Chapter 14 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
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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 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