfreadings, specificity, chemical hazards, and environmental impacts. A handy heavy metal ion test kit was constructed

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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.

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.

Lab

ora

tory

C

hem

ical

s

1. Potassium sodium

tartrate

2. Tartaric acid 3. Citric acid 4. Glycine 5. Leucine

Ho

use

hold

C

hem

ical

s

6. Cream of tartar 7. Baking powder 8. MSG 9. Aspartame 10. Gelatine

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

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Experiments


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.


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.


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.


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.


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


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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

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.

Glycine

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Cu2+

Ni2+

Co2+

Cr3+

Fe3+


Zn2+

Pb2+

Ag+

Hg2+

Pb2+

/ Ag+/ Hg

2+

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 Ni

2+ 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 Hg

2+ 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.

MSG MSG MSG MSG MSG

Baking powder Baking powder

Baking powder

Gelatine Gelatine

Gelatine

Leucine

Glycine Glycine Glycine Glycine Glycine

Cream of tartar Cream of tartar Tartaric acid

Tartrate Tartrate Tartrate Tartrate Citric acid

Documents

ST. PAUL’S CONVENT SCHOOL METALLIC …20Rainbow%20abstract.pdfreadings, specificity, chemical hazards, and environmental impacts. A handy heavy metal ion test kit was constructed