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

76 Manual of basic techniques for a health laboratoryThey have various capacities:— 2000ml— 1000ml— 500ml— 250ml— 200ml— 100ml— 50ml— 25ml.Volumetric flasks are more accurate than measuring cylinders. They should be usedfor the preparation of reagents.Fig. 3.51 Preparing sodium chloride solutionin a volumetric flaskFig. 3.52 Alternative method for preparingreagents using a volumetric flaskFor example: 1 litre of sodium chloride, 0.85% solution (reagent no. 53), is preparedby washing 8.5g of sodium chloride, dissolved in 100ml of distilled water ina 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 solutionpoured 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 liquidThe temperature at which liquids should be measured is etched on the flask (afterthe capacity figure; Fig. 3.53).Liquids expand with heat and contract with cold. Never measure hot liquids, orcold liquids just taken from the refrigerator.Fig. 3.53 Mark the temperatureat which thereagent should bemeasured on theflaskStoppersVolumetric flasks should have plastic stoppers; if these are not available use groundglass ones. Be careful not to lose them.CostVolumetric flasks are very expensive, so use them with great care.