Your buret probably has a Teflon stopcock, which does not require lubrication. Make sure it is tight enough to prevent leakage, but not so tight that it is hard to rotate. If your buret has a ground-glass stopcock, you may have to grease it. Apply a thin, uniform layer of stopcock grease (not silicone lubricant), using very little near the hole and taking care not to get any grease in the hole. Insert the stopcock and rotate it. There should be a uniform, transparent layer of grease, and the stopcock should not leak. If there is too much lubricant, it will be forced into the barrel or may work into the buret tip and clog it. Grease can be removed from the buret tip and the hole of the stopcock with a fine wire.
Next, fill the buret with the solution it will deliver. Fill it above the zero mark and open the stopcock to fill the tip. Check the tip for air bubbles. If any are present, they may work out of the tip during the titration and cause an error in reading. Work air bubbles out by rapidly opening and closing the stopcock to squirt the titrant through the tip, or by tapping the tip while the solution is flowing. There should be no bubbles; if bubbles are present in the barrel of the buret, it is likely dirty.
The initial reading is obtained by letting the buret drain slowly to the zero mark. Wait a few seconds to ensure the drainage film has caught up with the meniscus. Read the buret to the nearest 0.01 mL (for a 50-mL buret). The initial reading may be 0.00 mL or higher. The reading is best taken by placing your finger just behind the meniscus or by using a meniscus illuminator . The illuminator has a white and a black field, and the black field is positioned just below the meniscus. Avoid parallax error by taking the reading at eye level.
The titration is carried out with the sample solution in an Erlenmeyer flask. Place the flask on a white background, with the buret tip positioned within the neck of the flask. Swirl the flask with the right hand while manipulating the stopcock with the left hand or in whatever manner feels comfortable. This grip keeps a slight inward pressure on the stopcock so that no leakage occurs. The solution can be stirred more efficiently with a magnetic stirrer and stirring bar.
As the titration continues, the indicator changes color near the point where the titrant is added, because of local excesses. It quickly returns to the original color as the titrant spreads through the solution to react with the sample. As the end point nears, the return to the original color happens more slowly, because the dilute solution must be mixed more thoroughly to use up all the titrant. At this point, halt the titration and rinse down the sides of the flask with distilled water from the wash bottle.
A drop from the buret is roughly 0.02 to 0.05 mL, and the volume is read to the nearest 0.02 mL, so it is necessary to split drops near the end point. Do this by slowly turning the stopcock until a fraction of a drop appears at the buret tip, then closing it. Touch the fraction of the drop onto the wall of the flask and rinse it into the solution with the wash bottle, or move it with a glass stirring rod. At the end point, there will be an abrupt and "permanent" (lasting at least 30 s) shift in color when a fraction of a drop is added.
The titration is typically carried out in triplicate. Once the first titration is done, subsequent titrations may be accelerated by using the first to approximate the end point volumes. Alternatively, you can determine the approximate volume for the replicate titrations from the weights of the samples and the titrant's molarity. This will save time. The volume should not be calculated to closer than 0.1 mL, so as to prevent bias in the reading.
Once a titration is finished, never pour leftover titrant back into the original bottle; throw it away. If the titrant is not within pH 4 to 8 and does not appear on the short list of substances approved for drain disposal, discard it in a recycle container.
If a physical property of the solution, such as potential, is measured to identify the end point, the titration takes place in a beaker with magnetic stirring so that electrodes can be inserted into the solution.
Example
(a) Use Table to calculate the volume of the 20-mL pipet in Example 2.2 (steel
weights), from its weight in air. Assume the temperature is 23◦C. (b) Give the
corresponding volume at 20◦C as a result of glass contraction. (c) Compare with the
volume calculated using the weight in air with that calculated using the weight in
vacuum and the density in water .
Solution
(a) From Table 2.4, the volume per gram in air is1.0035mL at 23◦C:
19.994g× 1.0035 mL/g= 20.064 mL
(b) Theglasscontractionat20◦Crelativeto23◦Cis0.0015mL(0.000025 mL/mL/◦C
× 20mL× 3◦C),so the pipet volume at 20◦C is20.062mL.
