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

3. General laboratory procedures 73Cleaning and maintenanceFor details of cleaning and maintenance of centrifuges, see section 3.5.3.3.4 Measurement and dispensing of liquidsMany of the liquids handled in the laboratory are either infectious, corrosive orpoisonous. It is important for the prevention of accidents that the correct proceduresfor the measurement and dispensing of these liquids are clearly understoodand are followed conscientiously.Many of the new procedures for analysis require very small volumes of fluid andvarious pipetting and dispensing devices are available to enable small volumes tobe 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. Avolumetric flask measures a single volume of fluid, e.g. 1 litre, accurately.Small volumes of fluid (0.1–10ml) can be dispensed rapidly and accurately usingone of the following methods:● A fixed or variable volume dispenser attached to a reservoir made of glass orpolypropylene. Various volumes from 0.1 to 1.0ml and from 2.0 to 10.0ml canbe dispensed.● A calibrated pipette with a rubber safety bulb.3.4.1 PipettesTypes of pipetteGraduated pipettesGraduated 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 measuredis contained between the 0 mark and the tip.● A pipette with graduations not extending to the tip (B). The total volume is containedbetween the 0 mark and the last mark before the tip (this type is recommendedfor quantitative chemical tests).Various volumes can be measured using graduated pipettes. For example:— a 10-ml pipette can be used to measure 8.5ml;— a 5-ml pipette can be used to measure 3.2ml;— a 1-ml pipette can be used to measure 0.6ml.Fig. 3.44 A graduatedpipetteVolumetric pipettesVolumetric pipettes are intended to measure a precise volume with a high degreeof 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 themark. 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 isexpressed against the side of the recipient container. It should not be expelled.Fig. 3.45 Types of graduatedpipetteA: pipette withgraduations to the tip;B: pipette withgraduations notextending to the tip.