What criteria indicate that samples have been dried to a stable weight?

The correct choice indicates that a sample is considered to be dried to a stable weight when there is either a difference of less than 4% or a weight difference of less than 0.5mg between two successive measurements. This standard ensures that the drying process has effectively removed moisture and that the sample weight has reached a consistent level, indicating minimal further change.

Using this specific threshold allows analysts to ascertain that the drying process is thorough. A smaller percentage of difference (like 2% or less) can sometimes be too stringent for practical laboratory procedures, particularly if the mass of the sample is small. Furthermore, relying solely on a weight difference of 1mg might not provide the necessary assurance of stability when the sample size varies. Thus, the combination of both percentage and absolute weight criteria in choice B provides a reliable indicator of stability in weight during the drying of samples.