Metodo kjeldhal

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Leidy Tatiana Castillo Higuera: 20101150013
Group 4


In the following report it is explained in way it details the procedure to find the nitrogen percentage according to the method of Kjeldahl, in which are determined the total nitrogen in organic form and ammonia, not being included the nitrate. Basically we say that the method is based on the conversion ofthe nitrogen in sulfate ammonium, for treatment with sulfuric acid in hot. Later on you alkaline, is distilled and according to the corresponding formula is find the percentage of total nitrogen.

KEY WORDS: Nitrogen, digestion, distillation, titration, neutralization, indicator.


The nitrogen is presented in the nature in many organic compoundsand the nitrogen determination in many compound amino it is of particular importance.

All the proteins, vegetables or animals, contain nitrogen amino, which is nitrogen together to two atoms of hydrogen and one of carbon,--C—NH2.

The nitrogen in this form is determined by means of the Kjeldahl method and starting from the result is calculated the number of proteins multiplicand by anappropriate factor, in many cases for 6.5.

This method multiplied by Kjeldahl in 1883, is used in industrial laboratories, doctors and of investigation. The trivalent nitrogen so much organic as inorganic it is usually determined by this method or their respective modifications and certain previous treatments, they also allow to apply this method to the determination of many other nitrogen forms.BEGIN OF THE KJELDAHL METHOD

The Method developed by Kjeldahl consists of three stages:

1. Digestion: Conversion of the Nitrogen (coming from the proteins, for example) in ion ammonium.

2. Distillation: Separation for haulage with vapor of the ammonia and later solubilization in a acid solution of known concentration.

3. Valuation: Mensuration of the quantity of acid neutralizedby the dissolved ammonia, what indicates the quantity of present Nitrogen in the initial sample.

According to the origin of the sample, through a factor is related the quantity of opposing Nitrogen with the percentage of proteins that originated it.

Next it is explained each one of the parts corresponding to this method more detailedly.


The sample to analyze whose contentof proteins is ignored, it is attacked by a mixture of Sulfuric Acid to which is added a mixture of a salt (Sulfate of Potassium or of Sodium) containing a small proportion of a catalyst. At the present time, the most broadly used one is Sulfate of Copper, for not to be toxic and to be innocuous for the environment.

The attaché of the blended salt with the catalyst causes that the temperatureduring the digestion rises and increase the speed of rupture of the molecular connections, facilitating the complete conversion of the Nitrogen protean in ion Ammonium.

The technique consists then on placing the sample in the reaction tube, to add the acid together with the mixture of the salt and the catalyst. You proceeds then to increase the temperature by means of the direct heating of thetube that contains to the whole mixture, until reaching the 370*C; once reached that temperature stays it until achieving the complete digestion of the sample.

It is important to highlight that in this stage it becomes necessary to eliminate the sulfurous gases coming from the digestion of each one of the tubes.

For it is believed it a light aspiration current by means of hole, collecting andsolubilized the gases in an alkaline solution that avoids the contamination of the atmosphere in the labor environment

The control of the temperature is very important, since an inferior temperature to the one mentioned causes an incomplete digestion while a bigger temperature will cause loss of compound nitrogenous in the current of gases that you/they leave the tubes; in both cases, the...
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