Why are some medicinal and acid solution bottles brown in colour?

Bottles (containers) made of ordinary, clear, colourless glass and of plastics (organic glasses) are normally used to hold chemicals (medicinal solutions and reagents) in labs and hospitals. The main reasons for using such containers are chemical inertness, transparency, ease of handling and cleaning, and economical. But in certain cases, the containers will have to be coloured — brown, yellowish-green and black. This is to prevent the action of (Sun) light on the contained chemical which otherwise would be deleterious. Eg. Silver chloride when exposed to sunlight, if a trace of moisture were present, assumes a violet tint and finally turns black (decomposition into its elements). Hydrogen peroxide decomposes when exposed to the rays of light from a mercury lamp.

Certain chemical effects are produced by electromagnetic radiation (visible, ultra violet and the near infra red) of wavelengths within the range, say, 7000 to 1000 Angstrom.

Light may be regarded as waves characterised by their wavelengths (or by their frequency). The Sun is a source of visible light as well as ultra violet light. The addition of such radiant or light energy to a system produces electronically excited molecules that are capable of undergoing chemical reactions.When (Sun) light falls on any body, part of it is reflected, part may be transmitted and part may be absorbed. It is only the absorbed light that is effective in bringing about the above type of chemical action — `Grotthus-Draper' Law.

However, the converse of the law is not true. All absorbed light does not bring about chemical action. For, in such cases, the energy so absorbed, is converted mainly into thermal energy. It, therefore, follows that, wherever the above law is relevant, if the container itself absorbs the incident radiation considerably, the contained chemical will remain unaffected.

This is the principle of the `optical filters'. Therefore, by using coloured glasses — optical filters — , the spectral radiant energy can be altered, modulated and controlled precisely. Thus, the action of light on the contained chemical can be prevented. The colouring agents are selective. Thus, ferrous oxide imparts a yellowish green colour — strong infrared absorption; manganese dioxide imparts brown and violet red colour — ultraviolet absorption; iron, copper, chromium, manganese oxides give a black glass — light absorption.

Source : The Hindu