Buret. Pipet. Volumetric flask

The molar concentration scale is most appropriate in volumetric experiments using pipets, volumetric flasks, and burets. A disadvantage of this scale is that molar concentrations are temperature-dependent since solution density depends (usually weakly) on temperature. 

All measuring devices are potential sources of systematic errors. For example, pipets, burets, and volumetric flasks may hold or deliver volumes slightly different from those indicated by their graduations. These differences arise from using glass-... 

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.)... 

FIGURE 4.2 Drawings of the common glassware used in titrimetric analysis (not drawn to scale). Left, buret center, volumetric pipet right, volumetric flask. (From ChemClip Art 1000, a product of Molecular Arts Corporation, Anaheim, California. With permission.)... 

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. 

The use of clean glassware is of utmost importance when doing a chemical analysis. In addition to the obvious need of keeping the solution free of contaminants, the walls of the vessels, particularly the transfer vessels (burets and pipets), must be cleaned so that the solution will flow freely and not bead up on the wall as the transfer is performed. If the solution beads up, it is obvious that the pipet or buret is not delivering the volume of solution intended. It also means that there is a greasy him on the wall that could introduce contaminants. The analyst should examine, clean, and reexamine his or her glassware in advance so that the free how of solution down the inside of the glassware can be observed. For the volumetric flask, at least the neck must be cleaned in this manner so as to ensure a well-formed meniscus. 

Give both your acid and base solutions one final shake at this point to ensure their homogeneity. Rinse the buret with 5 to 10 mL of NaOH twice, and then fill it to the top. Open the stopcock wide open to force trapped air bubbles from the stopcock and tip. Allow this excess solution to drain into a waste flask. Bring the bottom of the meniscus to the 0.00-mL line. Using a clean 25-mL pipet (volumetric), carefully place 25.00 mL of the acid solution into each of the three flasks. Add three drops of phenolphthalein indicator to each of the three flasks. 

Define standard solution, volumetric flask, volumetric pipet, standardization, buret, stopcock, titration, titrant, substance titrated, equivalence point, indicator, end point, and 9automatic titrator. 

In some cases, the exam question will request a list of the equipment needed, while in other cases you will get a list from which to choose the items you need. Certain items appear in many experiments. These include the analytical balance, beakers, support stands, pipets, test tubes, and Erlenmeyer flasks. Burets, graduated cylinders, clamps, desiccators, drying ovens, pH meters, volumetric flasks, and thermometers are also commonly used. If you are not sure what equipment to choose, these serve as good guesses. Most of the remaining equipment appears in three or fewer experiments. 

A volumetric flask is calibrated to contain a particular volume of solution at 20°C when the bottom of the meniscus is adjusted to the center of the mark on the neck of the flask (Figure 2-9, Table 2-3). Most flasks bear the label TC 20°C, which means to contain at 20°C. (Pipets and burets are calibrated to deliver, TD, their indicated volume.) The temperature of the container is relevant because both liquid and glass expand when heated. 

To dilute a solution to a desired concentration, we first use a pipet to transfer the appropriate volume of solution to a volumetric flask, a flask calibrated to contain a specified volume. Pipets and burets are narrow tubes used to transfer liquids accurately (Fig. G.9). Then we add enough... 

Standardization of a solution using a primary standard pipet, buret, Erlenmeyer flasks, volumetric flask, wash bottle, analytical balance, drying oven, desiccator, support stand, pH meter... 

Assay Accurately weigh about 2 g of sample, previously dried at 150° for 4 h, and dissolve it in 100 mL of water and 5 mL of 2.7 N hydrochloric acid. Transfer the solution into a 250-mL volumetric flask, dilute to volume with water, and mix well. Pipet 50.0 mL of this solution into a suitable container, and add 50 mL of water. While stirring, preferably with a magnetic stirrer, add about 30 mL of 0.05 M disodium EDTA from a 50-mL buret, then add 15 mL of 1 N sodium hydroxide and 300 mg of hydroxy naphthol blue indicator, and continue the titration to a blue endpoint. Each milliliter of 0.05 M disodium EDTA is equivalent to 10.51 mg of C3H7Ca06P. 

Standards of Accuracy The capacity tolerances for volumetric flasks, transfer pipets, and burets are those accepted by the National Institute of Standards and Technology (Class A),1 as indicated in the accompanying tables. Use Class A volumetric apparatus unless otherwise specified in the individual monograph. For plastic volumetric apparatus, the accepted capacity tolerances are Class B.2... 

Volume may be measured reliably with a pipet, a buret, or a volumetric flask. 

Volumetric equipment is marked by the manufacturer to indicate not only the manner of calibration (usually TD for to deliver or TC for to contain ) but also the temperature at which the calibration strictly applies. Pipets and burets are ordinarily calibrated to deliver specified volumes, whereas volumetric flasks are calibrated on a to-contain basis. 

Figure 1-13 Some laboratory apparams used to measure volumes of liquids 150-mL beaker bottom left, green liquid) 25-mL buret (top left, red) 1000-mL volumetric flask center, yellow) 100-mL graduated cylinder right front, blue) and 10-mL volumetric pipet right rear, green).

Under what conditions are the molarity and molality of a solution nearly the same Which concentration unit is more useful when measuring volume with burets, pipets, and volumetric flasks in the laboratory Why ... 

The measurements chemists make are often used in calculations to obtain other related quantities. Different instraments enable us to measure a substance s properties The meter stick measures length or scale the buret, the pipet, the graduated cylinder, and the volumetric flask measure volume (Figure 1.8) the balance measures mass the thermometer measures temperature. These instruments provide measurements of macroscopic properties, which can be determined directly. Microscopic properties, on the atomic or molecular scale, must be determined by an indirect method, as we will see in the next chapter. 

Typical laboratory glassware used for volume measurement is shown in Figure 1.8. The volumetric flask is designed to contain a specified volume, and the graduated cylinder, pipet, and buret dispense a desired volume of liquid. 

Common laboratory equipment used for the measurement of volume. Graduated cylinders (a), pipets (b), and burets (c) are used for the delivery of liquids volumetric flasks (d) are used to contain a specific volume. A graduated cylinder is usually used for measurement of approximate volumes it is less accurate and precise than either pipets or burets. 

We have mentioned a few precautions in the use of volumetric flasks, pipets, and burets. Your laboratory instructor will supply you with detailed instructions in the use of each of these tools. A discussion of some general precautions and good laboratory technique follows. 

Pipets and burets should be rinsed at least twice with the solution with which they are to be filled. If they are wet, they should be rinsed first with water, if they have not been already, and then a minimum of three times with the solution to be used about one-fifth the volume of the pipet or buret is adequate for each rinsing. A volumetric flask, if it is wet from a previously contained solution, is rinsed with three portions of water only since later it wifi be filled to the mark with water. It need not be dry. 

Figure 1.7 Common laboratory volumetric glassware. From left to right are two graduated cylinders, a pipet being emptied into a beaker, a buret delivering liquid to an Erlenmeyer flask, and two volumetric flasks. Inset, In contact with glass, this liquid forms a concave meniscus (curved surface).

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