Measurement of composition Flashcards
Two systems used for measuring composition
- Photometric
- Electrochemical
Importance of composition measurement
- standard way of measuring dissolved solids
- ## we cannot always measure composition directly
Photometric methods
- absorbance
- turbidity
- refractive index
- Brix
Electrochemical methods
- Conductivity
- pH
- REDOX
A light source emits radiaiton through the medium
Transmitted radiation is measured on the detector side
A photodiode determines the intensity of the light and converts it to a photoelectric current
The intensity of light received is proportional to the concentration of the substances
Absorbance
The cloudiness of haziness of a fluid caused by suspended particles
Turbidity
Measurement via the light absorbing and light scattering propoerties of the fluid
Most suspended solids are made up on inorganic materials, though bacteria and algae can also contribute to the total solids concentration
Sources of turbidity
Anything drifting or floating in the water
Measure turbidity by using a lamp which emits a beam into the medium which scatters as it hits the particles.
The detector converts the received light intensity into an electric signal
Nephelometric turbidity sensors
They feature an LED lamp and a light detector that is positioned in a 90o angle. The lamp emits a beam into the medium which scatters as it hits the particles. The detector converts the received light intensity into an electric signal
Absorption equation
Absorption = extinction coefficient * concentration * OPL
Amount of oxygen needed to fully oxidise all the oxidisable substances in a sample and can be determined via absorbance.
The sample is first prepared by being heated and adding digesting reagents. After digestion process has occured, the sample changes colour. This colour change is proportional to the concentration of carbon compounds. It is measured photometrically and the final value is determined using preset absorption curves
Chemical oxygen demand measured via absorbance
FNU are measured with …
infrared light
NTU are measured with …
white light
Use the back scattered light method. LED lamp and two light detectors positioned at 90 and 135o. Solid particles in the medium cause the incident light emitted by the lamp to scatter
Total solids sensors
Optical so not a direct measure
refractive index formula
speed of light in vacuum/speed of light in the substance
The ….. the refractive index, the slower the light travels in that material
larger
The larger the refractive index, the ….. the light travels in that material
slower
The speed of light in a vacuum is fixed, but when light moves through any other medium, it ….. since it is constantly being absorbed and re-emitted by atoms in the material
slows
When light changes speed as it crosses a boundary from one medium into another its direction of travel ……….
changes
Work by measuring the angle of critical refraction
Refractometers
Evaluate the amount of dissolved substances in solutions. They measure a wide range of liquid systems from low to hgh viscosity. Portable devices are available that can be used on location. Up to in line process devices which are relatively expensive.
Refractometers
Amount of sucrose by mass in a pure water solution
Brix scale
One degree Brix is 1 gram of sucrose in 100 grams of solution.
Dissolution of sucrose and other sugars in water changes not only specific gravity but also optical properties in particular refractive index
Refractive index changes with …
Temperature
Instruments measure the process stream
temperature and refractive index and … the Brix value to what it would be at 20°C
correct
- based on the presence of ions
- analysis of water based systems
Electrochemical methods
Conductivity is linked directly to the ………………………
total dissolved solids
Works by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance
Conductivity
Conductivity of solutions ……. as temperature changes
changes
measure of transfer of hydrogen ions between chemical species in an aqueous solution
pH
Measures of transfer of electrons between chemical species
reduction potential
- Glass or non glass electrodes
- The pH electrode uses a hydrogen ion selective glass in contact with the solution which develops a potential proportional to the pH of the solution
- A reference electrode is designed to maintain a constant potential at any given temperature and serves to complete the pH measuring circuit within the solution
- REF electrode provides a known reference potential for the pH electrode. The difference in the potentials of the pH and reference electrodes provides a millivolt signal proportional to pH
pH potentiometric principle
Remove electrons from another substance and are reduced themselves
Oxidisers
Transfer electrons to another substance and are oxidised themselves
Reducers
Measure in volts of the affinity of a substance for electrons, its electronegativity is then compared with Hydorgen (which is set at 0).
Redox potential
A measure (in mV) of the tendency of a chemical substance to oxidise or reduce another chemical
Oxidation Reduction Potential
A positive ORP indicates that a substance is ….
an oxidising agent
A negative ORP indicates that a substance is ……
a reducing agent
ORP potential is … dependent
temperature
Determined by measuring the potential of a chemically inert electrode which is immersed in the solution. The sensing electrode potential is read relative to the reference electrode and the value is presented in millivolts (mV)
ORP measurement
What is used to measure ORP
ORp sensor
REDOX sensor
Usage of conductivity
Monitoring desalination
Nephelometric Turbidity Units
- NTU
- FNU
Brix measurement usages
- food industry for quality contro
Usages of ORP
- Water which contains a relatively high concentration of a redox active species.
- ORP measurements are used often to control disinfection
- detect metallic pollution in groundwater or surface water or to determine the content of wastewater effluent
- ORP to detect arsenic levels in drinking wells
measured potential is reflection of the combination of the effects of the dissolved species in the medium
non specific measurement
ORP is a ____ measurement
non specific
Because of this factor, the measurement of ORP in
relatively clean environmental water (ground, surface,
estuarine, and marine) has only limited value unless a
predominant redox-active species is known to be
present
Digital multiparameter transmitter (4-20mA) which can be used for multichannel simultaneous monitoring of: conductivity, disinfection, ORP, oxygen, sludge level, turbidity & pH
Multiparameter field device