(c) The density of water at 23◦C is 0.99754g/mL, so from the weight in vacuum:
20.015 g/0.99754 g/mL= 20.064 mL
The same value is obtained
Tolerances and Precision of Glassware
The National Institute of Standards and Technology (NIST) has established specific tolerances, or absolute errors, for various types of volumetric glassware, and a number of these appear on the back cover of the textbook. When volumes exceed roughly 25 mL, the tolerance falls within 1 part per thousand relative, though it becomes larger for smaller volumes.
The letter "A" marked on a volumetric flask, buret, or pipet signifies that the item meets Class A tolerances. This provides no information regarding the precision of delivery. Volumetric glassware that satisfies NIST specifications or has been certified by NIST is available, though at a considerably higher cost than uncertified glassware. Cheaper glassware might have tolerances twice those set by NIST. It is straightforward, nonetheless, to calibrate such glassware to an accuracy equal to or better than the NIST specifications .
Reading a 50-mL buret carries a precision of roughly ±0.02 mL. Because a buret is read twice, the overall absolute uncertainty can reach ±0.04 mL. The relative uncertainty varies inversely with the total volume delivered, so a titration performed with a 50-mL buret should use approximately 40 mL of titrant to attain a precision of 1 ppt. Smaller burets may be used when greater precision is needed at low volumes. Pipets likewise have a certain reading precision, though only one reading is needed for volumetric pipets.
Calibration of Glassware: For the Ultimate calibration of glassware, correcting for the buoyancy of the water used during calibration, that is, correcting to weight in vacuum. When the weight of the water in vacuum is divided by its density at the given temperature, the result is converted to volume.
calibration of glassware, correcting for the buoyancy of the water used during calibration, that is, correcting to weight in vacuum. When the weight of the water in vacuum is divided by its density at the given temperature, the result is converted to volume.
Techniques for Calibrating Glassware
Glassware is generally calibrated to five significant figures, which is the maximum precision you are likely to achieve when filling or delivering solutions. Therefore, your net weight of water must have five figures. If the glassware is larger than 10 mL, weighing to 1 mg is all that is required. This can be done conveniently with a top-loading balance instead of a more sensitive analytical balance. [Note: If the volume number is large regardless of the decimal, for instance 99, then four figures will be sufficient. See the Chapter 3 discussion on significant figures. For example, a 10-mL pipet may be calibrated and shown to actually deliver 9.997 mL. This is as accurate as if the pipet had been determined to deliver 10.003 mL; the last significant figure in both cases represents one part in 10,000.]
- Calibration of a Volumetric Flask. Weigh the clean, dry flask along with its stopper. Next, fill it to the mark with distilled water. There should be no droplets on the neck; if any are present, blot them with tissue paper. Bring the flask and the water to room temperature. Weigh the filled flask, then record the temperature of the water to 0.1 °C. The gain in weight corresponds to the weight in air of the water held by the flask.
- Calibration of a Pipet. Weigh a dry Erlenmeyer flask fitted with a rubber stopper, or a weighing bottle with a glass stopper or cap, depending on the amount of water to be weighed. Fill the pipet with distilled water (whose temperature you have recorded) and release the water into the flask or bottle using proper pipetting technique. Promptly stopper the container to prevent loss through evaporation. Weigh again to determine the weight in air of the water delivered by the pipet.
- Buret Calibration. Calibrating a buret follows a procedure similar to that for a pipet, except that multiple volumes are delivered. The internal bore of a buret is not perfectly cylindrical and is somewhat "wavy," so the actual volume delivered will differ both above and below the nominal volumes marked on the buret as larger volumes are delivered. Determine the volume at 20% full-volume increments (e.g., every 10 mL for a 50-mL buret) by filling the buret each time and delivering the nominal volume into a dry flask. (The buret is filled each time to minimize evaporation errors. You may instead make successive deliveries into the same flask, filling the buret just once and carrying out the deliveries quickly.) Because the volume delivered does not have to be exact, though it should be near the nominal volume, you can make deliveries fairly quickly, but allow roughly 10 to 20 s for the film to drain. Graph volume correction against nominal volume and connect the points with straight lines. Interpolate at in-between volumes using those lines. Usual volume corrections for a 50-mL buret can extend up to about ±0.05 mL.
