XVIII. NMR Spectroscopy
A.NMR Sample Preparation
1.1H NMR
Easily accessible instruments: Gemini 200, XL-200A, QE-300.
200 and 300 MHz
Deuterated solvents must be used in order to lock onto the sample. Approximately 0.4ml of sample in a 5mm NMR tube is necessary for the detector to observe the sample. A concentration of 0.005 to 0.01 M is usually high enough to get a decent spectrum.
2.31P NMR
Easily accessible instruments: XL-200A, QE-300
Deuterated solvents must be used to lock onto the sample, may beas little as 30% deuterated. Approximately 0.4 mL of sample in a 5mm NMR tube is needed. The sensitivity of 31P is approximately 16 times lower than 1H. Therefore, the sample must be more concentrated and/or more transients must be acquired.
3.13C NMR
Easily accessible instruments: Gemini 200, XL-200A, QE-300
Deuterated solvents must be used to lock on the sample. A 5mm tube can be used with the QE-300 with at least 30% deuterated solvent to lock on.
4.Problems and Cautions:
a.Particulate matter in the solution will cause problems in resolution. The solid should be removed by filtration or the solid can be collected in the top or the bottom of the NMR tube by using the centrifuge.
b."Good" NMR tubes must be used. "Good" tubes are labeled 528PP near the top of the tube. The other tubes are thin walled and could break in the probe and cause damage.
XIX. Purification Techniques
A.Sublimation
Compounds which have a sufficiently high vapor pressure below their melting points can often be purified by sublimation. The process involves the direct vaporization of the solid and the condensation of the vapor to the solid state on a cool surface, there is no appearence of the liquid state. Only a few compounds sublime at atmospheric pressure. Under reduced pressure and heating the types and numbers of compounds that can be sublimed are greatly increased (for the volatile compounds, like W(CO)6, static vacuum should be chosen). Sublimation, although slow, is particualarly good for small quantities.
An apparatus for sublimation is illustrated in Figure 19-1. The sample is placed in the bottom of the outer piece. The pressure may be held constant with a Cartesian diver manostat. In the absence of a Cartesian diver manostat, a dynamic or static vacuum straight from the vacuum line can be used. For volatile compounds, like W(CO)6, Fe(C5H5)2 and Ni(C5H5)2, a static vacuum should be used to avoid sublimation to the vacuum trap.
The temperature of an oil bath, which surrounds the outer piece to the level of the sample, is increased until the solid appears on the walls of the cold finger and is then held constant. When the sublimation is complete, the vacuum is released cautiously, the inner piece is withdrawn carefully, and the product is scraped from the walls of the cold finger.
B.Extraction
1.Liquid-Liquid Extraction
Liquid-liquid extraction is a simple and powerful method for separating mixtures of compounds on the basis of their relative solubilities in two immiscible liquid phases. A mixture containing substances with rather different solubility properties (e.g. a non-polar compound and a carboxylic acid) can readily be separated into its components by a few extraction steps.
1)Simple Extraction
Select a separatory funnel that is at least 50% larger in volume than the combined liquid phases with which it is to be filled. Check the ground glass stopper and the glass or teflon stopcock for free movement and a leak-free seal. Support the funnel in the vertical position with a clamp or padded ring, close the stopcock, and pour the solution to be extracted and the extracting solution into the funnel.
Swirl the open funnel gently to saturate the air in the funnel with solvent vapors, then insert the stopper. Grasp the funnel with one hand about the stopper and the other about the handle of the stopcock and the stem of the funnel. With the stem directed away from vulnerable objects, invert and release the internal pressure by opening the stopcock. Close the stopcock, shake the funnel gently and briefly, and again equalize the pressure. Repeat this procedure two or three times with more vigorous shaking. Return the funnel to its original positionn in the clamp or ring, remove the stopper, and allow the layers to settle (Figure 19-2).
After the layers have cleanly separated, draw off the lower layer through the stopcock. If droplets of the heavier phase adhere to the walls of the funnel or to the surface of the lighter phase, dislodge them by gentle swirling or by stroking with a glass rod.
