Abstraet-Pertechnetic and perrhenic acids behave as very strong acids, Ka ,~ l0 s. Their extensive dehydration to MzO7 in such media as 7 M sulphuric acid complicates a spectrophotometric comparison of acid strengths. Chloroform extractable TcO~C1 forms from pertechnetate in the presence of chloride ion and concentrated sulphuric acid. The (VII) state of this compound is confirmed and its spectrum described. No evidence of unusual technetium (VII) species, in aqueous media of 1 N base to 1 N acid, has been found. The red colour of concentrated aqueous HTcO4 is ascribed to a lower (VI) or (V) state. The existence in alkaline media of a technetate, TcOs g-, species has been reexamined. Some (IV) and (III) state species are partially characterized, but no Tc~Os could be isolated.

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Students unfamiliar with the concept of significant figures have difficulty making sense of this concept when put in the role of passive note-takers recording lists of rules. The focus of this paper is how best to introduce the concept of significant figures so that students find it meaningful before a stage is reached at which they become turned off. The approach described begins with measurements students are already familiar with from their life experiences and involves the students as active learners. This interaction is accomplished by means of demonstrations involving student volunteers, and question-and-answer exercises using an overhead projector, to measure the length of an object to increasingly greater precision.The implied relative uncertainty concept is presented as a means for determining the number of significant figures in any recorded measurement--a concept that avoids using any list of rules. The meaning of the uncertainty labels on beakers and flasks (often ±5%) and the cumulation of uncertainties are presented briefly; the latter illustrated by the need to read a buret at least twice and the manner in which the associated errors cumulate.

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