introduction to ph and optical ph sensing · what is ph? • water exists as a mixture of...
TRANSCRIPT
Introduction to pH and Optical pH Sensing
What is pH?• Water exists as a mixture of
continually associating and dissociating H+ and OH- ions
• Water molecules in solution may exist as:– Dissociated H+ and OH-
– H2O
– H3O+
• pH is a measure of the concentration of hydronium ions (H3O+) present in a solution
What is pH?
• pH = -log10(aH+)
• Low pH suggests higher hydronium ion activity, acidic
• High pH suggests higher hydroxide ion activity, basic
• Biological range typically considered pH 5 - 9
Where is pH a Concern?• Almost everywhere…
• All things Life Sciences related:
– Cell culture
– Blood and other bodily fluid analysis
– Bioreactor monitoring
• Marine Research and Monitoring:
– Seawater analysis
– Home and commercial aquariums
• Food and Beverage Processing
• Pharmaceutical Processing
• Soil Analysis
• General manufacturing of products
• Powerplant cooling water
• …everything shown here was made at a plant or facility that at some point in the process needed to monitor the pH of a stream
Methods of pH Sensing• pH Paper
– Simple method of visually determining pH– Not highly accurate, check for the general range– Uses pH responsive molecule (indicator dye) to
change color with range
• Electrodes– Industry standard– Milli-voltmeter with very high input impedance– Measures potential difference between
hydronium-sensitive electrode and reference electrode
• Optodes– Light is sent to a material containing pH
sensitive molecules, interaction observed via photodetector
– Fluorescence lifetime, fluorescence intensity, colorimetric absorbance
Dynamic Range
Accuracy Electronics / Cost
Limitations
pH Paper Very limited General visual check
None General visual check
Electrode pH 0 – 14 0.01 pH unit Moderate, hundreds of
dollars
Delicate assembly, salinity dependent,
chemical compatibility,
storage solutions
FluorescentOptode
Limited to about 4 pH
units
0.01 pH unit High cost electronics,
thousands of dollars
Photobleachingdrift, salinity
dependent, storage conditions, cost,
range
Colorimetric Optode
Limited to about 4 pH
units
0.01 pH unit Moderate to low cost electronics,
hundreds of dollars
Limited range, cost in some cases
Theory of Operation
Theory of Operation
• Light is generated at the light source
• Transmits through optical fiber to sensor chemistry (probe tip or cuvette wall)
• Based on the pH of the solution, a certain amount of light will absorb at 620 nm
• Partially absorbed light travels to detector
• Absorbance determined based on previously taken reference
• Correlated to pH value via ratiometric algorithm
Theory of Operation• Absorbance = log10(Io/I)
• As the color of the patch changes, the intensity seen by the detector at 620nm also changes
• This intensity change is calculated as an absorbance value
Theory of Operation
Theory of Operation
System Requirements
• Based on the theory we’ve reviewed, each pH system requires:
– Light Source: Light source that strongly emits at analytical wavelength (620 nm) and some baseline wavelength (510 nm or anything ≥750 nm) (Tungsten, LED(s), etc.)
– Spectrometer: Detector that supports same wavelengths as above (STS, Spark, Flame, etc.)
Smart pH Cuvettes
• Sensor chemistry is coated onto a patch installed on the inner wall of a standard PMMA cuvette
• Parts for complete system:
– Spectrometer: Must be able to detect 620 nm, and 510 nm or anything >750 nm
– Light Source: Must emit at 620 nm, and 510 nm or anything >750 nm
– Cuvette holder
– Fibers: If needed
– Smart pH Cuvettes
– OceanView and pH Kit Worksheet
Applications for Smart pH Cuvettes• Laboratory R&D:
– Permanent desktop setup for small samples
– Check cell culture buffers
– Check lake/river/ocean water samples
– Check pharmaceutical solutions (contact storage, eye care, etc.)
• Field Measurements:
– Portable setup for small samples
– Again, great for lake/river/ocean water testing
– Commercial pool or aquarium applications
Transmissive pH Dip Probes
• Sensor chemistry is applied to peel-and-stick transmissive patches that can be stuck to the lens of a T300/TP300 dip probe
• Parts for complete system:
– Spectrometer: Must be able to detect 620 nm, and 510 nm or anything >750 nm
– Light Source: Must emit at 620 nm, and 510 nm or anything >750 nm
– T300 or TP300 Sleeve (only the sleeve, no 300 μm fiber)
– RE-BIF-BORO: This fits inside T300 sleeve
– Pack of transmissive pH Patches (PH-BCG-TRANS)
– OceanView and pH Kit Worksheet
Applications for Transmissive pH Probes
• Handheld:
– General laboratory R&D, similar use as electrodes
– Field R&D, water samples, pools, aquariums
• Process Integrated:
– Probe can be SwageLok’d into a process line
– Offered in PEEK or Stainless Steel to meet application needs
– Power plant cooling water
– Food and beverage processing
– Pharmaceutical processing
– Seawater monitoring
Reflective pH Patches• Transmissive pH sensor
chemistry is overlaid with electroformed gold mesh, and is made as a peel-and-stick patch
• Mesh allows liquid diffusion, but prevents ambient light and sample color interference
• Gold is chemically and biologically inert
• Adds non-intrusive pH sensing to any clear vessel or flow line
• Uses specially designed probe to maximize signal to the detector
Reflective pH Patches
• Parts for complete system:
– Spectrometer: Must be able to detect 620 nm, and 510 nm or anything >750 nm
– Light Source: Must emit at 620 nm, and 510 nm or anything >750 nm
– R1000-4-ANGLE: Specially designed probe for these reflective pH patches
– Pack of reflective pH Patches (PH-BCG-REFLECT)
– OceanView and pH Kit Worksheet
Applications for Reflective pH Patches
• Fermentation
– Two very successful application notes have been done with wine and E. coli fermentation
• Seawater monitoring
• Mud and turbid sample analysis
• Any application calling for non-intrusive pH measurement
• Any application calling for optical pH measurement of turbid or colored samples
Limitations
• Limitations of the pH sensors include:
– Alkaline Earth Metal Compounds: Group 2 salts in aqueous solution don’t follow normal behavior, so let us know if the application is known to have these species
– Temperature: The acrylic substrates can be brought up to 60 °C. High temperature substrates are available, though indicator dye chemistry will only survive to 130-140 °C