Calibration of Volumetric Glassware

Transcription

1 Chemistry 119: Experiment 2 Calibration of Volumetric Glassware For making accurate measurements in analytical procedures, next in importance to the balance is volumetric equipment. In this section volumetric flasks, pipettes, and burettes are discussed. The experimental procedure includes gravimetric calibration of pipettes and volumetric flasks to provide checks on the accuracy of the markings as well as on proper technique. The few minutes required to learn correct use of this equipment will save time in the long-run and give better results. For example, a typical, inexperienced person using incorrect technique takes about 10 s to read a burette and will have a standard deviation in the readings of 0.02 ml or more. A person of the same experience using correct technique takes about 6 s and will have a standard deviation in the readings of less than 0.01 ml. Volumetric Flasks Volumetric flasks are designed to contain a specific volume of liquid. Since accuracy in their use is not highly dependent on technique, calibration often is unnecessary for work at the level of a part per thousand. The principal source of error is variations in temperature, which causes enough expansion or contraction of aqueous solutions to give errors on the order of 0.1 %/5 o C. Before use, a volumetric flask should be cleaned thoroughly. The solution to be diluted or the solid to be dissolved is then transferred to it with the aid of a funnel. Often it is more convenient for a solid to be dissolved in a beaker or an Erlenmeyer flask first, and then the resulting solution is transferred to the flask with rinsing. In the most accurate work the flask is filled to just below the mark and immersed in a water bath at the calibration temperature before final volume adjustment. Alternatively, the temperature of the solution may be taken and a correction made. In careful work droplets of solvent above the meniscus may be removed with a lintless towel after the meniscus has been adjusted but before mixing. Finally, the solution is mixed thoroughly by inverting, shaking, and turning upright at least 10 times. Pipettes Graduated (Mohr) pipettes are used for delivering small volumes of liquid with an accuracy of only about 1%. Some of these may be marked "TC" (To Contain) or with a frosted glass ring to indicate that they contain the amount specified, but that they may not deliver it. 1

2 Volumetric pipettes are used to deliver precisely a single, definite volume of liquid. These are usually marked "TD" (To Deliver). The tip must meet stringent requirements, because drainage time is controlled by the diameter of the tip. The amount of liquid delivered depends on how a pipette is used; accuracies of 1 part per 1000 (0.1%) can be attained readily, provided the pipettes are used in a reproducible manner. The proper use of a "TD" volumetric pipette will be discussed. As you read this, keep in mind the following points. First clean the pipette so that water drains smoothly from the interior surface. Rinse the interior by drawing a portion of the liquid to be pipetted into the pipette with aid of a suction bulb (NEVER USE YOUR MOUTH TO DRAW LIQUIDS INTO THE PIPETTE!!), and then tilt and turn the pipette until all the inner surface has been wetted. Discard this portion of solution and repeat the operation twice. Then draw solution above the mark, wipe the tip and stem of the pipette carefully to remove external droplets, and allow the solution to drain until the bottom of the meniscus - the line formed by the surface of the water on the glass - is even with the calibration mark. With your finger, hold the liquid at this point. Touch the tip momentarily against a beaker wall to remove excess solution. Then, move the pipette to the receiving vessel and allow the solution to drain freely. During drainage the pipette should be held vertically, with the tip in contact with the container. Keep the tip in contact with the container side - but not the liquid in the container - for about 5 s after free solution flow has stopped and then remove it. The remaining liquid in the tip is left there; do not blow out this portion! Rinse the pipette thoroughly after use. Avoid drawing liquid into the bulb. If the bulb is contaminated accidentally, thoroughly rinse and dry it before reuse. Volumetric pipettes marked "TD" deliver a known volume of water at a defined temperature, usually 20 or 25 o C, when used in a specified manner. The pipette will actually contain a small amount more than it delivers - it is normal for some small amount of liquid to remain in the pipette after delivery. Do not blow this out, or otherwise cause it to be delivered, as it will lead to a substantial error in your results! A manufacturer's markings are seldom in error by more than a part or two per thousand when a pipette is used correctly. Since the volume of liquid delivered depends on how the pipette is used, the technique needs to be carefully specified. The calibration exercise outlined below is recommended to compensate for individual variations in handling. Burettes Burettes are designed to deliver accurately measurable volumes of liquid, particularly for titrations. A 50-mL burette, the most common size, has 0.1 ml graduations along its length and can be read by interpolation to the nearest 0.01 ml. Parallax errors in reading are minimized by extension of every tenth graduation around 2

3 the tube. For reproducible delivery, proper design of the tip is important. Changes affecting the orifice will affect reproducibility; a burette with a chipped or fire-polished tip should not be employed for accurate work. Drainage errors are usually minimized if the tip is constricted so that the meniscus falls at a rate not exceeding 0.5 cm/sec. The accuracy of graduation marks on volumetric burettes depends on the diameter of the bore being uniform. The most convenient way to determine actual volumes is to weigh the amount of water delivered. Burettes can be purchased already calibrated and accompanied by a calibration certificate. Nevertheless, the calibration operation is best done by the user; it is simple and is an excellent means of learning to use the burette correctly. Because graduations on a 50 ml burette are 0.1 ml apart, volumes between the marks must be estimated. In this estimation the width of the lines must be taken into account. The thickness of a line on a 50-mL burette is usually equivalent to about 0.02 ml and so takes up one fifth of the distance from one mark to the next, as illustrated in Figure 2.1. Generally it is preferable to read the bottom of the meniscus and to take as the value for the given line the point where the meniscus bottom just touches the top of the line. For most people this "edge-to-edge" technique gives consistent results. Figure 2.1 shows readings of various meniscus positions when this system is used. Parallax, another source of error in reading a burette, occurs if the eye is above or below the level of the meniscus. This error can be minimized by use of the encircling markings on the burette as guides to keep the eye level with the meniscus. Figure 2.1. Enlarged sections of a 50-mL burette showing the meniscus at several positions; correct readings are given below each section. (The vertical scale is exaggerated over the horizontal for clarity.) The apparent position of the meniscus is significantly affected by the way it is illuminated. Lighting errors are minimized by use of a reading card consisting of a white card (e.g., an index card) with a dull black horizontal mark. The card is placed behind and against the burette so that the top of the black portion is flush with, or no more than a scale division below, the bottom edge of the meniscus. For some 3

