1 / 64

Chemical Variables Measurements

Chemical Variables Measurements. Lecture for Licentiate Course in Measurement Science and Technology. Marion Hermersdorf February 15. 2006. Time Schedule. 14:15h 45min Lecture Part 1 15:00h 15min Break 15:15h 45min Lecture Part 2 16:00h 15min Questions. Table of Content. Content.

daniel_millan
Download Presentation

Chemical Variables Measurements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chemical Variables Measurements Lecture for Licentiate Course in Measurement Science and Technology Marion Hermersdorf February 15. 2006 1 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  2. Time Schedule 14:15h 45min Lecture Part 1 15:00h 15min Break 15:15h 45min Lecture Part 2 16:00h 15min Questions 2 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  3. Table of Content Content Lecture Part 1 Introduction pH Measurements Lecture Part 2 Humidity and Moisture Measurements 3 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  4. Table of Content – Lecture Part 1 Content • Introduction • pH Measurements • Definition of pH • Electrochemical Methods of pH Measurement • Glass Membrane Electrode • pH-FET • Metal/metal oxide electrodes • Liquid Membrane Electrode • Optical Methods of pH Measurement • Indicator dyes • Indicator paper • Fiber-optic pH probes 4 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  5. Measurements in General Introduction The book defines … A little bit fuzzy, because … • e.g. • force -> capacitance -> frequency • temperature -> resistance • length -> time 5 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  6. Electronic Measurements Introduction In the following are only the sensing elements discussed not the conversion and further signal processing. Signalconversion and processing Sensingelement Physical value resistance heat NTC Wheatstone Bridge temperature capacitance force Capacitive MEMSpressure element Switched capacitor circuit pressure 6 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  7. Definition of pH pH Measurement pH = pondus Hydrogenii, literally: hydrogen exponent Most common interpretation: pH is used to specify the degree of acidity or basicity (also called causticity) of an aqueous solution. Historical definition: pH is defined as the negative logarithm of the hydrogen ion concentration in solution Later definition: pH is defined as the negative logarithm of the hydrogen ion activity in solution 7 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  8. pH Examples pH Measurement • Water: • Hydrochloric acid: • Sodium hydroxide: 8 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  9. pH Measurements pH Measurement Most widely performed measurement in chemical laboratories. Measurement principles: • Electrochemical Methods • Optical Methods 9 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  10. Electrochemical Methods of pH Measurement pH Measurement Electrochemical measurement of pH utilizes devices that transduce the chemical activity of the hydrogen ion into an electronic signal, such as an electrical potential difference or a change in electrical conductance. Methods: Glass membrane electrode pH-FET Metal/metal oxide electrodes Liquid membrane electrodes 10 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  11. Glass Membrane Electrode pH Measurement • Most widely used • Indicator and reference electrodes commonly combined into a single probe(combination electrode) Referenceelectrode Glass indicatorelectrode 11 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  12. Glass Indicator Electrode pH Measurement • Glass membrane about 0.1 mm thick • Glass membrane acts as a transducer of the pH 12 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  13. Reference Electrode pH Measurement • Stable and low resistance electrical contact between the external measuring circuit and the sample • Different kinds of reference electrodes: • most widely used is the silver/silver chloride electrode • another commonly used reference electrode is the calomel electrode (HgCl) for high precision, limited temperature • Selection of reference electrode dependent on: • Type of solution • Temperature range • precision 13 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  14. Potential vs. pH pH Measurement • ideally 59.16 mV per pH unit • Reference electrode introduces additional potential -> can be calibrated out 14 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  15. Measurement Circuit pH Measurement • Measured potential ranges in between a few 100 millivolts • extremely high resistance of the measurement electrode's glass membrane (100MΩ to more than 1000MΩ) • voltmeter with extremely high internal resistance needed (high input impedance amplifier with FET input stage) 15 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  16. Calibration pH Measurement 16 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  17. Temperature Compensation pH Measurement • temperature coefficient of approximately 0.3% per °C • most pH meters have provision for temperature compensation • meters equipped with automatic temperature compensation (ATC) use a platinum resistance thermometer 17 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  18. Selected Glass Membrane Electrodes pH Measurement 18 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  19. Selected Glass Membrane Electrodes pH Measurement pHC2401 pH electrode MI-506 Flexible pH Electrode 19 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  20. pH-FET Measurement Principle pH Measurement • Relatively recent development • Based on the use of an ion-selective field-effect transistor (ISFET) • pH-responsive membrane (instead of metal gate) • Advantages: • Inexpensive, • robust, • battery-powered, • pocket size • Especially used in food industry 20 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  21. pH-FET Operation pH