Foam-over

Two hundred grams of eleaned and dried crab shells (Note 1) ground to a fine powder is placed in a 2-1. beaker, and an excess of dilute (approximately 6 N) commercial hydrochloric acid is added slowly to the powdered material until no further action is evident. Much frothing occurs during the addition of the acid, and care must be exercised to avoid loss of material due to foaming over the sides of the beaker. After the reaction has subsided, the reaction mixture is allowed to stand from 4 to 6 hours to ensure complete removal of calcium carbonate. The residue is then filtered, washed with water until neutral to litmus, and dried in an oven at 50-60°. The weight of dried chitin is usually about 70 g., but with some lots of crab shells it may be as low as 40 g. 

Carbon dioxide is evolved copiously at this point and may cause the solution to foam over if the acid is not added cautiously. 

The solution is apt to foam over during neutralization. This may be prevented by adding to the foaming solution from time to time small amounts of w-butyl alcohol to break up the bubbles. 

There is considerable tendency for the benzophenone to foam over during the early part of the vacuum distillation and care must be taken to prevent this. 

Additional security against foaming over is provided by a glass tube that projects into the neck of the flask and is attached to a water pump. The checkers found it helpful to use a Hersh-berg stirrer with two wire blades the upper blade was adjusted so that its ends extended above the surface of the reaction mixture and into the foam. 

Foam fluids can be used in many fracturing jobs, especially when environmental sensitivity is a concern [1669]. Foam-fluid formulations are reusable, are shear stable, and form stable foams over a wide temperature range. They exhibit high viscosities even at relatively high temperatures [209]. 

When once the evolution of carbon dioxide sets in, the flame must be cut down so as to avoid the formation of a thick layer of froth which might foam over. 

About 3 hours is required to add the hydroxide solution. The rate of addition may be increased, but considerable foaming occurs during the decomposition, and caution should be taken that the hydroxide does not foam over into the traps. 

This method requires a foam dam to retain the foam over the primary and secondary seals or weather shield. This dam is normally 12-24 in (30-61 cm) high. Complete construction details of the foam dam may be found in NFPA 11. Figure 7-30 illustrates a typical foam chamber and dam for floating roof tank protection. 

In a 4-1. beaker equipped with a stirrer which can be operated above the liquid level to break the foam are placed 250 ml. of 12 N hydrochloric acid and 250 ml. of water. The filtrate obtained above is cooled to about 50° and is added to the hydrochloric acid in small portions and at such a rate that the mixture does not foam over. If efficient stirring is used in the foam layer, this addition can be carried out in 5 minutes. The product is isolated by filtration through a Buchner funnel and is washed on the filter, first with a 250-ml. portion of dilute hydrochloric acid (prepared by diluting 50 ml. of 12 N hydrochloric acid to about 250 ml.) to remove anthranilic acid, and then with 500 ml. of water. The product is sucked as dry as possible and is then spread in a thin layer and allowed to air dry for about 15 hours. When easily pulverizable, the material is transferred to an oven and dried for 3 hours at 100-120°. 

There is a vigorous evolution of carbon dioxide, and unless the acid is added slowly the contents of the flask may foam over. 

A. Sebaconitrile. A 3-1. three-necked flask (Note 1), equipped with a mechanical stirrer (Notes 2 and 3) and a thermometer which dips into the liquid, is heated in an oil bath to 160°. In the flask are placed 505 g. (2.5 moles) of commercial sebacic acid (Note 4) and 180 g. (3 moles) of urea (Note 5), and the melt is heated with stirring for 4 hours at about 160° (Note 6). The oil bath is removed, the surplus oil is wiped off, the flask is insulated (Note 7), and the temperature is then raised, as rapidly as foaming permits, to 220° by means of a triple burner and wire gauze. It is important to continue the stirring for at least 5 minutes after 220° is attained otherwise the mass will foam over during the subsequent distillation. 

Some cooling is necessary, or the rapid evolution of gas will cause the reaction mixture to foam over with consequent loss of material. 

Place the flask on a magnetic stirrer unit and stir vigorously while adding 25 ml of 1.0 m ethanolic sodium borohydride as rapidly as possible without allowing the contents of the flask to foam over. After about one minute, add 20 ml of glacial acetic acid or concentrated hydrochloric acid to destroy excess of sodium borohydride. 

Add 3.0 g (NH4)2C03, a little at a time, while gentle heating is continued. Decrease the heat if the solution threatens to foam over the top of the beaker. 

Remove the dichloromethane by evaporation in the hood. Be careful not to overheat the solvent, since it may foam over. The solid residue which remains after the solvent is gone is the crude caffeine. Reweigh the cooled flask (3). Calculate the weight of the crude caffeine by subtraction (4) and determine the percent yield (5). 

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