There are many useful analytical tools, such as photometric analysis, electrometric analysis, chromatography, mass spectrometry, thermal conductivity, and various physical property measurements (density and specific gravity), which can be used to determine the composition of mixtures. This paper includes a brief introduction of the important composition sensors, their working principle and operation. It also contains the selection criteria of composition sensors. After studying this document, we will be able to select an appropriate composition sensor for given conditions.
Types of testing
There are two basic types of testing used in the industry for composition measurement. These are as follows: 
In online testing the instrument is directly connected to line and provides a feedback through a transmitter. The instrument is exposed to the process conditions and it needs to be very accurate. The basic purpose of online composition sensor is to control a specific composition in a reactor or column etc. The on line testing is of two types in-line and slip stream testing.
In in-line testing the instrument is attached directly to process line. For stream slip testing a side stream is taken that run along the main process stream. The composition sensor is attached directly to this side stream and it will generate a signal that is used to control the composition.
In off-line testing, the sample is withdrawn from the process or reaction and is sent to quality control lab for analysis. The results are then sent to modify or maintain the process conditions to get a desired composition.
1.1-Comparison of on-line and off-line testing:
Online testing is very expensive then the off-line testing. On-line sensors generate quick and immediate feedback response to the changes in process conditions while there is a time lag in composition measurement using the off-line sensors. On-line testing forbids the continued production of wrong product, with an immediate response and correction of the flawed material but in case of off-line testing there may be accumulation of the wrong material.
These two methods are widely used in the industry but now trend is shifting towards online testing. Mostly the food industries are using the online testing. Although the capital cost of the on-line sensors is high but saving from the process would not only compensate the installation cost but also preclude the significant losses.
It is a technique used to separate and analyze the mixtures for composition measurement. It involves two phases, one mobile phase and other is stationary phase. Depending on the basis of the type of chromatography these two phases are combined in different ways. Chromatography is of two types i.e. preparative and analytical. In preparative chromatography the mixture is separated into its components while in analytical chromatography composition/proportion of mixture is measured.
Separation occurs in the column based on varying affinities of the components to bind column packing. A detector measures some physical property that relates to concentration. 
2.2-Classification of chromatography
There are many types of the chromatography depending on different basis. 
Basis: Mobile phase
(1) Liquid chromatography
(2) Gas chromatography
(3) Supercritical chromatography
Basis: Bed shape
(1) Column chromatography
(2) Planar chromatography
(3) Paper chromatography
(4) Thin layer chromatography
Simulated moving-bed chromatography
Basis: Stationary phase [3a]
(1) Ion exchange chromatography
(2) Size-exclusion chromatography
2.3-Advantages of chromatography
This technique can be used to find chemical composition of almost every compound.
These are very precise and accurate.
It is very accurate method for the identification of the drugs and sometimes used for quantitative analysis. 
2.4-Disadvantages of chromatography
Only used for known mixtures
Photometric analysis is a technique used for measurement of the intensity of visible light and other electromagnetic (EM) waves. By the measuring these values, the composition of sample mixtures can be found in numerous different ways. There are many photometric devices available for composition measurement in the chemical industry.
3.1-Classification of photometry
(a) There are two broad categories of photometers on the basis of methods used.
In these types of photometers, the light from the source is dispersed and a narrow spectral band is selectively directed to the sample and detector. A monochromator is the tool that performs this function. These devices can scan a spectrum and make measurements at different wavelengths. Due to this capability they are used for the analysis of multi component systems.
These use a narrow-band-pass filter to block out a large amount of undesired radiation. They make measurements at selected discrete wavelengths. The filter passes radiation at the selected reference and measures wavelengths. Non-dispersive photometers are used to detect only a single component in the stream.
Non-Dispersive Infrared (NDIR) analyzer is such a device that is frequently used to detect the concentrations of many gases in a mixed gas flow. It uses a reference non-absorbent gas such as Cl2, O2 and N2 and compares it to a sample gases like CO2, SO2, CO, CH4, etc. These analyzers use two beams of infrared radiation: an analyzing beam for the sample and a reference beam for the non-absorbent gas and.
(b)There are two main classes of photometry on the basis of the instruments used.
(b) Instruments using visible light
It (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. It is typically used in spectroscopic analysis to identify materials. Spectrometers have two broader classes
(a) Radiation: It uses heat, luminescence, and fluorescence to measure composition
(b) Absorption: This technique makes use of either ultraviolet or infrared waves to reveal composition.
