The system was controlled by an Apex AquaController (Neptune Systems, Inc.), which consisted of two 1000-watt 10,000-kelvin ReefLux®

metal halide lamps (Coral Vue, Inc.), a water circulation pump, protein skimmer, heater, thermometer, and two pH probes. The probes were standard pH electrodes (Neptune Systems, Inc.) with an internal Ag/AgCl reference. Both probes were calibrated in standard buffer solutions (pHNBS = 7.01 and pHNBS = 10.01; Milwaukee Instruments, Inc.). Immediately following this calibration, four pH instruments (the LED photometer, the narrowband spectrophotometer, and two electrodes) were used to monitor the pH of the aquarium water over a 16 h period (measurement interval = 30 min). The emission bandwidths Afatinib cost Epigenetic inhibitor of the LEDs in the photometer are substantial compared to the absorbance bandwidths of the L2 − (basic) and HL− (acidic) forms of mCP (Fig. 2). LED1 has an emission maximum at λ = 427 nm (near the HL− absorbance maximum of λ1 = 434 nm) with a full width half maximum (FWHM) of 66 nm. LED2 has an emission maximum at λ = 574 nm (near the L2 − absorbance maximum of λ2 = 578 nm) with a FWHM of 13 nm. Ideally, the peaks of the light sources should provide output at the two absorbance peaks of the indicator. In this case, to minimize the cost of instrument construction,

no monochromator was used and the match was approximate. A calibration was necessary to link absorbance ratios measured with the broadband photometer to absorbance ratios determined using a narrowband spectrophotometer. Calibration of the LED photometer was required to link the broadband measurements

of absorbance ratios (RB) to the original narrowband measurements (RN) on which the pHT and indicator characterizations of Eqs.  (4), (5), (6) and (7) are based ( Liu et al., 2011). The relationship between RN and RB, derived from data obtained in well-buffered solutions, is shown in Fig. 3. To a very good approximation, RN is a linear function of RB: equation(8) RN=(1.1892±0.0069)RB−(0.3079±0.012).RN=1.1892±0.0069RB−0.3079±0.012. Operationally, this equation is used to convert the photometer many measurements of RB for seawater samples to their corresponding RN values (i.e., the sample absorbance ratios that would have been reported by a narrowband spectrophotometer). These RN values are then used in Eq.  (4) to calculate the pHT of the seawater sample. This particular relationship (Eq. (8)) is specific to the photometer system used in our study. The function may vary somewhat for other systems, even those of nominally identical construction, because the electrical and optical characteristics of the components (i.e., LEDs and optical converter) may vary by producer and batch.