Measurement • Voltage applied to reference electrode (relative to silicon substrate) Charging of capacitor (electrode, solution, insulation layers and silicon substrate) Drain source current influenced 21 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  22. Selected ISFET Electrodes pH Measurement 22 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  23. Metal/Metal Oxide pH Sensors pH Measurement • Metal electrodes coated with an oxide • Operation at high temperatures and high pressures • Various shapes of electrode possible • Based on reduction of the metal oxide : • Near Nernstian response of -59mV per pH 23 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  24. Liquid Membrane Electrodes pH Measurement • Ion-selective electrode • Membrane is selectively permeable to ions of interest 24 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  25. Optical Methods of pH Measurement pH Measurement Use of organic dye molecules with pH-dependent spectral properties Methods: Indicator dyes Indicator paper Fiber-optic pH probes 25 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  26. Indicator Dyes pH Measurement • organic dye molecules are weak acids or bases • loss or gain of a proton changes the electronic structure of the molecule • measurable change in the manner in which the molecule interacts with light • interaction can be the absorption of light at a particular wavelength or fluorescence • pH of interest therefore dictates selection of the particular dye • Limitations of the human eye restrict detectable changes in color of ±1 pH unit. • Thus, an indicator with a pKa of 5 will display a color change if the solution in which it is dissolved changed from 4 to 6 pH units. 26 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  27. Indicator Papers pH Measurement • simple, rapid, and inexpensive means of measuring pH • strip of paper or plastic that has been impregnated with one or more absorption indicator dyes Litmus paper 27 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  28. Fiber-Optic pH Probes pH Measurement • often referred to as optrodes • most sophisticated pH sensors • indicator dye at the tip of a light guide • Challenge and dependency of fixating dye at tip • Advantage: • Usable in electrically noisy environment • New methods and techniques developed in recent years • Two main methods: • Absorption optrodes • Fluorescent indicator optrodes 28 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  29. Absorption Optrodes Principle pH Measurement • Measure the change in intensity of the light returned from the fiber tip • Two fibers necessary • Measurement at two wave lengths (one for reference) • Ratio of the scattered intensities at the two wavelengths is related to the pH 29 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  30. Fluorescent Indicator Optrodes pH Measurement • single fiber to both interrogate and collect signal-carrying light • amount of fluorescent pH indicator at the fiber tip must be maximized • due to the relatively small light intensities, the detector is typically a photomultiplier tube rather than a photodiode 30 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  31. Break .. after the break: humidity and moisture measurements 31 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  32. Table of Content – Lecture Part 2 Content Humidity and Moisture Measurements • Introduction • Humidity measurements in gases • Moisture measurements in liquids and solids 32 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  33. Introduction to Humidity and Moisture Humidity and Moisture Humidity and moisture have great economic importance • Storage of food and raw material • Optimum manufacturing conditions Water and water vapor can be found everywhere Humidity = water vapor in the air or any other gas Moisture = water in liquids and solids 33 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  34. Expressions for Humidity and Moisture Humidity and Moisture • Vapor pressure Ranges from a half to a few percent • Absolute humidity Mass of water vapor per unit volume • Relative humidity ratio of the actual vapor pressure and the saturation vapor pressure at a certain temperature • Dewpoint temperature is the temperature to which a gas must be cooled, at constant pressure, to achieve saturation • Mixing ratio mass of water vapor per unit mass of dry gas, usually expressed in grams per kilogram • Mole fraction ratio of the number of moles of water to the total number of moles • Concentration of water in liquids/solids Given in kg/kg or kg/volume 34 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  35. Characteristics of Humidity and Moisture Humidity and Moisture Saturation vapor pressure When the saturation vapor pressure is reached, any further addition of water vapor results in condensation. In the presence of air molecules at atmospheric pressure, the saturation vapor pressure is about 0.4% higher (enhancement factor). Equilibrium relative humidity Condition where there is no net exchange of water vapor between a moisture-containing material and its environment. Water activity the same condition like equilibrium relative humidity but expressed as a ratio instead of a percentage 35 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  36. Fundamental Behavior of Water Humidity and Moisture Water changes: • length of organic materials • conductivity and weight of hygroscopic material and chemical absorbents • impedance of almost any material • color of chemicals • refractive index of air and liquids • velocity of sound in air • electromagnetic radiation in solids • thermal conductivity of gases, liquids, and solids Water absorbs: • infrared radiation • ultraviolet radiation • microwave radiation 36 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  37. Measurement Methods of Humidity and Moisture Humidity and Moisture