Under best conditions a clean meniscus is formed and a rapid separation of phases can be carried out. Under unfavorable circumstances the two phases may form an emulsion that is slow to separate. Emulsions are most frequently encountered during the extraction of aqueous alkalis (especially with benzene-minimized by use of ammonium hydroxide as base) or certain salts (e.g. chromium salts). Some emulsions can be broken by occasional gentle swirling and patience. If this fails, there remain several alternatives:
1)Add a small portion of the organic solvent, swirl again gently and wait.
2)If finely divided solid is present and is preventing a clean separation of layers, filter the mixture through a pad of Celite.
3)Dissolve a strong electrolyte (e.g., NaCl or sodium sulfate) in the aqueous layer, or add an anti-forming agent. The electrolyte acts by both increasing the viscosity difference between the two phases and reducing the stability of the interlayer film.
4)Centifuge the mixture. This is a particularly convienient and effective technique in small-scale extractionns when a tabletop chemical centrifuge is available.
5)Consider changing the solvent on subsequent extractions.
After the two layers from an extraction have been separated, each layer should be washed with a fresh portion of the solvent used in the second layer. For example, in an ether-water partition, the ether layer should be washed with an additional small portion of water and the water layer should be washed with an addition small portion of ether. This final ether wash is combined with the ether layer, etc. The extraction of an organic product into an organic solvent represents only a portion of the procedure to be used in the purification process. The combined orgainc layers are usually washed with appropriate aqueous solution to remove undesirable impurities; and so on.
2)Separation of a Complex Mixture by Simple Liquid-Liquid Extraction
It is pointed out that various aqueous extractants will extract different classes of compounds from a mixture in an organic solvent. This can be used to separate a complex mixture, or to remove certain impurities from a desired compound. Figure 19-3 is a flow diagram for one commonly used procedure for separating an ethereal mixture into bases, strong and moderately weak acids, very weak acids (e.g., phenols), and neutral compounds.
The organic solution is extracted successively with aqueous extracting agents which remove certain classes of compounds. Extraction with each agent is carried out two or three times, with the total volume of extractant about equal to the volume of the organic solution. The compounds may be re-extracted into an organic phase by changing the pH of the aqueous solution so that the compounds are converted to their neutral forms. Each organic layer is then washed with water to remove inorganic substances. It is often desirable to do this final wash with saturated aqueous NaCl to reduce the amount of water left dissolved in the orgainc layer. The organic layer is then dried over a suitable drying agent and the drying agent is removed by filtration. The solvent is removed by distillation, crystalization, or some other suitable procedure.
2.Liquid-Solid (Soxhlet) Extraction
If the sample is particularly difficult to dissolve, continuous liquid-solid extraction with a Soxhlet extraction appparatus (Fig. 19-4) may be used. This technique is also useful in isolating natural products from their sources. The crushed sample is placed in the apparatus in a filter-paper thimble of appropriate size for the extractor used, the solvent is refluxed until the extraction is complete. Soxhlet extractors in which hot vapors flow around the thimble are also available, and may be useful if hot solvent is required for the extraction.
In some cases, one is lucky enough to have the extracted solid crystallize out in the flask as the extraction proceeds.
C.Chromotography
Chromotagraphy is used to separate the components of a mixture. Most forms of chromatography involve two phases; a stationary phase and a mobile phase. The mixture is dissolved in the mobile phase and this solution is allowed to pass over the stationary phase. Separations result from differences between the affinities of the individual components of the mixture for one of the phases. The distance that a component of the mixture moves along the stationary phase is expressed as some fraction of the distance that the solvent moves. This quanity, known as the Rf, can be calcualated as shown below:
Rf = (distance compound moves)/(distance solvent front moves)
1.Liquid-Liquid Partition Chromatography
Liquid-liquid partition chromatography may be performed in several ways. In thin-layer chromatograpohy (TLC) a glass plate is coated with a solid support and a small spot of sample is applied near one end of the coated plate. The spotted plate is placed in a closed chamber with solvent and the solvent is allowed to ascend from just below the point of application to near the top of the coating. Solvent is allowed to evaporate from the developed chromatogram and the plate is stained with iodine vapor or other reagents to reveal the location of spots of components. Very small amounts (e.g. 0.1 micromole) of a component can be separated from other components in a short time (5 to 10 minutes) on small glass slides (e.g. 3" long) that typically contain hundreds of theoretical plates. Alternatively, the method may be used for isolation on a larger scale (e.g. 100 mg).
