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 Time Schedule 2 14:15h 45min Lecture Part 1 15:00h 15min Break 15:15h 45min Lecture Part 2 16:00h 15min Questions Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Content Table of Content Lecture Part 1 Introduction pH Measurements Lecture Part 2 Humidity and Moisture Measurements 3 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Content Table of Content – Lecture Part 1 • 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 Introduction Measurements in General 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 Introduction Electronic Measurements Physical value Signal conversion and processing Sensing element resistance heat NTC temperature force pressure Capacitive MEMS pressure element Wheatstone Bridge capacitance Switched capacitor circuit In the following are only the sensing elements discussed not the conversion and further signal processing. 6 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Definition of pH 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 pH log H pH log aH logγ H pH Measurement pH Examples • Water: • Hydrochloric acid: H 2 O HCl H 3O Cl • Sodium hydroxide: H 2O NaOH Na 2H 2O OH 8 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement pH Measurements Most widely performed measurement in chemical laboratories. Measurement principles: 1. Electrochemical Methods 2. Optical Methods 9 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Electrochemical Methods of 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 pH Measurement Glass Membrane Electrode • Most widely used • Indicator and reference electrodes commonly combined into a single probe (combination electrode) 11 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Glass indicator electrode Referenc e electrode pH Measurement Glass Indicator Electrode • 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 pH Measurement Reference Electrode • 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 pH Measurement Potential vs. pH • 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 pH Measurement Measurement Circuit • 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 pH Measurement Calibration 16 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Temperature Compensation • 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 pH Measurement Selected Glass Membrane Electrodes 18 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Selected Glass Membrane Electrodes pHC2401 pH electrode MI-506 Flexible pH Electrode 19 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement pH-FET Measurement Principle • Relatively recent development • Based on the use of an ionselective 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 pH Measurement pH-FET Operation • 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 pH Measurement Selected ISFET Electrodes 22 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Metal/Metal Oxide pH Sensors • 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 pH Measurement Liquid Membrane Electrodes • Ion-selective electrode • Membrane is selectively permeable to ions of interest 24 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf pH Measurement Optical Methods of 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 pH Measurement Indicator Dyes • 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 pH Measurement Indicator Papers • 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 pH Measurement Fiber-Optic pH Probes • • • • • 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 pH Measurement Absorption Optrodes Principle • 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 pH Measurement Fluorescent Indicator Optrodes • 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 Break .. after the break: humidity and moisture measurements 31 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Content Table of Content – Lecture Part 2 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 Humidity and Moisture Introduction to Humidity and Moisture Water and water vapor can be found everywhere Humidity = water vapor in the air or any other gas Moisture = water in liquids and solids Humidity and moisture have great economic importance • Storage of food and raw material • Optimum manufacturing conditions 33 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Expressions for 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 Humidity and Moisture Characteristics of 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 moisturecontaining 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 Humidity and Moisture Fundamental Behavior of Water 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 Humidity and Moisture Measurement Methods of Humidity and Moisture Many different measurement methods. 