Abstract
Measurement of pH for oceanographic studies, particularly with respect to the role of the oceans in global warming, is becoming increasingly common. The present work addresses several key issues with the method of spectrophotometric pH measurement as applied to seawater media.
The basis of the spectrophotometric pH measurement method is the calculation of indicator dye speciation in the medium of interest, which coupled with the pKa' of the dye yields the pH of the solution with high precision.
New methods of data treatment are suggested, and critically assessed for improvements in precision obtained, and possible bias introduced. The "within samples" standard deviation has been reduced by up to a factor of 10 by the use of these methods with the spectra from relatively cheap charge-coupled detector spectrophotometers. The laboratory precision of a diode array spectrophotometer based pH measurement system utilising these methods was found to be ±0.0001 pH units, however improvements in the spectrophotometric method and sampling protocols are required before this precision can be maximally utilised.
Several underlying assumptions of the spectrophotometric pH measurement method have been analysed, and found to be slightly inaccurate, however the magnitude of errors introduced into the method has been calculated, and is not large. Methods for correcting some parameters of the system have been proposed and used to analyse published data.
The spectrophotometric pH measurement method has been extended from the seawater salinity range down into the freshwater range, which allows for pH measurements in estuarine systems with a degree of accuracy and precision not previously available with glass electrodes.
Photolysis of the indicator dyes has been observed and characterised, and impacts on the spectrophotometric pH method assessed. This has led to several recommendations on sampling protocol.
Numerous potential problems with the equipment and methods of spectrophotometric pH measurement have been identified, with protocols being implemented to prevent or remove the effects of such problems where possible.