The measurement of pH is typically performed on aqueous solutions of materials using a pH meter. The pH meter displays a unit-less number representing a log based measure of the hydronium ion concentration in the solution. The pH scale ranges from 0 to 14, with 7 as the neutral state. Solutions that measure over 7 are considered basic, such as milk or sodium hydroxide and those measuring under 7 are considered acidic, such as carbonated soft drinks and nitric acid. Buffers can be used to help stabilize a solution at a certain pH level, but it is still possible to overwhelm the system and shift the pH into an undesirable level. Additionally, solvents other than water can be classified as basic, acidic or amphiprotic (i.e. have both acidic and basic properties). The presence of one or more of these solvents in a formulation will impact the pH of the system, which could be a determining factor for performance or processability. Below are some typical instances in which pH may affect properties:
- In coatings or personal care items, pH can drive a thickener’s ability to develop or the efficacy of an active ingredient.
- Improper pH may reduce the strength of corrosion inhibitors, preservatives and adhesion properties of formulations.
- If pH is observed to shift during stability studies, the formulation may experience a shorter shelf life or product lifetime when marketed to consumers.
- It is well known that products applied to the skin have a narrow threshold at which pH is safe.