Kjeldahl flask

A very suitable apparatus for semi-micro steam-distillation, particularly for suspensions that are likely to bump badly, is showm in Fig. 44. This consists of a 50 ml. Kjeldahl flask, clamped at an angle of 45°, and fitted with a long glass tube for the inlet of steam. The Outlet-tube is bent twice, first at 135° and then at 45° as shown, and fitted into a small water-condenser. 

Two convenient forms of bath are shown ui Fig. 11,10, 2, a and 6. The former consists of a long-necked, round-bottomed flask (a longnecked Kjeldahl flask of 100 ml. capacity is quite satisfactory) supported by means of a clamp near the upper part of the neck. The thermometer is fltted through a cork, a section of the cork being cut away (see inset) so that the thermometer scale is visible and also to allow free expansion of the air in the apparatus. The bulb is about three-quarters filled with... 

Reactions under pressure are usually carried out in an autoclave. However, several simple vessels can be used for reactions at moderate pressure. A heavy walled Pyrex test tube or Kjeldahl flask drawn out and sealed with an oxygen torch makes a suitable container for many Diels-Alder reactions. The tube can be heated in an oil or water bath, but care must be exercised to protect against explosions. At the conclusion of the reaction, the tube is cooled to room temperature, the neck is scratched with a file or carborundum chip, and a hot Pyrex rod is touched to the scratch. A large crack in the neck should result, and the sealed top can be easily knocked off. 

Steel. Dissolve 5 g, accurately wdghed, of a steel in 1 1 nitric acid with the aid of the minimum volume of hydrochloric acid in a Kjeldahl flask. Boil the solution... 

Preparation of standard titanium solution. Weigh out 3.68 g potassium titanyl oxalate K2Ti0(C204)2,2H20 into a Kjeldahl flask add 8g ammonium sulphate and 100 mL concentrated sulphuric acid. Gradually heat the mixture to boiling and boil for 10 minutes. Cool, pour the solution into 750 mL of water, and dilute to 1 L in a graduated flask 1 mL = 0.50 mg of Ti. 

A novel TLC spectrofluorometric method for identification and determination of selenium in different food samples of animal and vegetable origin has been proposed [30]. The procedure involves the digestion of food sample (1 to 5 g) in the presence of cone. HNO3 (5 ml), 70% HCIO4 (10 ml), and FIjO (10 ml) in a 250-ml Kjeldahl flask reduction of Se(VI) into Se(IV) complexation of the isolated selenium with 23-diaminonaphthene (DAN) extraction of the resultant Se—DAN complex with cyclohexane and spectrofluorometric determination followed by confirmation of the presence of Se in the sample by TLC using thin layers of MN-300 cellulose powder. 

Francis [26] has recently described some modem methods for element determinations, including the oxygen flask technique and the determinations of C, H, O, N (Kjeldahl), halogens, S, P and F (ion-selective electrode). 

A spray trap should be placed between the flask and the condenser otherwise some isonitrosoketone will be carried over. The checkers Used a Kjeldahl Connecting Bulb, Cylindrical Type, illustrated as item 2020 in the Pyrex Catalog, L.P-21 for 1941. 

Rather than perform this analysis in laboratory glassware in a fume cupboard, special pieces of apparatus that hold the flasks and allow several Kjeldahl titrations to be carried out in parallel are employed. 

