hydroxide (Na(OH)2 or K(OH)2) because sodium and potassium fluorides are highly poisonous.

4.1.11 Extra Cleaning Tips

Cleaning Fritted Filters. Because of the nature of fritted glass, its cleaning requires special mention. Fritted glass is crushed glass (Ace Glass uses crushed glass fibers) separated by size, compacted together, then heated until the pieces stick together. Because the structure is loosely connected, the frit maintains its porous nature, which provides the frit its filtering abilities.

Because different materials get onto and in a frit, different cleaning processes are required to clean a frit. For this reason, storage is an important aspect of frit cleaning. If you know what got on a frit, you know how to clean it. Keeping a frit in a clean, dry, and dust-free area will simplify your cleaning needs. Before a fritted filter is first used, it should be rinsed with hot sulfuric acid and then with distilled water until the filtrate is neutral in pH. This procedure will remove any glass particles and dust that may be on the new frit.

After a frit has just been used, it is important to flush it immediately with distilled water in the opposite direction that it was used. Then proceed with a chemical cleaning. Once the fritted glass is cleaned, rinsed, and dried, store it in a clean, dry, and dust-free area. Some suggested contamination and appropriate cleaning solutions are as follows :

•Fats; Carbon tetrachloride

•Silver chloride; Ammonia

•Organic materials: Potassium permanganate

•General cleaning: Potassium permanganate^

Because of the effects of basic solutions on glass, never let a basic solution (such as NaOH or KOH) remain in contact with a fritted glass filter. Rinse with hydrochloric acid and then with distilled water until the filtrate is neutral in pH.

Removing Tungsten from Glassware. A dark film of tungsten is often deposited on the walls of components of glass vacuum systems having tungsten filaments. This deposit hinders observation within the glassware and can also cause an increase in wall temperature.

Although a solution of HF, or a mixture of HF and HNO3, can remove the tungsten stain, it also removes some of the glass, which can etch the glass and thereby increase pumping time due to increased surface area. If the HF solution is left too

*Treat the filter with a 1% aqueous solution of potassium permanganate followed by a few drops of concentrated sulfuric acid. The heat of the reaction will increase the oxidizing power of the mixture. tTreat the filter with a 1% aqueous solution of potassium permanganate followed by a few drops of concentrated sulfuric acid. The heat of the reaction will increase the oxidizing power of the mixture.

Flask

Beaker

Fig.4.4 Removing tar from a flask.

long, the glass can become dangerously thin for a vacuum system.

A tungsten coating can be removed quite successfully with a hypochlorite solution (Clorox bleach, for example). A black opaque coating can be removed in a few minutes, and, if the solution is warmed, the process is even faster. The solution can be reused many times.

Removing Tar from Distillation Flasks. As mentioned in the beginning of this section, the best time to clean glassware is immediately after it is used. The hardest glass pieces to clean are those that have been left for someone else to clean. Tar, however, is always hard to clean and is always left for last (which also makes it more difficult to clean). A simple solution for cleaning tar from the bottom of flasks is to invert the flask into a beaker of acetone and let the vapor of the solvent dissolve the tar (see Fig. 4.4). This technique can remove most of the tar deposit in a few hours. It can be hastened by heating the solvent in a steam bath (do not use an open flame).

The trick to this technique is getting the solvent into the neck of the upsidedown flask. One method is to lightly heat the flask before inserting it into the solvent. Then, as the flask cools, the solvent will be drawn into the flask. Perhaps the simplest method is to place a flexible tube into the neck to allow air out of the flask as it is inverted into the beaker (be sure the tube is not soluble in the solvent).

4.1.12 Additional Cleaning Problems and Solutions15

In the following cleaning procedures, all work should be done in a fume hood, hand and eye protection should be worn, and a lab apron is recommended. After cleaning, an item should be thoroughly rinsed in water followed by a rinse in distilled water. If an item needs to be dried, a final rinse in reagent-grade methanol, ethyl alcohol, or acetone can be used.

Iron Stains. Equal parts hydrochloric acid and water should be used.

Mercury Residue. Use hot nitric acid. The acid used should be collected and returned to your safety supervisor because it now contains a heavy metal and should not be rinsed down the sink under any circumstances.

250 Cleaning Glassware

Permanganate Stains. Use concentrated hydrochloric acid or a saturated solution of oxalic acid at room temperature.

Bacteriological Material. Soak the glassware in a weak Lysol® solution, or autoclave in steam. A number of sources recommend soaking the glassware in a 2% to 4% cresol solution. However, the EPA has recently identified cresol as a hazardous material. Because other options are readily available (see Sec. 4.1.1 through Sec. 4.1.9), there is no reason to use the material. If you have any cresol, contact a hazardous materials disposal firm in your area for removal.

Albuminous Materials. Soak in nitric acid.

Carbonaceous Materials. Soak in a solution of equal parts sulfuric and nitric acid. Be aware of the possible formation of dangerous compounds when organic material is present.

Magnesium Oxide Stains. Instead of HC1, try a 20% to 30% solution of sodium bisulfite (NaHSO3). Because an acid is not being used, SO2 is not liberated.14

4.1.13 Last Resort Cleaning Solutions15

The following solutions should only be used as last resorts for cleaning glassware because of the dangers each present. Each is meant to be used as a soaking solution for glassware and should be left in a fume hood at all times. After soaking, rinse off the solution with copious amounts of water. Before using any of the following materials, strongly consider whether the glassware is worth cleaning.

Alkaline Peroxide. This material is good for removing organics.

Place 100 g of NaOH (sodium hydroxide) into a 500-mL glass beaker. Slowly add 250 mL of water. This solution will get hot (120°C), bubble, and fizzle. Therefore gloves and eye protection are a must.

This material can etch glass if it is left to soak for an extended period of time, therefore you might wish to select an old, worn beaker in which to do the mixing. You must use a glass beaker because of the exothermic reaction that is created when the mixture is first mixed.

If this solution is in a container and allowed to get cold, it can begin to deposit out of solution. The solid can be removed by slowly adding water or a dilute acid.

Alkaline Permanganate. This material is good for removing siliceous deposits and acidic crud.

Place 500 g to 1 kg of NaOH (Sodium Hydroxide) in a 5-liter beaker. Add 4 V2 liters of water. Once dissolved, add 50 mg of KMnO4 (potassium permanganate). This solution will be purple and, after leaving it overnight to allow it to become active, will turn green.

This mixture can be used up to one year. It should be covered to reduce evaporation. During that time some water will evaporate leaving a brown crust of MnO^ This crust can be knocked back into the remaining solution. If too much water has evaporated, simply add water to original levels. If the solution leaves a brown deposit on glassware, the deposit can be removed with HC1 (hydrochloric acid).