Any hazardous waste cannot be simply thrown away. Such materials must be stored by the user and picked up by companies licensed to remove such materials. For the sake of this planet, do not ignore environmental laws.

4.1.9 Chromic Acid

Because of the long history of chromic acid use and because of the current environmental problems of heavy metals, chromic acid deserves separate comment.

Preparation. Mix 15-20 g K2Cr2O7 (potassium dichromate) and 50-100 ml water to dissolve. Then, slowly add 900 mL of concentrated H2SO4 (sulfuric acid). This solution gets very hot, so do not use a plastic container for mixing.

Use. Chromic acid effectively removes organic contaminants, and it has a long history of good service. Chromic acid is, by many standards, an excellent cleaner. By simply soaking your glassware in chromic acid, organic dirt and film deposits are removed. It is safe for glassware to be left in chromic acid for extended periods of time. It can be used over and over until it begins to turn green (it provides its own potency indicator). Chromic acid is safe for volumetric ware. It does not have the same problems of the base bath of dissolving the glass (and changing the volume) or of dissolving the ceramic lines on volumetric ware. There are, of course, safety precautions for its use because of its strong oxidation potential.

Chromic acid use is not recommended if you are doing experiments in conductivity because the chromic ion can coat the walls of the glassware.

Because chromic acid can be reused, it is often kept in large glass containers. It is important to keep a lid on the container that is (reasonably) air-tight. This containment is not so much for evaporation concerns, but rather because sulfuric acid can absorb 30% of its weight in water, including water from the atmosphere. As it absorbs atmospheric water, its volume changes, and it can overflow the container.

Disposal. Disposal is best done by reducing the dichromate to the insoluble chromium hydroxide with a sodium thiosulfate solution. The process for neutralizing 100 mL of chromate solution is as follows:

1. Add sodium carbonate (about 180 g) slowly (while stirring) until the solution is neutral to litmus paper (color goes from orange to green).

2.Re-acidify this solution with 55 mL of 3 M sulfuric acid (color returns to green).

3.While continuously stirring, add 40 g of sodium thiosulfate (Na2S2O3-5H2O). (The solution now goes to blue and cloudy.)

4.Neutralize the solution by adding 10 g of sodium carbonate. In a few minutes, a blue-gray flocculent precipitate forms.

5.Filter the solution through Celite or let stand for one week and decant the liquid. This liquid can be allowed to evaporate or be filtered through Celite. Regardless, the remaining liquid contains less then 0.5 ppm of chromium and can be washed down the sink (check local regulations).

246 Cleaning Glassware

grease was left on glassware that went through an oven (> 400°C), the nonvolatile remains of the grease are undoubtedly burnt into the surface. HF can be used to remove burnt silicon grease remains, decal identification markings, and some quantity of the glass. Thus, prolonged or repeated soaking of glassware in HF can easily remove enough of a glass item to make it useless. Volumetric ware should never be soaked in HF. It was found that cleaning a 100-mL flask with a 5% HF solution for five minutes increased the flask's capacity by 0.04 mL.

Pre-preparation. All greases and oils must be removed before treating with HF. Baking the glassware in an oven of at least 400°C is very effective for removing any organics and other volatile materials. However, such baking can burn inorganics into the glass surface, requiring more extensive soaking in HF which, in turn, could cause greater erosion of the glass.

Material. Hydrofluoric acid is available in plastic containers (in a concentration of 48%-52%). These containers should always be stored in a fume hood. Never store HF in glass containers.

Preparation. HF should not be used at full (bottle) strength (typically 48%). A 5% to 10% solution is best. Once prepared, it should be stored in a polyethylene or other strong plastic container that can be closed. Because HF is a glass stripper, do not store the acid solution in a glass container. HF, at any concentration, should always be stored in a fume hood.

