For many years, a type of goldenrod-colored paper has been used as an indicator to demonstrate acid–base behavior, turning yellowish in acidic conditions and reddish in basic conditions. This pH sensitive color change has been the basis of demonstrations involving making hidden messages and pictures using techniques such as printing (e.g., to make a “bloody” red handprint), spraying, wax resist, and electrolytic processes. Candidate dye compounds for the goldenrod paper that feature a pH sensitive yellow/red color change include Direct Yellow 4 and curcumin. Direct Yellow 4, curcumin, and the color-changing goldenrod paper also all exhibit additional changes to darker colors in very acidic solutions (pH < 0). This paper provides evidence that Direct Yellow 4 (or a closely related compound) and not curcumin is the dye present in the color-changing goldenrod paper. These color changes of the Direct Yellow 4 in the goldenrod paper, in conjunction with the fact that paper sheets can display two-dimensional images or shaped into three-dimensional objects, lead to novel ways to demonstrate acid–base and other chemical concepts to audiences. For example, paper flowers made from the goldenrod paper will change from yellow to red in basic solutions, to black in very acidic solutions, to off-white in sodium hypochlorite solutions, and to purple-black in iodine solutions.

中文翻译：

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基于变色金杆纸的演示扩展

多年来，一种金黄色彩色纸已被用作指示剂，以证明其酸碱行为，在酸性条件下呈黄色，而在碱性条件下呈红色。这种对pH敏感的颜色变化已成为演示的基础，该演示涉及使用诸如印刷（例如，制作“红色”手印），喷涂，防蜡和电解工艺等技术来制作隐藏的消息和图片。带有pH敏感的黄/红颜色变化的菊科植物用候选染料化合物包括直接黄4和姜黄素。直接黄4，姜黄素和变色金杆纸在非常酸性的溶液（pH <0）中也都显示出较深颜色的其他变化。本文提供了证据，直接黄4（或密切相关的化合物）而不是姜黄素是存在于变色金杆纸中的染料。菊科植物纸中直接黄4的这些颜色变化，以及纸张可以显示二维图像或成型为三维物体的事实，导致向观众展示酸碱和其他化学概念的新颖方法。例如，由菊科植物纸制成的纸花在碱性溶液中将从黄色变为红色，在强酸性溶液中变为黑色，在次氯酸钠溶液中变为灰白色，在碘溶液中变为紫色-黑色。结合纸张可以显示二维图像或成形为三维物体这一事实，导致了向观众展示酸碱和其他化学概念的新颖方法。例如，由菊科植物纸制成的纸花在碱性溶液中将从黄色变为红色，在强酸性溶液中变为黑色，在次氯酸钠溶液中变为灰白色，在碘溶液中变为紫色-黑色。结合纸张可以显示二维图像或成形为三维物体这一事实，导致了向观众展示酸碱和其他化学概念的新颖方法。例如，由菊科植物纸制成的纸花在碱性溶液中将从黄色变为红色，在强酸性溶液中变为黑色，在次氯酸钠溶液中变为灰白色，在碘溶液中变为紫色-黑色。