USP〈1331〉 USE AND CALIBRATION OF VOLUMETRIC APPARATUS

INTRODUCTION

In any laboratory, it is critical to ensure that volumetric apparatus used to perform analytical tests meet the laboratory’s requirements and comply with the relevant standard specifications over the time of use. Ensuring accuracy and precision will lead to a high level of confidence in measurement results obtained during use. This chapter also describes operation of volumetric apparatus. Failure to ensure accuracy and precision will impact the measurement uncertainty and reliability during use.

Calibration is a critical activity to be carried out prior to placing any type of volumetric apparatus into service. Therefore, this chapter provides guidelines on validating the original calibration of volumetric apparatus described in Volumetric Apparatus 〈31〉 and for periodic recalibrations as needed (1–4). For additional information about the relevance of cleanliness, see Cleaning Glass Apparatus 〈1051〉.

USER CONSIDERATIONS

Volumetric apparatus must be selected and used according to the desired limits of error and the manufacturer specifications, including accuracy class (1–2,4). Volumetric apparatus are available that are calibrated on a “to contain” or “to deliver” basis. In general, use of an apparatus should be consistent with the basis of calibration—apparatus calibrated to contain, should not be used to deliver, and apparatus calibrated to deliver should not be used to contain. In special cases (such as viscous syrups) where delivery from a “to contain” flask or transfer pipette is required, the “to contain” apparatus should be rinsed clean after draining, and the rinsed portions should be included in the quantitative transfer.

Temperature is an important factor when making accurate volume measurements because temperature impacts the behavior of the volumetric apparatus and also causes the liquid being measured to expand or contract (i.e., changes liquid density).

Volumetric apparatus made of single material (such as volumetric flasks or transfer pipettes) have a cubic thermal expansion coefficient that is knowable, and calibrations of these apparatus should be corrected to a stated reference temperature as shown in Equation 1. When used at a temperature that differs from the reference temperature, the user is able to apply a correction based on the temperature difference and the known expansion coefficient.

For apparatus made of multiple materials (such as a glass syringe with a metal plunger) or having a captive air volume (such as air-displacement pipettes), the thermal behavior of the apparatus can be complex and might not be easily corrected. When thermal behavior is unknown or correction is not possible, the potential impact should be evaluated and appropriate actions should be taken (e.g., calibration).

Thermal expansion of the liquid being measured should also be considered—an aqueous solution containing 1 g/L of solute at 25° C will contract on cooling and become a solution of greater than 1 g/L of solute at 20° C. When solutions are prepared, the temperature at the time of preparation should be recorded (e.g., if the liquid is equilibrated to the laboratory temperature, the temperature is adequate).

When mixing dissimilar liquids, ideal mixing (in the thermodynamic sense) should not be assumed—mixing 100 mL of ethanol into 900 mL of water does not result in 1000 mL of final solution. Careful attention is required when writing and following procedures so that the intended volume is described and measured.

Manufacturer information and documentary standards contain information regarding proper operator techniques to be used for calibration. To obtain the best results, a laboratory user should employ the same techniques as used during calibration. The calibration standards listed in Table 2 include recommended operating procedures, as well as recommendations for the training and evaluation of pipette operators (5).

Source form USP

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