Volumetric Techniques
Total Page：16
File Type：pdf, Size：1020Kb
REVISED 10/15 CHEMISTRY 1104L VOLUMETRIC TECHNIQUES Volume measurements are important in many experimental procedures. Sometimes volume measurements must be exact; other times they can be approximate. The main goal of today’s experiment is to investigate the use of volumetric equipment and to examine the preparation of solutions in chemistry. Many types of laboratory procedures require the transfer of a liquid from one container to another. If this transfer is to be done in a precise and quantitative way, then the liquid volume must be measured exactly. Volumetric equipment is designed for this purpose and is calibrated at the factory. Calibration means that the container is etched with a line or lines to show the volume or volumes, which the container holds when a liquid is added to the level of the line. Generally, the volume of a liquid is measured by using volumetric glassware, which includes graduated cylinders, volumetric and graduated pipets, volumetric flasks and burets. The specific type of glassware that you employ depends on the precision required for the volume measurement. Volumetric flasks, pipets, and burets can measure volumes very precisely and are commonly used in quantitative analysis methods. Glassware commonly used as containers, such as beakers and Erlenmeyer flasks, provide approximate measures only and can be used when precision is not a concern. Volumetric glassware is calibrated either to contain (TC) or to deliver (TD) the stated volume. Most glassware will have the initials TC or TD on them so you will know in which way they have been calibrated. Glassware is also calibrated to contain or to deliver a volume at a specific temperature, usually 20oC, because the volume of the container is slightly temperature dependent. Beakers and graduated cylinders are generally calibrated to contain while most pipets and burets are calibrated to deliver. For volumetric glassware marked TD, the remaining liquid in the tip is supposed to be left there; do not blow out this portion. Some of the most common types of volumetric glassware are shown in Figure 1. 1 Figure 1 Volumetric glassware. Reading Volumetric Glassware The surface of most liquids, in glass containers, form concave shapes. That is, when a container is filled with a liquid the liquid level is seldom horizontal over the entire surface but is curved down slightly. This downward curvature is called the liquid “meniscus”. The shape of the meniscus is determined by the relative strengths of the attractive forces between the liquid and the walls of the container, and the cohesive forces within the liquid itself. To obtain an accurate volume measurement, the level of the liquid contained in the glassware must be read correctly. Always read calibrated glassware by viewing the bottom of the meniscus at eye level. Viewing the meniscus above or below eye level will yield inaccurate and irreproducible measurements. See Figure 2. 2 FIGURE 2. Reading a meniscus correctly. A. GRADUATED CYLINDERS: Graduated cylinders are the least precise of all volumetric equipment and are designed to measure any liquid capacity up to the highest numbered calibration on the cylinder. Liquids cannot and should not be measured above this line. The volume of liquid is estimated to one more decimal place than the smallest division on the cylinder. For example, the volume of liquid in the cylinder below is read as 38.8 mL: 3 NOTE: THIS EXPERIMENT MUST BE DONE INDIVIDUALLY! Part A Procedure: Fill your graduated cylinder to the 50.0 mL mark with distilled water. For the last mL, use a medicine dropper to add water to adjust the liquid level so that the bottom of the meniscus touches the 50.0 mL calibration line. Record this volume on the data sheet. Transfer water from the graduated cylinder to completely fill a clean, dry test tube. Measure the volume of water remaining in the cylinder. Record this volume on the data sheet. Calculate the volume of the test tube. Dry the test tube and carry out a second measurement. For your second run, do not refill the graduated cylinder to the 50.0 mL mark; use your final reading from sample 1 as your initial reading for sample 2. B. BURETS The buret is a calibrated glass tube with a stopcock valve and tip on the lower end. Burets are calibrated and can normally deliver up to 50.00 mL or 100.00 mL. Major divisions at 1.00 mL intervals are numbered 0.00 at the top and 50.00 at the bottom, minor divisions divide these into 0.1 mL intervals. As with the pipet, the buret must be clean so that it drains leaving only a film of liquid