ISO 15181-4:2008 specifies the apparatus and analytical method for determining the amount of pyridine-triphenylborane (PTPB) that has been released from an antifouling paint into artificial seawater in accordance with the procedure given in ISO 15181‑1. It determines the PTPB concentration in the artificial seawater extract and gives the final calculation for the release rate of PTPB under the specified laboratory conditions.

This part of ISO 15181 is designed to allow the concurrent determination of PTPB and other biocides that can be released by a given antifouling paint (for example, zineb) through the analysis of separate sub-samples of an artificial seawater extract generated in accordance with the procedure given in ISO 15181‑1.

When used in conjunction with ISO 15181‑1, the practical limits on the quantitative measurement of release rates by this method are from 1,8 micrograms per square centimetre per day to 500 micrograms per square centimetre per day. The quantitative measurement of release rates below this range will require an additional concentration step described in the text or the use of an analytical method with a lower limit of quantitation for PTPB in artificial seawater.