Reference materials, as well as inter-laboratory comparisons (ILCs), are two essential metrology principles allowing the validation and evaluation of a measurement method. This paper describes the use of a reference material for seawater pH(T) measurements in an inter-laboratory comparison carried out with the French coastal observation service SOMLIT, and laboratories within the project CocoriCO(2). The interpretation of the ILC results demonstrated the capability of the spectrophotometric measurement method to provide compatible results with the reference value. The ILC data also allowed the estimation of an uncertainty budget of the spectrophotometric pH(T) measurement method following the ISO standard 21748 guidelines. The estimated standard uncertainty of the method is of 0.0046 in terms of pH(T). This level of uncertainty doesn't meet the requirements of the "climate" quality objective of the Global Ocean Acidification Observing Network for the monitoring of ocean acidification. However, some recommendations that may allow reducing this uncertainty are proposed in the paper.