Clock reactions are a perennial favorite among chemical demonstrations, with their bright colors and abrupt changes. Unfortunately the number of chemical systems that successfully produce a clock reaction effect is relatively small. The Landolt iodine clock reaction (1), which uses iodate and bi-sulfite with starch is indicator, and its many variations such as the "Old Nassau" (2) clock reaction, which adds mercuric ion to the Landolt formula, are probably the most popular of all clock reactions. The formaldehyde-bisulfite clock reaction (3), which can be used with a variety of pH indicators, is also popular, while other clock reactions are used much less commonly (4). The continued popularity of the Landolt clock stems from its ease of preparation and use of reagents that are generally available to the high school or college teacher.
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机译：时钟反应以其鲜艳的色彩和突变而备受化学演示的青睐。不幸的是，成功产生时钟反应效应的化学系统的数量相对较少。使用碘酸盐和亚硫酸氢盐与淀粉的Landolt碘钟反应（1）是指示剂，它的许多变化，例如“ Old Nassau”（2）钟反应，它在Landolt公式中添加了汞离子。在所有时钟反应中最流行。可以与多种pH指示剂一起使用的甲醛-亚硫酸氢盐时钟反应（3）也很流行，而其他时钟反应则很少使用（4）。 Landolt时钟的持续流行源于其易于制备和使用通常适用于高中或大学教师的试剂。
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