Why are my sections colourless after eosin?
In the same way that the presence of acid can intensify eosin, the presence of alkali will diminish or completely eliminate it. The reasons are much the same, but in reverse. Alkalis cause elevated pH levels, which diminish ionisation of amino groups and inhibit attachment of the dye. The extent to which it does this varies with the degree to which the pH is changed. The most likely source of alkali is the blueing agent. Many laboratories use either dilute ammonia or saturated lithium carbonate to blue alum hematoxylin stains. When these agents are used, it is imperative that the slides be completely immersed in the wash water for long enough to completely remove all traces of the blueing alkali. If even the smallest trace is left, it will dissolve in the eosin solvent and affect the staining. Special attention should be paid to this point, both as regards total immersion of the slides and the length of time washing is done. Remember also that the concentration of alkali is cumulative,
