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Chemical Variables Measurements

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

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

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

4 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 Small introduction to measurement principle (because I am first lecture in this course) Metrology Course –Lecture 1.ppt / / Hermersdorf

5 Measurements in General
Introduction The book defines … A little bit fuzzy, because … Book defines I think this is not very clear e.g. force -> capacitance -> frequency temperature -> resistance length -> time Metrology Course –Lecture 1.ppt / / Hermersdorf

6 Electronic Measurements
Introduction Signal conversion and processing Sensing element Physical value heat resistance NTC temperature Wheatstone Bridge I define electronic measurements now this way force capacitance Capacitive MEMS pressure element Switched capacitor circuit pressure In the following are only the sensing elements discussed not the conversion and further signal processing. Metrology Course –Lecture 1.ppt / / Hermersdorf

7 Definition of pH pH = pondus Hydrogenii, literally: hydrogen exponent
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 - interpretation - definition - Historically just used to make it easier to talk about the hydrogen concentrations 10-7 moles/liter = pH 7 - pH has no unit - [H+] is molar concentration of solvent protons in mol/liter - aH+ is the ion activity - gamma = activity coefficient Metrology Course –Lecture 1.ppt / / Hermersdorf

8 pH Examples Water: Hydrochloric acid: Sodium hydroxide: pH Measurement
Some compounds form weak acids or bases; Matter of dissociation grade Balance is temperature dependent Caustic = basicity Metrology Course –Lecture 1.ppt / / Hermersdorf

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

10 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 Chemical activity => electrical potential Metrology Course –Lecture 1.ppt / / Hermersdorf

11 Glass Membrane Electrode
pH Measurement Most widely used Indicator and reference electrodes commonly combined into a single probe (combination electrode) Glass indicator electrode Reference electrode First methods using the electrochemical principle Metrology Course –Lecture 1.ppt / / Hermersdorf

12 Glass Indicator Electrode
pH Measurement Glass membrane about 0.1 mm thick Glass membrane acts as a transducer of the pH Electrode = Silver wire buffer solution = KCl In glass :sodium or lithium, which act as electrical charge carriers When the membrane glass is immersed in aqueous solution, a gel-like hydrated layer on the order of 100 nm thick forms at the surface of the glass. Metrology Course –Lecture 1.ppt / / 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 Low resistive contact to sample (relative point) Electrode Metrology Course –Lecture 1.ppt / / Hermersdorf

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

16 Calibration pH Measurement
Metrology Course –Lecture 1.ppt / / 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 The graph shows two calibration lines. The original calibration was done with pH 4 and pH 10 buffers at temperature t1. Suppose the temperature changes to t2 . The pH analyzer automatically changes the slope by the ratio Metrology Course –Lecture 1.ppt / / Hermersdorf

18 Selected Glass Membrane Electrodes
pH Measurement Just to give an impression Range in general 0-14 Accuracy within 0.1 to 0.03 pH Cost around 100 to 300$ Metrology Course –Lecture 1.ppt / / Hermersdorf

19 Selected Glass Membrane Electrodes
pH Measurement pHC2401 pH electrode MI-506 Flexible pH Electrode Metrology Course –Lecture 1.ppt / / 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 pH responsive membrane show in picture (mistake) = gate electrode FET show in picture pH-responsive membrane material: silicon nitride, aluminum oxide, or tantalum oxide Metrology Course –Lecture 1.ppt / / 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 A = geometrical factors Vds = voltage applied to the drain Vg = voltage applied to the reference electrode VT = “threshold voltage” needed to produce mobile charge C2 = capacitance of the gate region. F = phase-boundarypotential at the interface between the sample and the pH-responsive insulating layer constant drain current also temperature compensation needed Metrology Course –Lecture 1.ppt / / Hermersdorf

22 Selected ISFET Electrodes
pH Measurement Overview Accuracies and range similar to glass membrane method Price higher Device smaller and rugged Metrology Course –Lecture 1.ppt / / 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 Metal oxide reduction Metrology Course –Lecture 1.ppt / / Hermersdorf

24 Liquid Membrane Electrodes
pH Measurement Ion-selective electrode Membrane is selectively permeable to ions of interest Shortly mention also … Icon selective electrode (e.g. selective for potassium) Done by membrane which is saturated with liquid ion exchanger (ion exchanger liquid: N-tridodecylamine) For blood pH measurements Metrology Course –Lecture 1.ppt / / Hermersdorf

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

26 Indicator Dyes organic dye molecules are weak acids or bases
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. Everybody knows from the school chemistry labs Change color depending on pH value Also fluorescence Table: … Interesting: eye limitation equilibrium constants Ka Metrology Course –Lecture 1.ppt / / Hermersdorf

27 Indicator Papers simple, rapid, and inexpensive means of measuring pH
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 Based on the indicator dyes Litmus paper Metrology Course –Lecture 1.ppt / / Hermersdorf

