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WATER STILLS AND DEIONIZERS

Water for laboratory use must be free from contamination’s that could interfere with the performance of the test.  The quality of water therefore depends on the type of the work to be carried out. Water is required for the preparation of   

1. Reagents and stains.
2. For cleaning and disposal.
3. For preparation of culture media.

The following methods can be used to obtain water of adequate purity and quantity:

1. Distillation using water stills
2. Filtration using gravity filters
3. Deionization  using  mixed bed ion- exchange  resin

Distilled or deionized water is recommended for analytical work

(a)  Distillation

In this method impure water is boiled  and steam produced is condensed on a cold surface  (condenser) to give chemically pure  distilled water  from which volatile organic and inorganic matter are removed .  Distillation however does not remove dissolved gases e.g. NH₃ , CO₂ and Cl.  

It also uses a lot of electricity and too much water goes to waste in the process of cooling the distillate, the condensed water is collected when it is hot and will require cooling before it is used for analytical work. Distilled water should be clear, colorless and odorless.

Water Still

Distillation process is carried out in water still. The condenser of the still should be preferably made of pure fussed quartz.

A considerable volume of water is required to operate a still; the water feeding the still should not be heavily contaminated. If really pure water is desired, then it may be advisable to distill the water for about three times.

In regions where there is hard water, the still elements and its walls acquire a scale and for effective distillation to take place, these formed scales on the walls must be removed regularly by using acids  to dissolve it .

A wide range of stills are available, the automatically operated ones are the most expensive ones and are capable of producing highly purified water for research and specialized laboratory work i.e. grade 1 and grade 2 water quality,

Use and Care of Stills

1. Read and follow the manufacturer’s instructions carefully.
2. Ensure there is a sufficient supply of cool running water to feed the condenser.
3. Do not allow the boiler to run dry
4. Collect the distillate in a clean plastic or glass.
5. Avoid storing the distillate in for more than few days, always keep the container capped.
6. Clean regularly the still, descale it in case you always hard water.
7. NB:  When buying a still, select one that can be easily cleaned without being dismantled.

(b) Deionization

In this process, impure water is passed through an ion exchange resin to produce ion free water. Deionized water is more purer than distilledwater and  it is sometimes called conductivity water because  it have such a   low electric  conductivity and is most suitable in conductivity measurements ,  it have almost neutral pH and is free from water soluble salts  ,  and  sterile.  It may however have some non-electrolyte contaminants and some organic extracts from the resins.  

Ion -Exchange Resins

Deionization process s faster and a lot of water is produced within a short period, deionizers are portable and cheap to operate. However, the water may not be sterile and not all non-electrolytes are removed.

De-ionizing resins are cost effectively used if already filtered water is used. The use of unfiltered water will lead to rapid contamination and exhaustion of the resin.

Resin beds may also contain both cation and anion resins and an indicator that changes color as the resin becomes exhausted. The filtered water trickles through the resin, is deionized, and passes through the small holes in the bottom of the tube into a collecting container.

Ion – exchange resins are of two types:

 (i) Cation – exchange resin  

(ii) Anion – exchange resin.

Maintenance of deionizer resins

1. Keep the deionizer tube out of sunlight to avoid deionization of  the indicator
2. When not in use , remove the deionizer tube  and store in dark place , these will protect the indicator  and also minimize contamination of the resin column
3. When first used after storage, discard the water passing through the column for the first five minutes.
4. Keep un- used resin in an opaque sealed airtight plastic bag or tin.

NB: Deionizer resin can cause irritation if allowed to enter the eyes or skin. It is therefore advisable to wear protective clothing e.g. goggles when filling the plastic tubes.  NB; Distilled water is free of inorganic materials, suspended impurities, and most organic contaminants.  Deionized water is free of inorganic materials and most suspended contaminants.