Why do marbles become paler on grinding? Reflectance, Spectroscopy, Color, and Particle size.

M. GABRIELA LAGORIO

JOURNAL OF CHEMICAL EDUCATION

American Chemical Society

Lugar: Wisconsin; Año: 2004 vol. 81 p. 1607 - 1611

0021-9584

The observed color of a material is due to interaction between light and matter. This interaction is usually more easily understood for transparent media than for solid opaque media. When dealing with solids some surprising phenomena occur in the observed colors upon grinding, which results for many materials at constant chromophore concentration. If a colored glass marble is ground, it will look paler. The same phenomenon takes place with copper sulfate, CuSO4.5H2O, which appears blue in large crystals and almost white as a powder. Oppositely, some compounds like potassium permanganate, KMnO4, increase their color at very small particle sizes. In this article, the theoretical aspects of the spectroscopy of solids will be presented to explain the physical principles involved in these observations. The concepts presented here may be useful for an advanced course in UV?vis spectroscopy involving solids. In the first part of this article, a qualitative description of the color-change problem is presented to rationalize what happens from a simple physical picture. In the second part of the article the topic is more rigorously presented and the discussion is directed to those who are interested in the quantitative dependence of diffuse reflectance on particle size. Finally color changes in nanoparticles are discussed. 4.5H2O, which appears blue in large crystals and almost white as a powder. Oppositely, some compounds like potassium permanganate, KMnO4, increase their color at very small particle sizes. In this article, the theoretical aspects of the spectroscopy of solids will be presented to explain the physical principles involved in these observations. The concepts presented here may be useful for an advanced course in UV?vis spectroscopy involving solids. In the first part of this article, a qualitative description of the color-change problem is presented to rationalize what happens from a simple physical picture. In the second part of the article the topic is more rigorously presented and the discussion is directed to those who are interested in the quantitative dependence of diffuse reflectance on particle size. Finally color changes in nanoparticles are discussed. 4, increase their color at very small particle sizes. In this article, the theoretical aspects of the spectroscopy of solids will be presented to explain the physical principles involved in these observations. The concepts presented here may be useful for an advanced course in UV?vis spectroscopy involving solids. In the first part of this article, a qualitative description of the color-change problem is presented to rationalize what happens from a simple physical picture. In the second part of the article the topic is more rigorously presented and the discussion is directed to those who are interested in the quantitative dependence of diffuse reflectance on particle size. Finally color changes in nanoparticles are discussed.