2.3.6 Reading Volumetric Ware

Fig. 2.16 By using the lines above and below the one you are sighting on, you can avoid parallax.

you read the top of the meniscus. Incidentally, if the glassware is dirty, a smooth meniscus cannot form and proper sighting is impossible; therefore, clean volumetric glassware is essential.

It may take a bit of practice to properly see the correct part of the meniscus line for accurate measurement. Fortunately, there are tricks and devices to facilitate the reading. For instance, if the graduation lines on the volumetric ware mostly encircle the tube, it is easy to line up your vision so that you can avoid parallax problems (see Fig. 2.16).

If the meniscus is difficult to see, you can make it stand out by placing a piece of black paper behind the glass tube, below the liquid line. The liquid picks up the dark color and, like a light pipe, the end of the liquid column will have a dark line. Alternatively, the paper can be folded around the tube and can be held by a paper clip (like a French cuff). This "paper cuff' can be raised or lowered easily, and it frees both hands. Black paper is inexpensive and easy to use, but if the paper gets wet it will need to be replaced. This problem is not major, but if it occurs midexperiment, it can be an inconvenience.

Another technique that does not require paper uses a black ring cut from flexible tubing. These rings can be purchased to fit a variety of tube sizes, but if you have a hose of the right size available, they are easy to make by cutting out a disk from black tubing or by slicing a disk from a black rubber stopper (and a hole) and then cutting a wedge out of either disk (see Fig. 2.17). By sliding this ring up or down so that it is below the top of the liquid column, the meniscus can be easily seen.

2.3.7 General Practices of Volumetric Ware Use

After using volumetric ware, wash if necessary and always rinse thoroughly, first with water and then with distilled water. Although it is not necessary to dry to deliver volumetric ware between measurements (if the same chemical is being

Fig. 2.17 A piece of black flexible tube that facilitates seeing the meniscus.

measured), it is necessary to dry the insides of to contain containers before use. Reagent grade acetone or methyl alcohol may be used to facilitate drying.

If using volumetric ware for gravimetric calibration, unintended weight (i.e., dirt or fingerprint oils) may alter the results of any weighing. Therefore, be very careful where you place volumetric ware on a bench and how you hold it. You may choose to use cotton gloves to prevent fingerprint marks or handle all glassware with tweezers or tongs.

Excess solution on the walls (inside or out) of volumetric ware adds weight to measurement, but are not considered part of the volume. Therefore, be careful not to splash your solution into the container. Such splashing can cause drops to settle on the side walls. To prevent splashing, place the tip of a burette or pipette against the walls of the receiving container so the liquid slides down the walls. This procedure should always be done above the calibration line. Wait a few minutes for any residual fluid along the walls to settle before making any final volume determination. Because fluids may cling to the walls of the container, it may take a few moments for everything to drain to its lowest level (for an exaggeration, think of honey). During the settling period, cap the container to limit the amount of fluid that would otherwise evaporate.

Do not leave alkaline materials in glass volumetric ware. Aside from the damage they may do to the volumetric ware, dissolved glass can affect the pH of your solution. In addition, a glass plug or stopcock may freeze (stick) in place if left in contact with an alkaline solution. This freezing prevents the stopcock or plug from being turned or removed. A long-standing alkaline solution can roughen a surface of the glass barrel even in a Teflon stopcock. The rough surface can later scratch the surface of the Teflon plug when it is rotated.

2.3.8 Calibrations, Calibration, and Accuracy

The volumetric marks on volumetric ware are called calibrations. How they were located on the volumetric ware is called calibration. All volumetric ware is calibrated to provide its stated volume at 20°C. The International Standards Organization has recommended that the standard volumetric temperature should be changed to 27°C. However, so far there has not been any significant movement toward this goal. The ASTM recommends that those labs in temperate climates that are unable to maintain an environment at 20°C should maintain one at 27°C.

There are many labs that not only are unable to maintain a 20°C temperature, but cannot maintain any temperature consistently. Temperature variations can create problems if you are attempting to do accurate work. The more accurate the work, the greater the need to maintain room temperature at 20°C. Alternatively, include the temperature of the liquid at the time of measurement in your experiment log. Then, at a later time, apply corrections to all measurements made to conform to ASTM 20°C measurements. However, do not waste your time doing unnecessary work. The correction for any given measurement may be irrelevant if either (1) its result is smaller than the tolerances of your equipment (2) it involves

a From the ASTM document E 542, Table 4, reprinted with permission.

*The term "reproducibility" refers to the maximum difference expected between two independent determinations of volume.

more significant figures than can be supported by other aspects of your work,* or

(3) you do not require that level of accuracy.

Another concern for accuracy is based on how accurately the user can read calibrations on the volumetric ware. The reproducibility of an individual user will be more consistent than the reading made by a variety of users. Therefore, if there will be a variety of users on any given apparatus, all who are likely to use it should make a series of measurements. This way, the individual errors can be calculated. The ASTM has analyzed the range of errors made by trained personnel, and the reproducibility of these results are shown in Table 2.9.

*For example, suppose you are calculating your car's "miles per gallon." It is a waste of time to measure the gas to hundredths of a gallon if your odometer can only read tenths of a mile.