Gauge flask

The solution of silver in nitric acid, and the evaporations of the salts with the acids, were made in a horizontal weighed round-bottomed flask heated on a sand bath, with the neck passing inside another flask. He used an empty flask counterpoise as a gauge flask . Penny s results have not been recalculated recently with a correct atomic weight of silver and reduction of the weights to a vacuum, but his value for nitrogen (14 026) found from the conversion of the sodium salts is rather better than Stas s (14 04). Penny also introduced the titration of ferrous iron with potassium chromate or dichromate, using potassium ferricyanide as external indicator. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Although a pressure gauge is more commonly used to measure the pressure inside a laboratory vessel, a manometer is sometimes used (Fig. 4.5). It consists of a U-shaped tube connected to the experimental system. The other end of the tube may be either open to the atmosphere or sealed. For an open-tube manometer (like that shown in Fig. 4.5a), the pressure in the system is equal to that of the atmosphere when the levels of the liquid in each arm of the U-tube are the same. If the level of mercury on the system side of an open manometer is above that of the atmosphere side, the pressure in the system is lower than the atmospheric pressure. In a closed-tube manometer (like that shown in Fig. 4.5b), one side is connected to a closed flask (the system) and the other side is vacuum. The difference in heights of the two columns is proportional to the pressure in the system. 

The lithium tetramethylpiperidide solution from Part A is cooled with a dry ice-acetone bath. When the solution of ethyl 1-naphthoate and dibromomethane has cooled to -74°C or below (internal solution temperature), addition of the dry ice-acetone cooled solution of lithium tetramethylpiperidide is begun. Addition is made via a double-ended (16 gauge) needle over a 40 to 50-min period using a slight positive nitrogen pressure in the 500-mL flask (Note 7). During this time, the addition rate is slowed or stopped as needed to maintain the reaction temperature below -67 C. 

As indicated by the title of this section, the manufacturers of such plants attempt to make them flexible for different applications by using a modular concept. Often, the basic unit consists of a condenser, a vacuum pump and a vacuum gauge, to which various drying systems can be added e. g. manifolds for flasks, ampoules or vials, baseplates with belljars or small chambers with temperature-controlled shelves. The following qualities could be important in selecting the most suitable laboratory plant ... 

The checkers used butylllthlura purchased from Lithium Alkyls which was standardized by titration with 2,5-dlmethoxybenzyl alcohol. The submitters used butylllthlum obtained from Aldrich Chemical Company, Inc. as a 1.55 M solution In hexanes which was measured by transfer to a septum-capped 250-mL graduated cylinder with a 15-gauge cannula and nitrogen back pressure. This solution was then transferred to the flask containing THF In the same manner. The THF must be cooled prior to addition of the butylllthlura. Stainless steel double-ended needles of the type used by the submitters are available from Aldrich Chemical Company, Inc. and Ace Glass, Inc. 

Propyne of 99.96% purity, purchased from Liquid Carbonic Company in a lecture bottle, was used without purification and was introduced to the flask by means of a Tygon tube which was attached to a 9-inch, 18-gauge hypodermic needle. 

Alternatively, the butyl lithium solution may be forced into the reaction flask by means of an 18 gauge cannula Inserted through the serum cap. 

The cannula is a stainless steel tube, 16 gauge, sharpened to a needle at both ends, and 60 cm long. One end is placed through a rubber septum into the flask containing the 1,2-dibromoethane solution, while the other end is positioned under the surface of the benzenethiolate solution. Control of the nitrogen pressure allows slow transfer of the benzenethiolate solution. 

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