Die vielen stabilen leicht verfügbaren, günstigen synthetischen blauen Pigmente, die seit dem Anfang des 18. und vor allem am Anfang des 19. Jahrhunderts hergestellt wurden, haben die notwendigen Voraussetzungen und Chancengleichheit in der Malerei geschaffen und den Impressionismus mit dem ganzen Farbspektrum und all seinen Nuancen erst ermöglicht.
The reaction of sodium with water is a spectacular and essential classroom demonstration. Many teachers want to show also the more violent reaction of potassium. We propose not to do so because explosions can happen even before the metal is in contact with water.In March 2006 a 62-year-old experienced chemistry teacher wanted to demonstrate the properties of potassium to his class at the secondary school of Goldach, Switzerland. The metal was stored under petroleum in a glass bottle. When he tried to take out a small piece with a knife this piece exploded and burning petroleum splashed on the teacher's chest. The bottle and the potassium in it remained intact. The teacher suffered from severe burns but none of the students were injured.As a consequence, a total ban of the use of potassium at the secondary schools of the Canton of St. Gallen was proposed by the chemistry professors of the School for Teacher Education. The chemistry teachers were urgently asked to dispose of all potassium stocks at their schools and not to use this chemical again.Numerous warnings about the dangers of potassium metal can be found in the literature, not only with regard of its reaction with water. The possible formation of oxide crusts is described in various places. Bretherick's Handbook of Reactive Chemical Hazards (1) and a recent article by DeLaHunt and Lindeman (2) lists potassium dioxide KO 2 (or superoxide), potassium peroxide K 2 O 2 , and potassium trioxide KO 3 as hazardous compounds. They are formed on exposure of the metal to air, during storage under petroleum, and even during storage under vacuum in polyethylene bags (3). They can react with the combustible storage solvent and with potassium metal itself. The contact of the strong oxidants with the strong reducer potassium, occurring at the moment of picking up a piece with tweezers or cutting it with a knife, can lead to heavy explosions.We strongly advise not to remove any crusts manually but to dispose of crusted samples. Potassium with crusts should not be used for classroom experiments. The reaction of alkali metals with water can be demonstrated with the less dangerous lithium and sodium just as well. Sodium does not develop peroxides spontaneously.Video demonstrations of the reaction of potassium (and other elements) with water are very instructive. They can be found in the CD-ROM "Periodic Table Live!" produced by JCE (4). Literature Cited (a) Urben, P. G. Bretherick's Handbook of Reactive ChemicalHazards, 7th ed.; Elsevier:
The biogenic substance E-indigo can form supramolecular, hydrophobic structures using self-organization. These structures show a low coefficient of friction as a gliding layer against polar surfaces. The formation of primary particles with platelet morphology based on hydrogen-bonded E-indigo molecules is ideal to produce the gliding layer. Structures with excellent gliding properties can be achieved by means of directed friction and high pressure, as well as through tempering. The resulting hard, thin gliding layer of E-indigo does not easily absorb dirt and, thus, prevents a rapid increase in friction. Field tests on snow, with cross-country skis, have shown promising results in comparison to fluorinated and non-fluorinated waxes. Based on quantitative structure–activity relationship (QSAR) data for E-indigo, and its isomers and tautomers, it has been demonstrated that both the application and abrasion of the thin indigo layers are harmless to health, and are ecologically benign and, therefore, sustainable.
Eine populäre Theorie vom “Blaumachen” am Montag im Zusammenhang mit der Küpenfärberei von Indigo scheint ein Irrtum zu sein. Der Montag war keineswegs ein bequemer Tag für die Blaufärber.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.