Monitors wavelengths of EM spectrum. The degree of absorption /radiation reveals molecular structure and concentration of a sample. The amount of the radiations absorbed is high if the compound is concentrated.This absorbance between samples varying with concentration linearly. When monochromatic light (light of a specific wavelength) passes through a solution there is usually a quantitative relationship (Beer’s law) between the solute concentration and the intensity of the transmitted light, that is, 
Where I sub 0 is the intensity of transmitted light using the pure solvent, when the colored compound is added the intensity of the transmitted light is I, the colored compound has composition c, l is the distance the light passes through the solution, and k is a constant.
The radiation spectrometers are often used in the biological or pharmaceutical industries. Also used for detection of the protein cells.
The absorption spectrophotometers can be used for any gas/liquid system. These are usually used to detect the presence of aromatics in the organic compounds.
These are highly sensitive and are very specific.
These are reliable.
These have large data bank for reference.
These are relatively inexpensive and can be used for a variety of substances.
Spectrophotometers are too specific for a given application.
Application is limited to certain electromagnetic range.
These can only be used for gas/liquid systems
3.3-Photometers using visible light
There are basically three types of the visible light photometers. These are as follows
Turbid meters 
Opacity monitors 
Refractrometer is an instrument that measures the refractive index. The refractive index can be calculated using Snellâ€™s law. The refractive index is related with composition using Gladstone-Dale relation (J. H. Gladstone and T. P. Dale, 1864).
These are used in veterinary medicine industry, to measure total plasma protein in the blood sample and specific gravity of urine. In gemology, it is used for the identification of gems.
Turbidity is a measure of cloudiness and haziness in the liquid caused by very small particles that cannot be seen without magnification. Turbidity can also be used for some transparent solids e.g. glass. In the production of plastics haze is defined as the percentage of light deflected more than 2.5 degree of the incoming light.
Opacity measures attenuation of light i.e. (scattering or absorption of light). In general when light strikes an interface between two materials some of the light is absorbed, reflected, scattered and rest is transmitted.
Refractrometers are used to monitor juice concentration in orange juice industry.
Turbidity meter detects the pollutants in liquids especially in water.
Opacity meter measures the smoke density in stack of furnace or heater.
These have relatively simple setup.
Calculations are simple.
As these are simple in construction these are relatively cheap.
These are ideal for small concentration of solids in fluids.
These are more susceptible to be fouled.
These photometers can only be used for transparent mixtures.
Photometer that uses visible light is less stable.
These are only used if solids are present.
These measure the composition by measuring the electrical conductivity. Insert the probe into the sample solution making sure the slot in the tip is fully immersed. Agitate the probe vertically. If the sample is not flowing or being stirred, make sure air bubbles are not trapped inside the slot. 
It works on the following principle
R=f (T) = f (k) = f (composition)
Where T is the absolute temperature, k is thermal conductivity
Conductivity probes are used in lab scale reactors, mixers etc.
These are employed to measure composition of electrolytes.
These may be used to find salinity of a liquid.
These are very cheap and simple in construction.
Conductivity probes are stable in operation and are accurate.
These corrosion resistant and are easily cleaned.
These are the online composition sensors.
Only used for small scale operation and cannot be used on large scale.
As conductivity depends on many other factors like temperature, structure etc. so these may affect the reading.
The calculations are complicated if the components present are large.
Electro analytical composition measuring techniques
There are many electro-analytical techniques used for composition measurement. A few of them are as follows 
pH sensors : These sensors measures the pH of the mixture that can be then converted into composition according to proper calibration. These canâ€™t distinguish between multiple acids or bases.
Ion specific: Ion specific sensors detect a specific ion. These are highly selective and only good for specific ions. These sensors are usually used in water treatment plants for measurement of fluorine.
Polarography : It is a subclass of voltammetry, the voltammetry of is an electro analytical technique used in analytical chemistry and many chemical industries. It measures the current as potential is varied. Polarography uses a mercury electrode whose potential varies. These are highly accurate and results are reproducible. The application is usually to measure oxygen concentration flue gases.
Physical property measurement sensors
These instruments measure some physical property that can then be related to composition of the mixture. These can be used for binary mixtures either liquid or in gaseous form and also for solutions of solids in liquids or gases.
There are many types of physical property measuring instruments. A few of them are briefly discussed here.
Liquid column: Liquid columns are of two types one open to atmosphere and other closed. These columns are manometers that measure pressure which can be related to composition. These are relatively cheap and simple. These can only used for simple mixtures and also not durable.
Other examples of physical property measuring sensors are as follows
Direct mass measurement
Radiation density gauges
After a thorough study of the composition sensors we came to the point that there is no general rule for the selection of composition sensors. A composition sensor is selected on the basis of nature of fluid, maintenance, operating conditions and safety conditions etc. Every instrument has its own limitations and advantages. So for each type of fluid there are different types of sensors are available.
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