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在试验中,我们常用KI——淀粉试纸检验溶液中的溶质是否能置换碘(作为氧化还原反应一类滴定的指示剂)或用碘证实淀粉的存在。这是利用了高中化学课本第一册中所说“淀粉溶液遇碘显蓝色”的性质。《有机化学实验》(高等学校教材)也指出:“淀粉在百万分之几时仍能给出碘试验的正性结果”。但在实验中常出现淀粉溶液(包括市售可溶性淀粉)在遇一定量碘时出现颜色后又立即消褪的现象。而且,当再加入较多量的碘时,颜色也不一定显蓝色,有时会出现紫蓝、紫、紫红、赭蓝,甚至是赭色等。这是什么缘故呢?
In the experiment, we used KI, a starch test paper, to test whether the solute in the solution can replace iodine (as an indicator of titration for redox reactions) or confirm the presence of starch with iodine. This is the use of the “starch solution in the case of iodine blue” in the first volume of the high school chemistry textbook. “Organic Chemistry Experiment” (Teaching Materials for Colleges and Universities) also pointed out: “A few millimeters of starch can still give positive results of the iodine test.” However, in the experiment, starch solutions (including commercially available soluble starches) often appear to disappear immediately after a certain amount of iodine. Moreover, when a large amount of iodine is added, the color is not necessarily blue, sometimes purple blue, purple, purple, indigo, and even black. What is the reason for this?