Chapter 13 - Analytical Instrumentation 1 Chapter 14 Analytical Instrumentation

Chapter 13 - Analytical Instrumentation

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Chapter 14Analytical Instrumentation


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pH Measurement and Control

• Many industrial applications require the control of the concentration of acids and bases in a chemical solution

• The analytical process that controls this is called pH control

• Within a chemical solution, the number of negative ions compared to positive ions determines whether the solution is acid or base


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pH Factors

• The relationship between negative and positive ions is called dissociation (ionization) constant


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Relationship of pH and

Hydrogen Ion Concentration


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pH Measurements

• Devices used to measure pH values detect the concentration of hydrogen ions

• Early techniques used litmus paper which changed color based upon the solution being acid or base

• Electronic sensors were developed to overcome the shortcomings of paper-based tests


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Electronic Sensors• Electronic sensors use two

electrodes and an amplifier to measure pH– The active or sensing probe

produces a voltage proportional to the the hydrogen-ion concentration

– The reference probe provides a signal against which the measuring electrode is compared

Sensing Probe Reference Probe


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Controlling pH

• In a pH control system, either a solution is too acidic or too alkaline

• The corrective ingredient added to a solution is called a reagent

• One objective of a pH control system is to minimize the amount of reagent added to the solution, eliminating overshoot


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Batch Systems

• A batch system normally uses a tank for the solution for storage

• Solution entering the tank is called influent

• Solution exiting the tank that has been treated is referred to as effluent

• Batch pH control is used for relatively small volumes


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Continuous Systems• Many continuous systems use a

tank • A reagent is added to correct the

pH of a solution using either an on/off or proportional controller

• A disadvantage of using a tank is a long delay time

• In applications requiring pH values between 4 and 10, a static mixer is used


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Conductivity• Any process that involves liquids requires flow

• The liquid used in these processes is referred to as a process stream

• In many applications, the purity of water or the concentration level of solutions is measured or controlled

• This can be done by measuring the conductivity of a solution

• Conductivity refers to the ability of material to pass electric current

Formula to determine conductance

G = 1

R


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Factors Affecting Conductivity

• Concentration of an ingredient dissolved in water, ranging from zero to very high

• The type of electrolyte contained in a dissolved ingredient

• The temperature of the liquid


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Conductivity Probes

• Two types of probes used are:– Electrode probe

– Inductive probe

Conductivity Probe


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Conductivity Inductive Probe

• The conductivity inductive probe uses to toroidal coils

• One coil is connected to an oscillator, the other coil is used as a pickup

• The the current induced in the pickup coil is directly proportional to the conductivity of the solution


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Conductivity Values


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Combustible Gases

• Some gases are dangerous if they escape from their container– Hydrogen– CO– Hydrogen sulfide– Methane, propane, butane, ethane

• Analytical sensors are used to detect these gases


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Thermo-Conductivity Detector

• Every gas has the ability to conduct heat

• The type of gas and its concentration determine the rate at which heat is conducted

• The TCD uses a bridge , measuring chamber, and a reference chamber


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Hydrocarbon Gases

• Combustion occurs when hydrocarbon fuels are ignited

• When they burn completely, the by-products are water and carbon dioxide

• CO2 and CO can be detected with an infrared gas analyzer


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Optical Gas Analyzer

• An infrared gas analyzer consists of six major elements:– Infrared light source– Sample chamber– A prism– Two optical filters– Two detectors– Electronic converter


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Combustion Analyzers and Control

• Energy for many industrial process is produced by combustion

• Combustion uses a combination of gases and fuel• Two types of fuel

– Hydrocarbon

– Combustible gas


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Humidity

• Humidity is defined as the amount of moisture in the air

• Humidity control is important in many industrial applications

• Humidity affects hygroscopic materials. Hygroscopic means the ability to absorb and retain moisture


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Quantitative Measures of Humidity

• There are three different quantitative measures of humidity– Absolute

– Relative

– Dew Point


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Absolute Humidity

• Defined as the mass of water vapor present in a particular volume of atmosphere

• Absolute humidity value is expressed as the ratio of the mass of water vapor to the volume occupied by the air-water vapor mixture

W = Pw

Pa

W = Absolute Humidity

Pw = Mass Density of Water

Pa = Mass Density of Air


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Relative Humidity

• Defined as the actual amount of water vapor present as compared to the maximum amount of water vapor the air can hold at a given temperature


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Absolute Humidity Sensor

• The most common type of device to measure absolute humidity is the aluminum oxide sensor

• Essentially, the aluminum oxide sensor functions as a capacitor with the value of capacitance dependent upon humidity


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Dew Point Measurements

• Dew Point is defined as the temperature at which the air becomes saturated

• When air is cooled at a constant pressure, condensation of vapor will occur at the dew point temperature

• Three common techniques to measure dew points are:– Manual chilled mirror– Adiabatic expansion sensing– Optical chilled mirror


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Optical Chilled-Mirror Hygrometer

• Components include:– Gold or rhodium-

plated copper mirror

– Thermoelectric cooler

– High-intensity LED

– Optical detector


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Relative Humidity Detectors

• Psychrometric Detector - uses two identical thermometers called a dry bulb and a wet bulb

• Hygrometric Detector - measures the change in dimension of hygroscopic materials

• Electronic Capacitance Detector - A common RH sensor that is constructed as a small capacitor with a hygroscopic polymer acting as the dielectric


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Sampling Measurement Systems• Whenever the

temperature of the air being tested is above the level the sensor can be exposed, it must first be cooled before a reading can be taken

• A sampling system like the one on the right offers a solution

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Chapter 13 - Analytical Instrumentation 1 Chapter 14 Analytical Instrumentation