Measurement and dispensing of liquids

Many of the liquids handled in the laboratory are either infectious, corrosive or poisonous. It is important for the prevention of accidents that the correct procedures for the measurement and dispensing of these liquids are clearly under- stood and are followed conscientiously.

Many of the new procedures for analysis require very small volumes of fluid and various pipetting and dispensing devices are available to enable small volumes to be measured with great precision.

Large volumes can be measured using a measuring cylinder or a volumetric flask. A measuring cylinder measures various volumes of fluid but is not very accurate. A volumetric flask measures a single volume of fluid, e.g. 1 litre, accurately.

Small volumes of fluid (0.1–10 ml) can be dispensed rapidly and accurately using one of the following methods:

● A fixed or variable volume dispenser attached to a reservoir made of glass or polypropylene. Various volumes from 0.1 to 1.0 ml and from 2.0 to 10.0 ml can be dispensed.

● A calibrated pipette with a rubber safety bulb.

1. Pipettes

Types of pipette

Graduated pipettes

Graduated pipettes have the following information marked at the top (Fig. 3.44):

— the total volume that can be measured;

— the volume between two consecutive graduation marks. There are two types of graduated pipette (Fig. 3.45):

● A pipette with graduations to the tip (A). The total volume that can be measured is contained between the 0 mark and the tip.

● A pipette with graduations not extending to the tip (B). The total volume is contained between the 0 mark and the last mark before the tip (this type is re- commended for quantitative chemical tests).

Various volumes can be measured using graduated pipettes. For example:

— a 10-ml pipette can be used to measure 8.5 ml;

— a 5-ml pipette can be used to measure 3.2 ml;

— a 1-ml pipette can be used to measure 0.6 ml.

Fig. 3.44 A graduated pipette

Fig. 3.45 Types of graduated pipette

A: pipette with graduations to the tip; B: pipette with graduations not extending to the tip.

Volumetric pipettes

Volumetric pipettes are intended to measure a precise volume with a high degree of accuracy.

There are two types of volumetric pipette (Fig. 3.46):

● A pipette with a single graduation mark (A), which is intended to be filled to the mark. After discharge of the contents, the pipette is allowed to drain for 15–45 seconds, according to its size (marked on the bulb) and the last drop is expressed against the side of the recipient container. It should not be expelled.

● A pipette with two graduation marks (B) may be more accurate in skilled hands. It is less reliable when used by an inexperienced person because it is easy to overrun the lower graduation mark when discharging the contents

Fig. 3.46 Types of volumetric pipette

A: pipette with a single graduation mark; B: pipette with two graduation marks.

Hold the pipette vertically to check that the liquid reaches the desired graduation mark (G in Fig. 3.47). This mark should be level with the bottom of the meniscus formed by the liquid. The tip of the pipette should be held against the side of the receptacle while the fluid is discharged.

Plastic bulb pipettes

Plastic bulb pipettes are cheap and very useful for transferring volumes of liquid such as serum or disinfectant.They are available with different tips and can be obtained with calibrations marked on the stem.

They can be reused after disinfection and washing but cannot be autoclaved.

Micropipettes

Micropipettes with disposable tips are frequently used to measure small volumes.They are available in a variety of volumes, ranging from 5 ml to 1000 ml. Used tips are disposed of directly into disinfectant using an ejector mechanism.The micropipettes have two positions of the plunger operated by thumb (Fig. 3.48).The first position is used to pick up the sample and the second to expel the sample from the tip into a tube or well.

Fig. 3.48 A micropipette with a disposable tip

T: disposable tip

Micropipettes must be calibrated and maintained according to the instructions of the manufacturer.

Calibrated dropping pipettes

Ordinary calibrated dropping pipettes often deliver 20 drops per ml of distilled water, thus 1 drop = 0.05 ml. Hold the dropping pipette absolutely vertical to expel the drops (Fig. 3.49).

Fig. 3.49 Using a dropping pipette

Calibration of dropping pipettes (Fig. 3.50)

Using a volumetric pipette (see page 74), measure 1 ml of water into a small tube.

Draw the water into the dropping pipette to be calibrated. Count the number of drops delivered from the millilitre of water. Repeat the procedure three times to check the accuracy.

Precautions

Pipetting by mouth is dangerous and should not be done. It can cause the following:

— infection

— burns

— poisoning

— cuts.

Always use a rubber safety bulb (see Fig. 3.50) with the pipette instead.

2. Volumetric flasks

Volumetric flasks are graduated to measure a certain volume when filled to the graduation mark.

They have various capacities:

— 2000 ml

— 1000 ml

— 500 ml

— 250 ml

— 200 ml

— 100 ml

— 50 ml

— 25 ml.

Volumetric flasks are more accurate than measuring cylinders.They should be used for the preparation of reagents.

Fig. 3.51 Preparing sodium chloride solution in a volumetric flask

Fig. 3.52 Alternative method for preparing reagents using a volumetric flask

For example: 1 litre of sodium chloride, 0.85% solution (reagent no. 53), is prepared by washing 8.5 g of sodium chloride, dissolved in 100 ml of distilled water in a beaker, into a 1000-ml flask through a funnel and diluting to the 1000-ml mark (Fig. 3.51). The solution should be shaken before use.

Alternatively, the substance(s) can be dissolved in a small container and the solution poured into the flask along a glass rod (Fig. 3.52). Fill to the graduation mark. (This method is recommended for the preparation of titrated chemical reagents.)

Temperature of the liquid

The temperature at which liquids should be measured is etched on the flask (after the capacity figure; Fig. 3.53).

Liquids expand with heat and contract with cold. Never measure hot liquids, or cold liquids just taken from the refrigerator.

Fig. 3.53 Mark the temper-ature at which the reagent should be measured on the flask

Stoppers

Volumetric flasks should have plastic stoppers; if these are not available use ground glass ones. Be careful not to lose them.

Cost

Volumetric flasks are very expensive, so use them with great care.

3. Burettes

These are graduated glass tubes with a glass stopcock at the lower end. Burettes are filled from the top with the liquid to be measured (Fig. 3.54). They are of 10 ml, 20 ml, 25 ml and 50 ml capacity.

Fig. 3.54 Filling a burette

Maintenance of burettes

The stopcock and tap should be kept well greased. To grease a clean stopcock properly, apply the tiniest smear of petroleum or silicone jelly with a finger tip down the two sides of the stopcock away from the capillary bore. Then insert the stopcock in the burette and rotate it until a smooth covering of the whole stopcock is obtained. Keep the top plugged or covered (Fig. 3.55).