Land Contamination: Technical Guidance on Special Sites: Acid Tar ...

5.3 Assessment of Investigation DataThe assessment of investigation data is conventional in approach and reference should bemade to the Model Procedures.As pH values can be below the conventional scale (of 1 to 14), a comment on the definitionand interpretation of pH is made in Box 5.1.Box 5.1 What does a pH of below 1 mean?pH is defined as the negative of the logarithm of the hydrogen ion activity.i.e. pH = -log 10 [H + ] aA solution of pH 1 is thus ten times the acidity of a solution of pH 2 and a solution ofpH 0 is ten times as acidic as a solution of pH 1. Although the pH scale in normal useruns from 1 to 14, from the above equation it is clear that concentrated solutions of acidcan have negative pH values, i.e. the pH scale can extend through zero to negativevalues.A 1 mole per litre solution of sulphuric acid corresponds to 98 g of H 2 SO 4 in 1 litre(roughly 10% by weight) and has a pH of about -0.3 (minus 0.3). It can be seen thatvery concentrated sulphuric acid solutions (65% by weight in some acid tars) will havean even more negative pH (although the approximation that hydrogen ion activityequals hydrogen ion concentration breaks down in very concentrated solutions).The pH values that may be found on acid tar contaminated sites are very much moreacidic than those likely to be found on most other types of sites.R&D <strong>Technical</strong> Report P5-042/TR/04 31