Canister and contents.
The canister contained 1140 mg of a mixture of two solid substances, shown above. One of the solids consists of small, clear, colourless solid spherical particles that to the eye appear to be white in colour; the other consists of irregularly sized and shaped chunky black particles. The canister has a snap-off re-closable lid, which is perforated with a regular grid of holes each about 0.185 mm square (photomicrograph left below).
Initially the canister was at the bottom of the tablet bottle (Graphic 1 right above) but as tablets were removed, it eventually moved to the top and fell out when tablets were removed from the bottle by tipping it sideways.
- What is the nature of the clear, colourless solid spherical particles? For what purpose is it generally used? For what specific purpose is it present in the ASA tablet bottle?
- What is the nature of the irregularly sized and shaped chunky black particles? For what purposes is it used? For what specific purpose is it present in the ASA tablet bottle?
This topic was researched in 2007, and was brought back to mind by a friend who began taking a daily low-dose ASA tablet. She had difficulty with the safety cap on the ASA bottle, so she transferred the contents of her first bottle to a bottle with a simple screw cap. In doing so the canister was no longer at the bottom of the bottle as in Graphic 1 but at the top of the bottle as in Graphic 2 (above). As soon as she began to take out tablets the canister fell out almost every time she tipped the bottle sideways. She was not happy having to repeatedly handle the canister to replace it in the bottle, due to the likelihood of microbial contamination.
When my friend consulted me about the canister, I explained to her why the canister was put into the bottle, and why, since the canister was now at the top of the bottle, it was falling out. So she posed this question to me: Should the canister be kept in the bottle? Answer: Yes. I was then asked if there was a way to put the canister back at the bottom of the bottle, without taking anything out, or putting anything into the bottle, or handling the tablets? After a bit of thought, Answer: Yes. It did not take long; I did it by applying an odd physical phenomenon with a strange ‘common’ name that is very important to a surprising number of industries. But perhaps there are other ways to do this.
Describe how to move the canister from the top to the bottom of the bottle below the tablets without opening the bottle. If appropriate, name and describe the phenomenon applied.
Go to page 14 for the answers to the quiz questions and the challenge along with a discussion.
Answers to the ASA tablet bottle quiz and challenge
The canister from the ASA bottle in 2007 bore the legend with the initials S-CPP which stands for the Süd-Chemie Corporation - Performance Packaging Division.
The two substances in the canister are silica gel1 and activated carbon.1 Dried silica gel is primarily used as a drying agent. Activated carbon, also known as activated charcoal has catalytic uses. But as a fine powder it is used to absorb and remove organics from water, and as large chunks it is used to absorb and remove organic vapours from the air. My article published in the Crucible Online, Science Teachers’ Association of Ontario, September 2007, summarizes the production, uses and purchase availability of these substances and describes a few experiments that are possible with them. Send me an email to get a copy. These substances are safe to handle with gloves and are very useful when discussing diffusion of molecules in the gas and liquid states.
Silica gel and activated carbon are placed in the ASA bottle because acetylsalicylic acid (‘aspirin’)1 is not chemically inert. If exposed to moisture, over time it breaks down to form salicylic acid and acetic acid. Salicylic acid is the actual active agent of the pharmaceutical, but it is far more toxic and corrosive than ASA.
acetylsalicylic acid + H2O → salicylic acid + acetic acid
A sealed bottle of ASA tablets that is kept past its expiry date may have an odour of acetic acid (vinegar) when opened. The breakdown of ASA is much more rapid in the presence of acid or base, and at a higher temperature. The silica gel prevents the breakdown by absorbing water molecules, and the activated carbon adsorbs any acetic acid molecules that may have been produced.
The phenomenon I employed to move the canister is ‘the brazil nut effect’. Wikipedia calls this ‘granular convection’.1 It is also known as ‘segregation’.2 When a heterogeneously sized collection of solid particles, whether the same substance or several, is shaken in gravity, the big particles move ‘up’ relative to the small particles. You can see this in a can of mixed nuts, a box of mixed cereal, or in a bag of sugar crystals: the larger objects are near the top, the dust is at the bottom.
The sequence of actions required to employ this phenomenon is illustrated by the graphics below, reading from left to right. Cap the bottle firmly (A). Invert the bottle (B). The canister is now at the bottom of the upside down bottle. Shake the bottle up and down and side to side for a minute or two, firmly but not violently. The larger canister will be moving upwards relative to the smaller tablets inside the inverted bottle due to segregation (C). Stop and again invert the bottle. The canister will now be closer to the bottom than at the start (D). This procedure is not damaging to low-dose aspirin tablets, which have a hard enteric coating,1 but may not be advisable for softer more crumbly tablets. The procedure described temporarily cures the problem, but continued tipping of the bottle soon moves the canister to the top again, and it begins to fall out once more.
Segregation is a technical challenge in all industries where blending of solids is required, including the food, chemical, pharmaceutical, polymer and mineral industries. In these industries, ‘industrial mixers and blenders’1 are used to mix or blend a wide range of materials. I have been reading about this topic while researching an article about the variability of active ingredient content of pharmaceutical tablets. Segregation is one of the major factors working against uniform content of the powder blends present in most tablets.
- Wikipedia.org for: silica gel; activated carbon; aspirin (acetylsalicylic acid); granular convection; enteric coating; industrial mixers and blenders.
- Richard Holdich, Fundamentals of Particle Technology, Chapter 12: (free download) http://www.particles.org.uk/particle_technology_book/