Commercially Accessible Microwave Acid Digestion Bombs

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Perhaps the most typical varieties of electrical equipment found in a laboratory are the units used to produce the heat wanted to impact a response or separation. These include ovens, sizzling plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, sizzling-tube furnaces, scorching-air guns, and microwave ovens. The use of steam-heated devices fairly than electrically heated units is mostly most popular each time temperatures of a hundred °C or less are required. Because they do not present shock or spark risks, they are often left unattended with assurance that their temperature will never exceed 100 °C. Use steam that's generated by items which can be devoted to laboratory use. Steam generated for general facility use could comprise contaminants that could interfere with laboratory work. Take quite a few common precautions when working with heating devices within the laboratory. If utilizing a variable autotransformer (variac), you should definitely wire (or rewire) new or current tools, as illustrated in Figure 7.3, earlier than use. However, temperature controllers with constructed-in safety interlock functionality are available from business sources and are most well-liked to variable autotransformers. Enclose the actual heating ingredient in any laboratory heating device in a glass, ceramic, or insulated metal case to prevent a metallic conductor or laboratory personnel from by accident touching the wire carrying the electric present. Such a building minimizes the danger of electric shock and of by accident producing an electrical spark close to a flammable liquid or vapor (see Chapter 6, section 6.G.1). It additionally diminishes the possibility that a flammable liquid or vapor will come into contact with wires at temperatures which may exceed its ignition temperature. As a result of many household appliances (e.g., sizzling plates and house heaters) do not meet this criterion, do not use them in a laboratory. Resistance gadgets used to heat oil baths shouldn't contain naked wires. If any heating gadget turns into so worn or damaged that its heating component is uncovered, either discard the gadget or repair it before it is used again. Schematic diagram of a correctly wired variable autotransformer. Use laboratory heating units with a variable autotransformer to control and restrict the input voltage to some fraction of the entire line voltage, usually 110 V. If a variable autotransformer is just not wired on this method, the change on it may or might not disconnect both wires of the output from the 110-V line when it is switched to the off position. Also, if this wiring scheme has not been adopted, and especially if the grounded three-prong plug shouldn't be used, even when the potential difference between the 2 output lines is simply 10 V, every output line could also be at a relatively excessive voltage (e.g., 110 V and a hundred V) with respect to an electrical floor. As a result of these potential hazards exist, each time laboratory personnel use a variable autotransformer with an unknown wiring scheme, prudent apply assumes that both of the output lines carries a potential of one hundred ten V and is able to delivering a lethal electric shock. The exterior cases of all variable autotransformers have perforations for cooling and ventilation, and some sparking could happen every time the voltage adjustment knob is turned. Therefore, locate these devices the place water and other chemicals cannot be spilled onto them and where their movable contacts is not going to be uncovered to flammable liquids or vapors. Mount variable autotransformers on walls or vertical panels and outside laboratory chemical hoods; don't simply place them on laboratory benchtops. Electrical enter strains, together with strains from variable transformers, to nearly all laboratory heating units have a potential of one hundred ten V with respect to any electrical floor; all the time view these strains as potential shock and spark hazards. Connections from these strains to a heating gadget must be each mechanically and electrically safe and fully coated with insulating materials. Do not use alligator clips to connect a line cord from a variable autotransformer to a heating gadget, particularly to an oil bath or an air bath, as a result of such connections pose a shock hazard. In addition they may slip off, creating an electrical spark and, maybe, contacting different metal elements to create an extra hazard. Make all connections by using, ideally, a plug-and-receptacle mixture, or wires with insulated terminals firmly secured to insulated binding posts. At any time when an electrical heating gadget is used, either a temperature controller or a temperature-sensing device should be used that may flip off the electric energy if the temperature of the heating system exceeds some preset restrict. Similar control units are available that can turn off the electric energy if the circulation of cooling water by a condenser is stopped owing to the lack of water strain or loosening of