Manual of basic techniques for a health laboratory - libdoc.who.int

24 Manual of basic techniques for a health laboratory2.4.1 Clean waterTo check whether the water supply is clean, fill a bottle with water and let it standfor 3 hours. Examine the bottom of the bottle. If there is a deposit, the water needsto be filtered.FilteringUsing a porous unglazed porcelain or sintered glass filterThis type of filter can be attached to a tap. Alternatively, it canbe kept immersed in a container of the water to be filtered (Fig.2.28).Important: Filters of this type must be dismantled once a monthand washed in boiling filtered water.Fig. 2.28 Filtering water using a porous unglazedporcelain or sintered glass filterUsing a sand filterA sand filter can be made in the laboratory. You will need thefollowing (see Fig. 2.29):— a filter reservoir (a large container such as a metal drum, abig earthenware pot or a perforated bucket)— sand (S)— gravel (G).Note: Water that has been filtered through a sand filter is almostfree of particles, but it may contain water-soluble chemical compoundsand bacteria.Storage of waterIf water is scarce or comes from a tank or well, always keep alarge supply in reserve, preferably in glass or plastic containers.Decant water that has been stored before filtering it.Fig. 2.29 Filtering water using a sand filterG: gravel; S: sand.Water supplyIf there is no running water in the laboratory, set up a distributoras follows (see Fig. 2.30):1. Place the container of water on a high shelf.2. Attach a length of rubber tubing to the container so that thewater can flow down.3. Clamp the rubber tubing with a Mohr clip or a small screwclamp.2.4.2 Distilled waterDistilled water is free from nonvolatile compounds (e.g. minerals)but it may contain volatile organic compounds.Fig. 2.30 A water distributorPreparationDistilled water is prepared using a still, in which ordinary wateris heated to boiling point, and the steam produced is cooled as itpasses through a cooling tube where it condenses to form distilledwater.