The Pipet

TABLE 4.1 Steps Involved in Transferring a Solution with a Volumetric or Transfer Pipet 

Step 2 Evacuate the pipet bulb (by squeezing), and while keeping it evacuated, seat the bulb opening over the top opening of the pipet. 

Step 4 Fill the pipet to well past the calibration line by releasing the squeezing pressure, as in step 4. Reevacuate the bulb if necessary. 

Step 5 Quickly remove the bulb and seal the top of the pipet with your index finger. 

Step 6 Keeping your index finger in place, remove the tip from the solution and wipe with a towel. To avoid 

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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. 

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. 

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,... 

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 ... 

If the buoyancy correction is ignored, the pipet s volume is reported as... 

Schwartz has published some hypothetical data for the titration of a 1.02 X ICr" M solution of a monoprotic weak acid (pXa = 8.16) with 1.004 X ICr M NaOH. " A 50-mL pipet is used to transfer a portion of the weak acid solution to the titration vessel. Calibration of the pipet, however, shows that it delivers a volume of only 49.94 ml. Prepare normal, first-derivative, second-derivative, and Gran plot titration curves for these data, and determine the equivalence point for each. How do these equivalence points compare with the expected equivalence point Comment on the utility of each titration curve for the analysis of very dilute solutions of very weak acids. 

FIG. 20-4 Equipment used in the pipet method of size analysis. 

By this procedure about 80-85% of the mineral oil was removed. Because some sodium hydride is lost in the pipetting procedure, an excess is initially employed. 

Sweden produced a disproportionate number of outstanding chemists in the eighteenth and nineteenth centuries. Jons Jakob Berzelius (1779-1848) determined with amazing accuracy the atomic masses of virtually all the elements known in his time. In his spare time, he invented such modern laboratory tools as the beaker, the flask, the pipet, and the ringstand. 

Dry heat sterilisation is used for equipment that can withstand high temperature and dry heat but cannot withstand wet or steam autoclave. This method is often used for glassware as it dries and sterilises in one operation. The pipets must be wrapped in dustproof aluminum foil or placed in metal pipette cans. The can lids are removed during heating and replaced after sterilisation, that is before any dust can get in the can. Disposable items are not recommended for dry heat sterilisation. This method may only be good for permanent reusable glass pipettes. 

Step 3 - The pipet is reinserted into the liquid, resulting in apposition of the hydrocarbon tails of the attached monolayer to those of the original monolayer, forming a bilayer (27) ... 

A homemade combination scraper-coUector used in my laboratory can be made from a Pasteur pipet (229 mm x 7 mm OD Fisher Scientific Co., Pittsburgh, PA Catalog No. 13-678-6B) [52]. The pipet is cut with a file 60 mm from the top and 65 mm from the tip to produce a pipet that is approximately 100 mm long, which is then plugged with glass wool. The pipet tip is attached to a vacuum pump or... 

Jars containing the same amount of hand wash solution as used to collect the entire hand wash from the test volunteer should be fortified. The samples are fortified with the appropriate amount of active ingredient solution using a 1-mL volumetric pipet, blowing out the remaining solution in the pipet. The solutions are capped, shaken, and placed immediately in a freezer or dry-ice cooler. 

Br > Cl > I. This conductance was not time-dependent. On the other hand, Cl conductance stimulated by the Ca2+ ionophore A23187 (2.5 pM) or elevation in the free Ca2+ levels in the pipet from 100 to 500 nM was time-dependent and exhibited a nonlinear current-voltage relationship that was outwardly rectifying. The permselectivity of this Ca2+-stimulated conductance was 1 > Br > cr, distinguishing it from the cAMP-stimulated Cfr conductance. As both calcium ionophore A23187 and cAMP were shown to elevate the Cl conductance of the cornea, it is quite likely that these two types of Cr channels are present in the corneal epithelium [96,106,114],... 

The pipet is designed to deliver an exact volume of liquid the volumetric flask is designed to hold an exact volume of liquid and the buret is designed to deliver precisely measurable volumes of liquid. (Not drawn to scale.)... 

The electrophysiological technique used to measure changes in membrane capacitance is the patch clamp [5,6] in the whole-cell recording mode, where the plasma membrane patch in the pipet is ruptured. In another configuration of the patch clamp, the plasma membrane patch is maintained intact. In this case, small currents due to the opening of individual channels can be measured in the membrane patch. The whole-cell patch clamp... 

A student blew the last drops of solution from the pipet into the volumetric flask when transferring commercial bleaching solution to the flask. 

The three volumetric glassware products we will discuss are the volumetric flask, the pipet, and the buret. Let us study the characteristics of each type individually. 

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