GENERAL PROCEDURES

Cleaning glassware

Glassware should be scrupulously clean. However, clean does NOT mean dry! Clean means that the glassware does not contain any contaminants.

Clean glass will sheet deionized (DI) water rather than retaining droplets on the walls. To get the glass clean, use soap and a copious amount of tap water for an initial cleaning.(Remember, the soap itself can be a contaminant! So use only if absolutely necessary.) When glassware appears clean, rinse clean glassware with SMALL amounts of deionized water three times. (“Small” is about 1/10 the entire volume or 5 mL, whichever is smaller.) Coat all areas which may contact solution in a rinse. This not only conserves costly DI water, but is actually more effective than filling the entire container.

Before using, rinse glassware cleaned in the procedure above three times with the solution it will soon contain (or solvent, is the glassware is used to make a solution). After this procedure, the glassware will contain only your solution so it is therefore "clean".

If soap and tap water is not sufficient to clean the glassware, trade glassware with TA so that the TA can use more extreme cleaning techniques.

Use of Volumetric Glassware

Volumetric glassware measures volume precisely, usually to four significant figures. Volumetric glassware includes: volumetric flasks, burets and volumetric pipets. It does not include graduated cylinders, beakers or graduated pipets. Volumetric glassware is costly and should be treated with care. Volumetric glassware must be used properly to obtain this level of precision. Glassware may be labeled “TD” which means “to deliver” (typical of pipets) or “TC” which means “to contain” (typical of volumetric flasks). It is calibrated such that the specified volume is delivered or contained when the measuring line is at eye level and the bottom of the meniscus is at the line. For example, a 200 mL volumetric flask contains 200 mL of solution, but could not be used to deliver 200 mL of solvent to another container.

While the glassware should be “clean”, it does not have to be dry (see above). This is fortunate since volumetric glassware must NEVER be put into an oven. It will change the calibration. In addition, solutions should not be stored in volumetric glassware. Difficult to remove precipitates that may form or reactions with the glass may occur when the solution is in contact with the glass for a long period of time. If you will be using a solution for more than one day, transfer it to a storage bottle.

Funnels are recommended in the transfer liquids to avoid spillage and waste. However, when you use a funnel, be aware that funnel may hold a larger volume than the receiving vessel. Once you have added solution to a funnel, you have committed to adding it to the receiving flask. Adding too much solution causes it to overflow. Therefore, add solution slowly and in small increments. If solution does not flow quickly through a funnel, it is often because there is no exit for the air in the flask. This problem can be avoided by lifting the funnel slightly either by hand or folding a small piece of paper over a portion of the rim of the receiving flask. Funnels may also be a source of contamination and should, therefore, be cleaned and rinsed with the solution they will contain before using. (See procedure for cleaning glassware above, it is the same one!)

Pipet. Currently there are several types of pipets. All of them are designed to deliver a precise volume. The use of a pipet depends on the type of pipet and type of bulb used. In this class you will be using a standard volumetric pipet and bulb and the following procedure refers to that type.

1)Use bulb to draw up a small amount of solution FROM A BEAKER. Rinse entire pipet with this solution and discard into waste. 2) Repeat twice more with fresh solution. 3)Use bulb to draw fresh solution up past the measuring line. 4)Quickly remove bulb and replace with INDEX finger. 5)Continue stoppering the pipet with index finger and wipe outside of pipet to remove drops from outside. 6) Loosen index finger (do not remove) and allow liquid level in pipet to drop so that when the line is at eye level, the bottom of the meniscus is just at the line. If the meniscus goes below the line, draw up more solution past to past the line and repeat from step 5. (you do not have to allow all the solution to flow out, stopper with index finger while repositioning and just add the extra!) 7)Touch the end of the pipet to the wall of the beaker at a slight angle. This will remove any dangling drops. 6)Allow pipet to drain, by gravity, into the receiving container. 7)Touch tip of pipet to container to remove dangling drops. You are done.

NEVER, EVER, EVER: put pipet in an oven, blow out or shake out any remaining liquid (it is supposed to be there), or pipet from a volumetric flask. Avoid using thumb to control volume. If the liquid gets into the bulb, trade TA for a new bulb and clean the pipet before attempting to use it again.

