VOLUMETRIC TECHNIQUES

Transcription

1 REVISED 10/14 CHEMISTRY 1101L 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 20 o C, 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

2 Buret Volumetric Flasks Graduated Pipet Cylinder FIGURE 1. Common Volumetric Equipment 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

3 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

4 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 ml or ml. Major divisions at 1.00 ml intervals are numbered 0.00 at the top and 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

5 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 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

6 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

7 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 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. Molarity is defined as the number of moles of solute per liter of solution. Molarity is used to describe the concentrations of solutions, examples include: 6.0 M HCl and 3.0 M NaOH. REMEMBER THAT 1 L = 1000 ml!! moles of solute Molarity(M ) volume of solution(l) The solute is dissolved in enough solvent to make a specific volume of solution. 7

8 PREPARATION OF A SOLUTION WITH A SPECIFIC, KNOWN CONCENTRATION 1) PREPARATION OF A SOLUTION FROM A SOLID A solution of known concentration may be prepared by weighing out a specific amount of solute and dissolving this solute in an appropriate amount of solvent. In this experiment you will be assigned a specific Molarity of a solution to prepare and must calculate the mass of solute needed to prepare ml( L) of this solution. The amount of solute needed depends on both the Molarity of the solution and the amount of solution(volume). The larger the volume required, the more solute that must be used to prepare a solution of a given concentration. moles solute volume of solution(l) Molarity(M) Remember that molarity is defined as the moles of solute/l of solution. So the above relationship can be rewritten to show the cancellation of units. moles solute moles of solute volume of solution(l) L of solution By multiplying volume by molarity you can see that the L of solution cancels out leaving you moles of solute. Once the moles of solute is know the mass, in grams, can be determined by using the molar mass which relates the amount of material in grams to a unit quantity(1 mole) of solute. mass of solute mass of solute moles of solute mole of solute 2) DILUTION OF A CONCENTRATED REAGENT Another method used to prepare a solution of known concentration is to carry out the dilution of a stock solution. A known amount of stock solution is combined with a known amount of solvent and the new, lower concentration is calculated. Consider the following example: Calculate the molarity of the resulting dilute solution when ml of a stock a M hydrochloric acid solution is diluted to a final volume of ml through the addition of distilled water? 8

9 The calculation is as follows: M Stock V Stock = M dilute V dilute {DILUTION FORMULA} mol/l HCl) ml = M dilute ( ml) M = M dilute Note that adding an equal volume of water to a solution of known concentration resulted in a ½ dilution. In this experiment you will be given a stock solution of known concentration(m stock ) and asked to calculate the volume of stock(v stock ) needed to prepare a dilute solution with a specific concentration(m dilute ) and volume(v dilute ). D. VOLUMETRIC FLASKS Volumetric flasks are commonly used in solution preparation, to prepare solutions of known concentration. They are available with capacities ranging from 3.00 to ml. Volumetric flasks are normally calibrated to contain (TC) a stated volume of liquid at a standard temperature, usually 20 o C. 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. This type of solution is called a standard solution. It is usually prepared by dissolving a weighed amount of solute in a known volume of solvent. The solid reagent (solute) is transferred to the volumetric flask by rinsing the weigh boat out with the solvent being used and transferring it to the volumetric flask. The weigh boat is rinsed several times with the solvent to ensure that the entire solid has been transferred to the volumetric flask. In this way, the solute is transferred quantitatively (none of the solid is left behind in the weigh boat). The flask is then filled half-way with solvent, stoppered and swirled until all of the solid has dissolved. The solvent is then added until the meniscus is near the calibration mark. The final amount of liquid is added to the flask, using an eye dropper, until the bottom of the meniscus is touching the calibration line. It is important that the solution is then mixed well so that it is of uniform concentration. The flask should then be stoppered and inverted several times. Note that if the volumetric flask is filled with solvent so that the meniscus sits above the 9

10 calibration line, the volume of liquid in the flask is not known, and the exact concentration of this solution is also not known. NOTE: THIS EXPERIMENT MUST BE DONE INDIVIDUALLY! Part D Procedure: The instructor will give you the exact concentration and amount of the standard solution which you are required to prepare. Calculate the mass of solute needed to make up the solution. Use the example problem on page 8 to help with your calculations. Before proceeding, check your calculations with the lab instructor. Weigh out the desired amount of solute and add it to the volumetric flask, then dissolve in the solvent and fill the flask up to the calibration mark, as demonstrated by the lab instructor. Label the flask (include the concentration and your name). The accuracy of your standard solution and thus your grade will be determined by measuring the Absorbance of visible light by your sample in the UV-visible spectrophotometer set up in the lab. To use the spectrophotometer bring your sample up to one of the instruments. Rinse the cuvette supplied with a little distilled water and with a little of your solution to be tested. This is to ensure that there is no contamination from the previous user and that the measured absorbance and thus concentration is accurate. Wipe the outside of the cuvette with a kimwipe, insert into the spectrophotometer as demonstrated by your instructor and close the sample lid. Determine the Absorbance and record into your data sheet. 10

11 E. DILUTION OF A STOCK SOLUTION The reagent (solute) is transferred to the volumetric flask by pipetting a specific amount of a stock solution (solution of known concentration) into a volumetric flask. The flask is filled with the solvent until the meniscus sits on the calibration line. NOTE: Molarity volume (in Liters) = moles solute. The equation used for the dilution of stock solutions is as follows: M Stock V Stock = M dilute V dilute where M stock and V stock are the molarity and volume of the stock solution required and M dilute and V dilute are the molarity and volume of the resulting diluted solution. The flask is filled half-way with the solvent, and swirled to ensure mixing. The solvent is then added until the meniscus is near the calibration mark. The final amount of liquid is added to the flask, using an eye dropper, until the bottom of the meniscus is touching the calibration line. It is important that the solution is then mixed well so that it is of uniform concentration. The flask should then be stoppered and inverted several times. NOTE: THIS EXPERIMENT MUST BE DONE INDIVIDUALLY! Part E Procedure: Obtain from the lab instructor the concentration of the standard solution and the dilution (volume of solution) required for the solution that you are to prepare. Carry out the appropriate calculations on the data sheet. Before proceeding, check your calculations with the lab instructor. The lab instructor will provide you with a standard solution; DO NOT USE THE STANDARD YOU PREPARED IN PART D. Transfer some of the standard 0.20 M solution to a small 50 ml beaker and take back to your bench. Make up the dilute solution as required. You should choose the appropriate pipet and use it to dispense the concentrated solution directly into a volumetric flask of the correct size required. Make up to the mark with distilled water, stopper and invert several times to mix well. 11

12 The accuracy of your standard solution and thus your grade will be determined by measuring the Absorbance of visible light by your sample in the UV-visible spectrophotometer using the same procedure as in Part D. Volumetric Techniques Quiz Hints: 1. Be able to identify the various pieces of volumetric equipment used in this experiment and know how to use each piece correctly. 2. Know how to read a meniscus for each of the pieces of volumetric equipment used in this experiment, and report the volume with the correct number of significant figures/decimal places. 3. Be able to calculate the molarity of a solution given mass of solid, chemical formula of solid and solution volume. 4. Be able to carry out a dilution calculation (see example page 8-9) Be able to explain how you would prepare the solution from #5 step by step. 12