Use. It is important to wear gloves and safety glasses and to work in a fume hood!! If you do not have all of these items for safe use of HF, do not use HF as a cleaning material! Pour the diluted HF from its plastic storage container into a plastic soaking tank (like a polypropylene beaker). Let the glassware soak in the HF for anywhere from 30 seconds to two minutes. Rinse for at least one minute (or five full flushes) with water. Then rinse with distilled or deionized water. The HF can be reused many times until a decrease in effectiveness is obvious.

Safety Considerations. Always wear rubber gloves with long arm covers when working with HF. It is hard to overemphasize how dangerous HF can be. One of the dangers in getting dilute HF on your skin is that it may initially be no more noticeable than water. If you get HNO3 on your skin, you know it instantly by the burning sensation. That burning sensation is telling you something is terribly wrong. Because of the seemingly inconsequential aspects of HF, you might be inclined simply to rinse off any suspected HF. However, even with some washing, you may not successfully remove all of the HF. If any HF gets on your skin, wash with copious amounts of soap and water for 10 minutes. If you get any HF under a fingernail, scrub thoroughly with a fingernail brush!! The treatment for an HF burn under the fingernail invariably includes removal of the fingernail. Once in the skin, HF spreads throughout all the surrounding tissue. Hydrofluoric acid burns can look similar to those of fire burns, but the development is in slow motion and can eat through to the bone. Remember, HF travels through body tissues doing damage to those tissues as it travels. If HF gets in the eye, blindness is highly probable. Always wear protective goggles when using HF.

The fumes from HF are also very dangerous and can cause burn blisters on the

linings of the lungs. Although not pleasant, HF is not as sharply irritating to breathe as other strong acids (such as HNO3), and you may not think to be repelled. Always use HF in a fume hood.

One safety trick is to add to your total HF solution about one or two percent nitric acid. This addition will not assist the cleaning properties to any significant amount, but it will add more effective warning characteristics to both the odor and any accidental skin contact. This practice is similar in concept to the addition of hydrogen sulfide to methane.

Disposal. Although HF is an acid, it cannot simply be neutralized and washed down the sink because the fluorine in the acid will attack any aluminum in the plumbing. Thus, HF, like chromic acid, requires special handling. A 5% solution of HF can be safely treated by mixing with a 1:1 ratio of the following solution :

2 g aluminum nitrate (A1(NO3)3-2H2O) 0.8 g Ca(OH)2

1 liter of water

After mixing the HF and the above neutralizing solution in a 1:1 ratio, the resultant mixture can then be poured down the sink.

Extra Tip: For an extra-tough stain, use a mixture of 3 mL of storage bottle strength (49%-52%) hydrofluoric acid to 100 mL of concentrated nitric acid. This solution should be used for only a minute or two at most and must be done in a fume hood. Use the same safety considerations that you would use for standard hydrofluoric acid.

Hydrofluoric acid alternative. Hydrofluoric and perchloric acid both aggressively attack glass. Because of the speed of attack on glass as well as the significant danger with working with either acid, Occasionally it may be safer (and easier) to a less agressive glass stripper. Ammonium hydrogen difluoride ((NH4)HF2) (a.k.a. ammonium bisulfate) has shown to be an appropriately aggressive glass stripper that provides safer (longer) "soaking" time and is much safer to use. To prepare a solution of ammonium bisulfate for glass "cleaning," dissolve approximately 240 - 250 grms of ammonium bisulfate into four liters of distilled water. It must be stored in a plastic (polyethylene is best) or metal container.

Soaking times can range from ten minutes to a half hour depending on the degree of contamination and the freshness of the solution. All grease must be removed prior to any soaking as this solution will not be able to remove the grease, and the grease will prevent the solution from making contact with the glass. Just because this is considered a "safer" glass stripper, it doesn't mean safety considerations can be ignored. Gloves, eye protection, proper clothing are still a must for using this material.

This solution can be reused till it shows significant signs of impaired performance. It can be disposed of by mixing in some slacked lime (calcium hydroxide Ca(OH)2) till any fizzing stops. The calcium and fluoride will combine to form solid CaF2 which is inert and safe to throw out. Do not use sodium or potassium