Kjeldahl reactions

As an example of the benefits of microscale work, consider the t)q)ical Kjeldahl reaction, which uses mercury as a catalyst. The mercury waste produced by this procedure creates a difficult disposal problem. Converting to micro-Kjeldahl equipment and quantities reduces the waste by 90%, which could result in a reduction of several liters of waste per day in laboratories that routinely run Kjeldahl reactions. 

Determination. Weigh a suitable quantity of the sample, defatted if necessary, into the Kjeldahl reaction flask of the apparatus (Fig. 7), add 5 ml of 20 per cent sodium hydroxide solution and 15 ml of redistilled water and swirl until the sample is evenly dispersed. Add a piece of copper foil, in. square (previously washed with concentrated hydrochloric acid, distilled water and redistilled water and dried at 100°), 10 mg of ceric sulphate and about twenty small glass beads. Add, carefully, 80 ml of 18N sulphuric acid, mix until homogeneous, and then carefully add 5 g of potassium permanganate that has been recrystallised from redistilled water, dried at 100° and pulverised with an agate pestle... 

Oxidative degradation of [B qH q] and [B22H22] C to boric acid is extremely difficult and requires Kjeldahl digestion or neutral permanganate. The heat of reaction obtained from the permanganate degradation leads to a calculated heat of formation for [B qH q] (aq) of 92.5 21.1 kJ/mol (22.1 5.0 kcal/mol) (99). The oxidative coupling of both [B qH q] and has been studied ia some detail (100). 

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. 

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. 

In a back titration, an excess amount of standardized reagent is reacted with an unknown amount of a component of interest. When the reaction is complete, the remaining unused reagent is titrated and the amount of component of interest is determined by difference. In the Kjeldahl procedures described next, ammonia is distilled into an add of known concentration. When all of the ammonia has been distilled, the remaining unreacted add is titrated. The... 

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. 

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],... 

A procedure given by Zahn and Wiirz (89) was followed. 0.5 gm of the carbon was agitated with 25 ml of a 1 if solution of DNFB in dimethyl-formamide (DMF) for 20 hours. In order to neutralize the HF set free, 10 ml of a 1% aqueous solution of NaHCOj were added. The reaction products were washed and extracted successively with DMF, dilute HCl, methanol, and ether. Nitrogen determinations, Kjeldahl as well as Dumas determinations, gave identical results. Treatment with p-nitrobenzoylchloride followed standard procedures. 

There are two approaches to the analysis. In the first, the extractable ammonium-N is determined by a Kjeldahl distillation, and this is subtracted from the value for ammonium plus nitrate (and nitrite) determined by a further separate Kjeldahl distillation preceded by reduction with nascent hydrogen produced by the reaction of Devarda s alloy (45% Al, 50% Cu, 5% Zn) in strongly alkaline solution ... 

On nitrided aluminophosphates, AlPON, Massinon et al. [206] observed on a series of six samples with increasing nitrogen contents a good correlation between the catalytic activity in the Knoevenagel condensation reaction and the amount of surface NH, species (1 < x < 4) quantified by the Kjeldahl method. The authors suggest that those species are not the only active species and evoke an additional role of the nitride ions in the reaction [206] on the other hand, Benitez et al. [207] suggest hydroxyls linked to aluminum cations in the vicinity of terminal P-NH2 groups as basic centers. 

The choice between oxidation by the Kjeldahl or the Schoniger technique is not clear-cut. Both methods will usually yield quantitative reactions, both will occasionally give incomplete combustion and low results, depending on the family of substances analysed and on the slight modifications introduced into the procedures in different laboratories. 

The following reactions takes place in the Kjeldahl flask, 2NaN3 + HaS04 - 2HN3 + NaaS04, and in the receiver, HN3 + NaOH- NaN, + HjO... 

In the Kjeldahl nitrogen determination (Reactions 7-3 through 7-5), the final product is a solution of NHJ ions in HCI solution. It is necessary to titrate the HCI without titrating the NH ions. 

Bound nitrogen includes all nitrogen-containing compounds, except N2, dissolved in water. Kjeldahl nitrogen analysis, described in Section 7-2, is excellent for amines and amides but fails to respond to many other forms of nitrogen. An automated combustion analyzer converts almost all forms of nitrogen in aqueous samples into NO, which is measured by chemiluminescence after reaction with ozone 17... 

The C and N Balances. The sum of the carbon and nitrogen contents of the solid and of the liquid phases was obtained by combustion for C and by Kjeldahl for N. It reproduces the initial content before reaction in all cases within the limits of the experimental error ( 0.3% for C and N). Since hydrocarbons are detected in the gas phase, this means that their total C content is smaller than the experimental error effecting the combustion analysis. 

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