Chemistry in your cupboard

Extension - ‘How Science Works’ material

Once the alginate raft has been produced, it will not remain in the stomach indefinitely.

It is largely broken up mechanically by the action of stomach contractions. However, a pharmaceutical chemist might have to consider other possible chemical reasons for its breakdown before being sure that mechanical destruction was its principal fate.

These might include:

• loss of calcium ions that cross link the alginate molecules to form the raft, by absorption of these ions through the stomach wall, for example
• hydrolysis of some of the saccharide bonds that hold the alginate linear polymers together

In fact, neither of these mechanisms is believed to be significant in dispersing the raft in practice.

Experimental work

The alginate raft can be demonstrated simply and effectively as follows. Take a 100 cm³ conical flask with a narrow neck and fill it to the 100 cm³ mark with 0.1 mol dm^-3 hydrochloric acid. The acid will reach almost to the base of the neck of the flask. The body of the flask represents the stomach, the neck of the flask the oesophagus and the acid the stomach contents.

Add to the acid about 10 cm³ of Gaviscon Liquid. This is quite viscous, but the exact amount is not critical. The Gaviscon initially sinks to the bottom of the flask where a gel forms on contact with the acid. Over a period of a minute or two bubbles of carbon dioxide gas can be seen forming on the surface of the Gaviscon and these carry layers of the gel to the surface of the acid where they form a plug in the neck of the flask.

After about five minutes a plug of gelatinous foam will have formed which blocks the neck of the flask and which may rise out of its mouth. At this stage the flask may be inverted (over a sink) and the plug will remain in place holding in the contents of the flask, Figure 8.

Figure 8: Demonstration of how alginate raft works

After the experiments the plug (and the acid) may be washed down the sink with hot water, the plug having been broken up with a test tube brush. With larger quantities, resulting from a class experiment, it may be worth sieving off the gel and disposing of it with the solid waste to avoid the possibility of blocked sinks.

Safety

• Wear eye protection.
• Your school/college should be consulted before carrying out this activity. This activity is covered by model (general) risk assessments widely adopted for use in UK schools such as those provided by CLEAPSS, SSERC, ASE and DfES. Bear in mind, however, that these may need some modification to suit local conditions.