chemical reaction

Chemical Reaction

The chemical reaction campaign has come to an end now that the new European Chemicals law, REACH, has come into force.

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds between atoms, and can often be described by a chemical equation.

However, now the law is in place there is much to be done to ensure that it really does protect us from the worst chemicals; the following web sites can tell you more about what is happening now.

ChemTrust

Greenpeace

Chemicals Health Monitor Project

International Chemicals Secretariat

The "substitute it now" campaign against the worst chemicals

WWF

Scanning Probe Microscopy SPM

 Scanning Probe Microscopy (SPM): 

The scanning probe microscopy is a general term for a wide variety of microscopic 

techniques, which measure the morphology and properties of surfaces on the atomic scale.  

This includes the following: 

Scanning Tunneling Microscopy (STM) – which studies the surface topography and electronic 

structure, Atomic Force Microscopy (AFM) – which studies the surface topography, surface 

hardness and elastic modulus, Lateral Force Microscopy (LFM) – which studies the relative 

frictional properties,  Scanning Thermal Microscopy (SThM) –  which studies the thermal 

conductivity, Magnetic Force & Electric Force Microscopies (MFM & EFM) – which study 

the magnetic and electric properties. 

The techniques of STM and AFM are discussed below, since these are widely used: 

   

Principle:  The general principle for all the scanning probe microscopes is that a sharper 

probe (or a very fine tip) is used to scan the surface of the sample with much lower force and 

obtain the topography and morphology information. 

Scanning tunneling microscope: When a sharp tip made of a conducting material is brought 

close to a  conducting sample, overlapping of the electron clouds between the two surfaces 

will occur. If a potential is given between them a current of electrons is formed, which is 

often referred as “tunneling”  current,  and the effect is known as “tunneling” effect.  This 

effect is largely depended on the distance between the tip and the sample material. Hence, if 

the scanning tip is controlled by a high precision motion device made of piezo-electric 

material, the distance between the tip and the sample can be measured during a scanning 

through a feedback loop control of the piezo-electric element. By this way the sample can be 

scanned with sub-angstrom precision.   

Atomic force microscope:  This technique operates by measuring the forces between the 

sample and the tip, and the sample need not be a conducting material. Here, the tip is brought 

close enough to the sample surface to detect the repulsive force between the atoms of the tip 

material and the sample. The probe tip is mounted at the end of a cantilever of a low spring 

constant and the tip-to-sample spacing is held fixed by maintaining a constant and very low 

force on the cantilever. Hence, if the tip is brought close to the sample surface, the repulsive 

force will induce a bending of the cantilever. This bending can be detected by a laser beam, 

which is reflected off the back of the cantilever. Thus by monitoring the deflection of the 

cantilever, the surface topography of the sample can be tracked. Since the force maintained 

on the cantilever is in the range of inter-atomic forces (about 10-9 

Newton), this technique 

derived the name “atomic force” microscopy. 

AFM operates at two modes: 

Repulsive or contact mode – which detects the repulsive forces between the tip and sample; 

Attractive or non-contact mode – which detects the van der waals forces that act between the 

tip and sample. 

Instrumentation:  

Scanning tunneling  microscope:  It mainly consists of a scanner, probe motion sensor 

composed of piezo-electric material, micro probe, etc. 

Atomic force microscope:  It mainly consists of a scanner, cantilever, laser source, photo-

diode detector, micro-probe, etc.

Applications: Both STM and AFM find applications widely in material sciences especially 

for surface studies on a nano scale range. While STM finds its applications in the 

characterization of surface structure (including the electronic structure), AFM finds its 

applications in measuring the  hardness of materials. Sometimes, AFM can be used in the 

study of “depth profile” of the deposited oxide layer on to a material.  

Disadvantages: A limitation to STM is that it can study only the conducting samples, since 

the technique is based on the tunneling current between two conducting areas. Hence, it 

doesn’t lend itself to the study of non-conducting materials. In fact, the AFM had been 

developed to encounter this  problem. These methods require  special sample preparation 

techniques, which are tedious, like, thin sectioning, electo-polishing, various mechanical 

cutting and polishing techniques, etc. 

Transmission Electron Microscopy TEM

Transmission Electron Microscopy (TEM): 

Principle:    In this technique, a beam of high-energy electrons (typically 100 - 400keV) is 

collimated by magnetic lenses and allowed to pass through a specimen under high vacuum. 

