Purified indicator should be used in the initial instrument calibration and all subsequent pHT measurements. Differences between seawater pH values determined Maraviroc supplier using the broadband LED photometer (pHT(B)) and the narrowband benchtop spectrophotometer (pHT(N)) are shown in Fig. 4a. These samples covered a typical range of surface seawater conditions: 7.6 ≤ pH ≤ 8.2, 30 ≤ S ≤ 36.2, and 15 °C ≤ t ≤ 30 °C. The average difference between the prototype and

research-grade measurements was 0.001 (n = 136). The standard deviation (SD = ± 0.008) can be considered as an index of photometer measurement accuracy relative to conventional state-of-the art spectrophotometric procedures. The precision of the broadband measurements was ± 0.002 (at pHT(B) = 7.991; n = 6). Fig. 4b and c shows that no systematic pH deviations were

observed for measurements obtained over a sizable range of salinity and temperature. Although the LED photometer was not designed for high-precision open-ocean work, we tested its performance at sea (relative to the performance of a standard seagoing spectrophotometer) in order to evaluate (a) its durability Selleck EPZ 6438 in a demanding shipboard environment and (b) its accuracy over the full range of pHT values encountered in a surface-to-deep vertical ocean profile. The DIY photometer worked properly during the research cruise without any issues. Fig. 5 shows vertical profiles of seawater pHT(B) and pHT(N) measured at a sample station in the northeastern Gulf of Mexico (sea surface to 1450 m depth). The results are generally in good agreement. Average ∆pHT for the station profile was − 0.001 (SD = 0.006, n = 14). A second field test was conducted in an aquarium setting. Fig. 6 shows temporal changes in the pH of a saltwater reef aquarium as measured by four different instruments: the LED photometer, a research-grade spectrophotometer, and two glass pH electrodes designed for aquarium use. Over the course of the 16 h monitoring period (Fig. 6), all of PLEK2 the instruments showed a similar temporal pattern of aquarium chemistry, with pH increasing over the course of illumination, then decreasing in the dark. In terms of absolute pH values, however,

the four instruments differed. The identical potentiometric probes reported pHNBS values that differed by as much as 0.05 from each other and by as much as 0.2 from the pHT measured spectrophotometrically. The nearly constant offset of approximately 0.2 units is due to the pH scale established by the standard buffers supplied with the aquarium electrodes. The buffers were of low ionic strength, with pH values reported on a scale different from the total hydrogen ion concentration scale of the spectrophotometric measurements (Dickson and Millero, 1987, Dickson, 1993 and Millero, 1995). Values of pHT obtained using the LED photometer showed good agreement with those obtained using the narrowband spectrophotometer. Average ∆pHT was − 0.008 (SD = 0.006, n = 32).