Water for laboratory use

The medical laboratory needs an adequate water supply for its work. It requires: 

— clean water 

— distilled water 

— demineralized water (if possible) 

— buffered water (if possible). 

1. Clean water 

To check whether the water supply is clean, fill a bottle with water and let it stand for 3 hours. Examine the bottom of the bottle. If there is a deposit, the water needs to be filtered. 

Filtering

Using a porous unglazed porcelain or sintered glass filter

This type of filter can be attached to a tap. Alternatively, it can be kept immersed in a container of the water to be filtered (Fig.2.28).

Important: Filters of this type must be dismantled once a month and washed in boiling filtered water.

Fig. 2.28 Filtering water using  a porous unglazed porcelain or sintered glass  filter

Using a sand filter

A sand filter can be made in the laboratory. You will need the following (see Fig. 2.29):

— a filter reservoir (a large container such as a metal drum, a big earthenware pot or a perforated bucket)

— sand (S)

— gravel (G).

Note: Water that has been filtered through a sand filter is almost free of particles, but it may contain water-soluble chemical com- pounds and bacteria.


Storage of water

If water is scarce or comes from a tank or well, always keep a large 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  filter

G: gravel;  S: sand.

Water supply 

If there is no running water in the laboratory, set up a distributor as 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 the water can flow down. 

3. Clamp the rubber tubing with a Mohr clip or a small screw clamp.

Fig. 2.30 A water distributor

2. Distilled water 

Distilled water is free from nonvolatile compounds (e.g. miner- als) but it may contain volatile organic compounds. 

Preparation 

Distilled water is prepared using a still, in which ordinary water is heated to boiling point, and the steam produced is cooled as it passes through a cooling tube where it condenses to form dis- tilled water.

The following types of still are available:

— copper or stainless steel stills (alembics)

— glass stills

— solar stills.

They are heated by gas, kerosene, electricity or solar energy, depending on the type of still.

Copper or stainless steel alembics (Fig. 2.31)

1. Fill the reservoir (R) with the water to be distilled.

2. Connect the cold-water tube (T) to a tap.

3. Heat the reservoir with a Bunsen burner (B) or kerosene heater.

The still can produce 1 or 2 litres of distilled water per hour, depending on the efficiency of the heating system.

Glass stills (Fig. 2.32) 

Glass stills are more fragile, but almost always produce purer water than metal stills. The distillation method is the same. Make sure that the running water circu- lates freely round the condenser (C). The water can be heated in the flask by the electric element (E).

Fig. 2.33 Components of a solar still

Solar stills (Fig. 2.33) 

For laboratories in remote areas and with limited resources, a simple solar- powered water still can be easily constructed using a clean plastic container with two compartments (one large and one small) and a large surface area, over which is placed a glass cover in a sloping position. 

The water is poured into the large compartment from which it is evaporated by the sun. It condenses on the glass cover and drops into the small compartment. The small compartment has an outlet at the bottom through which the distilled water can pass into a glass bottle placed underneath the container. 

In tropical climates 2–7 litres of distilled water can be produced daily from a solar still with a surface area of 1 m2.

Important:

● Collect the distilled water in a glass or plastic container.

● Do not distil the last quarter of the water heated; it contains residues.

Quality control

The pH of distilled water is normally between 5.0 and 5.5 (i.e. it is acid).

Use a 1.7% solution of silver nitrate (AgNO3) (reagent no. 49) to check for the absence of chloride compounds (e.g. calcium chloride).

Put in a beaker:

— 10 ml of distilled water;

— 2 drops of nitric acid;

— 1 ml of silver nitrate solution.

The water should remain perfectly clear.

If a slight whitish turbidity appears, the distillation process should be repeated.

Uses

Distilled water is used for the preparation of reagents and as a final rinse for some glassware before drying.

Important:

● Do not use commercial distilled water (the type sold for filling car batteries) for the preparation of laboratory reagents.

● Freshly prepared distilled water is preferable; if this is not available, use distilled water stored in glass or plastic containers, which should be washed periodically.

● Always use distilled water prepared the same week.

3. Demineralized water

Principle

Demineralized water is free from ions but not necessarily free from organic com- pounds.

Preparation

Demineralized water is prepared by passing ordinary water through a column of ion-exchange resin. The apparatus consists of a long cartridge filled with ion- exchange resin granules. The water filters through the column of granules, which retain all the mineral ions (i.e. all the dissolved mineral salts). Some demineralizers have two cartridges through which the water passes successively (Fig. 2.34).

1. Check that the cartridge is completely filled with ion-exchange resin granules.

2. Connect the inlet tube of the apparatus to the water supply (a tap or a small tank placed above the apparatus). In some models the water flows in at the top of the column, in others it flows in at the bottom.

3. Let the water flow in slowly.

4. Collect the demineralized water in a closed container.

Fig. 2.34 A demineralizer

Quality control 

Apparatus with a control dial 

The dial registers the resistivity of the water resulting from the presence of ions. The more complete the demineralization, the higher the electrical resistivity of the water. 

1. Check that the control system is fitted with a battery in good working order. 

2. To check that the battery is charged, press the button marked “zero test”; the needle on the dial should swing to zero (Fig. 2.35(a)). 

3. Let water flow into the cartridge. 

4. When demineralized water begins to flow out at the other end, press the button marked “water test”.The needle should register a resistivity of over 2 megaohms/ cm (2 MW/cm) (Fig. 2.35(b)). 

