Building on the work of the BONUS PINBAL innovation project, SPECTROPHABS will clarify open questions regarding long-term stable and traceable environmental monitoring of the pH value in the Baltic Sea through spectrophotometric pH measurement, and develop a suitable monitoring concept. This will allow to make statements on possible acidification, especially in the German Baltic Sea areas. The long-term goal is the operationalisation of spectrophotometric measurements of the pH-value in the Baltic Sea and thus the establishment of a long-term stable monitoring of the pH-value in a complex brackish water system. To approach this goal i) the currently commercially available devices for spectrophotometric pH measurement in seawater will be tested and evaluated, ii) a strict uncertainty consideration as a function of the influencing variables will be carried out, iii) possibilities for dealing with the problem of the lack of availability of the purified indicator dye meta-cresol purple will be investigated, (iv) routine measurements during monitoring cruises in the Baltic Sea and by using a MARNET station will be demonstrated; and (v) a revision of the corresponding HELCOM guideline for the determination of pH in seawater based on the project results will be proposed.