Brief presentation of the developed pH instrument
The general principle of the instrument developed within BONUS PINBAL is the measurement of the pH value of a continuous sample stream through an injection of a concentrated indicator solution into the stream. The indicator injection leads to a concentration gradient of the dye in the sample water, which is needed for the subsequent detection and recording of the absorbance spectra at high frequency and decreasing dye concentration. These spectra are used for calculation of the pH value of the water sample. The starting point of the development was the scientific instrument and data processing described in Aßmann et al., Ocean Sciences, 7, 597 ff., 2011, and the know how from the development of an automated wet-chemical total alkalinity instrument by partner CONTROS.
The indicator is stored in user-friendly cartridges that allow for a fast and clean plug-and-play exchange of the reservoirs by the operator. The cartridges are placed within the system ensuring a stable temperature of the indicator solution. A second cartridge allows for a regular cleaning by flushing of the relevant parts such as the tubing and the optical path of the cuvette at defined intervals with diluted acid.
The instrument can be run on a continuous water supply as well as in a discrete sample mode. The measurement principle is identical in both cases, with a pre-set number of measurements foreseen in the discrete sample mode. In both cases, a dark and a blank spectrum is recorded before each injection of indicator dye into the sample water.
The prototype development has reached a high degree of maturity (Fig. 3) in terms of userfriendliness (touch screen, basic mission planner, different measurement routines), dye perturbation correction (optimized correction function), autonomy and interfacing (low maintenance while in operation, remote control via serial connection). The precision in the main pH working range for Baltic Sea conditions (pH = 7…8.5) is within approx. ±0.002 pH units. The accuracy of the prototype (offset of 0.008 pH units compared to a separate reference system owned by IOW) is in the same order of magnitude as of the reference system itself, estimated to be approx. ±0.005 pH units (Carter et al., L&O Methods 11, 16-27, 2013). This performance results from several amendments made throughout the project and in particular in response to the final field experiment, addressing the time span between thermal equilibration, dye injection, and the measurement, as well as the range of dye concentrations used for the calculation of the pH of the measured solution.
