ISO 6387:1983

A 0,5 % (m/m) aqueous soap solution is prepared, and after maintaining this at testing temperature for 24 h, an aliquot portion of this solution is taken. This aliquot portion is mixed with a dilute solution of the dispersing agent (surface active agent) and subsequently with a specified volume of water of known calcium hardness. The mixture is maintained for 1 h at the testing temperature and then the calcium soap present in a aliquot portion of the lower layer is titrated with a standard volumetric solution of hydrochloric acid in the presence of bromcresol green as indicator.