Evaluation of indicator-based pH measurements for freshwater over a wide range of buffer intensities

Two different sulfonephthalein indicators, cresol red (CR) with a pKa of approximately 8.3 and bromothymol blue (BTB) with pKa of approximately 7.4, were tested for an analysis of freshwater over a broad range of pH and total alkalinity values. Measurements from an autonomous sensor system using a 1 cm optical path length were compared to those using a 10 cm path length on a benchtop spectrophotometer. The indicator pH perturbation was quantified with a thermodynamic model and nonlinear least-squares analysis. The laboratory study found that the perturbation-corrected pH differed between the 1 cm (large indicator perturbation) and 10 cm (small indicator perturbation) optical path length measurements from -0.017 to +0.15 with a median of +0.0041 pH units for CR and from -0.015 to +0.026 with a median of -0.0008 pH units for BTB. Precision was +0.0005-0.013 and +0.0001-0.0027 pH units for the 1 and 10-cm-path-length measurements, respectively. The autonomous sensor was deployed for 14 days in a local creek. Simultaneous glass pH electrode measurements had a large negative and drifting offset (-0.15 to -0.40 pH units) compared to the indicator-based measurements. This study is the first in situ comparison between potentiometric and spectrophotometric pH methods in a freshwater system.

A submersible autonomous sensor for spectrophotometric pH measurements of natural waters

An autonomous sensor for long-term in situ measurements of the pH of natural waters is described. The system is based upon spectrophotometric measurements of a mixture of sample and sulfonephthalein indicator. A simple plumbing design, using a small, low-power solenoid pump and valve, avoids the need for quantitative addition of indicator. A approximately 50-microL slug of indicator is pulled into the sample stream by the pump, and subsequent pumping and mixing results in a section of indicator and sample where absorbance measurements can be made. The design also permits direct determination of the indicator pH perturbation. Absorbances are recorded at three wavelengths (439, 579, and 735 nm) using a custom-built 1.7-cm path length fiber-optic flow cell. Solution blanks are obtained by periodically flushing the cell with sample. Field tests were performed in a local river over an 8-day period. The in situ accuracy, based on comparison with laboratory spectrophotometric pH measurements, was -0.003 pH unit (n = 16), similar to the measurement precision. No drift was observed during the 8-day period. The absorbance ratio used to calculate pH, in combination with a simple and robust optical design, imparts an inherent stability not achievable with conventional potentiometric methods, making the design feasible for long-term autonomous pH measurements.

Aluminium fractionation in a simulated rat stomach: an in vitro study

This study is part of a project dealing with the absorption of aluminium in rats from acidified drinking water. The hypothesis of the project was that 'labile' forms of aluminium in water might be more available for absorption than aluminium in food. To investigate this hypothesis, the distribution of species of the metal within the rat stomach must be considered. So far, few attempts in this direction have been made. The distribution of aluminium forms in vitro was studied, simulating the conditions present in a rat stomach. The in vitro set-up is based on methods used for drug release studies. The results show that only part of the aluminium (29-54%) added in 'labile' forms at a concentration of 4.0 mg l-1 Al was detected as total dissolved aluminium (Altot) after incubation in a simulated rat stomach. The levels of 'quickly reacting aluminium' (Alqr) (approximately the sum of Al and its monomeric hydroxo and sulfato complexes) were very low (< 0.2 mg l-1), but difficult to quantify precisely in this matrix. The pattern of the in vitro results was confirmed by measurements in pooled rat stomachs from in vivo experiments. There are obviously considerable amounts of ligands present in rat feed that bind strongly to aluminium and thereby affect the distribution of metal species inside the rat stomach. It is believed that phosphoserine, present in the protein casein, is an important ligand for aluminium complexation in this case. The results show that studies of Al absorption should be accompanied by fractionation measurements within simulated or real gastric systems.