In paper chromatography filter paper is the support. This method is considerably slower than TLC and gives poorer resolution in most cases. It is still used, especially in biochemical studies, not only because of the huge volume of useful technical information accumulated on particular classes of compounds before the advent of TLC, but also because it is effective in combination with another separation technique, paper electrophoresis.
2.Column Chromatography
A vertical cylindrical tube is packed with a slurry of a solid support (such as silica, alumina, florisil, etc.) containing a relatively polar stationary phase and a mobile phase. Excess liquid is drained off and the sample, in a small volume of mobile phase, is applied to the top of the column. The column is then eluted with fresh mobile phase or eluant. Since it is much more convienient to collect fractions emerging from the bottom of the column than to develop a column, remove the support and examine it, elution is the standard procedure for column chromatography. The composition of the eluant can be varied in discrete steps to elute components that have widely different distribution coefficients (K) between the stationary and mobile phases. Changing the eluant allows components with initially low values of K to take on new higher values, where efficiency of separation is better. This type of stepwise elution may be further refined by continuously mixing a more polar solvent into the relatively non-polar eluant that is applied to the top of the column initially; this procedure is known as gradient elution. Frequently, gradient elution can be used to separate the components of highly complex mixtures, either on a large (>10g), intermediate, or small (<10 mg) scale.
3.Ion-exchange Chromatography
This is a type of column chromatography in which the support consists of fine beads of a resin that has an affinity for cations (cation exchanger) or anions (anion exchanger) that is chiefly the result of the presence of electrically charged groups of opposite sign within the resin beads. The method has been particularly successful for separation of the variety of ionizable components that occur in biological organisms.
4.Gel Filtration
Although gel filtration is not a type of sequential extraction, it is a distribution process that is similar to column chromatography in procedure and analysis. A vertical column is packed with a slurry of fine beads that contain pores of submicroscopic dimensions. Excess liquid is drained off, a small volume of sample is applied to the top of the column and the column is eluted with the same solvent in which the column was packed. Solute molecules which are too big to enter the pores pass down the column ahead of those solute molecules which are small enough to pass through the pores to the solvent space within the beads. Thus components can be separated according to molecular size. Gel filtration may be used with substances that are soluble in water or in relatively polar solvents.
5.Gas-liquid Partition Chromatography, GLC, GLPC
A small (1-50 μL) liquid sample is vaporized at one end of a long (e.g. 4') tube packed with an involatile liquid on an inert solid support. An inert gas (nitrogen or helium) is passed through the column at a constant temperature or over a steadily rising range of temperatures (e.g. 50-200°C; cf. gradient elution) and at a constant rate (e.g. 1 mL/sec). The volatile components are eluted by the inert carrier gas (mobile phase) and are detected as they emerge from the tube. Concentration is automatically plotted as a function of time. The entire procedure for a single run is sufficiently rapid (10-30 min) and powerful (ca. 1200-2500 theoretical plates in a 4' column) that methods are constantly being devised to make volatile derivatives of complex solid mixtures so that the mixtures may be analyized by this preferred, quanitative method. Like TLC this method may be used convieniently for the isolation of 100 mg quanities of pure substances.
D.References
1.K.B. Wilberg, "Laboratory Technique in Organic Chemistry," McGraw-Hill Book Co., Inc., New York, N.Y., 1960, pp. 149-165.
2.F.L.J. Sixma and H. Wynberg, "A Manual of Physical Methods in Organic Chemistry," John Wiley and Sons, Inc., New York, N.Y., 1964, pp. 19-104.