28 Fiber-Optic pH Probes often referred to as optrodes
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 More sophisticated Still indicator dye (tip of light guide) Challenge to fixate dye Metrology Course –Lecture 1.ppt / / 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 Own light source now Two wave length (reference) Scattered light => amplifying necessary Metrology Course –Lecture 1.ppt / / 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 Fluorescent dye Laser to excite the dye Max dye concentration PMT photomultiplier tube Metrology Course –Lecture 1.ppt / / Hermersdorf

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

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

33 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

34 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 vapor pressure kPa Metrology Course –Lecture 1.ppt / / Hermersdorf

35 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 moisture-containing material and its environment. Water activity the same condition like equilibrium relative humidity but expressed as a ratio instead of a percentage Metrology Course –Lecture 1.ppt / / 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 Sensors are based on at least one of these characteristics Metrology Course –Lecture 1.ppt / / Hermersdorf

37 Measurement Methods of 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 Select method by defining on following requirements Metrology Course –Lecture 1.ppt / / 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 Introduce “heavy” measurement methods Then small scale and low cost Metrology Course –Lecture 1.ppt / / 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 Metrology Course –Lecture 1.ppt / / 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 Metrology Course –Lecture 1.ppt / / 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

42 Psychrometer Principle: Two thermometers ventilated by the humid air
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 pressure ew: saturated vapor pressure A: psychrometer coefficient P: total atmospheric pressure Metrology Course –Lecture 1.ppt / / 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) Metrology Course –Lecture 1.ppt / / Hermersdorf

44 Lithium Chloride Dewpoint Meter (2)
Humidity and Moisture Advantages: Simple sensor Relative cheap Rugged Disadvantages: Flow rates between 0.05 and 1 m/s Response time in order of minutes Lower limit at bout 11% r.h. Metrology Course –Lecture 1.ppt / / 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 resistive - impedeance hygroscopic medium: conductive polymer, salt, treated substance => polymere relative humidity sensors Metrology Course –Lecture 1.ppt / / 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 Dunmore sensor on the very right Metrology Course –Lecture 1.ppt / / 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 Also based on polymer => sometime confusion with resistive method Metrology Course –Lecture 1.ppt / / 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) Metrology Course –Lecture 1.ppt / / Hermersdorf

49 Relative Humidity Accuracy vs. Dew Point Accuracy
Humidity and Moisture Just to give and idea about 2% r.h. accuarcy Vaisila DryCap (+-2°C) Metrology Course –Lecture 1.ppt / / 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 because heat is retained by water vapor in the atmosphere Desert air cools down fast during night Humid air at the sea stays warm over night absolute humidity Metrology Course –Lecture 1.ppt / / Hermersdorf

51 Coulometric Method Principle: Advantages: Disadvantages:
Humidity and Moisture Principle: A phosphorous pentoxide absorbs water The water is electrolyzed The resulting current is representing a defined amount of water 1mA = μg 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 Shortly some more less widely used methods: Metrology Course –Lecture 1.ppt / / Hermersdorf

52 Crystal Oscillator Principle: Advantages: Disadvantages:
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

53 Infrared Method Principle: Implementation: Advantages: Disadvantages:
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

54 Comparison of Humidity Sensors
Humidity and Moisture Primary Standards. These systems rely on fundamental principles and base units of measurement. Transfer/fundamental. Instruments in this category operate on fundamental principles and can provide good, stable, and repeatable results, but if they are not properly used, the instruments can give incorrect results. Secondary devices. These devices are nonfundamental and must be calibrated against a transfer standard or other fundamental system. Metrology Course –Lecture 1.ppt / / Hermersdorf

55 Measurement of moisture in Liquids and Solids
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

56 Gravimetric Method Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

57 Karl Fischer Method Chemical method
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

58 Infrared Techniques Reflectance of the surface indicate moisture
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

59 Microwave Absorbance Humidity and Moisture Microwave absorbance is depending on the water incorporated’ Water absorbs strongly in 1-2 GHz and 9 to 10GHz range Metrology Course –Lecture 1.ppt / / Hermersdorf

60 Nuclear Magnetic Resonance Method
Humidity and Moisture 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 Metrology Course –Lecture 1.ppt / / Hermersdorf

61 Other Methods Neutron Moderation Time Domain Reflectory
Humidity and Moisture 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. Metrology Course –Lecture 1.ppt / / Hermersdorf

62 Mechanical Hygrometer
Humidity and Moisture 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. Last but not least Still in use in some museum Metrology Course –Lecture 1.ppt / / Hermersdorf

63 Questions ? Metrology Course –Lecture 1.ppt / / Hermersdorf

64 Thank You ! Metrology Course –Lecture 1.ppt / / Hermersdorf

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