1. Minimum range of operation Over-specification can be expensive 2. Exposure of the sensor to the measurement environment Danger of condensation 3. Accuracy needs In general expected accuracies not better than 2% r.h. of 0.5°C 4. Response time 5. Calibration frequency 37 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Measurement of Humidity in Gases 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 Humidity and Moisture Gravimetric Method • 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 Humidity and Moisture Precision Humidity Generator • Three practical methods: 1. Two flow method One dry stream of air, one test stream (known temp.) -> humidity = rates of flows 2. Two temperature method 3. Two pressure method • Advantage: • • 40 Accuracy close to gravimetric method Disadvantage: • Stationary device • Expensive • Big device Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Condensation Dewpoint Hygrometer • 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 41 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf LAB-EL DP-373 Humidity and Moisture Psychrometer • 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 pressure ew: saturated vapor pressure A: psychrometer coefficient P: total atmospheric pressure 42 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Lithium Chloride Dewpoint Meter (1) • 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 Humidity and Moisture Lithium Chloride Dewpoint Meter (2) • Advantages: 44 • Disadvantages: • Simple sensor • Flow rates between 0.05 and 1 m/s • Relative cheap • Response time in order of minutes • Rugged • Lower limit at bout 11% r.h. Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Resistive Humidity Sensors (1) • 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 Humidity and Moisture Resistive Humidity Sensors (2) • 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 Humidity and Moisture Capacitive Humidity Sensors (1) • 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 Humidity and Moisture Capacitive Humidity Sensors (2) • 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 Humidity and Moisture Relative Humidity Accuracy vs. Dew Point Accuracy Vaisila DryCap (+-2°C) 49 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Thermal Conductivity Humidity Sensor • 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 Humidity and Moisture Coulometric Method • Principle: • A phosphorous pentoxide absorbs water • The water is electrolyzed • The resulting current is representing a defined amount of water • 1mA = 0.0935μg H2O/s • The sample stream of air must be very accurate • Advantages: • No calibration needed • Especially suited for low humidity • Disadvantages: • 1 minute response times • The sample stream of air must be very accurate 51 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Crystal Oscillator • Principle: • Surface of a quartz crystal is coated with a hydroscopic material • Resonant frequency of the quartz is a function of the mass of the quartz • Alternately exposed to humid and dry air • Advantages: • Lowest humidity measurable • Disadvantages: • Expensive • 1 minute response time 52 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Infrared Method • Principle: • Water absorbs radiation in the infrared region • Implementation: • Gas is lead through a optical path between an infrared source and detector • Another path through a reference gas • Advantages: • Wide range measurements • Response time less than 1s • Disadvantages: • Pressure dependency • Expensive 53 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Comparison of Humidity Sensors 54 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Measurement of moisture in Liquids and Solids Gravimetric method Karl Fischer method Infrared techniques Microwave absorbance Nuclear magnetic resonance method Neutron moderation Time domain reflectory Frequency domain technique Thermal conductivity measurement Water activity 55 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Gravimetric Method • Difference in weight before and after a drying process • Assumption: loss of weight only based on water loss • Problem with volatile components and crystal water 56 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Karl Fischer Method • Chemical method • Karl Fischer reagent controlled added to liquid • Electrodes measure the current through the liquid • Sudden change in current indicated usage of all water • Karl Fischer reagent is a mixture of iodine sulfur dioxide pyridine methanol 57 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Infrared Techniques • Reflectance of the surface indicate moisture • Surface has to be representative • Calibration for each material necessary • Wave length bands of 1.45, 1.94 and 2.94μm 58 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Microwave Absorbance • Microwave absorbance is depending on the water incorporated’ • Water absorbs strongly in 1-2 GHz and 9 to 10GHz range 59 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Nuclear Magnetic Resonance Method • Hydrogen atoms in a permanent magnetic field have some defined orientation • To change the orientation a defined amount of energy is needed • At a right frequency the hydrogen atoms resonate • The energy needed for the resonate state is proportional to the hydrogen/water atoms/molecules 60 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Other Methods • Neutron Moderation Neutron of high energy are slowed down by hydrogen atoms • Time Domain Reflectory Propagation velocity of electrical pulses Water content in soils • Frequency Domain Technique Similar to TDR • Thermal Conductivity Measurement Thermal conductivity related to water content Heat pulses and then cooling measured • Water Activity Material enclosed in measuring chamber developed after some time an equilibrium of relative humidity. 61 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Humidity and Moisture Mechanical Hygrometer • Principle change of length of certain materials dependent of the humidity • Use of human hair, textiles, or plastic fiber • Accuracy up to 2% r.h. (in the range of 35% to 95%) commonly 5% r.h. 62 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Questions ? 63 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Thank You ! 64 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf