Non-intrusive Reflective pH Sensing in Tubes of Muddied/Sediment Water
Goal
There was an interest in non-intrusively monitoring pH in tubes containing wet soil samples. This was significant for a number of reasons; this was the first time the reflective patches were to be tested in something other than a cuvette, on a curved surface rather, and this was also the first time an analyte solution was observed that contained sediment or turbidity.
A reflective pH patch was affixed to the inner wall of a small 7mL tube, and was held in place using a ring stand and clamp. A 600um bifurcated reflective probe was connected to an LS-1 light source with a blue filter, as well as a USB2000 spectrometer with grating #1, 200um slit, and no lens installed. The first feasibility test used clear buffers instead of the muddied buffers, in order to observe the performance. The experimental setup is pictured below, along with the plot of the resulting titration.
Although the curves showed a slight distortion, they followed the expected trend and produced a linear calibration plot. Dirt was obtained and mixed in with the buffer solutions; the experimental setup is shown below.
This yielded a very interesting titration plot, unlike any that had been seen before:
Typically for reflective pH patches, the peak is observed at 620nm and the baseline correction is done at 509nm. In this case, however, we see the peak had shifted closer to 635nm, and the 509nm region was distorted completely. As a result, 750nm was used as the baseline correction wavelength, which produced an incredibly linear calibration plot, as seen below.
This application has been critical in determining the effect of turbidity on the absorbance curves for the reflective pH patches, and has concluded that the correct wavelengths for acquisition and baseline need to be chosen based on the nature of the analyte solution.
Tags: sediment water, soil