the water provide hose to a condenser. Independent temperature sensors must be used for the temperature controller and shutoff units. Fail-safe devices, which may be both purchased or fabricated, can stop the more serious problems of fires or explosions that will arise if the temperature of a reaction increases considerably due to a change in line voltage, the unintended lack of reaction solvent, or lack of cooling. Use fail-secure devices for stills purifying reaction solvents, as a result of such stills are sometimes left unattended for important durations of time. Temperature-sensing gadgets absolutely should be securely clamped or firmly mounted in place, maintaining contact with the item or medium being heated always. If the temperature sensor for the controller isn't correctly situated or has fallen out of place, the controller will continue to provide energy till the sensor reaches the temperature setting, creating a particularly hazardous scenario. Oil bath fire on account of a unfastened temperature sensor. A researcher walking past a laboratory seen a flame burning behind the closed sashes of the chemical fume hood. He determined that the oil in an oil bath was burning. Scorching plates, oil baths, and heating mantles that may melt and combust plastic materials (e.g., vials, containers, tubing) can cause laboratory fires, and the area across the gear needs to be cleared of these hazards prior to use. Bear in mind that dry and concentrated residues can ignite when overheated in stills, ovens, dryers, and other heating devices. Electrically heated ovens are commonly used in the laboratory to remove water or different solvents from chemical samples and to dry laboratory glassware. Never use laboratory ovens to prepare meals for human consumption. Buy or assemble laboratory ovens with their heating components and their temperature controls physically separated from their interior atmospheres. Small family ovens and related heating devices normally don't meet these necessities and, consequently, should not be utilized in laboratories. With the exception of vacuum drying ovens, laboratory ovens hardly ever stop the discharge of the substances volatilized in them into the laboratory environment. The volatilized substances could also be current in sufficient focus to kind explosive mixtures with the air contained in the oven (see Chapter 6, part 6.G). This hazard can be reduced by connecting the oven vent on to an exhaust system. Muffle furnace hearth. A laboratory specializing within the evaluation of paint samples was asked to analyze pigmented polypropylene. Step one of the analytical protocol called for ashing the sample in a muffle furnace. Do not use ovens to dry any chemical pattern that has even average volatility and may pose a hazard due to acute or chronic toxicity until particular precautions have been taken to make sure steady venting of the environment contained in the oven. (See Vignette 7.2.) Thus, do not dry most natural compounds in a traditional unvented laboratory oven. To avoid explosion, don't dry glassware that has been rinsed with an organic solvent in an oven till it has been rinsed once more with distilled water. Probably explosive mixtures will be formed from unstable substances and the air inside an oven. Bimetallic strip thermometers are most well-liked for monitoring oven temperatures. Don't mount mercury thermometers via holes within the tops of ovens with the bulb hanging into the oven. If a mercury thermometer is broken in an oven of any sort, close the oven and turn it off immediately to avoid mercury publicity. Keep it closed until cool. Take away all mercury from the cold oven with using applicable cleansing gear and procedures (see Chapter 6, part 6.C.10.8). After removal of all visible mercury, monitor the heated oven in a laboratory chemical hood until the mercury vapor focus drops beneath the threshold limit worth. Laboratory scorching plates are often used when solutions are to be heated to 100 °C or greater and the inherently safer steam baths can't be used because the supply of heat. As beforehand famous, use solely hot plates that have completely enclosed heating components in laboratories. Though virtually all laboratory hot plates at the moment bought meet this criterion, many older ones pose an electrical spark hazard arising from both the on/off switch located on the new plate, the bimetallic thermostat used to regulate the temperature, or each. Usually, these two spark sources are located within the lower a part of the recent plate in a area where any heavier-than-air and presumably flammable vapors evolving from a boiling liquid on the new plate would are inclined to accumulate. In principle, these spark hazards are alleviated by enclosing all mechanical contacts in a sealed container or through the use of strong-state circuitry for switching and temperature management. Nevertheless, in practice, such modifications are troublesome to include into a lot of the hot plates now in use. Warn laboratory personnel of the spark hazard related to these sizzling plates. Arrange any newly purchased scorching plates to avoid