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Equipment

Water Still

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Fig 1: Merit W4000 still
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Summary

An electrically heated unit for making distilled (deionised) water.

Operation

Water stills are available through regular lab supply companies and vary considerably in both price and output.

The output rate of the unit is usually expressed in litres per hour and the heating element in watts or kilowatts (1000 watts). Most run on standard mains electricity and need a steady flow of cold water and open drain for excess water flow.

The ways in which water stills work are generally very similar. Water flows into a tank where it is heated by an electrical element. As the water reaches boiling point, steam is produced which rises into another chamber and is cooled by tubes carrying cold running water. The steam condenses and drips into a collection vessel.

Most modern stills aimed at school use are pretty much self contained units designed for moderate usage. Budget units start at around £400-£500. These units are well suited for school use providing around four litres per hour. Higher output units providing around eight litres per hour will cost from £1000-£2000.

Over time, limescale may be found inside the unit, especially in chalky areas. This can be cleaned by using dilute hydrochloric acid although refer to the unit manual before cleaning the unit with acid as sensitive parts may be damaged. Many units can be taken apart easily and running the acid through the glass parts usually remove any scale problems. Soaking the glass tubing in an acid bath for a few hours can remove heavy scale.

Most water still manuals contain instructions on cleaning and repairing the unit. Heating elements can fail over time and can be replaced at a cost from manufacturers. Always check the warranty before replacing any part and do not undertake any electrical repairs unless you are competent to do so.

As these units can sit in the preproom for many years without maintenance, check any tubing for cracks and electrical cable for wear and tear every six months or so.

Safety

 CautionThe contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.