on its inner surface, no droplets. Fill the buret, with ~ 10 mL of stock solution and tilt the buret so that the liquid comes into contact with all inner surfaces except the very top. Drain some of the “washing” solution out through the tip of the buret and while gently rotating the buret, pour the remaining liquid out of the top of the buret into the appropriate waste container. Fill the buret through the top until the buret is filled above the 0.00 mL calibration line. Open the stopcock fully to drain out a small portion of the solution. THERE MUST BE NO AIR BUBBLES IN THE BURET – then lower the liquid level in the buret so that the initial buret reading is at 0.00 mL. 4 NOTE: THIS EXPERIMENT MUST BE DONE INDIVIDUALLY! Part B Procedure: Fill the buret to the 0.00 mL mark with distilled water, as demonstrated by the lab instructor. Record this initial buret reading on the data sheet. Transfer water from the buret to completely fill a clean, dry test tube. Record this final buret reading on the data sheet. Calculate the volume of water added to the test tube. Dry the test tube and carry out a second measurement. For your second run, do not refill the buret to the 0.00 mL mark; use your final reading from sample 1 as your initial reading for sample 2. Record all volume measurements to two (2) decimal places when using a buret. Remember: When using a buret do not allow the liquid level to fall below the 50.00 mL calibration mark as the volume in the tip of the buret is not known and will introduce error into the experiment. C. Volumetric Pipets A volumetric pipet has one calibration mark and is designed to deliver one fixed volume. Various capacities (i.e. 5 mL, 20 mL, 50 mL etc.) are available. Pipets are filled using a rubber bulb to supply the suction needed to draw the liquid into the pipet. Do not pipet by mouth!! Do not push the pipet into the bulb. The bulb should not actually be fitted over the neck of the pipet. This would most likely cause the liquid to be sucked into the bulb. Rather, squeeze all of the air out of the bulb, and, merely press the opening of the bulb against the opening of the pipet to apply suction force. Keep the tip of the pipet under the surface of the liquid. Slowly release the pressure on the bulb so that the liquid is drawn into the pipet. When the level of the liquid is above the calibration mark, take care as the last portion of the pipet often fills very fast. Remove the bulb and place your index finger over the tip of the pipet to prevent the liquid level from falling. Gently release the pressure of your index finger and allow the liquid level to slowly fall until the bottom of the meniscus touches the calibration line. Allow the liquid to drain freely into the container; do not force the liquid out of the pipet using the pipet bulb. 5 Before using a pipet to transfer a precise volume of liquid, the pipet must be rinsed first with deionized water and then with the stock solution. An unrinsed pipet should never be inserted into a container of stock solution. Instead pour a small amount of solution into a beaker and use this to rinse the pipet. To rinse, draw in about one-fifth of the pipet’s volume, twirl the pipet horizontally a few times to coat the inner glass surface and then discard the liquid. The pipet is ready to use. 6 NOTE: THIS EXPERIMENT MUST BE DONE INDIVIDUALLY! Part C Procedure: Choose a volumetric pipet. Record the size of pipet that you are using on the data sheet. Fill a 250 mL beaker with distilled water. Weigh a clean, dry 50 mL beaker and record the mass on the data sheet. Fill the volumetric pipet with distilled water until the meniscus sits on the calibration line and drain the liquid into the weighed 50 mL beaker. Reweigh the beaker and water. Record the mass on the data sheet. Determine the mass of water added to each beaker and the volume based on the mass. Note: Assume the density of water is 1.0000 g/mL so 1 g = 1 mL. PREPARATION OF SOLUTIONS Many types of solutions are prepared for laboratory use. These solutions require specific preparations, and to understand these preparations you must be familiar with the many ways by which chemists express concentrations. Some common expressions of concentrations are molarity, molality, parts per million, weight percent (or mass percent), and weight-volume percentage. These various expressions have different uses and functions and the one that you employ is simply a matter of convenience or convention. This laboratory course will use molarity. MOLARITY Molarity is a concentration term that relates the amount of solute present in a unit volume(1 L) of solution.