The Volumetric 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. 

The calibration line is affixed so that the indicated volume is contained rather than delivered. Accordingly, the legend TC is imprinted on the base of the flask, thus marking the flask as a vessel to contain the volume indicated, as opposed to to deliver (TD) the volume. The reason this imprint is important is that a contained volume is different from a delivered volume, since a small volume of solution remains adhering to the inside wall of the vessel and is not delivered when the vessel is drained. If a piece of 

FIGURE 4.3 Left, a 1000-mL volumetric flask. Right, a close-up view of the neck of the flask showing the single calibration line. 

FIGURE 4.5 Close-up views of the labels found on a 1000-mL volumetric flask. On the left, the stopper size (22) is evident, and on the right, besides the capacity of the flask (1000 mL), the TC designation and the 20°C specification are evident. 

FIGURE 4.6 A tapered ground glass stopper may be used with a flask with a ground glass opening. A tapered plastic stopper may also be used. 

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To prepare the solution we measure out exactly 0.1500 g of Cu into a small beaker. To dissolve the Cu we add a small portion of concentrated HNO3 and gently heat until it completely dissolves. The resulting solution is poured into a 1-L volumetric flask. The beaker is rinsed repeatedly with small portions of water, which are added to the volumetric flask. This process, which is called a quantitative transfer, ensures that the Cu is completely transferred to the volumetric flask. Finally, additional water is added to the volumetric flask s calibration mark. 

Shake the volumetric flask vigorously to allow the material to dissolve and ensure homogeneity. 

Step 1 Calculate the amount of solute, n, in the final, dilute solution of volume Vtina. (This is the amount of solute to be transferred into the volumetric flask.)... 

A 50-g homogenized plant sample is weighed into a centrifuge tube and blended at 7000 rpm with 100 mL of methanol-water (7 3, v/v) for 10 min. The resulting mixture is centrifuged at 5000 rpm for 5 min. The supernatant is collected in a 200-mL volumetric flask with suction. In the case of strawberry and peach, the supernatant is filtered through a glass filter (17G-3) previously packed with 10 g of Celite 545. The residue is re-extracted with 50 mL of the same aqueous methanol in the same manner as described above, and the supernatant is collected in a 200-mL volumetric flask. The volumetric flask is filled to the mark with the same aqueous methanol. 

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.)... 

We have a second standard uncertainty for the volume of the volumetric flask. This estimate was obtained by making replicate measurements of the volume. The two standard uncertainties relating to the volume must be combined to produce a single value. This is achieved by a straightforward application of equation (6.12) ... 

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

If a student blew drops of commercial bleaching solution into the volumetric flask, then that student has introduced another set of chemicals from the moisture of their mouth into the solution, altering the experiment and altering the results. 

Conveniently, the volumetric flask can be purchased in a variety of sizes, from 5 mL up to several liters. Figure 4.4 shows some of the various sizes. 

One problem that exists with the volumetric flask, because of its unique shape, is the difficulty in making prepared solutions homogeneous. When the flask is inverted and shaken, the solution in the neck of the flask is not agitated. Only when the flask is set upright again is the solution drained from the neck and mixed. A good practice is to invert and shake at least a dozen times to ensure homogeneity. 

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... 

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. 

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. 

The solutions are diluted to a known volume in the volumetric flask. Measurements of the absorbance are made with a spectrophotometer. 

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. 

All weighed samples are converted to moles by using the molar mass, and the moles are divided by the volume of the volumetric flask in liters to yield molarity. 

Pour the resulting solution quantitatively, into the funnel placed in the mouth of the volumetric flask with the help of a glass rod and a sharp jet of water from a wash-bottle by holding the beaker with the right hand and the guiding rod with the left hand,... 

Remove the funnel, swirl the contents of the volumetric flask and make up the volume upto the mark,... 

Save the filter paper with residue for Zr detn. Fill the volumetric flask to the mark and pipet a 50-ml aliquot into a 400-ml beaker for the chromate detn. Dilute to 250 ml, and add 10 ml of dilute sulfuric acid (1 4) and 10 ml of phosphoric acid (1 1). Add a measured excess consisting of ca 30 ml of standardized 0.05N ferrous ammonium sulfate soln and 6 to 8 drops of Na diphenylamine sulfonate indicator soln (0.2 g in 100 ml of w). Titrate the excess ferrous iron with 0.05N std K dichromate soln, adding the dichromate slowly with stirring, until the pure grn color changes to gray-grn. Then add the dichromate one drop at a time until the first tinge of purple or violet-blue appears... 

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