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To recognise the end point of acid-base titrations, colour indicators are still frequently used. Colour indicators can cause systematic errors, and the theoretical evaluation of these errors is given in many textbooks. Random errors caused by the transition range of an indicator can also be estimated with the help of pH-logc i diagrams. However, very often, undergraduate students have problems to correctly interpret the colours of the different species of an indicator (its acidic and alkaline forms) and colour changes during titration. Frequent questions are of the type: ''Is it still blue, or is it already red?'' The aim of the present text is to show how carefully a suitable colour indicator has to be chosen, and what colour changes happen at the inflection point of a titration. Typical acid-base titrations like the alkalimetry of hydrochloric acid, sulphuric acid, oxalic acid, and the acidimetry of borax at two concentration levels using eleven common colour indicators are compared with potentiometric end point determination. Photographs were taken of solutions containing the indicators during the course of titration, i.e., at different pH values. These photographs were used to prepare a colour map for each indicator. Such colour maps are very helpful to find the most suitable indicator for a certain reaction, and also to estimate the maximum accuracy.

No abstract

A semi-automatic sieving apparatus for separation of small samples (50-500 mg) of granular material into several uniform grain-size fractions is described. The achieved separations are reproducible, yielding more than 95% of the initial sample. Such separations are needed for investigations of surface phenomena in very fine powders. It is demonstrated that the apparatus is very suitable to handle highly radioactive materials, keeping the radiation doses very low for the operator. The separations into several fractions are faster, cleaner, and more reproducible than those obtained in ordinary hand-sieving techniques.

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