electrical sparks. Along with the spark hazard, outdated and corroded bimetallic thermostats in these units can ultimately fuse shut and ship full continuous present to a sizzling plate. This risk may be prevented by wiring a fusible coupling into the road inside the hot plate. On many brands of mixed stirrer/sizzling plates, the controls for the stirrer and temperature control will not be easily differentiated. Care must be taken to distinguish their features. A fire or explosion could happen if the temperature reasonably than the stirrer pace is increased inadvertently. Heating mantles are generally used to heat round-backside flasks, reaction kettles, and related response vessels. These mantles enclose a heating element in layers of fiberglass cloth. As long because the fiberglass coating just isn't worn or broken and no water or different chemicals are spilled into the mantle (see part 7.C.1), heating mantles pose minimal shock hazard. They're usually fitted with a male plug that fits into a feminine receptacle on an output line from a variable autotransformer. This plug mixture offers a mechanically and electrically secure connection. At all times use heating mantles with a variable autotransformer to regulate the input voltage. Never plug them straight into a 110-V line. Educated laboratory personnel ought to watch out to not exceed the enter voltage really helpful by the mantle producer. Larger voltages will trigger a mantle to overheat, melting the fiberglass insulation and exposing the naked heating ingredient. Some heating mantles are constructed by encasing the fiberglass mantle in an outer metal case that gives bodily protection in opposition to harm to the fiberglass. If such metallic-enclosed mantles are used, good observe is to ground the outer metallic case both by using a grounded three-conductor cord from the variable autotransformer or by securely affixing one end of a heavy braided conductor to the mantle case and the other end to a identified electrical ground. This apply protects the laboratory personnel against an electric shock if the heating aspect inside the mantle brief-circuits against the metal case. Putting the heating mantle on a laboratory jack and holding the flask or container being heated by clamps attached to a separate ring stand or grid work is the really helpful process. This allows for rapid elimination of heat in the case of overheating or exothermicity. When using oil, salt, or sand baths, take care not to spill water and different risky substances into the baths. Such an accident can splatter hot materials over a large space and trigger severe accidents. Electrically heated oil baths are often used to heat small or irregularly shaped vessels or to maintain a relentless temperature with a stable heat supply. For temperatures below 200 °C, a saturated paraffin oil is often used; for temperatures up to 300 °C, a silicone oil must be used. Care have to be taken with sizzling oil baths not to generate smoke or have the oil burst into flames from overheating. All the time monitor an oil bath by using a thermometer or other thermal sensing gadget to ensure that its temperature doesn't exceed the flash level of the oil getting used. For the same reason, fit oil baths left unattended with thermal-sensing devices that flip off the electric power if the bath overheats. Heat these baths by an enclosed heating factor, corresponding to a knife heater, a tubular immersion heater resembling a calrod, or its equal. The enter connection for this heating aspect is a male plug that fits a feminine receptacle from a variable autotransformer (e.g., Variac) output line. Alternatively, a temperature controller can be used to manage the temperature of the bath exactly. Temperature controllers are available that provide quite a lot of heating and cooling choices. Thermocouples utilized by controlling gadgets must be clamped securely in place to take care of contact with the medium or object being heated at all times. Oil baths have to be properly combined to make sure that there aren't any scorching spots around the elements that take the surrounding oil to unacceptable temperatures. This drawback will be minimized by inserting the thermoregulator pretty close to the heater. Comprise heated oil in either a metal pan or a heavy-walled porcelain dish; a Pyrex dish or beaker can break and spill hot oil if struck by accident with a tough object. Mount the oil bath rigorously on a stable horizontal help similar to a laboratory jack that can be raised or lowered simply without danger of the bath tipping over. Always clamp gear high enough above a scorching plate or oil bath that if the reaction begins to overheat, the heater will be lowered immediately and changed with a cooling bath with out having to readjust the clamps holding the equipment setup. Never help a bath on an iron ring due to the larger chance of unintentionally tipping the bath over. Present secondary containment in the event of a spill of hot oil. Put on proper protective gloves when handling a scorching bath. Molten salt baths, like scorching oil baths, offer the benefits of excellent