Teflon vessels

Zollinger, 1981). In the presence of less than 5 ppb of 02 it obeys first-order kinetics in glass vessels, but zero-order kinetics in Teflon vessels. With between 60 and 100 ppb of 02, a fast initial reaction slackens off after about 15% conversion autocatalysis is observed on exposure to air, but in 100% 02 there is again a first-order reaction. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

Teflon vessels are particularly suitable since they are transparent to MW and are resistant to attack by chemicals or solvents (both organic and aqueous). The high melting point of Teflon (320 °C) enables reactions to be performed relatively safely, although vessels have been known to expand or explode at high pressures, particularly when the temperature is also high [1, 7]. 

The pressure generated in a reaction vessel, and hence the rate enhancement, depends on a number of factors including the MW power level, the volatility of the solvent, the dielectric loss of the reaction mixture, the size of the vessel and the volume of the reaction mixture [7, 20]. Gedye et al. [20] found that, in the esterification of benzoic acid with a series of aliphatic alcohols (Scheme 4.1) in closed Teflon vessels, the most dramatic rate enhancements were observed with methanol (the most volatile solvent). 

The effect of the size of the reaction vessel on the rate of M W-heated SN2 reaction of 4-cyanophenoxide ion 1 and benzyl chloride 2 in methanol (Scheme 4.2) was investigated by reacting identical amounts of reagents in Teflon vessels of different sizes. 

The heating rate, and hence the rate of pressure increase, also depends on the volume of the reaction mixture [20], When the volume is small, the pressure increases as the volume of the reaction mixture increases. However at a certain volume this trend is reversed and a larger volume heats more slowly. For example when water was heated in a 150-mL Teflon vessel, the greatest rate of pressure increase occurs when 15 mL water are heated using a power of 560 W. The volume at which this maximum heating rate, however, varies for different solvents. For example it occurs at 20 mL for 1-propanol. 

In this early work, pressure was measured by connecting the cap of a pressure-re-lease Savillex [24] Teflon vessel to a pressure gauge outside the MW oven and the final temperature estimated by pointing an infrared sensor at the mixture in the vessel immediately after the heating was completed. 

With KF in the presence of a phase-transfer agent (18-crown-6) silyl ketene acetals react with aldimines to give -lactams within a few minutes under the action of micro-wave irradiation in closed Teflon vessels (Eq. 55). 

Reactions were performed in sealed thick-walled glass tubes or in Teflon acid-digestion vessels, in domestic microwave ovens [13]. Teflon vessels can be used at pressures up to 14 atm, at temperatures below 250 °C, and are resistant to most commonly used chemicals, although they deform at temperatures >250 °C. 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

The REE analysis was carried out in twenty two sand samples by using 0.1 g of dried sample (mesh 200) and digested with strong acid. Digestion was performed in teflon vessels using 4 ml of HCI 04 and 10 ml HF. This mixture was heated and residue dissolved in distilled water. 

CAUTION. Fluorine is a strong oxidizer and a very corrosive material. An appropriate vacuum line made from copper or Monel in a well-ventilated area should be constructed for working with this element. The reactions themselves were carried out in Teflon vessels. If elementary precautions are taken, work with fluorine is relatively simple. 

Closed vessel microwave digestion for bones, teeth, hair, and soil Specially designed closed pressurized Teflon vessels may be used for microwave digestion. Teflon is transparent to microwaves, which enhances the effect of the acids by raising the temperature and pressure within the vessel. In addition the closed vessels will retain any volatile components (e.g., Si) in solution. It must, however, be emphasized that any sealed vessels must only be heated if they have been designed for the purpose. Examples of applications include Kingston and Walter (1992), Baldwin et al. (1994), Sheppard et al. (1994), and Tamba et al. (1994). 

For the determination of arsenic by conventional inductively coupled plasma atomic emission spectrometry the samples were digested in closed Teflon vessels, similar to the technique described by Brzezinska et al. [126]. 

