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