3.K. Randerath, " Thin-Layer Chromatography," 2nd Ed., Academic Press, New York, N.Y., 1966.
4.H.G. Cassidy, "A Technique of Organic Chemistry," A. Weissberger, Ed., Vol. 5, "Adsorption and Chromatography," Interscience Publishers, Inc., New York, N.Y., 1951; ibid, Vol. 10, "Fundamentals of Chromatography," Interscience Publishers, Inc., New York, N.Y., 1957.
5.J.G. Kirchner, "Technique of Organic Chemistry," E.S. Perry and A. Weissberger, Eds., Vol. 12, "Thin-Layer Chromatography," Interscience Publishers, New York, N.Y., 1967.
XX. High Vacuum Line
The high vacuum line is a mercury-free and grease-free vacuum line capable of vacuum to < 1 micron (0.001 mm Hg). This line is specifically designed for the freeze-pump-thaw degassing of photolysis samples and for bulb-to-bulb distillations.
A.Freeze-Pump-Thaw Degassing
Section 2, Part L "UV-Vis Sample Preparation" contains a description and a diagram on how to use the high vacuum line for freeze-pump-thaw degassing.
B.Bulb-to-Bulb Distillation
A bulb-to-bulb distillation set-up is shown in Figure 20-1. Flask A contains the solution to be distilled. With the stopcock closed, freeze the solution in flask A with liquid N2. Open the stopcock. When a vacuum of ~ 1 micron has been attained close the stopcock. Remove the liquid N2 bath from flask A and allow the contents of A to thaw. Refreeze the contents of flask A with liquid N2 and open the stopcock. When a vacuum of ~1 micron has been attained close the stopcock. Remove the liquid N2 bath from flask A and put it under flask B. Allow flask A to warm to room temperature. The volatiles in flask A will distill over and collect in B. When the distillation is done introduce N2 gas through the stopcock and remove the set-up from the line.
C.Do's and Don'ts
There are several things to remember when operation the high vacuum line:
--do not introduce mercury into the line.
--do not introduce grease into the line (most grease fluoresces).
--do not pull solvents into the line or over into the liquid N2 trap (this will decrease the ultimate vacuum).
--limit the exposure of the vacuum line to air and water (water adsorbs to the glass of the line and is difficult to remove; thus limiting the ultimate vacuum).
--do not overtighten the Cajon fittings which are on the stainless steel flexible tubing sine the tubing can buckle and cause a leak.
--finally, treat this line like any other vacuum line -- with care!
XXI. Solvents and Baths for Heating and Cooling
It is often necessary to maintain a temperature different from that of the laboratory. In cases where precise control is needed, a liquid bath is usually superior to other methods. For temperatures above that of the room, any liquid tht is nonviscous at the temperature desired and boils at least 20 to 300 above that temperature can usually be used. Of course, other factors such as expense, toxicity, and flammability are important. Silicon oil is the most commonly used liquid for heating baths at high temperature or wide ranges of temperatures. The tables list some mixtures of common laboratory liquids that can be used to very low temperatures, yet remain liquid at room temperature. These mixtures can also be used as solvents at these low temperatures. For temperatures down to about -50o, there are many common organic liquids that can be used.
While most laboratories have several means for achieving and maintaining temperatures above room temperature (e.g., bunsen burners and hot plates), a cooling apparatus is much less common. The last two tables in this section offer convenient ways to cool a system to a specified temperature without the need for expensive aparatus.
A.Salt-Ice Cooling Mixtures
Table 21-1 lists salt/ice cooling mixtures that can be obtained by mixing the salt (at about room temperature) with water or ice at the specified temperature in the amount noted. In actual practice, these temperatures are often difficult to reach and may depend on the rate of stirring and on how finely crushed the ice is.
