• Low molecular weight organic acids concentrations were high in the coastal seawater.
• Low molecular weight organic acids can reduce the pH value of the seawater.
• Software of Org·TCO2TA can more accurately calculate the pH of the coastal seawater.
Effects of low molecular weight organic acids (LMWOAs) on the pH value of seawater were investigated in the surface seawater of the Jiaozhou Bay, China. The new software of Org·TCO2TA was developed to calculate the pH values of seawater based on the alkalinity (Alk) equation where organic acid Alk (Org-Alk) was separated into LMWOA Alk (LMWOA-Alk) and humic acid Alk (HA-Alk). In the calculations, all dissociation constants of organic acids were from previous literature. In our study, the average concentration of total LMWOAs was 14.5 ± 11.2 μmol·kg−1 SW. pH values from the Org·TCO2TA software were closer to the pH values from spectrophotometric measurement than those from the CO2SYS program, indicating pH values can be influenced by high concentrations of LMWOAs in coastal seawater of the Jiaozhou Bay. Although the differences still existed between the pH values from the spectrophotometric method and the calculated pH values from the Org·TCO2TA software due to the influence of various factors, including the analytical errors of dissolved inorganic carbon and nutrients, the new software can calculate the pH values of coastal seawater more accurately by considering the effects of LMWOAs.
Lyu L.-N., Lu D., Sun C., Ding H., Yu L.-M. & Yang G.-P., 2019. A new software of calculating the pH values of coastal seawater: considering the effects of low molecular weight organic acids. Marine Chemistry 211: 108-116. Article (subscription required).