Why is sodium hydroxide used to correct the pH of both the acid & alkaline solutions in ISO 105-E04 & AATCC TM15?

ISO 105-E04 & AATCC TM 15 are standards which are used to determine the colour fastness of textiles to perspiration.

Specimens are soaked in two different solutions – acid and alkaline - and then placed in a perspirometer and incubated for a set time.

Once dried, the specimens are assessed for change in colour and staining of the adjacent fabric.

The acidic solution must be pH5.5.

The alkaline solution must be pH8.0.

Once each solution has been made up, the pH should be checked. 

Both solutions are formulated with a lower pH than the target pH to allow for fine adjustment using sodium hydroxide – an alkaline with pH13.

When sodium hydroxide is added to either an acid or alkaline solution, the pH of that solution will rise to become a weaker acid or a stronger alkaline (pH7 = neutral).

Add a few drops of sodium hydroxide at a time, thoroughly mixing and rechecking the pH until the correct level is reached.