Transmissive pH Probe
Instructions
Overview
Ocean Optics’ fully integrated pH systems provide full spectral analysis to help eliminate errors from
dye leaching or from changes in turbidity, temperature, and ionic strength. Inherent calibration based
on the physical properties of the immobilized indicator dye eliminates the need for frequent
calibration. The ratiometric algorithm provides accurate and reproducible measurements at a high
resolution.
The Transmissive pH Probe’s form factor provides very fast response with high signal strength,
though is limited to clear, non-turbid solutions. These sensors are compatible with aqueous solutions,
ethanol/methanol solutions, ammonia, peroxides, and sodium hypochlorite solutions.
Transmissive Probes and Patches
The pH sensory substrates use a sol gel sensing material coated onto a transmissive patch. The
immobilized indicator dye(s) are encapsulated into the sol gel matrix, allowing for the diffusion of
ions while preventing leaching of the dye. The sensors provide very accurate measurements in the
biological range, from pH 5 to 9. Advantages of these probes over traditional potentiometric devices
include faster response time, easy storage and no maintenance, and low cost. These are especially
useful for monitoring low conductivity samples such as boiler water, where electrode devices fail.
Software Interface
The Transmissive pH Probes can be used with a desktop system or the Jaz handheld spectrometer. The
desktop system uses a special SpectraSuite software module that allows for easy calibration,
convenient pH readings, customizable data logging, and comprehensive export of data and calibration
values.
PH-BCG-TRANS-01-0210 1
pH Probe Instructions
Transmissive pH Probe Benefits
Chemistry
- Proprietary Organically Modified Sol Gel (ORMOSIL) formulation engineered to
maximize immunity to ionic strength sensitivity
- Compatible with some organic solvents (i.e. acetone, alcohols, aromatics)
material chosen over typical polymer method, allowing for a faster response
- time, versatility in the desired dopants, greater chemical compatibility, flexible coating,
and enhanced thermal and optical performance
- Indicator molecule allows high resolution measurement in biological range (pH 5-9)
Simplified algorithm takes analytical and baseline wavelengths into account to reduce errors
caused by optical shifts
Sterilization
- Transmissive patches are autoclavable
- Transmissive patches are gamma and ethylene oxide sterilizable
Maintenance
- Long-term storage
- Dry or wet storage
- Single point reset
Applications
- Organic solvents
- Low ionic strength cooling water
- Biological Processes
- Environmental
Durable design allows probe to survive conditions/shocks that would cause glass electrodes to
break
◊ Sol-Gel
Fiber Optic pH Sensor System Components
The Fiber Optic pH Sensor system consists of the following:
Transmissive pH probe (T300-UV/VIS or VIS/NIR)
Ocean Optics VIS-NIR spectrometer (or Jaz Sensor module) that covers 350–1100 nm (slit
options 25, 50, 100, 200)
SpectraSuite Spectrometer Operating Software for reading values
Light source (LS-1 Tungsten Halogen Light Source or a white LED)
Transmissive pH patches, pack of 5 (PH-BCG-TRANS)
Calibration requires recording spectra in high and low pH samples, as well as in at least one pH
standard solution (such as a NIST-traceable buffer).
2 PH-BCG-TRANS-01-0210
pH Probe Instructions
For field measurements, the Ocean Optics handheld Jaz spectrometer offers an easy and portable
solution. An SD card contains a script that allows you to use the factory calibration or a complete
calibration, shows live pH values and provides the ability to save data right onto the SD card.
Probe Storage/Lifetime
Probes can be stored dry at room temperature for any amount of time. As they are used, the patches
may slowly leach indicator dye from the sensing material. As a rule,
at pH 11 falls below 0.1, the patch should be discarded and replaced
1). The patch’s lifetime depends on frequency of use, harshness of the samples it is exposed to, the
temperature of samples, and other environmental factors.
once the maximum absorbance
(assumes a reference of pH
Nature of Samples
The analyte solutions being measured should have a pH within the biological range (pH 5-9) for
accurate readings. Data obtained from analyte solutions that register values above or below this range
should not be considered valid within the specifications of the sensors. Aqueous solutions,
ethanol/methanol solutions, peroxides, ammonia, and sodium hypochlorite solutions are all compatible
with the sensor material. Samples being used with the transmissive probe should be optically
transparent, having no turbidity or sediment present. It is also ideal to have analyte solutions that are
colorless, though colored liquids can be compensated for.
Response time is dependent on the ionic strength of the solution, with higher salinity samples
responding notably faster. For example, using the calibration buffers of pH 5 – 8 will show a 90%
response in 10 seconds or less, but when pure D.I. water is being measured, more time is needed to
equilibrate at a final value. Make sure that the probe is sufficiently submersed into the analyte
solution, and that there are not bubbles present. To ensure no bubbles remain, shake the probe back
and forth through the liquid to knock them free. Likewise, more accurate results will be obtained if the
probe is rinsed once or twice with the analyte solution after calibration. This removes any residual
buffer solution that may contaminate your sample.