Many different measurement methods. • Minimum range of operation Over-specification can be expensive • Exposure of the sensor to the measurement environment Danger of condensation • Accuracy needs In general expected accuracies not better than 2% r.h. of 0.5°C • Response time • Calibration frequency 37 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  38. Measurement of Humidity in Gases Humidity and Moisture Gravimetric method Precision humidity generator Condensation dewpoint hygrometer Psychrometer Lithium chloride dewpoint meter Resistive humidity sensor Capacitive humidity sensor Thermal conductivity humidity sensors Coulometric method Crystal Oscillator Infrared method Mechanical hygrometer 38 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  39. Gravimetric Method Humidity and Moisture • Most fundamental way of measuring the amount of water vapor in a moist gas • Operation principle: • The water vapor is frozen out by a cold trap • Or absorbed by a chemical • Advantages: • very accurate 0.1% to 0.2% or 0.1°C dew point (used for primary standards) • Disadvantages: • difficult and laborious to use • very expensive • not portable 39 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  40. Precision Humidity Generator Humidity and Moisture • Three practical methods: • Two flow method One dry stream of air, one test stream (known temp.) -> humidity = rates of flows • Two temperature method • Two pressure method • Advantage: • Accuracy close to gravimetric method • Disadvantage: • Stationary device • Expensive • Big device 40 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  41. Condensation Dewpoint Hygrometer Humidity and Moisture • Air is cooled down until saturation temperature (constant pressure) • Saturation temperature (dewpoint) is detected • Practical means: • A mirror/inert substance is cooled down • Air is passed over • Condensation is detected (visual, electrical or acoustical) • Accuracies around 0.5°C • Advantage: • Contamination of the mirror • Measurement of another condensable vapor instead of water LAB-EL DP-373 41 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  42. Psychrometer Humidity and Moisture • Principle: • Two thermometers ventilated by the humid air • One thermometer surrounded by a wet cloth • The other thermometer measures the air temperature t • The energy needed to evaporate water from the wet cloth to the air cools the “wet”-thermometer down by tw e: vapor pressureew: saturated vapor pressureA: psychrometer coefficientP: total atmospheric pressure 42 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  43. Lithium Chloride Dewpoint Meter (1) Humidity and Moisture • Principle: A hygroscopic soluble salt, e.g. LiCl, added to water decreases the equilibrium saturation humidity • Implementation • Sleeve fabric with a LiCl solution is put between two electrodes • Electrodes heat up fabric until resistance between fabrics increases sharply (= dry fabric) • Cooling down => LiCl in fabric “sucks” in water out of humid gas => temperature of fabric cools down very fast • At one point the LiCl reaches equilibrium saturation relative humidity => this can be measured by a “stabilized” temperature curve • This temperature point (b) can be transformed into a relative humidity (see figure) 43 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  44. Lithium Chloride Dewpoint Meter (2) Humidity and Moisture • Disadvantages: • Flow rates between 0.05 and 1 m/s • Response time in order of minutes • Lower limit at bout 11% r.h. • Advantages: • Simple sensor • Relative cheap • Rugged 44 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  45. Resistive Humidity Sensors (1) Humidity and Moisture • Principle: • Relative humidity is a function of the impedance/resistance of a hygroscopic medium • Implementation: • Noble metal electrodes • Substrate coated with conductive hygroscopic medium • Medium absorbs water => resistance decreases • AC excitation voltage for resistance measurement to prevent polarization (30Hz to 10kHz) • Resistance => impedance • Rectify to dc voltage 45 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  46. Resistive Humidity Sensors (2) Humidity and Moisture • Advantages: • No calibration needed • Small • Fast responding • Do not dissipate heat • Life expectancy >>5 years • Disadvantages: • Significant temperature dependency • Condensation problem (=> new improving developments) • Historical First sensor of this type in 1940: Dunmore type 46 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  47. Capacitive Humidity Sensors (1) Humidity and Moisture • Principle: Relative humidity is proportional to dielectric constant of polymer or metal oxide => change in capacitance about 0.2 to 0.5pF for 1%r.h. • Implementation: • Substrate (glass, ceramic or silicon) • Between two electrodes a thin-film polymer or metal oxide • Coating with porous metal electrode => protection from contamination and condensation 47 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  48. Capacitive Humidity Sensors (2) Humidity and Moisture • Advantages: • Use of semiconductor processes (signal conditioning circuit included) • Small • Low cost • Widely used • Disadvantages: • Calibration needed (or laser trimmed) 48 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  49. Relative Humidity Accuracy vs. Dew Point Accuracy Humidity and Moisture Vaisila DryCap (+-2°C) 49 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

  50. Thermal Conductivity Humidity Sensor Humidity and Moisture • Principle: measure the absolute humidity by quantifying the difference in thermal conductivity of dry air and humid air dry air has a greater capacity to sink heat (e.g. desert) • Implementation: • Two matched NTC thermistors in a bridge circuit • One is hermetically encapsulated in dry nitrogen 50 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

More Related