Power of hydrogen - pH
pH is an abbreviation meaning “power (or potential) of hydrogen”. It is a convenient way to express the concentration or more precisely the activity of the hydrogen ion in solution. The pH-scale is logarithmic, yielding a manageable expression for the strength of an acidic or alkaline solution.
pH = - log [aH+]
Solutions with a pH lower than 7, and thus a hydrogen ion activity higher than 10-7 moles/liter are acidic, solution with a pH higher than 7, and thus a hydrogen ion activity lower than 10-7 moles/liter are alkaline. pH and the underlying hydrogen activity is an important parameter in the many chemical processes, those occurring in engine coolants are no exceptions. The pH value will determine for example whether or not the inhibitors in a coolant are sufficiently soluble and stable, stabilizers operate in their optimal range, metallic and non-metallic material compatibility can be guaranteed and in general optimal performance can be achieved.
pH plays a crucial role in corrosion protection, with an optimal, slightly alkaline pH between 7 and 9 ideal for modern day cooling system alloys. Historically the pH of engine coolants was significantly higher due to the predominance of ferrous alloys that were more effectively protected at higher pH values by the older additive technologies (e.g. phosphate, amine and borates containing coolants). Nowadays, additive packages are capable of providing excellent protection for ferrous metals at significantly lower pH values, while in addition they also ensure protection of aluminium alloys, which is negatively affected by pH values higher than 9.
To get things right from the start, care should be taken to choose the correct water quality when diluting a coolant, as such only water with a pH between 4 and 10 should be used.
The pH of a coolant is seldom static, several processes may affect it. Coolants differ in their intrinsic capacity to resist pH changes when acid or alkaline is added on purpose, a property related to the parameters of, “reserve alkalinity” and “reserve acidity” and in more general terms “buffer capacity” (see Reserve Alkalinity). Nevertheless, there is overwhelming evidence that it is not this buffer capacity that is the most parameter affecting pH stability, but rather the capacity of the coolant additive technology to cope with the operational conditions in the cooling system. Glycol breakdown, often assisted by some of the older additive technologies generates acids that will decrease the pH, while aluminium corrosion will increase the pH by consumption of hydrogen ions leading to a continuous rise in pH and even increase in the aluminium corrosion rate.