heat switch, commonly have the next operating range (e.g., 200 to 425 °C), and should have a excessive thermal stability (e.g., 540 °C). The response container used in a molten salt bath should have the ability to withstand a really speedy heat rise to a temperature above the melting point of the salt. Care must be taken to keep salt baths dry, because they are hygroscopic, a property that may cause hazardous popping and splattering if the absorbed water vaporizes throughout heating. Hot air baths could be helpful heating devices. Nitrogen is most popular for reactions during which flammable materials are used. Electrically heated air baths are frequently used to heat small or irregularly formed vessels. Due to their inherently low heat capacity, such baths normally should be heated considerably above the specified temperature (≥100 °C) of the vessel being heated. Buy or construct these baths so that the heating factor is totally enclosed and the connection to the air bath from the variable autotransformer is both mechanically and electrically secure. These baths may be constructed from metal, ceramic, or, less desirably, glass vessels. If a glass vessel is used, wrap it totally with heat-resistant tape so that if the vessel breaks by accident, the glass shall be contained and the naked heating factor is not going to be exposed. Fluidized sand baths are normally most well-liked over air baths. Tube furnaces are sometimes used for high-temperature reactions under lowered strain. The right selection of glassware or metallic tubes and joints is required, and the procedures ought to conform to safe follow with electrical equipment and evacuated apparatus. Laboratory heat guns are constructed with a motor-pushed fan that blows air over an electrically heated filament. They're often used to dry glassware or to heat the higher components of a distillation apparatus during distillation of high-boiling level supplies. The heating ingredient in a heat gun sometimes becomes purple-hot during use and, necessarily can't be enclosed. Additionally, the on/off switches and fan motors are not often spark-free. Moreover, heat guns are designed to pull lab air into and throughout the pink-sizzling heating components, thereby increasing the ignition threat. For these reasons, heat guns virtually all the time pose a critical spark hazard (see Chapter 6, part 6.G.1). By no means use them near open containers of flammable liquids, in environments the place appreciable concentrations of flammable vapors may be present, or in laboratory chemical hoods used to take away flammable vapors. Family hair dryers may be substituted for laboratory heat guns only if they've three-conductor line cords or are double-insulated. Any handheld heating machine of this kind that can be utilized in a laboratory should have GFCI safety to make sure towards electric shock. Use microwave ovens specifically designed for laboratory use. Domestic microwave ovens should not acceptable. Microwave heating presents several potential hazards not generally encountered with other heating methods: extremely rapid temperature and strain rise, liquid superheating, arcing, and microwave leakage. Microwave ovens designed for the laboratory have built-in security options and operation procedures to mitigate or get rid of these hazards. To avoid exposure to microwaves, by no means operate ovens with the doors open. Don't place wires and different objects between the sealing floor and the door on the oven's front face. Keep the sealing surfaces completely clear. To avoid electrical hazards, the oven should be grounded. If use of an extension cord is important, use only a 3-wire cord with a rating equal to or greater than that for the oven. To reduce the danger of fireplace in the oven, do not overheat samples. The oven have to be carefully watched when combustible materials are in it. Don't use metallic containers or metallic-containing objects (e.g., stir bars) in the microwave, as a result of they can cause arcing. On the whole, do not heat sealed containers in a microwave oven, because of the hazard of explosion. If sealed containers have to be used, choose their supplies fastidiously and the containers correctly designed. Commercially available microwave acid digestion bombs, for example, incorporate a Teflon sample cup, a self-sealing Teflon O-ring, and a compressible stress-relief valve. Do not exceed the producer's loading limits. For such purposes, correctly vent the microwave oven using an exhaust system. Inserting a big item, akin to a laboratory microwave or an oven, inside a chemical fume hood isn't really helpful. Heating a container with a loosened cap or lid poses a major threat. Microwave ovens can heat material (e.g., solidified agar) so shortly that, although the container lid is loosened to accommodate growth, the lid can seat upward against the threads and the container can explode. Screw caps have to be removed from containers being microwaved. If the sterility of the contents must be preserved, screw caps may be changed with cotton or foam plugs.


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