When immersed in solution, the film dyes may leach very slowly over time and will have to be
replaced. The film response rate is limited by diffusion of ions into the material; therefore increasing
stirring speed and ionic strength tend to increase the response rate.
pH Probe Set Up
The following procedures describe how to connect and calibrate the transmissive pH probe using a
VIS-NIR spectrometer, a light source and SpectraSuite software. See your spectrometer, light source
and SpectraSuite manual for more detailed installation information (available at
http://www.oceanoptics.com/technical/operatinginstructions.asp).
PH-BCG-TRANS-01-0210 3
pH Probe Instructions
Installing the pH Sensor System
► Procedure
Perform the steps below to install the pH Sensor components:
1. Install SpectraSuite software on your computer.
2. Connect the spectrometer to your computer using the supplied USB cable.
3. Install the light source as specified in its instructions.
4. Attach the fibers between the spectrometer, probe, and light source. The transmissive probe
has two identical legs.
5. Turn on the light source and allow it to warm up for the period specified in the light source
instructions.
6. The cap for the transmissive probe screws off. The probe uses a thin sensing film with
adhesive on one side. This can be affixed to the lens of the probe. A second patch can be
affixed to the mirror as well to dramatically increase signal strength and pH resolution.
Calibrating the pH Sensor System
The pH sensor patches include a pre-calibrated pK value determined at the factory. This value was
originally obtained at 22°C, and it is recalculated using the temperature compensation algorithm based
on the temperature that was entered in SpectraSuite’s Calibration Wizard. Using the Factory
Calibration method is ideal for being able to start making pH measurements quickly, though it is less
accurate than performing an Independent Calibration. The specifications listed for the probes assume a
complete Independent Calibration, as this eliminates the errors seen from temperature and other
environmental differences.
Using Factory Calibration
► Procedure
1. Open SpectraSuite and select File | New | New Sol Gel pH Measurement.
2. Click the Calibration Wizard button to begin the calibration.
3. Select the spectrometer to use and click
4. Select
4 PH-BCG-TRANS-01-0210
Use Factory Calibration. Click the box for Using Reflective Probe, and then click
Next. The Enter Experimental Parameters screen appears.
Next.
pH Probe Instructions
5. Enter your Experimental parameters: Acquisition Wavelength, Baseline Wavelength, and
approximate
Ambient Temperature. Click Set, then click Next.
Note
For pH probes that perform in the biological range (pH 5 – 9), the Acquisition
Wavelength
6. Enter the value for pK that came with your probe pH patch. Then click Next.
7. Take a low pH reference spectrum at pH 1.0. To do this,
a. Submerge the probe tip in pH 1 buffer and shake back and forth to remove bubbles.
b. Wait 30 seconds, then click Acquire. The spectrum shown should have a maximum
intensity around 80% of the saturation level. Most Ocean Optics spectrometers have a
saturation limit of 4000 counts. If saturation is occurring, reduce the integration time
appropriately. If the signal is too low, increase the integration time until the intensity at
620nm is at least over 1000 counts.
c. You can click
time; the last time you press the button will be the last reference that is saved.
d. Once the signal looks strong, make sure you’ve updated the acquisition and then click
Next.
8. Take a dark spectrum. To do this, block the light source and click
Then click
in the dark before acquiring the spectrum.
is 620nm and the Baseline Wavelength is 509nm.
Acquire repeatedly to update the spectrum after adjusting the integration
Acquire Dark Spectrum.
Next. Be sure to allow enough time for the system to complete an entire scan while
9. Unblock the light source.
PH-BCG-TRANS-01-0210 5
pH Probe Instructions
10. Take a high reference spectrum for pH 11.0. To do this, submerge the probe tip in pH 11
buffer and shake back and forth to remove bubbles. It’s a good idea to rinse the probe with
buffer before submersion to ensure residual buffer from before does not contaminate your
sample. Allow 30 seconds, and then click
Acquire. When complete, click Next.
11. Depending on the value for pK you previously entered, the wizard will ask you to expose the
probe to either pH 5 or pH 8. For pK values less than 6.5, pH 8 is used; for pK value greater
than 6.5, pH 5 is used. Submerge the probe tip in the requested buffer and shake back and
forth to remove bubbles. Allow 30 seconds, click
Acquire, and then click Finish.
12. You are now ready to take pH measurements. See
Taking pH Measurements.
Performing an Independent Calibration
► Procedure
1. Open SpectraSuite and select File | New | New Sol Gel pH Measurement.
2. Click the
3. Select the spectrometer to use and click
4. Select
then click
5. Enter your Experimental parameters:
approximate
Calibration Wizard button to begin the calibration.
Next.
Perform Independent Calibration. Click the box for Using Reflective Probe, and
Next. The Enter Experimental Parameters screen appears.
