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Chapter 14 Analytical Instrumentation

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Title: Chapter 14 Analytical Instrumentation


1
Chapter 14Analytical Instrumentation
2
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

3
pH Factors
  • The relationship between negative and positive
    ions is called dissociation (ionization) constant

4
Relationship of pH and Hydrogen Ion Concentration
5
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

6
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
7
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

8
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

9
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

10
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
11
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

12
Conductivity Probes
  • Two types of probes used are
  • Electrode probe
  • Inductive probe

Conductivity Probe
13
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

14
Conductivity Values
15
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

16
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

17
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

18
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

19
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

20
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

21
Quantitative Measures of Humidity
  • There are three different quantitative measures
    of humidity
  • Absolute
  • Relative
  • Dew Point

22
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 Absolute Humidity Pw Mass Density of
Water Pa Mass Density of Air
23
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

24
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

25
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

26
Optical Chilled-Mirror Hygrometer
  • Components include
  • Gold or rhodium-plated copper mirror
  • Thermoelectric cooler
  • High-intensity LED
  • Optical detector

27
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

28
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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