About 0.1-2.0g of wet sediment was placed into a Teflon vessel and 3mL of concentrated nitric acid, 0.5mL concentrated perchloric acid, and 4mL concentrated hydrofluoric acid were added. The closed vessels were kept at room temperature for lh. The samples were then placed in a pressure cooker and heated for lh on a hot plate at a temperature of 300°C. After cooling, the vessels were uncapped and the samples evaporated to 2mL on a hot plate at 250°C. After cooling, 3mL concentrated nitric acid was added. To complex the fluorides, lg boric acid was added to each sample. The solutions were transferred to lOOmT volumetric flasks and adjusted to volume with deionized water. Inorganic arsenic standards, having the same acid content as the samples, were used for calibration. 

Acid digestion with a mixture of nitric, perchloric and hydrofluoric acids in sealed Teflon vessels, as described by McLaren et al. [136]. 

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

The procedure for separating Sb-119 from an alpha-irradiated tin target has been described elsewhere (10,11). The amounts of cobalt and antimony coexisting with the nuclides are estimated to have been about 400 ng/mCi and 300 ng/mCi, respectively, i.e., to have been much smaller than that required for monolayer coverage of 30 mg of the hematite sample. About 10 cm3 of an aqueous solution containing 1 - 2 mCi of divalent Co-57 or 0.1 - 1 mCi of pentavalent Sb-119 was adjusted to an appropriate pH value in a Teflon vessel with a 0.5 mm-thick Teflon window at the bottom, and about 30 mg of hematite powder was added to the solution. The suspension was shaken for 30 min at room temperature. After settling of the powder at the bottom of the vessel, the pH was remeasured. 

Selection of digestion vessels the size of the digestion vessels depends on the sample volume. Standard vessels are 25, 40, SO, 100 and ISO ml glass. For extreme trace analysis, quartz and Teflon vessels are available. Vessels can also be supplied with dust covers and reflux extensions. 

Decomposition in closed Teflon vessels at medium pressure (up to 8 bar) with microwave heating [43-44]. Microwave Digestion System MDS-81D (CEM Corporation, North Carofina, USA Floyd Inc., South Carolina, USA). 

The Dutch Total Availability Leaching test (NEN 7341) was used to operationally quantify the elemental mass fraction available for leaching in the samples. The procedure involves two sequential extractions the first were conducted at a pH of 7.0 and LS of 100. For examination of mass fractions available for leaching, 8 g of sample was added to 800 mL of distilled, deionized water and stirred in a capped Teflon vessel. 

To a mixture of KH4F5 (1.1 mmol), DBH (1.1 mmol), and CH2C1, (3mL) in a Teflon vessel was added a solution of an alkene (sec Table 2) in C H,CI2 (2 mL). The mixture was stirred for 4h at rt and then quenched with aq NaHC03 and Na2S,03. The aqueous phase was extracted with CH2C12 (3x5 mL) and (he combined extracts were dried (MgS04). After evaporation of the solvent, the product was purified by chromatography (silica gel). 

Acetone absorbs microwaves, so it can be heated in a microwave oven. Hexane does not absorb microwaves. To perform an extraction with pure hexane, the liquid is placed in a fluoropolymer insert inside the Teflon vessel in Figure 28-8.18 The walls of the insert contain carbon black, which absorbs microwaves and heats the solvent. 

A Teflon vessel was charged with l-phenyl-4-vinylpyrazole 1 (170 mg, 1 mmol) and dimethyl acetylenedicarboxylate 2 (426 mg, 3 mmol) and then closed and the reaction mixture irradiated in a domestic oven at 780 W for 6 min (final tempera-... 

General Procedure for Preparation of Triazones 5a-f. 264 mg (3 mmol) /V,/V-di-metliylurea, 1 g paraformaldehyde, 3 mmol primary amine 4a-f and 2 g montmorillonite K-10 were irradiated by microwave in a Teflon vessel. The reaction mixture was filtered and washed with water. The organic phase was separated and dried with Na2S04 and concentrated by vacuum distillation. Purification of the... 

---