One of the greatest advantages of using an optrode for pH measurement is that the sol-gel matrix allows a much wider range of sampling options and form factors, from transmissive probes to pH-sensitive cuvettes and self-adhesive pH patches. We can also apply our pH sensors as a coating on Petri dishes, microtiter plates, flow cells, or other media where the sample volume may be limited.
All of these form factors have a response time of roughly 30 seconds, though shorter response times may be possible, depending on salinity, temperature, and liquid flow at the sensor interface. The salinity of the solution has the greatest impact on response time. While the accuracy of our pH sensors is not affected by salinity, the time needed to equilibrate and read a stable pH value is determined by the ionic strength of the solution. Higher salinity samples respond more quickly than lower salinity samples.
When working with any of these form factors, it is important to remember that absorbance is being measured, so any movement or change in the optics will affect the absorbance reading and therefore the pH value calculated. It is recommended to leave all optics and sensors in place once calibrated and throughout measurement, including cuvettes.
Transmissive pH Dip Probes
These probes are good for process lines that need continuous monitoring of pH, or for handheld spot-checks in the lab or in the field. They are composed of a high-stability bifurcated fiber (RE-BIFBORO-2) in a T300 or TP300 dip probe sleeve and cap, with a transmissive pH sensor patch applied to the lens. The disposable patches are peel-and-stick, allowing them to be replaced on-site as needed.
These probes are available as kits in stainless steel or in PEEK. Stainless steel probes can corrode in some solutions, notably seawater, making PEEK a good general-purpose choice. The components can also be purchased separately. A 5 mm or 10 mm pathlength tip is recommended to reduce the risk of bubbles being trapped in the probe tip and affecting measurements.
|Sleeve & cap material
|Transmissive patches (5)
Light travels down the bifurcated fiber, is transmitted through the pH patch, through the sample solution in the probe cap, and is reflected by a mirror in the cap back through the sample and pH patch back into the fiber for measurement by the spectrometer.
Transmissive probes are good for process lines that need continuous monitoring of pH, or for handheld spot-checking in the lab or in the field. They can be Swagelok’d or pressure-fitted into a standard ¼” fitting, which is ideal for process lines and various reaction vessels. Additionally, the patches can be purchased in custom shapes or sizes and applied directly to process vessels for in-situ monitoring of pH.
Typical handheld applications include general laboratory use and R&D as a direct replacement for pH electrodes, as well as field R&D, water sample testing, pools, and aquariums. They have also been integrated into process applications like monitoring of power plant cooling water, food and beverage processing, pharmaceutical processing, and seawater.
Note that transmissive probes are only suitable for samples that are clear and non-turbid. Since the light travels through the sample as well as the pH sensor patch, any color or sediment in the sample will affect the absorbance measurement and invalidate the pH reading.
Smart pH Cuvettes
As an alternative to using a transmissive probe, we have coated the pH sensing material onto the inner walls of standard cuvettes. These are available as plastic (PMMA) cuvettes for general purpose use, or quartz cuvettes for high temperature applications. The PMMA cuvettes themselves can be considered semi-disposable, as they can be used multiple times, but they are not intended for long-term permanent use.
| Part Number
Smart pH cuvettes must be used in a cuvette holder, with fibers to route light from the light source and to the spectrometer. The CUV-UV and direct attach CUV-FL-DA cuvette holders are the most popular choices, though any cuvette holder could be used. A pair of 100 µm or 200 µm core fibers should result in good signal levels (consult the fiber section below for more information on fiber-spectrometer combinations).
To simplify configuration, we offer convenient smart pH cuvette kits. For desktop spectrometers like the USB4000, the SC-PH-DESKTOP-KIT2 contains a CUV-UV cuvette holder, two 200 µm fibers (QP200-025-VIS-NIR), and an 8-pack of coated PMMA cuvettes (SC-PH-VIS1M-SAM). For the Jaz spectrometer, the SC-PH-JAZ-KIT2 is more compact, with a direct-attach CUV-FL-DA cuvette holder, a single 200 µm fiber (QP200-025-VIS-NIR), an SD-MEMORY card for storing data onboard, and an 8-pack of coated PMMA cuvettes (SC-PH-VIS1M-SAM).
Smart pH Cuvettes are ideal for small volume samples and for quick desktop readings, as they are inexpensive and disposable, and have a small footprint. They are often used as a permanent desktop setup for small samples in testing or R&D labs to check cell culture buffers, lake/river/ocean water samples, pharmaceutical solutions like contact storage, eye care, etc., and to test low-conductivity samples such as boiler water. In the field, they are an ideal portable setup for small samples to test environmental water samples, commercial pools, and aquariums.
As with the transmissive dip probe, smart pH cuvettes are only recommended for samples that are clear and non-turbid. Since the light travels through the sample as well as the pH sensor patch, any color or sediment in the sample will affect the absorbance measurement and invalidate the pH reading.
Smart pH cuvettes can be reused simply by washing, with up to 50 uses or more for a single cuvette. The cuvette should be disposed of when absorbance at pH 11 drops to less than 0.1 (relative to a pH 1 reference). High temperatures, high pH, and high pressure, as well photobleaching from the light source will destabilize and reduce the lifetime of the cuvette. All of our pH sensors are very stable at low pH values.
Reflective pH Patches
Samples that are colored or turbid pose an issue for transmissive pH sensors. We have solved that problem by developing a reflective pH sensor that can be applied to the interior of a transmissive container and read with a reflection probe. It consists of our existing transmissive pH sensor material, overlaid with a very fine gold mesh and a top piece of adhesive to create a peel-and-stick patch.
The electoformed gold mesh is liquid-permeable, allowing diffusion of hydrogen ions into the sensor material while acting as a mirror to reflect light back to the detector. Since the light being measured is reflected at the gold mesh and never transmits the sample itself, the sensor is immune to the effects of ambient light, sample color, and turbidity.
The patches themselves come in a package of 5 (PH-BCG-REFLECT), all self adhesive and cut in ½”x½” squares. They are also available in custom sizes and shapes down to 5×5 mm for use on the transmissive wall of a vessel or process line to provide non-intrusive pH sensing.
A specially designed reflection probe is used with reflective pH patches to maximize signal to the detector, allowing measurements to be made through container or vessel walls. The RE-BIFBORO-2 probe uses a randomized bifurcated borosilicate fiber bundle to provide uniform illumination and collection of light, and maximizes signal from the patch.
Reflective pH patches are a clear choice for any application calling for non-intrusive pH measurement. They work very well in bioreactors and flasks for monitoring beer, wine and E. coli fermentation processes. This novel design makes it easy to add a pH sensor to a spin-flask, beaker, fish tank, process line, or any other transmissive vessel. They are almost entirely immune to ambient light and sample color, and are ideal for mud and turbid sample analysis, as well as seawater monitoring.