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Media Volume

Media volume is often an area which is overlooked particularly with regard to the volume of the water changing with temperature. The USP states that the volume of the media should be within 1% of the required volume specified within the individual monograph. The USP also states that all volumes are measured at room temperature and not at 37°C.

The most accurate method to measure any volume is by weight. A good laboratory balance will be able to give a reading of 990g ±0.01g. This is far more accurate than using Grade A glassware which typically has an accuracy of ±2.5ml. The media will also expand then it is heated from room temperature to 37°C by approximately 4.5ml (1000ml of water at 20°C is 998.2g, 1000ml at 37°C is 993.2g). If you add these errors together you reach a value of ± 7ml which leaves little room for error. If the media is measured warm there may be problems with re-gassing and the fact that the glassware is calibrated to work at 20°C.

The best way to avoid these errors is to ensure that the media is measured by weight at the test temperature and that the standard is made at the same temperature. Both these liquids are likely to expand or contract at similar weights thus eliminating minimal inherent errors.

Media evaporation is also important. The actual rate of evaporation will be dependant on the temperature and relative humidity of the laboratory. In dry environments the rate of evaporation can be as high as 15ml over two hours. To prevent this, vessel covers should be used during the test and also during the media warming.

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