Burets. Rinse the buret, including the area below the valve, three times with a small volume of titrant. Fill the buret using a funnel which has been rinsed with titrant. Remove the funnel from top of buret and remove any air bubbles from the tip before making any readings and performing a titration. Always record both initial (even if it is zero, so why work too hard?) and final volumes by reading the bottom of the meniscus, at eye level, to two decimal places. This requires that the last decimal place be estimated. You may estimate a zero, but you must have two decimal places. Getting the buret at eye level may require removing the buret from the buret stand to do the reading. This is a GOOD thing, do you really believe the stand holds the buret level? If an initial or final reading is not within the graduation (or not recorded), it is impossible to get an accurate volume and the experiment must be repeated.

Be sure to rinse all the titrant into the reacting solution, so that the volume actually reacting is the one being recorded.

Volumetric Flasks. Before using a volumetic flask, rinse the flask with the solvent and capped, turn upside down. If the flask leaks, try a different cap. If the flask leaks while mixing your solution, you will have to discard the solution and make a new one, so it is best to test for leaks early and with solvent.

Introduce your solute to the volumetric flask using a funnel. Using a stirring rod can help direct flow of liquid and prevent spills when pouring from a beaker or similar container. Use a wash bottle to rinse all the solute from funnel, stirring rod and first container (if appropriate) a minimum of FOUR times after it looks like all the solute has been transferred.

If your solute is originally in the form of a solid, a powder funnel should be used or the solid should first be dissolved in a small beaker (or both).

After all the solute has been transferred, add enough solvent to reach almost to the neck of the flask. Mix solution completely (for at least 2 minutes), by inverting the tightly capped flask and shaking then returning it to an upright position (do this repeatedly). When the solution is completely mixed, add solvent until the bottom to the meniscus reaches the line. The last portion should be added dropwise to prevent overshooting the mark. Mix again.

IF AT ANY TIME THE VOLUME OF SOLUTION GOES ABOVE THE MARK OR SOLUTE IS SPILLED, THE ENTIRE SOLUTION MUST BE DISCARDED AND YOU MUST START OVER.

Weighing.

Balances. You will be expected to weigh by difference for all experiments. DO NOT TARE THE BALANCES. Instead, record the mass of the weighing bottle, tap out the amount you need into a receiving flask and reweigh bottle. This means your receiving flasks can be wet and you may use the final weight of the first trial and your initial weight of the second trial! Most receiving flasks are too heavy for the balances anyway. No chemicals should be placed directly on the balance pan.

Most directions call for you to “accurately weigh about” some value. This means that while you must record accurately (four decimal places with an analytical balance) the mass of substance used, it does not have to be exactly the mass called for. It should, however, be in that range. An appropriate range is about ±1 of the last digit called for in the directions. For example, to “accurately weigh about 0.4 g”, 0.4551 g or 0.3762 g is probably acceptable, but 0.5187 g would not be.

Drying. To get accurate masses, most substances must be dried to remove any water absorbed from the atmosphere then stored in a dessicator so that it will not reabsorb the water. The drying conditions will be specified in the procedure. Substances should be dried in weighing bottle which must be labeled with your initials and the substance. Anything which goes in the oven should be labeled with a permanent marker. Labeling tape will make a real mess when heated. The weighing bottles may be put in beakers to prevent tipping. Caps should be heated with the bottles, but NOT on the bottles. (How else will the water escape?) After heating the caps should be placed on the bottle. This will make contamination and spillage less likely.

In addition, substance must be weighed at ambient temperature. This generally requires 30 minutes of cooling in a dessicator before weighing.

pH Meters

General reminders. The sensing portion of a pH electrode is a special type of glass located on the bottom of the electrode. The response is due to an equilibrium between the glass and the solution. Thus, that part of the electrode must be completely submerged and treated with care due fragile glass. In addition, time must be allowed for the glass and solution to come to equilibrium. The time required will increase with the difference between the solutions measured. There is a salt bridge above the sensing device which must also be submerged for the electrode to work properly.

The electrode should be submerged in deionized water or pH 7 buffer when not in use. If allowed to dry out, the electrode becomes unusable. Sometimes the electrode might be renewed if submerged in deionized water for a minimum of 24 hours.

The electrode must also be rinsed before transferring to a new solution so that the old solution is not transferred to the new one. The best readings are obtained for solutions which are quiescent rather than ones which are stirred (although this is not required). Measurements of pH from a stirred solution should never be compared to those from a quiescent one.

Calibration. The electrode should always be calibrated before use (a minimum of once a day). A two point calibration, (using two different buffers) where the buffers bracket the range to be measured is preferred. A reading is only as good as the calibration. Standard electrodes and buffers are good to two decimal places. More precise measurements require both more precise electrodes and more precise buffer. Directions for calibration and electrode use are included with your electrode.