5. If the needle stops at a point below 2 MW/cm or stays at zero, the cartridge of ion-exchange resin granules has been used for too long and must be replaced or reactivated. 

The apparatus may indicate the resistivity (MW/cm) or the reciprocal value, the conductivity (cm/MW or Siemens, S). 

Apparatus without a control dial 

Using an indicator paper, determine: 

— the pH of the water supply flowing into the apparatus, and 

— the pH of the demineralized water that flows out at the other end. 

If the pH remains the same (usually below 6.5), the resin is no longer active. Demineralized water should have a pH between 6.6 and 7.0. 

An additional check can be made using a 1.7% solution of silver nitrate (reagent no. 49). Pass a weak solution of sodium chloride (cooking salt) through the resin, then carry out the test described in section 2.4.2 for the quality control of distilled water. If a slight whitish cloudiness appears, the resin must be replaced.

Fig. 2.35 Measuring the  resistivity of demineralized water

Change of colour in resin 

If the resin changes colour (e.g. it turns black), consult the instructions for use supplied by the manufacturer. 

It may need to be reactivated or replaced, as described below. 

Replacement or reactivation of ion-exchange resin 

This can be done in one of the following ways, depending on the model: 

● The cartridge is replaced by another filled with ion-exchange resin granules. 

● The column of the apparatus is refilled with ion-exchange resin or a mixture of two resins. 

● The exhausted ion-exchange resin is reactivated by passing a solution of ammo- nia through the apparatus. Follow the instructions supplied by the manufac- turer. 

Uses 

Demineralized water can be used for: 

— rinsing glassware before drying; 

— preparing almost all the reagents used in medical laboratories, including stains. 

4. Buffered water 

Distilled water is usually acid and demineralized water becomes acid on exposure to the air. For a number of laboratory procedures (preparation of stains, etc.) the pH of the water has to be around 7.0 (neutral water) and has to be kept neutral. This is achieved, if possible, by dissolving buffer salts in the water (buffered water).

Materials and reagents

● Measuring cylinders, 10 ml and 1000 ml

● Volumetric flask, 1000 ml

● Universal indicator paper (for measuring pH from 1 to 10)

● Indicator paper of limited pH range: for the 5.0 –7.0 range and for the 6.0 – 8.0 range

● Distilled (or demineralized) water

● Acetic acid, 5% solution (reagent no. 1), diluted 1:10 with distilled water

● Disodium hydrogen phosphate (Na2HPO4·2H2O), hydrated

● Phenol red, 1% solution (reagent no. 42)

● Potassium dihydrogen phosphate (KH2PO4), anhydrous

● Sodium carbonate, 0.2% solution (reagent no. 51).

Method

1. Weigh out accurately 3.76 g of disodium hydrogen phosphate.

2. Transfer the chemical to a 1000-ml volumetric flask through a funnel (Fig. 2.36).

3. Rinse out the weighing container into the volumetric flask several times with water. Rinse the funnel into the flask.

4. Weigh out accurately 2.1 g of potassium dihydrogen phosphate and proceed as in steps 2 and 3.

5. Add a little more water and mix the solution until the chemicals are dissolved.

6. Fill the flask to the 1000-ml mark with water.

7. Replace the flask stopper and mix the solution well.

8. Store the solution in a white glass reagent bottle and keep in a refrigerator.

Fig. 2.36 Transferring disodium hydrogen phosphate into  a volumetric flask

9. Dip a strip of the universal indicator paper into the buffer solution and compare the colour obtained with that shown on the standard chart (Fig. 2.37). Read off the pH unit given for the colour that matches the test paper most closely. 

10. According to the result obtained, select a strip of indica- tor paper for the corresponding limited range. For exam- ple, if the pH is 6, use indicator paper for the range 5.0 –7.0. If the pH is 7.5, use indicator paper for the range 6.0–8.0. 11. Repeat the test, using the paper for the corresponding lim- ited range. Read off the pH of the buffer solution on the standard chart.

Fig. 2.37 Checking  the  pH using  universal indicator paper

12. If the pH is between 7.0 and 7.2, the buffered water is satisfactory. If it is below

7.0, the water is acidic. If the water is acidic, make a fresh solution, using dis-

tilled water that has been boiled for 10 minutes in an uncovered round flask

(this gets rid of the carbon dioxide).

13. If the water is still acidic after boiling:

—  add five drops of phenol red solution for every litre of water;

—  neutralize by adding sodium carbonate solution, one drop at a time, until

the water turns pink (Fig. 2.38).

14. If the water is alkaline (pH above 7.2):

—  add five drops of phenol red solution for every litre of water;

—  neutralize by adding acetic acid solution, one drop at a time, until the water turns orange (Fig. 2.39).

Fig. 2.38 Correcting  the  pH of acidic buffered water

Fig. 2.39 Correcting  the  pH of alkaline buffered water

If neither disodium hydrogen phosphate nor potassium dihydrogen phosphate is available, neutralize distilled or demineralized water directly, as shown in steps

12–14 above.

Note: The pH can also be corrected by adding small quantities of the buffer salts:

●   Disodium hydrogen phosphate can be used to increase the pH if the water is acidic (pH below 7.0).

●   Potassium dihydrogen phosphate can be added to reduce the pH if the water is alkaline (pH above 7.2).