Table 21-1
Substance | Initial Temperature (oC) | g/100g H2O | Final Temperature (oC) |
Na2CO3 NH4NO3 NaC2H3O2 NH4Cl NaNO3 Na2S2O3●5H2O CaCl2●6H2O KCl KI NH4NO3 NH4Cl NH4NO3 NH4SCN NaCl CaCl2●6H2O H2SO4(66.2%) NaBr H2SO4(66.2%) C2H5OH(4o) MgCl2 H2SO4(66.2%) CaCl2●6H2O CaCl2●6H2O | -1 (ice) 20 10.7 13.3 13.2 10.7 -1 (ice) 0 (ice) 10.8 13.6 -1 (ice) -1 (ice) 13.2 -1 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) 0 (ice) | 20 106 85 30 75 110 41 30 140 60 25 45 133 33 81 23 66 40 105 85 91 123 143 | -2.0 -4.0 -4.7 -5.1 -5.3 -8.0 -9.0 -10.9 -11.7 -13.6 -15.4 -16.8 -18.0 -21.3 -21.5 -25 -28 -30 -30 -34 -37 -40.3 -55 |
B.Low-Temperature Baths
Two types of systems are shown in Table 21-2 (See following page). One involves pouring liquid nitrogen* (bp-196o) into a solvent by stirring until a slush is formed. The temperature may be maintained by periodically adding nitrogen to maintain the slush. These data are taken from R.E. Randeau, J. Chem. Eng. Data, 11, 124 (1966). The second system involves addition of small lumps of dry ice to the solvent until a slight excess of dry ice coated with frozen solvent remains. Again, temperature may be maintained by periodically checking the bath's temperature by low-temperature thermometer and adding more dry ice. These data are taken for the most part from A.M. Phipps and D.N. Hume, J. Chem.Educ., 45, 664(1968). The latter reference describes the use of the dry ice method in mixtures of ortho and meta xylene of varying compositions.
*CAUTION: Skin contact with liquid nitrogen may lead to a frostbite burn. An occasional droplet of nitrogen, such as is encountered when filling a Dewar, often does not freeze the skin because of an insulating film of gaseous nitrogen which forms immediately. However, the skin is readily frozen if the liquid nitrogen is held on a spot by clothing which is saturated with the refrigerant, or by any other means which leads to extended contact.
Table 21-2
System | oC | System | oC |
p-Xylene/N2 p-Dioxane/N2 Cyclohexane/N2 Benzene/N2 Formamide/N2 Aniline/N2 Cycloheptane/N2 Benzonitrile/N2 Ethylene glycol/CO2 o-Dichlorobenzene/N2 Tetrachloroethane/N2 Carbon tetrachloride/N2 Carbon tetrachloride/CO2 m-Dichlorobenzene/N2 Nitromethane/N2 o-Xylene/N2 Bromobenzene/N2 Iodobenzene/N2 Thiophene/N2 3-Heptanone/CO2 Acetonitrile/N2 Pyridine/N2 Acetonitrile/CO2 Chlorobenzene/N2 Cyclohexanone/CO2 m-Xylene/N2 n-Butyl amine/N2 Diethyl carbitol/CO2 n-Octane/N2 Chloroform/CO2 Chloroform/N2 Methyl iodide/N2 | 13 12 6 5 2 -6 -12 -13 -15 -18 -22 -23 -23 -25 -29 -29 -30 -31 -38 -38 -41 -42 -42 -45 -46 -47 -50 -52 -56 -61(-77) -63 -66 | Carbitol acetate/CO2 t-Butyl amine/N2 Ethanol/CO2 Trichloroethylene/N2 Butyl acetate/N2 Acetone/CO2 Isoamyl acetate/N2 Acrylonitrile/N2 Sulfur dioxide/CO2 Ethyl acetate/N2 Ethyl methyl ketone/N2 Acrolein/N2 Nitroethane/N2 Heptane/N2 Cyclopentane/N2 Hexane/N2 Toluene/N2 Methanol/N2 Diethyl ether/CO2 n-Propyl iodide/N2 n-Butyl iodide/N2 Cyclohexene/N2 Isooctane/N2 Ethyl iodide/N2 Carbon disulfide/N2 Butyl bromide/N2 Ethyl bromide/N2 Acetaldehyde/N2 Methyl cyclohexane/N2 n-Pentane/N2 1,5-Hexadiene/N2 i-Pentane/N2 | -67 -68 -72 -73 -77 -77 -79 -82 -82 -84 -86 -88 -90 -91 -93 -94 -95 -98 -100 -101 -103 -104 -107 -109 -110 -112 -119 -124 -126 -131 -141 -160 |
C.Special Liquids for Low Temperatures
Special solvent mixtures with very low freezing points but with boiling points above or near room temperature are contained in Table 21-3. These mixtures are good solvents at low temperatures and when used as such often freeze substantially lower than the value reported here. Since most of these substances are highly chlorinated and/or can be obtained deuterated, they make excellent low-temperature nmr solvents.
