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

1 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

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

Lecture Part 1

Introduction

pH Measurements

Lecture Part 2

Humidity and Moisture Measurements

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

Content


Measurements in General

The book defines …

A little bit fuzzy, because …

e.g.

• force -> capacitance -> frequency

• temperature -> resistance

• length -> time

Introduction


Electronic Measurements

In the following are only the sensing elements discussed not the conversion and further signal processing.

Physical value

Sensingelement

Signalconversion and

processing

temperature NTCresistance

Wheatstone Bridgeheat

pressureCapacitive MEMSpressure element

capacitance

Switched capacitor circuitforce

Introduction


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

HlogpH

HlogγlogpHHa

pH Measurement


pH Examples

• Water:

• Hydrochloric acid:

• Sodium hydroxide:

ClOHHClOH 32

OHO2HNaNaOHOH 22

pH Measurement


pH Measurements

Most widely performed measurement in chemical laboratories.

Measurement principles:

1. Electrochemical Methods

2. Optical Methods

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

pH Measurement


Glass Membrane Electrode

• Most widely used

• Indicator and reference electrodes commonly combined into a single probe(combination electrode)

Glass indicatorelectrode

Referenceelectrode

pH Measurement


Glass Indicator Electrode

• Glass membrane about 0.1 mm thick

• Glass membrane acts as a transducer of the pH

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

pH Measurement


Potential vs. pH

• ideally 59.16 mV per pH unit

• Reference electrode introduces additional potential -> can be calibrated out

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)

pH Measurement

Marion Hermersdorf

potentiometric or null-balance instrumentIn a potentiometric instrument, a precision adjustable voltage source is compared against the measured voltage, and a sensitive device called a null detector is used to indicate when the two voltages are equal


CalibrationpH 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

pH Measurement


Selected Glass Membrane ElectrodespH Measurement


Selected Glass Membrane Electrodes

MI-506 Flexible pH

Electrode

pHC2401 pH

electrode

pH Measurement


pH-FET Measurement Principle

• 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 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

pH Measurement


Selected ISFET ElectrodespH 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

pH Measurement


Liquid Membrane Electrodes

• Ion-selective electrode

• Membrane is selectively permeable to ions of interest

pH Measurement


Optical Methods of pH Measurement

Use of organic dye molecules with pH-dependent spectral properties

pH Measurement

Methods:

Indicator dyes

Indicator paper

Fiber-optic pH probes


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.

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

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

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

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

pH Measurement


Break

.. after the break:

humidity and moisture measurements


Table of Content – Lecture Part 2

Humidity and Moisture Measurements• Introduction

• Humidity measurements in gases

• Moisture measurements in liquids and solids

Content


Introduction to 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

Humidity and Moisture


Expressions for Humidity and Moisture

• Vapor pressureRanges from a half to a few percent

• Absolute humidityMass of water vapor per unit volume

• Relative humidityratio of the actual vapor pressure and the saturation vapor pressure at a certain

temperature

• Dewpoint temperatureis the temperature to which a gas must be cooled, at constant pressure, to achieve

saturation

• Mixing ratiomass of water vapor per unit mass of dry gas, usually expressed in grams per kilogram

• Mole fractionratio of the number of moles of water to the total number of moles

• Concentration of water in liquids/solidsGiven in kg/kg or kg/volume



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



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



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



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



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



Precision Humidity Generator

• Three practical methods:

1. Two flow methodOne dry stream of air, one test stream (known temp.) -> humidity = rates of flows

2. Two temperature method

3. Two pressure method

• Advantage:• Accuracy close to gravimetric method

• Disadvantage:• Stationary device

• Expensive

• Big device



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

LAB-EL DP-373



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

pressureA: psychrometer coefficientP: total atmospheric pressure



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)



Lithium Chloride Dewpoint Meter (2)

• 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



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



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



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



Capacitive Humidity Sensors (2)

• Advantages:• Use of semiconductor processes

(signal conditioning circuit included)

• Small

• Low cost

• Widely used

• Disadvantages:• Calibration needed (or laser

trimmed)



Relative Humidity Accuracy vs. Dew Point Accuracy

Vaisila DryCap (+-2°C)



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



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



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



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



Comparison of Humidity SensorsHumidity 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



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



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 iodinesulfur dioxidepyridinemethanol



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



Microwave Absorbance

• Microwave absorbance is depending on the water incorporated’

• Water absorbs strongly in 1-2 GHz and 9 to 10GHz range



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



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.



Mechanical Hygrometer

• Principlechange 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.



Questions ?


Thank You !

Documents

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