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Purity of water – a consideration of distillation

Published: 29 July 2019 - Sarah Mead

Distillare – to trickle down in minute drops. By Dr J J Lock and Dr J Bates

A little over 1,800 years ago Alexander of Aphrodisias described the process of water distillation as the condensing and collecting of vapour. At that time, it is known that distillation was being used as a method to turn sea water into drinking water. Today, distillation is used for various applications in laboratories who require high purity water, in the form of single distilled or double distilled water.

Water is one of the best solvents known to science and has an almost unique ability to dissolve, to a greater or lesser extent, almost every chemical compound. Even the most “insoluble” materials will, in fact, dissolve to a level of a few micrograms per litre, sufficient to interfere with many laboratory procedures. Water that has not been distilled can be loaded with impurities that are either dissolved or suspended in solution. Common contaminants in tap water fall into seven major categories: suspended solid particles, colloids, dissolved inorganic salts, dissolved organic compounds, microbes, pyrogens, and dissolved gases. These contaminants come from a variety of sources and each causes its own problems in the laboratory (Table 1). The levels of these impurities can vary according to where and when the water is sourced. 

Table 1: Problems encountered when using non-distilled water

 

There are many methods used in the purification of water from potable tap water to a standard suitable for use as a laboratory reagent. Some of the most commonly used technologies are distillation, deionisation, and reverse osmosis. Each of these has different capabilities in terms of which contaminants it will remove effectively (Table 2).

 

Table 2: Systems in relation to impurity removal

 

Why choose distillation?

There are several good reasons why scientists continue to specify water stills when choosing a system to produce general purpose water or to feed a polisher for ultra-pure water production:

Simple – there is little to go wrong with a water still apart from element failure.

Versatile – has the broadest capabilities of any system.

Reliable – less affected by variation in supply than other methods.

Efficient – gives a very good quality product. A double still can produce pyrogen-free water.

Visible – it is easy to see what is going on in a still and to check that everything is working as expected.

Choosing a Water Still

Stuart® is a long-established name in the design and manufacture of water stills having been involved in the development of water purification equipment for more than 50 years. This experience is reflected in a range of water stills, deionisers and filters for laboratory use in schools, universities, hospitals and in industry. The water stills under the Stuart brand include the Merit, Distinction and Aquatron®. These models are versatile and easy to install as they do not require the installation of infrastructure. The glassware of the water stills is constructed from borosilicate rather than Pyrex®, with tests on the distillate universally passing all quality control tests. 

Merit W4000 - A simple glass water still with a chromium plated heating element with built-in twin safety thermostats. It produces 4 litres/h of good quality single distilled water. This water still is manual and economically viable for applications in schools and small laboratories.

Distinction D4000 – An economical borosilicate glass water still with silica sheathed heating element. It produces 4 litres/h of high quality single distilled water and includes an automatic reservoir level control which turns off the heating element and prevents overflow when the collecting reservoir is full. It has two-built-in thermal cut-outs for safety to protect against over-heating in the event of feed water failure or loss of water from the boiler.

Aquatron® range – This is a range of fully-automatic cabinet water stills of all-glass construction with borosilicate glass boiler and condenser, and silica sheathed heating element. Models A4000 and A8000 produce high quality distillate at a rate of 4 and 8 litres/h respectively, while the model A4000D produces 4 litres/h of double distilled water for more demanding applications. The reservoir level control automatically switches off the still when the collection vessel is full and re-starts it again once the distilled water is used. The Aquatron® can also be fed by a pre-treated supply such as a deioniser or RO unit to avoid the need to descale the still and improve the water quality.

Single or double distilled?

The act of distillation leaves these impurities behind as the water boils and turns into steam which is later condensed back to a liquid state. Different levels of purity are required for different purposes and the water used must be determined to be fit for the purpose it is intended. Many applications may require additional treatments such as removal of nucleases for molecular biology applications. A short list of common applications and their associated water standards is given below to give some idea of which type of water may be required by any specific individual (Table 3).

Table 3: Common applications according to level of distillation

To conclude

Water distillation has been a well utilised process of water purification since at least 200 AD. As a method it has advantages over other techniques, including a capacity to remove impurities that cannot be achieved through deionisation and reverse osmosis. It is generally simple, reliable and efficient, and as exemplified by the Stuart range can be undertaken with a variety of water stills depending on the level of purity, automation, and volume required by an individual.

 



 
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