Basic Laboratory Operations
A desiccator is a vessel, usually of glass but occasionally of metal, which is used to equilibrate objects with a controlled atmosphere. Since the desiccator usually stands in the open, the temperature of this atmosphere generally approaches room temperature. It is normally the humidity of this atmosphere which is of interest. Objects such as weighing bottles or crucibles, and chemical substances, tend to pick up moisture from the air. The desiccator provides an opportunity for such materials to come to equilibrium with an atmosphere of low and controlled moisture content so that errors due to the weighing of water along with the objects can be avoided. A common type of desiccator is shown in Fig. 1.
The nature of the drying agent placed in the bottom of the desiccator determines the equilibrium partial pressure of water vapor in the desiccator space.
Table 1 show the efficiencies of various chemical desiccators. Common drying agent were studied by passing down volumes of wet nitrogen over the material, condensing the residual water in a liquid nitrogen trap, and weighing the water. The most powerful desiccant is not necessarily the best for a given application. Phosphorus pentoxide, for example, has a tendency to acquire a surface glaze as it picks up water, which prevents the bulk of the material from being effective. Calcium chloride, while rather poor with regard to equilibrium water vapor pressure, is inexpensive, has a fairly high capacity, and is adequate for most analytical work.
Table 1. Efficiency of Chemical Desiccants
Capacity must be distinguished from equilibrium vapor pressure in describing desiccant. A desiccant may have a high capacity (that is, it may be able to pick up a large weight of water vapor per unit weight of desiccant), but still leave much moisture in the air at equilibrium.
After reagents or objects such as crucibles have been dried in the oven, or perhaps at even higher temperatures, they are usually cooled to room temperature in the desiccator, a partial vacuum is created, and care must be taken in opening the vessel lest a sudden rush of air blow material out of a crucible or disturb the desiccant itself. For this reason, and also because glass is a very poor conductor of heat, it is usually best to allow a very hot object to cool well toward room temperature before it is placed in the desiccator. After a hot object has been placed in the desiccator, it is well to cover the vessel in such a way as to leave a small opening at one side. This allows air displaced by the warm object to recenter as the object cools, and hence minimized the tendency to form a vacuum. The desiccator is completely closed during the final stages of cooling,
The desiccator cover should slide smoothly on its ground-glass surface. This surface should be lightly greased with a light lubricant such as Vaseline (never stopcock grease!). Needless to say, the desiccator should be scrupulously clean and should never contain exhausted desiccant. After filling the desiccant chamber, beware of dust from the desiccant in the upper part of the desiccator.
Some common types of pipets are shown in Fig. 2. The transfer pipet is used to transfer an accurately-known volume of solution from container to another. It is filled by gentle suction to about 2 cm above the each line [Fig. 3(a)], using an aspirator bulb for toxic or volatile liquids.
The tip of the pipet should be kept well below the surface of the liquid during the filling operation. The forefinger is then quickly placed over the top of the pipet [Fig. 3(b)], and the solution is allowed to drain out until the bottom of the meniscus coincides with the etched line. Any hanging droplets of solution are removed by touching the tip of the pipet to the side of the beaker, and the stem is wiped with a piece of tissue paper to remove drops of solution from the outside surface. The contents of the pipet are then allowed to run into the desired container, with care being taken to avoid spattering. With the pipet in a vertical position, allow the solution to drain down the inner wall for about 30 sec after emptying, and then touch the tip of the pipet to the inner side of the receiving vessel at the liquid surface. A small volume of solution will remain in the tip of the pipet, but the pipet has been calibrated to take this into account; thus this small final quantity of solution is not to be blown out or otherwise disturbed. Pipets with damaged tips are not to be trusted.
Measuring pipets are graduated much like burets and are used for measuring volume of solution more accurately than could be done with graduated cylinders. However, measuring pipets are not ordinarily used where high accuracy is required.
Two types of micropipets are also shown in Fig. 2. The so-called lambda pipets are available in capacities of 0.001 to 2 mL, where 0.001 mL = 1 lambda. They are filled and emptied using a syringe. Those calibrated to contain a certain volume are rinsed with a suitable solvent. Those calibrated to deliver are not rinsed, but the last drop is forced out of the pipet with the syringe. Hamilton Microliter Syringes are widely used for delivering small volumes in such operations as gas chromatography. They can be brought equipped with stainless steel tips for use in injecting a sample into a closed system. The syringe shown in Fig. 2 has a capacity of 0.025 mL (25 microliters, or 25 lambdas) and the smallest divisions correspond to 0.0005 mL.
