The equilibrium constant K2T is a function only of solution salinity, temperature, and pressure and is thus independent of the configuration of the optical instrumentation. In contrast, STA-9090 the molar absorptivity terms are generally a function not of solution chemistry but of

instrument configuration. The values of the ε ratios (Eq.  (3)) therefore depend on whether they are determined using a narrowband or broadband instrument (within the class of narrowband spectrophotometers—i.e., bandwidths on the order of 2 nm or less—instrumental differences are insignificant.). In the original development of high-precision spectrophotometric methods for measuring seawater pHT (Clayton and Byrne, 1993), Eqs. (4), (5), (6) and (7) were determined using monochromatic light (bandwidth ~ 1 nm) to assess the relative concentrations of the

unprotonated and protonated (L2 − and HL−) forms of indicator. Subsequent characterizations of purified indicator were likewise conducted using narrowband spectrophotometers. For purified mCP (Liu et al., 2011): equation(4) pHT=−logK2Te2+logRN−e1/1−RN·e3/e2where click here RN = 578AL2 − / 434AHL− (this RN is equivalent to the R of Eq.  (3) of Liu et al.). The corresponding (narrowband) ei coefficients are a function of temperature (T) and salinity (S): equation(5) e1=−0.007762+4.5174×10−5Te1=−0.007762+4.5174×10−5T equation(6) e3/e2=−0.020813+2.60262×10−4T+1.0436×10−4(S−35)e3/e2=−0.020813+2.60262×10−4T+1.0436×10−4S−35at a measurement pressure of 1 atm. The

equilibrium constant term of Eq.  (2) is given as: equation(7) −logK2Te2=a+bT+clnT−dTwhere a=−246.64209+0.315971S+2.8855×10−4S2b=7229.23864–7.098137S−0.057034S2c=44.493382–0.052711Sd=0.0781344. This characterization is appropriate for 278.15 ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40. Fig. 1 illustrates the structure of the DIY LED photometer (part list, circuit schematic, and source code can be found in supplementary material.). For the light source module, LED1 (MV5B60, Everlight) and LED2 (LTL1CHKGKNN, Lite-On) were used to generate light with outputs centered near 434 nm and 578 nm, the wavelengths of maximum absorbance of the acidic and basic forms of mCP. The emission spectra of both LEDs were measured with a USB-4000 spectrophotometer (Ocean Sclareol Optics, Inc.). The detector module is based on a light-to-voltage optical converter TSL257 (TAOS Inc.), which combines a photodiode and a transimpedance amplifier on a single monolithic complementary metal–oxide–semiconductor (CMOS) integrated circuit. The system can be powered by either 4 AA batteries or 5V DC from a standard USB port. A 100 mL PYREX® (Corning Inc., USA) screw-cap round glass bottle seated within a foam nest serves as the sample bottle, reaction chamber, and optical cell (path length = 5.6 cm). The photometer measures 90 × 90 × 100 mm and weighs 370 g. During each measurement, the two LEDs are activated alternately, and the signals obtained from each LED are sent to the microcontroller via a simple 1 s RC filter.