Literature toxicity data cover experiments at temperatures from 6 to 28 C and pH values from 7 to 8.3. The unionised ammonia concentration as a fraction of the total will vary from 0.1 to 10% at the extremes of the above conditions. It is possible to inter-convert
both unionised and total ammonia to values at a standard condition,e.g. 20 C and pH 8, as has been done in Table 2 and it was these data that were used in the SSD. Since the bulk of the toxicity derives from unionised ammonia, there might be some logic in