Acquisition Wavelength, Baseline Wavelength, and
Ambient Temperature. Click Set, then click Next.
Note
For pH probes that perform in the biological range (pH 5 – 9), the Acquisition
Wavelength
6 PH-BCG-TRANS-01-0210
is 620nm and the Baseline Wavelength is 509nm.
pH Probe Instructions
6. Take a low pH reference spectrum at pH 1.0. To do this,
a. Submerge the probe tip in pH 1 buffer and shake back and forth to remove bubbles.
b. Wait 30 seconds, then click
intensity around 80% of the saturation level. Most Ocean Optics spectrometers have a
saturation limit of 4000 counts. If saturation is occurring, reduce the integration time
appropriately. If the signal is too low, increase the integration time until the intensity at
620nm is at least over 1000 counts.
c. You can click
time; the last time you press the button will be the last reference that is saved.
d. Once the signal looks strong, make sure you’ve updated the acquisition and then click
Next.
7. Take a dark spectrum. To do this, block the light source and click
Then click
in the dark before acquiring the spectrum.
8. Unblock the light source.
9. Take a high reference spectrum for pH 11.0. To do this, submerge the probe tip in pH 11
buffer and shake back and forth to remove bubbles. It’s a good idea to rinse the probe with
buffer before submersion to ensure residual buffer from before does not contaminate your
sample. Allow 30 seconds, and then click
10. Follow the wizard and repeat Step 9 for pH buffers 5, 6, 7, and 8 (follow on-screen prompts).
Again, it is a good idea to rinse the probe tip with buffer before submersion to prevent sample
contamination. Then, click
Next. Be sure to allow enough time for the system to complete an entire scan while
Acquire repeatedly to update the spectrum after adjusting the integration
Acquire. The spectrum shown should have a maximum
Acquire Dark Spectrum.
Acquire. When complete, click Next.
Finish.
11. You are now ready to take pH measurements.
Taking pH Measurements
Now that you have finished calibrating your pH sensor system, you can take pH measurements in the
biological range.
► Procedure
1. Submerge the probe tip into analyte solution for pH measurement in the biological range. The
pH value appears on the screen in the
Current pH field (upper right corner).
PH-BCG-TRANS-01-0210 7
pH Probe Instructions
2. Click the Run/Stop button to toggle data acquisition appearing in the lower table on the
screen. Data is recorded at the time interval you specify in the
Time Increment (sec) field.
3. Click one or all of the following buttons:
Reset: Clears the table and restarts the run time.
Export: Opens a window to save your data in a format that can be opened with Microsoft
Excel or a text program such as WordPad. The exported data file contains all of the
variables that you have entered and have been calculated, along with a time stamp for data
acquisition and save, the time-resolved pH data, and complete spectra for all reference and
calibration buffers used.
Export Calibration: Opens a window to save your calibration data. This creates a file
containing the reference spectra and other variables that can later be loaded via the
Calibration Wizard, allowing for very quick setup.
8 PH-BCG-TRANS-01-0210
Algorithms Used
pH Calculation
pH Probe Instructions
…where Abs
SlopepKpH
is the sample absorbance at 620nm with baseline correction, and Abs
Sample
log*
Abs
11
Sample
AbsAbs
SamplepH
is the
pH11
absorbance at pH 11 at 620nm with baseline correction.
Temperature Compensation
When you select Use Factory Calibration in SpectraSuite, the value for pK is adjusted via the van’t
Hoff equation based on the current temperature you entered:
11
*480
log
12
epKpK
*480
log
12
epHpH
TT
12
11
TT
12
Resetting pK and Slope
An x-y plot is made using data obtained from intermediate buffers 5 through 8. The x-axis is of the
term:
log
Abs
11
Sample
AbsAbs
SamplepH
…for each of the buffers. The y-axis shows the pH value of the buffers. This generates a plot such as
the one shown below:
PH-BCG-TRANS-01-0210 9
pH Probe Instructions
Performing a linear fit gives a line with pK equal to the y-intercept and slope equal to the slope. In the
example chart above, the new pK value would be 6.2977 and the new slope value would be 1.7248.
Specifications
Specification Ocean Optics pH Probe Value
pH Range 5 – 9
Temperature Range -5 – 80°C
Accuracy <1% of reading across range
Resolution 0.02 pH
Response Time (t90) 30 seconds
Response time increases with decreasing ionic strength
Calibration Requirements (minimum) 3 buffers
Sterilization Gamma, EtO, Autoclave
Chemical Compatibility Aqueous, alcohols, some organic solvents, peroxides,
ammonia, sodium hypochlorite
Chemicals to Avoid Concentrated acids
Drift (Continuous Stability) 1% per day
Discrete Stability (Lifetime) 50 uses or more, dispose when absorbance at
pH 11 < 0.1 (assumes pH 1 reference)
Storage Conditions Dry or wet storage
10 PH-BCG-TRANS-01-0210
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