Reserve alkalinity
What is Reserve Alkalinity?
Reserve Alkalinity (RA) is the number of millilitres, to the nearest 0.1 ml, of 0.100 N hydrochloric acid required for the titration to a of pH 5.5 of a 10 ml sample of an undiluted, unused coolant, antirust or coolant additive, and unused or used solutions thereof. The method is described in ASTM D 1121. The original intent of the characteristics was to provide an indication of the coolant's resistance to acidification. Through use a coolant would be rendered more acidic. And as a result, the coolant performance would be negatively affected, resulting in enhanced corrosion.
Influence of technology on RA
Vintage coolant specifications called for an RA of a minimum of 10 ml. In recent years, coolant technology has been developed that provides superior corrosion and other cooling system protection at pH and RA values dramatically different from those that the industry was accustomed to in the past. Acknowledging the capabilities of the new technology, industry standards (such as ASTM) and OEMs have often removed the specific RA requirement from their specifications.
The value itself is dependent on the actual coolant technology that is used. Some of the advanced technologies in the marketplace today exhibit low RA values. These values should not be interpreted as weaknesses or as a suggestion of compromised quality. Indeed the reverse is true the best and newest technologies often have a lower RA chemistry. Traditional and hybrid coolant technologies tend to have higher values for the Reserve Alkalinity. OAT and lobrid coolants tend to have lower values for the Reserve Alkalinity. The use of a buffer will also affect the Reserve Alkalinity value. It is therefore critical that you know the coolant technology used in the product before evaluating coolant quality and the associated Reserve Alkalinity.
Influence of amount of inhibitors on RA:
The end pH value of 5.5 for Reserve Alkalinity was chosen because this point could easily be measured for coolant technologies at the time, and the repeatability was maximal. This good repeatability is due to the inflection point in the titration curve. This was the point where the pH would suddenly change significantly for a set addition of HCl. This also indicated the end of the buffer plateau provided by the available coolant inhibitor technologies. The pH buffer range is dependent on the inhibitors selected. The inflection point of 5.5 is valid only for certain inhibitors. The Reserve Alkalinity then becomes an indication of the total amount of these inhibitors present in the solution and the Reserve Alkalinity could be interpreted as a measure of additisation of the coolant.
Newer inhibitor systems are no longer necessarily based on the use of these inhibitors. The newer OAT-based coolants use mixtures of organic acids, which results in a significant reduction in the Reserve Alkalinity values. This does not affect the corrosion protection provided.
