T-300 and TP-300 pH Probe Stability in Seawater
An investigation was performed to determine the long-term stability of the T-300 probe in seawater, as well as a comparative analysis with a Ross pH electrode. Seawater was obtained locally from a Dunedin, FL marina, and the test was performed isothermally in an environmental chamber at 25°C. An Ocean Optics USB2000 spectrometer was used with a 200?m slit and grating #2 with no lens installed.
A 2-meter 35?m bifurcated borosilicate bundle was used inside a T-300 sleeve, and an LS-1 tungsten halogen light source was used. All components were placed inside an environmentally controlled chamber, along with a second beaker of seawater being monitored by a Ross pH electrode. The beakers were sealed with Parafilm®, and the probes kept a tight seal with the film to prevent evaporation. The experimental setup is pictured below:
Galvanic corrosion became apparent with the use of the stainless steel sleeve, most likely due to improper sealing of the mirror’s metal from the steel. This led to accumulation of rust and sediment on the mirror, steadily causing a continued tilt in the absorbance curve. The typical pH algorithm looks at two wavelengths, an analytical wavelength and a baseline correction to account for vertical offset.
Though, a tilt in the absorbance spectra skews these values and yields incorrect measurements. To correct for this, an accompanying algorithm was used to eliminate this tilt effect, bringing the absorbance spectra back to its appropriate position, knowing that 750nm and 509nm (isosbestic point) should be pegged at zero absorbance. The algorithm is shown below, followed by the plot showing the corrected absorbance curve:
Knowing that it is required to monitor three wavelengths in order to account for the tilt, a long-term experiment was run to monitor pH patch stability. Analysis was performed that calculated the continuous pH assuming the original algorithm alone using a 750nm baseline correction, as well as a 509nm (isosbestic point) correction, and also using the tilt correction algorithm that looks at all three wavelengths. The plot of these three trends is shown below:
Clearly the original approach of using a single wavelength for baseline correction is not feasible for long-term measurement; rather, we see that the tilt algorithm is able to account for these absorbance curve distortions and produces a very coherent trend. The probe originally reads sea water as being roughly pH 8.2, and then drifts downwards to just above pH 6, where it equilibrates. This drift is most likely due to galvanic corrosion reactions occurring, which alter the pH of the system.
Ocean Optics also makes a transmissive probe of the same design using chemically inert peek material. This experiment was repeated using this alternate model, the TP-300, again with the Ross pH electrode running in parallel.
The accumulation of sediments and particles on the mirror and/or lens led to the absorbance curve tilt phenomenon previously seen, though this occurs much more slowly than the stainless steel probe. The system equilibrates after roughly 20 hours, and remains at a constant pH for the following 30 hours, showing the potential for strong long-term stability. A comparative plot shows the readings of the optical pH probe and the Ross electrode, with the electrode’s values being recorded manually at five discrete times:
The drift of the Ross pH electrode is slightly over 0.03 pH unit per day, which is roughly 15 times more drift than the specification reported in the user manual. While the Ocean Optics pH probe showed enhanced stability over the Ross electrode, the measurement was cut short due to light source failure. The LS-1 light source bulbs are offered in 900 hour and 10,000 hour lifespans; the longer-use bulb is recommended for long-term measurements. LED(s) may also be used with these sensors as another alternative. This study has been vital in determining a number of aspects and limitations for this optical pH probe system, some notable points include:
- The peek TP-300 probe should be used over the stainless steel version for pH measurement in seawater or high salinity environments; corrosion occurs extremely quickly with the stainless steel and sediment accumulates on the mirror.
- As sediments form on the mirror and/or lens, the absorbance curve may show a tilting distortion; this can be corrected via a dynamic algorithm that looks at three wavelengths to eliminate the effect.
- The light source should be relatively new and stable when performing long-term measurements. As the light source begins to die, there is a non-uniform intensity decay that causes an incorrect pH reading drift, followed by the light source finally dying completely. LED’s may be used as an alternative.
Algorithms Used:
pH Calculation:
Absorbance Spectra “Tilt” Correction:
Tags: seawater