4 solutions, such as permanganate and iodine, the bottom of the meniscus may be difficult to see; in such cases the top may be read. Before using a burette, clean it thoroughly with soap and a burette brush, taking care not to scratch the interior surface. Rinse it well with distilled water; the burette is clean when water drains from the inside surface uniformly without the formation of droplets. Store a burette filled with distilled water and with the Teflon stopcock nut loosened slightly. Before use, rinse the burette at least twice with titrant solution and tighten the Teflon stopcock nut only enough to prevent leakage of solution. After filling with titrant, make certain no air bubbles are present in the tip. Bring the solution level to or slightly below the zero mark, remove the drop adhering to the tip, and wait a few seconds for solution above the meniscus to drain before taking the initial readings. Calibration of a Pipette and Volumetric Flask Calibration of volumetric equipment is carried out by weighing the amount of water contained or delivered. Since the density, or weight per unit volume, of water changes about 0.03 %/ o C at 25 o C, the temperature of the water used in the calibration must be known. Because the density of water is relatively low, a correction must be made for buoyancy. This force corresponds to the net weight of the air displaced by the water, flask, and weights. The density of dry air being g/ml and that of stainless-steel weights 7.8 g/ml, the net correction amounts to about g/ml of water weighed. Although errors in placement of burette and pipette markings by the manufacturer are usually small, they cannot be neglected in careful work. For example, if a 10-mL pipette actually delivers ml, an error of 1 part per 1000 will be introduced unless the true value is employed. If a glass vessel is used at a temperature other than that at which it is calibrated, a small correction factor is also required to take into account the expansion of glass. With borosilicate glass this factor amounts to only about 1 part in 10,000 for a 5 o C change, and can be ignored for our work. Apparatus 50-mL Erlenmeyer flask with stopper 50-mL volumetric flask 25-mL pipette thermometer Chemicals distilled water 4

5 Procedure: 1. Pipette Calibration Ask the laboratory instructor for any supplementary instructions, and then clean (with pipe cleaners if necessary) and calibrate the pipette (25 ml) provided. Following the details of pipette use given earlier in this section, weigh the water delivered by the pipette. Use a stoppered 50-mL Erlenmeyer flask as a receiver, and perform all weighings on a balance to the nearest milligram. Calculate the true volume as in the example that follows. Duplicate calibrations should agree to within ml. Use the average value of at least three calibrations for all subsequent measurements with the pipette. Example 2.1 Temperature of water 26 o C Weight of flask + water g Weight of flask g Apparent weight of water delivered g True weight of water delivered (see a) g True volume of pipette (see b) ml a. The weight of air displaced by the water is ml x = g (with the approximation that the density of water is unity). The weight of air displaced by the stainless-steel weights used to weigh the water is (9.965/7.8)(0.0011) = g. The net buoyant effect of air on the water weighed is = g. The true weight of water delivered is = g. b. At 26 o C, 1 g of water occupies ml (consult text or other reference). The true volume of g of water at 26C is x = ml. 5

6 2. Calibration of Volumetric Flask Clean the 50-mL volumetric flask thoroughly. As the flask is to be calibrated to contain (TC) a known volume of liquid, it is imperative that the flask be completely dry when weighed empty. Remove as much water as possible from the flask by inverting it and allowing it to drain for about ten minutes. Then, place the flask in an 80 o C oven for three minutes (NO LONGER!), remove, let cool to room temperature and weigh the dry flask and its stopper. (For careful work, volumetric glassware should never be heated to high temperatures as hysteresis on cooling may result in changes of volume) Now transfer sufficient water to the volumetric flask to bring the meniscus to the mark. Stopper and weigh again. Repeat the entire operation twice and obtain the average apparent mass of water contained in the flask. Applying the proper corrections, obtain the true volume contained in the volumetric flask with standard deviation. Report Report all data obtained for calibration of the pipette and volumetric flask. Include calculations analogous to those shown in example 2.1. Compute the standard deviation of the calibrated volumes assuming that all indeterminate error arose from the volumetric operations and weighing. Answer the following questions: 1. Are the true volumes of your pipette and volumetric flask within Class A tolerance (consult the text for tolerances)? 2. For both the pipette and volumetric flask, is the deviation of the true volume from the nominal (labeled) volume greater or less than your error? Discuss. 3. Suppose that the error in maintaining and determining the temperature of the water was ± 1 o C. Repeat the computation of the volume of the pipette, propagating the uncertainties of determining the mass of water delivered (your standard deviation) and that of the density of water (based on ± 1 o C). 4. Perfect conditions for performing an experiment never exist. List and discuss the relative importance of experimental conditions that may have affected your calibrations. Possibilities: reproducibility of weighing, constancy of laboratory temperature, cleanliness The water used in your calibration was quite pure. Suppose, however, that impure water containing some salt was inadvertently used instead of pure water and that the impure water had a density of g/ml at the temperature of your calibration. Recalculate the volume of your volumetric flask. 6

7 This experiment has been adapted from a laboratory manual authored by Professor S. D. Brown. Last revision: 8/22/97 7