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METHODS TO MEASURE COMPOSITION: Analytical Technique Optical Concentration Measurement Electrochemical Composition Measurement Gas Analysis Technique Kinetic Methods

ANALYTICAL TECHNIQUE CHROMATOGRAPHY This technique includes the chromatography, mass spectrometry and measure of physical properties like density, viscosity, etc CHROMATOGRAPHY It is of the following two types: Liquid Chromatography Gas Chromatography

Liquid Chromatography The basic principle is based on the concentration equilibrium of the components of interest, between two immiscible phases. 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.

Gas Chromatography The basic principle is same but here gas is used as carrier 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.

Mass spectrometers The mass spectrometer is a device that is used for analyzing gases as well as liquid compositions. 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.

Physical properties measurement In this, we find some physical properties like density, viscosity, etc which in turn give us the composition. Some methods are described following: Density Measurement Viscosity Measurement (viscometer)

OPTICAL CONCENTRATION MEASUREMENT This technique includes these following four techniques to measure composition: Spectroscopy Colorimeter Flame Photometry Refractrometer

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

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.

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.

Electrochemical Composition Measurement This technique includes these following three techniques to measure composition: pH Measuring Devices Redox Measurement Conductivity analyzers

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 concentration A 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.

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

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.

GAS ANALYSIS TECHNIQUE This technique includes these following two techniques to measure composition: Gas Analysis by Chemical Absorption Gas Analysis by Infrared Technique

Gas Analysis by Chemical Absorption 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 Infrared Technique 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

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.

DISCUSSION AND QUESTIONS