Direct spectrophotometric pH measurements of seawater using the sampling bottle

Spectrophotometric pH measurements of seawater has become a widespread and routine measurement for many laboratories worldwide. However, dye perturbation correction and sample handling remain two significant sources of uncertainty in the measurement metrology. Here, we assessed the quality of spectrophotometric pH measurements made directly in a 250 ml borosilicate media bottle where the samples were collected. This approach does not move the solution, and allows for multiple additions of dye for each sample, addressing the two shortcomings listed above. Furthermore, temperature is measured rather than controlled, simplifying the equipment needs. During the C3PO19 research cruise along the Central California coast, this 'spec in a bottle' was compared to spectrophotometric measurements made using a standard benchtop system using a syringe pump and temperature controlled 10-cm cuvette. The two methods showed great agreement, with a mean difference of 0.001 ± 0.005 (1 std. dev, n = 94), over a sample pH range of 7.35 to 8.03.