Table 21-3
Substance | Vo. % | Mp (oC) | Substance | Vol. % | Mp(oC) |
Carbon tetrachloride Chloroform | 49 51 | -81 | Chloroform Methylene chloride Ethyl bromide trans-1,2-Dichloroethylene Trichloroethylene | 14.5 25.3 33.4 10.4 16.4 | -150 |
Chloroform Trichloroethylene | 31 69 | -100 | Methyl chloride Dimethyl ether | 25 75 | -154 |
Chloroform Methylene chloride Carbon tetrachloride | 27 60 13 | -111 | n-Pentane Methyl cyclohexane n-Propanol | 64.5 24.4 11.1 | <-180 |
Chloroform trans-1,2-Dichloroethylene Trichloroethylene Ethyl bromide | 20 14 21 45 | -139 |
|
|
|
D.Liquid Media for Heating Baths*
The data in Table 12-4 are taken mostly from R. Egly, "Heating and Cooling," in Vol. 3, Part 2, of Weissberger, ed., Technique in Organic Chemistry, Interscience, New York, 1957, p. 152. The percentages refer to weight percent. For other useful high-temperature baths see fused salts, page 40.
Table 21-4
Medium | Mp(oC) | Bp (oC) | Useful Range (oC)a | Flash Point (oC) | Comments |
H2O | 0 | 100 | 0-80 | None | Ideal in limited range |
Ethylene glycol | -12 | 197 | -10-180 | 115 | Cheap; flammable; difficult to remove from apparatus |
DCb 330 silicone oil | <-148 | - | 30-280 | 290 | Becomes viscous at low temperature |
20% H3PO3, 80% H3PO4 | <20 | - | 20-250 | None | Water soluble; nonflammable; corrosive; steam evolved at high temperature |
Triethylene glycol | -5 | 287 | 0-250 | 156 | Water soluble; stable |
DCb 550 silicon oil | -60 | - | 0-250 | 310 | Noncorrosive; can be used to 400o in N2 atmosphere |
Glycerol | 18 | 290 | -20-260 | 160 | Supercools; water soluble; nontoxic; viscous |
Paraffin | ~50 | - | 60-300 | 199 | Flammable |
aUseful and safe range for bath open to atmosphere. b Dow Corning.
*CAUTION: Do not heat sealed containers. Pressure build-up might cause an explosion
XXII. Waste Disposal
A.Liquids
a.Nonflammable, nontoxic, water soluble liquids may be flushed down the drain.
b.Flammable, toxic, water insoluble liquids and metal containing solvents should be placed in a specially labelled container in the hood. The categories of waste include:
1. Non-halogenated organic wastes
2. Halogenated wastes
3. Non-precious metal wastes
4. Precious metal wastes: individual containers for Ru, Os, Rh, Ir, Pd, Pt, Au
5. Very toxic wastes: individual containers for Hg, Tl, Cd, etc. These should be treated with sulfide and pH adjusted to 7.
c.When solvents and compounds are added to waste jars, the amound and compounds added must be listed on the forms hanging near that waste jar.
B.Waste Alkali Metals and Hydrides
Under no circumstances should such substances be discarded in the sink or in waste container. All the compounds should be destroyed completely by reacting with isopropanol, followed by water.
C.Mercury Waste
Waste mercury, broken mercury thermometers and all mercury compounds should be placed in a container designated for them and must not be discarded in the sink. If the mercury was used in sodium amalgam reaction, isopropanol should be added to destroy sodium first.
