Cross-linking polymers | alginate worms

This practical provides a fun look at cross-linking and a chance to explore some of the many and varied uses of sodium alginate. Sodium alginate is a polysaccharide of repeating monosaccharide units, each containing a carboxylate ion:

The naturally occurring form of this polymer is alginic acid (the protonated form), which can be extracted from brown seaweed and kelp. It usually has a molar mass of around 240 000. In this experiment, the sodium ions of sodium alginate are replaced with calcium ions. Since each calcium ion can bond to two carboxylate groups, the ions can cross-link the polymer chains, which results in the formation of an insoluble, gel-like substance.

If the cross-linked alginate is placed in a solution of sodium ions, these replace the calcium ions and the gel-like worms fall apart again.

Equipment required

Per pair or group of students:

• 5 cm³ sodium alginate suspension – see notes
• Dropping pipette
• 2 x 150 cm³ beakers
• 100 cm³ saturated sodium chloride solution – made in distilled water
• 100 cm³ calcium chloride solution – 1 g calcium chloride per 100 cm³ distilled water
• Labels for the beakers or pens to label the glass
• Eye protection.

Health, safety and technical notes

• Read our standard health and safety guidance 
• Solid calcium chloride is an irritant. A solution of the suggested concentration is not an irritant.
• Wear eye protection.

Notes

To make the alginate suspension, add 2 g sodium alginate to 100 cm³ distilled water.

Do not use tap water, especially in a hard water area.

It is best to make the suspension at least a day in advance and allow it to stand overnight so that it becomes homogeneous.

For this experiment, the results are clearer if a few drops of food colouring are added. This can either be done when making the suspension or students could do it themselves and perhaps be offered a choice of colours.

Gaviscon® can be used as an alternative to sodium alginate. This will give coloured and opaque worms as a result of the presence of other ingredients in the antacid, which can be quite fun. Any leftover Gaviscon® cannot be used medicinally.

It is also possible to cross-link alginate with other ions. Both nickel(II) chloride and copper(II) chloride can be used.

Nickel gives pale green and copper pale blue worms. These worms do not break down in the presence of sodium chloride and so are not suitable for this experiment but could be used to create a colourful demonstration. If you carry out such a demonstration, be aware that nickel salts are sensitisers.

Avoid skin contact and wash your hands after use. If the ‘worms’ are not removed from the calcium chloride straight away they can take a long time to break down in the sodium chloride. If left in the calcium chloride, the worms feel very different – they are much harder.

The Gaviscon® worms break down faster in the sodium chloride solution than the sodium alginate ones. It is also possible to try to make worms in sodium chloride solution. The Gaviscon® will form rather pathetic worms which disintegrate in time.

The sodium alginate does not form worms at all if squirted straight into sodium chloride.

Disposal

The solutions can be filtered or strained, and the worms put in the rubbish bin. The easiest way to do this is with a tea strainer.

What to do

• Put the calcium chloride solution into one of the beakers and the sodium chloride solution into the other. Label the beakers clearly.
• Using the pipette, squirt the sodium alginate or Gaviscon® into the calcium chloride solution. You are aiming to make ‘worms,’ although you can make beads if you prefer.
• Remove a few of your worms straight away and put them into the beaker of sodium chloride solution.
• Swirl both beakers gently and observe what happens to the worms in each one.
• You can remove and squeeze the worms as well as observing their appearance. You will need to wait a few minutes for all the reactions to be complete.