The transmitted beam and a number of diffracted beams can form a resultant diffraction 

pattern, which is imaged on a fluorescent screen kept below the specimen. The diffraction 

pattern gives the information regarding lattice  spacing and symmetry of the structure under 

consideration. Alternatively, either the transmitted beam or one of the diffracted beams can 

be made to form a magnified image of the sample on the viewing screen as bright-and dark-

field imaging modes respectively, which give  information about the size and shape of the 

micro-structural constituents of the material. High-resolution image, that contains 

information about the atomic structure of the material, can be obtained by recombining the 

transmitted beam and diffracted beams together.    

Instrumentation:    It comprises of a tungsten filament or LaB6 or a field emission gun as 

source of electron beam, objective lens, imaging lens, CCD camera, monitor, etc.   

Applications:  Transmission electron microscopy is used to study the local structures, 

morphology, and dispersion of multi-component polymers, cross sections & crystallization of 

metallic alloys and semiconductors, microstructure of composite  materials, etc. The 

instrument can be extended to include other detectors like Energy Dispersive Spectrometer 

(EDS) or Energy Loss Spectrometer (ELS) to study about the local chemistry of the material 

similar to SEM technique.   

Disadvantages:  The instrumentation is complicated and needs high vacuum. Sample 

preparation is very time consuming. Some materials, especially polymers, are sensitive to 

electron beam irradiation which results in the loss of crystallinity and/or mass. 

Scanning Electron Microscopy SEM

Scanning Electron Microscopy (SEM): 

Principle:  In this technique, an electron beam is focused onto the sample surface kept in a 

vacuum by electro-magnetic lenses (since electron possesses dual nature with properties of 

both particle and wave an electron beam can be focused or condensed like an ordinary light) 

The beam is then rastered or scanned over the surface of the sample. The scattered electron 

from the sample is then fed to the detector and then to a cathode ray tube through an 

amplifier, where the images are formed, which gives the information on the surface of the 

sample. 

Instrumentation:  It comprises of a heated filament as source of electron beam, condenser 

lenses, aperture, evacuated chamber for placing the sample, electron detector, amplifier, CRT 

with image forming electronics, etc. 

Applications: Scanning electron microscopy has been applied to the surface studies of metals, 

ceramics, polymers, composites and biological materials for both topography as well as 

compositional analysis. An extension (or sometimes conjunction to SEM) of this technique is 

Electron Probe Micro Analysis (EPMA), where the emission of X-rays, from the sample 

surface, is studied upon exposure to a beam of high energy electrons. Depending on the type 

of detectors used this method  is classified in to two as:  Energy Dispersive Spectrometry 

(EDS) and Wavelength Dispersive Spectrometry (WDS). This technique is used extensively 

in the analysis of metallic and ceramic inclusions, inclusions in  polymeric materials, 

diffusion profiles in electronic components.   

Disadvantages:  The instrumentation is complicated and needs high vacuum for the optimum 

performance.

Analysis Through Microscopy

ANALYSIS THROUGH MICROSCOPY 

The techniques described here are not for the simple, ordinary optical microscopes which use 

light for the magnification. These employ electron beam and mechanical probes to magnify 

the surfaces under study. 

X-Ray Diffractometry XRD

 X-Ray Diffractometry (XRD):

Principle:  In this technique the primary X-rays are made to fall on the sample substance

under study. Because of its wave nature, like light waves, it gets diffracted to a certain angle.

This angle of diffraction, which differs from that of the incident beam, will give the

information regarding the crystal nature of the substance. The wavelength of the X-rays can

be varied for the application by using a grating plate.

Instrumentation :  It consists of X-ray tube for the  source, monochromator and a rotating

detector.

Applications : The diffraction of X-rays is a good tool to study the nature of the crystalline

substances. In crystals the ions or molecules are arranged in well-defined positions in planes

in three dimensions. The impinging X-rays are reflected by each crystal plane. Since the

spacing between the atoms and hence the planes can’t be same or identical for any two

chemical substances, this technique provides vital information regarding the arrangement of

atoms and the spacing in between them and also to find out the chemical compositions of

crystalline substances. The sample under study can be of either a thin layer of crystal or in a

powder form. Since, the power of a diffracted beam is dependent on the quantity of the

corresponding crystalline substance, it is  also possible to carry out quantitative

determinations.