Volume measurement automatic pipettes

Figure 12.6 illustrates the outline of such an analysis. An automatic pipette extracts a preset volume of the liquid sample (or solution) from a cup presented to it on a turntable. The measured sample is mixed with the reagents in the appropriate proportions, and propelled through the instrument by the peristaltic proportioning pump. This pump operates by means of moving bars, attached to a chain drive, which sequentially compress the plastic sample and reagent tubes to drive the liquids forward through the instrument. The incorporation of a succession of air bubbles at... 

Figure 12.7 illustrates the outline of such an analysis. An automatic pipette extracts a preset volume of the liquid sample (or solution) from a cup presented to it on a turntable. The measured sample is mixed with the reagents in the... 

Volume measuring means the exact determination of a defined volume of a liquid (or of a powder mixture in the case of preparing capsules). Devices for measuring of volumes for pharmacy preparations include graduated pipettes (traditional or automatic), syringes and graduated cylinders. Beakers, Erlenmeyer flasks and medicine bottles are not fit for volume measurement, even if they are... 

The imprecision of the read-out can be made as small as possible by choosing a graduated pipette, syringe or measuring cylinder with a nominal capacity as near to the volume to be measured as possible. For example a volume of 0.5 mL should be measured by a 0.5 mL automatic pipette or with a 1.0 mL traditional graduated pipette. A volume of 21 mL should preferably be measured with a 25 mL graduated pipette or a measuring cylinder of a nominal volume of 25 mL. 

First we need to examine the assertion that in analytieal ealibration the concentrations are effectively error-free while the responses sueh as measurement results include error. While this is widely true, in some instanees it is at least questionable. In absorptiometry, for example, absorbanees are often repeatable to within 1 part in 500. Had the calibrators been prepared with small-volume automatic pipettes, random errors might easily amount to one part in 100. This clearly violates the usual assumptions so, prima facie, p-mode calibration should be used. Moreover, if the errors on both axes are quite small, there is little to choose between the two methods. In fact, the on-board calibration found in most modern instruments tends to use p-mode calibration (but fails to inform the... 

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