Spilled mercury should be cleaned up as best as possible. The area of the spill is then treated with sulfur. The mercury/sulfur mixture is then collected and disposed. Alternatively, spilled Hg can be frozen with liquid N2, swept up and disposed.
D.Dirty Glassware
Chemicals clinging to vials, test tubes, pipets and broken glassware should be rinsed off before they are discarded into trash can specifically indicated.
E.Dirty Papers
Papers with chemical and/or metal contaminants should be rinsed off before diposal in a trash can.
F.Glassware with Specific Chemical Contamination
1.Isocyanides
The contaminated glassware should be soaked in a bleach bath and rinsed well with water before being cleaned in the base bath. The used bleach bath may be poured down a hood sink after use.
2.Phosphorus Compounds
Glassware containing volatile phosphorus compounds can be treated with acid and/or placed in the hood to allow the volatile phosphorus compounds to evaporate.
For further information consult the safety books in the library.
XXIII.Glassblowing
A.The Torch
Before lighting the hand torch the oxygen tank is opened at the top valve with the regulator set for zero pressure. The regulator is then set to give about 10 psi and both the oxygen valve and the gas valve on the torch are opened momentarily to clear out the air in the hoses. The gas is then turned on, the flame is lit and then oxygen is added to produce a blue flame with a small cone. The flame should not "blast" excessively; if higher temperatures or a larger flame are needed, use a larger tip (or a bench torch or cross fires). When adjusting the flame or turning it off, avoid reducing the gas too much while the oxygen is on, or the flame may "flash back", to burn destructively inside the torch, or go out with a nerve-shattering "pop" that may produce a catatonic reaction in those within earshot. Turn off the oxygen first, blow out the gas flame, then turn off the gas. Turn on the oxygen again, turn the regulator to zero pressure, then shut off the oxygen at the tank valve.
B.Knife or file
For making a scratch on a glass tube preparatory to making a break, a knife with a tungsten carbide edge is best. The usual tool for this job, however, is an ordinary three-cornered file. For best results the file should be relatively new. A file dull enough that it scratches glass only with difficulty should be thrown away and replaced, or else ground on an abrasive grinding wheel to produce a renewed cutting edge. (In grinding, care must be taken to keep the file from heating up so much as to lose temper.)
C.Eye protection
The bright glow from heated glass is at best uncomfortable to the eyes and obscurant of the details of the work and at worst potentially damaging to the eyes. Except perhaps for minor jobs (bending and fire-polishing), blowing of Pyrex glass should be done with protective glasses on, usually made with violet-tinted "Didymium" glass which cuts out the strong yellow component (largely sodium D) of the light from the heated glass. Those who wear prescription glasses can place "clip-on" protective glasses over them. (For working Vycor or fused silica with the oxyhydrogen torch, greater protection is needed.)
D.Sealing a Glass Tube
Before the glass is to be worked is should be reasonably clean; in particular it should be free of grease, dust and evaporated residues. Glass tubing and rod as they come form the stockroom are ordinarily clean enough after they are wiped with a towel on the outside and (in the case of tubing) after the ends are wiped out for one or two inches with a pipe cleaner, or else broken off and discarded (Figure 23-1). It is better to push a wad of tissue through the entire length.
Glass tubing 5-8 mm in diameter is the most convienient for storage of X-ray crystal samples, microanalysis samples and small quanities of air sensitive materials. The glass tubing is initially cut to the desired length then one end is sealed by "working" the glass tubing. In the case of small diameter tubing, this involves simply melting the end by rotating the glass in the flame. The glass is "cooled" by turning off the oxygen supply and allowing the glass to remain in the "cooler" flame about one minute. The sample can then be placed in the tube and the tube, under nitrogen, sealed with a septum. At this point, the tube and the sample may be placed under a small static vacuum. To seal the tube, it is rotated in the flame. The partial vacuum forces the walls to collapse and the septum end can be removed (Figure 23-2).