Comparing light- and heavy-duty detergents

There are essentially two types of detergent for washing clothes – light-duty for finer fabrics, and heavy-duty for more robust fabrics subject to heavier soiling. One important difference between these is the pH they produce in the washing water – light-duty detergents are weakly alkaline, while heavy-duty detergents are moderately alkaline.

In this practical, students investigate the differing effects light- and heavy-duty detergents have on soiled fabrics using standardised washing conditions.

Finer fabrics may use dyes which are stable when washed with light-duty detergents, but may be badly affected by the use of heavy-duty detergents. This can be investigated using samples of dyed woollen cloth.

This set of experiments may be organised as an enquiry into why detergent manufacturers insist that you should use different types of detergent for different kinds of fabric. The series of experiments involved may need 30–60 minutes, depending on the range of experiments carried out by each working group. Since the method will be essentially the same, no matter what the group is setting out to investigate, it is reasonable to allocate different tasks to different working groups as a way to economise on time.

Equipment

Apparatus

• Eye protection
• Beakers, 1 dm³, x2
• Stirrers (see note 4 below)
• Tongs or a glass stirring rod
• Bunsen burner
• Tripod and gauze
• Thermometer, 0 - 110°C (see note 5)
• Heat resistant mat or Stirrer-hotplate
• Squares of standardised soiled cloth, x2 (see note 6)
• Full-range indicator paper strips, x2

For extension work on dyes

• Dyed wool samples (see notes 7 and 8)

Chemicals

• Detergent solution, light-duty, 250 cm³ (see note 3 below)
• Detergent solution, heavy-duty, 250 cm³ (see note 3)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout. Do not sit down when heating and stirring hot detergent solutions. 
• Detergent solution – modern detergents come in a bewildering variety of formulations. For this experiment, the suggested light-duty detergent is Dreft powder, and the suggested heavy-duty detergent is Daz powder. Liquid or tablet versions, and other makes, can be used but some research may be needed to establish which category they fit into and whether the different formulations makes them appropriate for this experiment. The two solutions should be made up in bulk from powder at a concentration of 0.25% w/v (25 g per 10 dm³).
• The washing trials in the beakers need controlled agitation, and should be the same for each beaker. If resources allow, this can be done using stirrer hotplates to heat and stir using a magnetic follower. More likely, this will have to be done using two glass stirring rods, one for each beaker, for each group. 
• The thermometer may be liquid-in-glass or electronic, but must be capable of reading safely up to 110 °C.
• Standardised soiled cloth samples may be prepared by taking a large piece of clean white cotton cloth (for example, an old sheet or shirt) and using a roller to apply a thin, even coating of a mixture of lampblack (fine carbon powder) and olive oil. Squares of this soiled cloth, 5 x 5 cm, may then be cut for the experiment.
• Wool samples dyed with non-fast dyes are needed. Old woollen clothes, already washed repeatedly, are unlikely to show much difference on washing in the two detergents. New woollen cloth in a strong colour may be available, though again may not show much change if dyed with a fast dye. Trials may need to be done to find samples that will fade noticeably on washing in a heavy-duty detergent. One possible source may be remnants of balls of strongly coloured knitting wool. Trial runs will be necessary for dye-fastness experiments to ensure a reasonable difference in outcome can be seen. A longer wash time may be needed.
• It may be advisable to carry out trial runs before the lesson to establish whether a four minute wash is sufficient to show a significant difference between the two detergents. If it is not, try extending the wash times by one minute intervals until the difference is clearly seen. This will depend on how heavily soiled the cloth was made initially.

Procedure

1. Pour about 250 cm³ of each detergent solution into separate 1 dm³ beakers. Using the strips of indicator paper, measure and record the pH of each solution.
2. Warm one solution to 50–60 °C over a low Bunsen flame, or on stirrer-hotplate.
3. Drop a square of soiled cloth into the beaker, noting the time.
4. Stir the contents gently and steadily, establishing a technique that can be repeated similarly with the second beaker. If using a glass rod, use a to-and-fro motion.
5. After 4 minutes, remove the cloth from the beaker using tongs or a glass rod and allow to drain.
6. When both samples have been washed, compare the results.
7. If testing the colour-fastness of wool dyes, repeat 1–6. with fresh samples of detergent solution in clean beakers.

Teaching notes

The differences between light-duty and heavy-duty detergents depend on the presence in the latter of alkali metal phosphates, which raise the pH of the washing water to a value around 10 (only pH 8 for light-duty). They may also contain oxidising agents.

Preliminary trials may be needed before the lesson to establish the best conditions, including timing, to produce clear differences between the washed soiled samples, and, if appropriate, the dyed wool samples.

It used to be possible to obtain dyed wool samples in the past from the dye manufacturers. Since most of these dye manufacturers have changed their owners or even gone out of business, it is not possible to recommend a supplier or a particular dye. If samples of lissamine green (also used as a food colorant, E142) or lissamine blue can be obtained, home-dyed wool samples can be produced that will show clear differences.