Pipetting

Volumetric transfer pipets Measuring and serological pipets ... 

Accuracy tolerances for volumetric transfer pipets are given by ASTM standard E969 and Federal Specification NNN-P-... 

Analytical chemists use a variety of glassware to measure volume, several examples of which are shown in Figure 2.4. The type of glassware used depends on how exact the volume needs to be. Beakers, dropping pipets, and graduated cylinders are used to measure volumes approximately, typically with errors of several percent. 

Pipets and volumetric flasks provide a more accurate means for measuring volume. When filled to its calibration mark, a volumetric flask is designed to contain a specified volume of solution at a stated temperature, usually 20 °C. The actual vol-... 

Common types of pipets and syringes (a) transfer pipet (b) measuring pipet (c) digital pipet (d) syringe. 

Proper means of reading the meniscus on a volumetric flask or pipet. 

Three important precautions are needed when working with pipets and volumetric flasks. First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Dirt and grease on the inner glass surface prevents liquids from draining evenly, leaving droplets of the liquid on the container s walls. For a pipet this means that the delivered volume is less than the calibrated volume, whereas drops of liquid above the calibration mark mean that a volumetric flask contains more than its calibrated volume. Commercially available cleaning solutions can be used to clean pipets and volumetric flasks. 

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

Finally, before using a pipet or volumetric flask you should rinse it with several small portions of the solution whose volume is being measured. This ensures that any residual liquid remaining in the pipet or volumetric flask is removed. 

Preparing a solution of known concentration is perhaps the most common activity in any analytical lab. The method for measuring out the solute and solvent depend on the desired concentration units, and how exact the solution s concentration needs to be known. Pipets and volumetric flasks are used when a solution s concentration must be exact graduated cylinders, beakers, and reagent bottles suffice when concentrations need only be approximate. Two methods for preparing solutions are described in this section. 

A sample of an ore was analyzed for Cu as follows. A 1.25-g sample of the ore was dissolved in acid and diluted to volume in a 250-mb volumetric flask. A 20-mb portion of the resulting solution was transferred by pipet to a 50-mb volumetric flask and diluted to volume. An analysis showed that the concentration of Cu in the final solution was 4.62 ppm. What is the weight percent of Cu in the original ore ... 

Substituting known volumes (with significant figures appropriate for pipets and volumetric flasks) into equation 2.4... 

Balances, volumetric flasks, pipets, and ovens are standard pieces of laboratory instrumentation and equipment that are routinely used in almost all analytical work. You should be familiar with the proper use of this equipment. You also should be familiar with how to prepare a stock solution of known concentration, and how to prepare a dilute solution from a stock solution. 

Balance Capacity (g) Measurement Error Pipet Range Volume (mL or ixL)=> Measurement Error ( %)... 

Determinate measurement errors can be minimized by calibration. A pipet can be calibrated, for example, by determining the mass of water that it delivers and using the density of water to calculate the actual volume delivered by the pipet. Although glassware and instrumentation can be calibrated, it is never safe to assume that the calibration will remain unchanged during an analysis. Many instruments, in particular, drift out of calibration over time. This complication can be minimized by frequent recalibration. 

Experimentally Determined Volumes Delivered by a 10-mL Class A Pipet... 

Suppose that you need to add a reagent to a flask by several successive transfers using a class A 10-mL pipet. By calibrating the pipet (see Table 4.8), you know that it delivers a volume of 9.992 mL with a standard deviation of 0.006 mL. Since the pipet is calibrated, we can use the standard deviation as a measure of uncertainty. This uncertainty tells us that when we use the pipet to repetitively deliver 10 mL of solution, the volumes actually delivered are randomly scattered around the mean of 9.992 mL. 

If the uncertainty in using the pipet once is 9.992 0.006 mL, what is the uncertainty when the pipet is used twice As a first guess, we might simply add the uncertainties for each delivery thus... 

It is easy to see that combining uncertainties in this way overestimates the total uncertainty. Adding the uncertainty for the first delivery to that of the second delivery assumes that both volumes are either greater than 9.992 mL or less than 9.992 mL. At the other extreme, we might assume that the two deliveries will always be on opposite sides of the pipet s mean volume. In this case we subtract the uncertainties for the two deliveries,... 

So what is the total uncertainty when using this pipet to deliver two successive volumes of solution from the previous discussion we know that the total uncertainty is greater than 0.000 mL and less than 0.012 mL. To estimate the cumulative effect of multiple uncertainties, we use a mathematical technique known as the propagation of uncertainty. Our treatment of the propagation of uncertainty is based on a few simple rules that we will not derive. A more thorough treatment can be found elsewhere. ... 

The class A 10-mL pipet characterized in Table 4.8 is used to deliver two successive volumes. Calculate the absolute and relative uncertainties for the total delivered volume. 

Using the tolerance values for pipets and volumetric flasks given in Table 4.2, the overall uncertainties in Ma and Mb are... 

A standard solution of Mn + was prepared by dissolving 0.250 g of Mn in 10 ml of concentrated HNO3 (measured with a graduated cylinder). The resulting solution was quantitatively transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10-mL aliquot of the solution was pipeted into a 500-mL volumetric flask and diluted to volume, (a) Express the concentration of Mn in parts per million, and estimate uncertainty by a propagation of uncertainty calculation, (b) Would the uncertainty in the solution s concentration be improved... 

A solution can be diluted by a factor of 200 using readily available pipets (f-mL to fOO-mL) and volumetric flasks (fO-mL to fOOO-mL) in either one, two, or three steps. Limiting yourself to glassware listed in Table 4.2, determine the proper combination of glassware to accomplish each dilution, and rank them in order of their most probable uncertainties. 

A 10-mL volumetric pipet was calibrated following the procedure just outlined, using a balance calibrated with brass weights having a density of 8.40 g/cm. At 25 °C the pipet was found to dispense 9.9736 g of water. What is the actual volume dispensed by the pipet ... 

---