The reaction was performed in flame-dried modified Schlenk (Kjeldahl shape) flask fitted with a glass stopper or rubber septum under a positive pressure of argon. Trifluoromethanesulfonic anhydride (1.4 equiv) was added to a solution of giycosyl donor (0.191 mmol, 1 equiv) and diphenyl sulfoxide (2.8 equiv) in a mixture of toluene and dichloromethane (8 ml, 3 1 vol/vol) at — 78 °C. The reaction mixture was stirred at this temperature for 5 min and then at —40 °C for 1 h. At this time, 2-chloropyridine (5.0 equiv) and the giycosyl acceptor (3.0 equiv) were added sequentially at —40 °C. The solution was stirred at this temperature for 1 h, then at 0 °C for 30 min and finally at 23 °C for lh before the addition of excess triethylamine (10 equiv). The reaction was diluted with dichloromethane (100 ml) and was washed sequentially with saturated aqueous sodium bicarbonate solution (2 x 100 ml) and saturated aqueous sodium chloride (100 ml). The organic layer was dried (sodium sulfate) and concentrated. The residue was purified by silica gel flash column chromatography. 

Figure 10.7. Setup for performing Kjeldahl analysis of soil. On the left are two different types of Kjeldahl flask to the right of the flask is a heating block used to heat the test tube-shaped flask during digestion and on the right, a steam distillation unit.

This is the basis for a common method for the determination of ammonia in soil.1 Soil is suspended in water and placed in a Kjeldahl flask. The suspension is made basic by the addition of a strong (5-50%) sodium hydroxide solution, and the flask is immediately attached to a steam distillation setup. Steam distillation of the suspension carries the released ammonia to an Erlenmeyer flask, catching the distillate in a standardized acid solution that is subsequently back titrated via acid-base titration. The amount of ammonia in soil can be calculated from the end point of the titration. This procedure is similar to a standard Kjeldahl determination and can be carried out using the same equipment, although no digestion is needed. 

It is also possible that the analyte must be calculated from a gaseous product of another reaction. In this case, one would want the gas to react with the titrant as soon as it is formed, since it could escape into the air because of a high vapor pressure. Thus an excess of the titrant would be present in a solution through which the gas is bubbled. After the gas-forming reaction has stopped, the excess titrant in this bubble flask could be titrated with the back titrant and the results calculated. This latter experiment is one form of the Kjeldahl titration. 

A laboratory that runs Kjeldahl analyses routinely would likely have a special apparatus set up for the distillation. One variation of this apparatus commercially available is shown in Figure 5.13. A baffle is placed on the top of the Kjeldahl flask and subsequently connected to a condenser, which in turn guides the distillate into a receiving vessel, as shown. The ammonia is then distilled into the receiving vessel. The receiving vessel contains an acid for reaction with the ammonia. 

Cindy Wagner-Wiebeck sets a Kjeldahl flask in place in a fume hood dedicated to Kjeldahl sample digestion. 

Weigh between 0.10 and 0.13 g of the macaroni sample on the analytical balance using weighing paper. Add this sample to a clean, dry 100-mL Kjeldahl flask to avoid having sample particles stick to the neck of the flask. Set aside for a few minutes (inside a beaker so that it stays upright) while you do step 4. 

Set the 250-mL beaker in place on the platform of the distillation apparatus in the fume hood such that the glass tip of the condenser is under the surface of the solution in the beaker. Make sure that the stopcock on the upper left of the distillation unit (the addition stopcock) is closed. Also make sure that the other stopcock (the siphon stopcock) is closed. Add the contents of the Kjeldahl flask to the funnel above the addition stopcock. Open this stopcock to deliver the solution fairly... 

A flour sample was analyzed for nitrogen content by the Kjeldahl method. If 0.9819 g of the flour was used, and 35.10 mL of 0.1009 NHC1 was used to titrate the boric acid solution in the receiving flask, what is the percent nitrogen in the sample ... 

Possible hazards introduced by variations in experimental techniques in Kjeldahl nitrogen determination were discussed [1]. Modem variations involving use of improved catalysts and hydrogen peroxide to increase reaction rates, and of automated methods, have considerably improved safety aspects [2], An anecdote is given of the classic technique when sodium hydroxide was to be added to the sulphuric acid digestion and was allowed to trickle down the wall of the flask. It layered over the sulphuric acid. Gentle mixing then provoked rapid reaction and a steam explosion [3],... 

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