Chapter 13 - Analytical Instrumentation 1 Chapter 14 Analytical Instrumentation

Chapter 13 - Analytical 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

Chapter 13 - Analytical Instrumentation 3 pH Factors The relationship between negative and positive ions is called dissociation (ionization) constant

Chapter 13 - Analytical Instrumentation 4 Relationship of pH and Hydrogen Ion Concentration

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 12 Conductivity Probes Two types of probes used are: –Electrode probe –Inductive probe Conductivity Probe

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 14 Conductivity Values

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 17 Hydrocarbon Gases Combustion occurs when hydrocarbon fuels are ignited When they burn completely, the by- products are water and carbon dioxide CO 2 and CO can be detected with an infrared gas analyzer

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 21 Quantitative Measures of Humidity There are three different quantitative measures of humidity –Absolute –Relative –Dew Point

Chapter 13 - Analytical Instrumentation 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 P w = Mass Density of Water P a = Mass Density of Air

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 26 Optical Chilled- Mirror Hygrometer Components include: –Gold or rhodium- plated copper mirror –Thermoelectric cooler –High-intensity LED –Optical detector

Chapter 13 - Analytical Instrumentation 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

Chapter 13 - Analytical Instrumentation 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