Fig. 3. (A) Filling pipet - liquid drawn above graduation mark
and (b) use of forefinger to adjust liquid level in pipet.
The National Bureau of Standards specifies 20¡É as the standard temperature for calibration of volumetric glassware. The use of such glassware at other temperatures leads to errors. However, the errors are normally small, and pipets can be used at "room temperature" without special precautions except in work of highest accuracy.
Pipets with dirty inner walls from which solutions do not drain properly can obviously give rise to errors. It is advisable to clean pipets with cleaning solution at frequent intervals.
A common form of buret is shown in Fig. 4(a). The buret is used to deliver accurately-known but variable volumes. mostly in titrations, The stopcock plug is made of either glass or Teflon. The Teflon stopcock required no lubrication, but the glass plug should be lightly greased with stopcock grease (not one containing silicones.). If too heavy a coating is applied the stopcock may leak and also some of the grease may plug the buret tip. To lubricate a stopcock, remove the plug and wipe old grease away from both plug and barrel with a cloth or paper tissue. Make sure the small openings are not plugged with grease (pipe cleaners are helpful in this event).
Fig. 4. (A) Buret and (b) method of grasping stopcock.
Then spread a thin, uniform layer of stopcock grease over the plug. keeping the application especially this in the region near the hole in the plug. Finally, insert the plug in the barrel and rotate it rapidly in place, , applying a slight inward pressure. The lubricant should appear uniform and transparent, and no particles of grease should appear in the bore.
Buret must be cleaned carefully to assure a uniform drainage of solutions down the inner surfaces. "Cleaning solution" may be used for this purpose, applied hot for a few minutes or overnight at room temperature. Detergents are also satisfactory, especially if used in conjunction with a long-handled buret should be filled with distilled water and capped (paper cups or small breakers are convenient) to prevent the entry of dust.
It is poor practice to leave solutions standing in burets for long periods. After each laboratory peroid, solutions in burets should be discarded, and the burets rinsed with distilled water and stored as suggested above. It is especially important that alkaline solutions not stand in burets for more than short periods of time. Such solutions, which attack glass, cause stopcocks to "freeze", and the burets may be ruined.
The beginner must be very cautions in reading burets. In order to become familiar with the graduations and adept at estimating between them, much practice is needed early in the laboratory work. An ordinary 50-mL buret is graduated in 0.1-mL intervals and should be read to the nearest hundredth of a milliter. An aqueous solution in a buret (or any tube) forms a concave surface referred to as a meniscus. In the case of solutions that are not deeply colored, the position of the bottom of the meniscus is ordinarily read (the top is taken if the solution is so intensely colored that the bottom cannot be seen, e. g., with permanganate solutions). It is most helpful to cast a shadow on the bottom of the meniscus by means of a darkened area on a paper or card held just behind the buret with the dark area slightly below the meniscus. Great care must be taken to avoid parallax errors in reading burets: the eye must be on the same level with the meniscus. If the meniscus is near a graduation that extends well around the buret, the right eye-level can be found by seeking a position so that the graduation mark seen at the back of the buret just below the meniscus serves the same purpose.
Before a titration is started, it must be ascertained that there are no air bubbles in the tip of the buret. Such bubbles register in the graduated portion of the buret as liquid delivered if they escape from the tip during a titration, and hence cause errors. When a solution is delivered rapidly from a buret, the liquid running down the inner wall is somewhat detained. After the stopcock has been closed, it is important to wait a few seconds for this "drainage" before taking a reading.
Occasionally a "freak" buret with flagrant errors in the graduations appears on the market; for example, a buret has been found which has, on one section of the scale, eleven small 0.1-mL divisions between two adjacent milliliter marks. Even where no readily detectable error exists, the possibility of improper calibration is always present. For very careful work, the analyst often calibrates his volumetric glassware himself.
In performing titrations, the student should develop a technique that permits both speed and accuracy. The solution being titrated, generally in an Erlenmeyer flask, should be gently swirled as the titirant is delivered. One way to accomplish this, while retaining control of the stopcock and permitting ease of reading the buret, is to face the buret, with the stopcock on the right, and operate the stopcock with the left hand from behind the buret while swirling the solution with the right hand [Fig. 4(b)]. The thumb and forefinger are wrapped around the handle to turn the stopcock, and inward pressure is applied to keep the stopcock seated in the barrel. The last two fingers push against the tip of the buret to absorb the inward pressure.