OFFLINE COMPOSITION
MEASURING SENSORS
PREPAIRED BY:07-CHEM-66
1
METHODS TO MEASURE COMPOSITION:
Analytical Technique
Optical Concentration Measurement
Electrochemical Composition Measurement
Gas Analysis Technique
Kinetic Methods
2
ANALYTICAL TECHNIQUE
CHROMATOGRAPHY
It is of the following two types:
Liquid Chromatography
Gas Chromatography
This technique includes the chromatography, mass
spectrometry and measure of physical properties like
density, viscosity, etc
3
Liquid Chromatography
A tube is filled with a suitable fine
powdered solid, the stationary
phase.
small amount of sample mixture is
added at the top.
Then sample is taken up by
mobile phase, which goes
through the column by gravity,
carrying the various constituents
of the mixture along with it.
The basic principle is based on the concentration
equilibrium of the components of interest, between two
immiscible phases.
4
Gas Chromatography
In this analyzer that consists of a
column packed with a material
through which the sample of the
gas to be analyzed is passed. At
the end of the column, a thermal
conductivity measuring system
is attached. As the sample moves,
the component of the sample are
separated because of their affinity
for the packing of the column.
The basic principle is same but here gas is used as carrier
5
Mass spectrometers
It first reduces gas or liquid sample
to a very low pressure by ejecting
it from the nozzle of the jet. Then
the sample is ionized, accelerated
and separated into its constituent
components according to the
respective charge to mass ratios.
The mass spectrometer is a device that is used for
analyzing gases as well as liquid compositions.
6
Physical properties measurement
1. Density Measurement
2. Viscosity Measurement
(viscometer)
In this, we find some physical properties like density, viscosity, etc which in turn give us the composition. Some methods are described following:
7
OPTICAL CONCENTRATION MEASUREMENT
This technique includes these following four techniques to measure composition:
Spectroscopy
Colorimeter
Flame Photometry
Refractrometer
8
Spectroscopy
According to the Quantum theory, the energy
state of atom or molecule is definite and for any
change in state of the atom requires a specific
amount of energy. If with the help of external
force, the state of atom is changed, the photons
of specific wavelength and frequency are
emitted and by finding the frequency or
wavelength of these photons, we find the
composition. This technique is known as
spectroscopy
9
Colorimeter
This is the method in which we use the human
eye as instrument. In this technique we compare
the color of the unknown composition solution
with the standard colors. This technique is
majorly used for the liquids.
10
Refractrometer
This involves the measurement of refractive
index of optically transparent material. When
light is passed between two different media
having different densities it will be refracted
according to the snell’s law according to which:
Where α1, α2 are angles of incidence &η1, η2
are refractive indexes of the two media.
As the composition of a sample changes its
refractive index changes, this change in
refractive index changes the critical angle
occurring at interface between glass prism &
sample and hence find the composition.
11
ELECTROCHEMICAL COMPOSITION MEASUREMENT
This technique includes these following three techniques to measure composition:
pH Measuring Devices
Redox Measurement
Conductivity analyzers
12
PH MEASURING DEVICES
The pH is normally measured by chemical indicators or by
pH meters.
The final color of chemical indicators depends on the
hydrogen ion concentrationA pH sensor normally consists
of a sensing electrode and a
reference electrode to form
an electrolytic cell.
The electrodes are connected
to a differential amplifier,
which amplifies the voltage
difference between the
electrodes, giving an output
voltage that is proportional to
the pH of the solution.13
Redox Measurement
Redox (oxidation, reduction) potential measurement is similar to pH measurement. In this technique, the ions are electrically charged. The electrodes of platinum and calomel are immersed in the solution whose composition is to be measured. An electrical potential is produced which is proportional to the relative concentration of the oxidized and reduced ions. With the help of this electrical potential, we find the respective composition of the elements present in the mixture
14
Conductivity analyzers
The electrical conductivity of any liquid is its ability to conduct
electricity
The conductivity of a conducting liquid is very sensitive to the
presence of the small quantities of electrolytes.
Conductivity cells measure the conductivity when current is
passed between two electrodes immersed in the sample &
relate this conductivity to the resistance offered to the flow of
the current.
15
This technique includes these following two techniques to measure composition:
Gas Analysis by Chemical Absorption
Gas Analysis by Infrared Technique
GAS ANALYSIS TECHNIQUE
16
The basic principle of chemical absorption in gas analysis is that the volume of the constituent gas is determined by finding the reduction in the volume of the sampled gas at constant temperature and pressure when the mixture is absorbed in an absorbent. The more likely gasses that are majorly used in chemical industries are carbon dioxide (CO2), carbon monoxide (CO) and oxygen (O2) is measured by this chemical absorption method. The absorbent used for each gas is different and its selection depends upon the nature of the specie
Gas Analysis by Chemical Absorption
17
Infrared type elements are suitable for analyzing certain components in the process streams. But this technique cannot be used by the components that absorb infrared radiations. The main and most significant advantage of this that the output signals produced are electrical signals and can be transmitted to the several hundred feet to the receiving instrument where it is displayed as a volume percentage of the total composition. In this technique, we get the compositions by the respective peaks from the graph
Gas Analysis by Infrared Technique
18
KINETIC METHODS
Kinetic methods involve the measurement of chemical
reactions or processes in a time dependent manner.
From the rate equation ;
We can deduced the result
In this way, a calibration curve can be generated against
which an unknown sample can be measured for the content
of specie A.
19
DISCUSSION AND QUESTIONS
20