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Preparation of Solutions, Part 2

Source: General Chemistry Lab, St. Mary's College of Maryland

The Volumetric Flask

Volumetric flasks are commonly used in solution preparation. To use one correctly, you must know how the volumetric flask has been calibrated. It is normally calibrated to contain (TC) a stated volume of liquid (known to four significant figures) at a stated temperature (usually 20 ^oC). When it is properly filled, the bottom of the meniscus should be level with the calibration line on the flask. This will ensure that the flask contains the specified volume. Some flasks are also calibrated to deliver (TD) a specific volume, however, this application is less commonly used. When the flask is properly filled to the TD calibration mark, it will deliver the specified volume to another container. (See Figure 2-10.)

Preparation of an Aqueous Solution Using a Pure Solid Solute

1. Properly clean a volumetric flask that has the desired volume (See Cleaning Laboratory Glassware.) and add distilled water until the flask is about half-full.
2. Using a plastic weighing dish or watch glass, weigh out the calculated amount of solute to within 0.01 g or better.
3. Quantitatively transfer the solid to the flask by using a funnel to guide the solid into the flask, or alternatively, fold the corners of a weigh boat into a pouch and pour the solid directly into the flask.
4. Rinse the residual solid from the weigh boat or watch glass and funnel into the flask with several portions of distilled water. This will ensure that all the solid is transferred to the volumetric flask. Wash down the neck of the flask and swirl the solution until the solid is dissolved.
5. Add distilled water until the level of the liquid almost reaches the mark on the flask.
6. Stopper the flask and hold the stopper firmly in place while you invert the flask to mix the solution. Mix the solution for at least three minutes to ensure thorough mixing. Let the flask stand for about 30 seconds (until the water drains down the sides of the flask).
7. Carefully add distilled water with a dropper or wash bottle until the bottom of the meniscus is level with the calibration line on the flask. Invert and swirl again for a few seconds.
8. Transfer the solution to a clean, dry, plastic bottle. If the bottle is not dry, rinse the bottle with small increments of 10-20 mL of the solution. Discard the rinse solution and then transfer the bulk of the solution to the bottle.
9. Label the bottle with the name and concentration of the reagent, the date, and your initials. Using this method, the concentration of the reagent is known at least to two or three significant figures.

 Quantitative Dilution of a Standardized Reagent

1. Calculate the volume of standard reagent needed to make the dilute solution.
2. Add distilled water to the appropriate volumetric flask until it is about half-full.
3. Use a pipet of appropriate size to transfer the required amount of standard to the volumetric flask.
4. Next, add distilled water until the level of the liquid almost reaches the calibration line on the flask.
5. Stopper the flask and hold the stopper firmly in place while you invert the flask to mix the solution. Mix for at least three minutes to ensure thorough mixing. Let the flask stand for about 30 seconds (until the water drains down the sides of the flask).
6. Carefully add distilled water with a dropper or a wash bottle until the bottom of the meniscus is level with the calibration line on the flask. Invert and swirl again for a few seconds.
7. Transfer the solution to a clean, dry, plastic bottle. If the bottle is not dry, rinse the bottle with small increments of 10-20 mL of the solution. Discard the rinse solution and then transfer the bulk of the solution to the bottle.
8. Label the bottle with the name and concentration of the reagent, the date, and your initials. Using this method, the concentration of the reagent is known at least to three or four significant figures.

Approximate Dilution of a Concentrated Reagent

Concentrated reagents such as acids have approximate concentrations, and therefore, do not require quantitative dilution.

1. Measure the calculated volume of concentrated reagent in a graduated cylinder.
2. Add distilled water to a clean, but not necessarily dry, plastic bottle of appropriate size until it is about half-full.
3. Next, slowly pour the concentrated reagent from the cylinder into the bottle. (See sidebar Handling Chemical Reagents.) Remember, concentrated acid is always added to a large volume of water to avoid spattering of the solution.
4. Swirl the contents of the bottle and add distilled water until the level of the solution reaches the rim of the bottle just below the neck. Hold the cap of the bottle in place and invert the bottle to mix the contents. Mix for at least three minutes to ensure thorough mixing.
5. Label the bottle (see Figure 2-9) with the name and concentration of the reagent, the date, and your initials. Using this method, the concentration of the reagent is known to only one significant figure.

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