Volumetric pipet calibration

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 ... 

Graduated cylinders are not as precisely calibrated as burets or volumetric pipets. Briefly explain why it is acceptable to measure the K1 and HC1 solutions used in the titration with graduated cylinders rather than with pipets or burets. 

As indicated previously, most pipets are pieces of glassware that are designed to deliver (TD) the indicated volume. Pipets come in a variety of sizes and shapes. The most common is probably the volumetric pipet, or transfer pipet, shown in Figure 4.8. This pipet, like the volumetric flask, has a single calibration... 

A volumetric pipet, because the diameter of the tube where the calibration fine is located is narrower. 

A 25-mL Class A volumetric pipet is certified by the manufacturer to deliver 25.00 0.03 mL. The volume delivered by a given pipet is reproducible, but can be anywhere in the range 24.97 to 25.03 mL. The difference between 25.00 mL and the actual volume delivered by a particular pipet is a systematic error. It is always the same, within a small random error. You could calibrate a pipet by weighing the water it delivers, as in Section 2-9. 

Release liquid from the pipet until the bottom of the meniscus is directly on the fill line. Transfer the pipet to the inside of the second container and release the liquid. Hold the pipet vertically, allow the solution to drain until the flow stops, and then wait an additional 5 to 10 seconds. Touch the tip of the pipet to the inside of the container to release the last drop from the outside of the tip. Remove the pipet from the container. Some liquid may still remain in the tip. Most volumetric pipets are calibrated as TD (to deliver), which means the intended volume is transferred withoutfinal hlow-oul, i.e., the pipet delivers the correct volume. 

Volumetric and measuring pipets are filled to a calibration mark at the outset the manner in which the transfer is completed depends on the particular type. Because an attraction exists between most liquids and glass, a small amount of liquid tends to remain in the tip after the pipet is emptied. This residual liquid is never blown out of a volumetric pipet or from some measuring pipets it is blown out of other types of pipets (Table 2-2). 

Measuring pipet A pipet calibrated to deliver any desired volume up to its maximum capacity compare with volumetric pipet. Mechanical entrapment The incorporation of impurities... 

Most volumetric pipets are calibrated to deliver with a certain small volume remaining in the tip. This should not be shaken or blown out. In delivering, the pipet is held vertically and the tip is touched on the side of the vessel to allow smooth delivery without splashing and so that the proper volume will be left in the tip. The forces of attraction of the liquid on the wall of the vessel will draw out a part of this. 

If high precision is important, a volumetric pipet or other high-precision device must be used for the transfer of to the container and a volumetric flask must be used for the container. Also, if too much water is added by mistake, such that the solution level is above the required calibration line, the solution must be discarded and the entire process repeated. Removing solution from the container to correct such a situation is not an acceptable method for accurately preparating solutions. The reason for this is that the solution being removed likely contains some solute, and thus the concentration of the solute would change in an indeterminable way if some were removed. 

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-... 

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. 

To dilute a stock solution to a desired concentration, we first use a pipet to transfer the appropriate volume of stock solution to a volumetric flask, a flask calibrated to contain a specified volume. Then we add enough solvent to increase the volume of the solution to its final value. Toolbox G.l shows how to calculate the correct initial volume of stock solution. 

Some volumetric glassware products have a large A imprint on the label. This designates the item as a class A item, meaning that more stringent calibration procedures were undertaken when it was manufactured. Class A glassware is thus more expensive, but it is most appropriate when highly precise work is important. This imprint may be found on both flasks and pipets. 

Some pipets are calibrated TC. Such pipets are used to transfer unusually viscous solutions such as syrups, blood, etc. With such solutions, the wetness remaining inside after delivery is a portion of the sample and would represent a significant nontransferred volume, which translates into a significant error by normal TD standards. With TC pipets, the calibration line is affixed at the factory so that every trace of solution contained within is transferred by flushing the solution out with a suitable solvent. Thus, the pipetted volume is contained within and then quantitatively flushed out. Such a procedure would actually be acceptable with any TC glassware, including the volumetric flask. Obviously, diluting the solution in the transfer process must not adversely affect the experiment. 

Prepare calibration standards that are 0.5, 1.0, 3.0, 5.0, and 10.0 ppm N in 50-mL volumetric flasks by diluting the stock standard with distilled water. After diluting to the mark, pipet 1.0 mL of 1 N HC1 into each flask and shake well. 

Prepare calibration standards that are 0.05, 0.10, 0.15, and 0.20 mg/mL in caffeine from the stock standard. Use 25-mL volumetric flasks and measuring pipets or pipetters suggested by your instructor. Calculate the concentration of sodium benzoate in each of these solutions. 

The final concentration of each reactant is calculated from the final volume and the volume and concentration of the solution pipeted into the volumetric flask. The calibration curve is used to find the equilibrium concentration. Using the balanced chemical equation, the equilibrium concentrations of the other substances may be calculated. 

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