OFFLINE COMPOSITION

MEASURING SENSORS

PREPAIRED BY:07-CHEM-66

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METHODS TO MEASURE COMPOSITION:

Analytical Technique

Optical Concentration Measurement

Electrochemical Composition Measurement

Gas Analysis Technique

Kinetic Methods

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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

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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.

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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

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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.

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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:

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OPTICAL CONCENTRATION MEASUREMENT

This technique includes these following four techniques to measure composition:

Spectroscopy

Colorimeter

Flame Photometry

Refractrometer

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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

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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.

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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.

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ELECTROCHEMICAL COMPOSITION MEASUREMENT

This technique includes these following three techniques to measure composition:

pH Measuring Devices

Redox Measurement

Conductivity analyzers

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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

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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.

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This technique includes these following two techniques to measure composition:

Gas Analysis by Chemical Absorption

Gas Analysis by Infrared Technique

GAS ANALYSIS TECHNIQUE

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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

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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

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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.

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DISCUSSION AND QUESTIONS

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