UNIVERSITY DISTRITAL FRANCISCO JOSE OF CALDASFACULTY OF SCIENCES AND EDUCATIONMASTER IN CHEMISTRY2013AUTORS: Diana Roa Garca* Jesenia Rodrguez Zapata*DETERMINATION OF NITROGEN IN SOIL SAMPLEINTRODUCTIONNitrogen is an essential element in the life processes of many organisms, as it is part of the main biomolecules of all living things, it is for this reason that it is one of the most abundant elements on earth and constitutes 78% of the atmosphere, despite these high rates the amount of nitrogen present in the soil is low because it depends directly on the dynamics and biogeochemical cycling. This nitrogen can reach the ground thanks to the fixing organic inputs and bacterial activity from the air, In this experimental practice is to study the level of nitrogen in a soil sample by the micro method Kjeldahl.Key words: nitrogen, digestion, distillation, titration.

THEORETICAL FRAMEThe nitrogen can reach the ground from organic inputs and bacterial attachment, once you set the ground is used by plants, animals and microorganisms that incorporate it into their tissues. The cycle is completed once these organisms die, so the nitrogen re-enters the soil, this process always begins with simple organic compounds (NH4 +, NO2-, NO3-, N2, NH3) and ends with complex organic compounds; after returning decomposition stage simple compounds.If there is lack of nitrogen in the soil, microorganisms will strongly affected growth and on the contrary there is abundant levels of the same, accelerated bacterial growth and decay, increasing ammonia production problems and usable nitrogen escaping into the atmosphere; most soils the relationship between C and N is close to 10:1 maintaining balance, normal nitrogen content varies from 0.05 to 2% in its various forms.The Kjeldahl method comprises three main phases:* Digestion of the sample: The sample is subjected to digestion by heating with sulfuric acid and a mixture of salts which accelerate and facilitate the oxidation of organic matter and the conversion of all forms of nitrogen in N3 +, which in acid medium radical are as ammonium (NH4 +) thus leading to organic forms of nitrogen mineral forms.* Distillation: Once it has all the nitrogen (NH4 +) the sample is exposed to a strong base of sodium hydroxide to form ammonium hydroxide, which when subjected to heat decomposes into ammonia (NH3) and water.* Rating: The ammonia released by the reaction is collected in a known volume of boric acid is titrated with hydrochloric acid or sodium hydroxide as the case and by comparison with a white determines the amount of acid that reacted with NH3.MATERIAL AND EQUIPMENT* Soil sample dried and ground with a mortar.* Analytical balance.* Kjeldahl flasks.* Beakers.* Specimens.* Digester.* 1 L volumetric flask* 1 L Erlenmeyer flask* Boiling stones.* Pipette.* Burette.* Universal support clip.SOLUTIONS AND REAGENTS*Mp=0,2 g*H2SO4= 5 ml concentrated*K2SO4= 2 g*CuSO4= 0,1 g*NaOH= 20 ml 35%*H3BO3= 25 ml 4%PROCEDUREDetermination of nitrogen by the Kjeldahl methodDigestionWeigh soil sample of 0.2 g, previously spray, place it on a filter paper, fold the paper being careful not to lose any of the samples and place it in a dry Kjeldahl flask, taking care that the sample does not adhere to walls or flask neck.For samples that containing nitrates. Add 5 mL of concentrated sulfuric acid, 2 g of K2SO4 and 0.1 g CuSO4. Stir gently to mix thoroughly. And we proceed to have the pipe in the digester until the solution clarifies (digestion end when the sample color is blue-green light, the process will take approximately 120 minutes).

DistillationKjeldahl flask immediately connect to the coolant as quickly as possible to prevent loss of this ammonia and a steam trap so that the tip of the condenser is immersed in an Erlenmeyer flask containing 25 ml of boric acid solution to 4 % w / v, 2-3 drops of methyl red to 0.2% w / v.Are added pumice granules to prevent bumping, leans the ball and add 20 mL of sodium hydroxide solution 35% w / v, or enough to make alkaline medium and check that all joints are well done. The digested mixture will become dark (blue-gray or dark brown). If it changes color add more NaOH.Then heated until all the distilled ammonia (150 mL of distillate minimum) to the flask containing boric acid and tested with litmus paper pink until ammonium negative result. With drawing the distillate outlet tube, rinsing the exterior of the tube in the distillate to prevent entrainment of steam.Titration_ DegreeTitration with methyl red indicator. Distillate is titrated with 1 N HCl solution, using 2 drops of methyl red as indicator until the color changes from yellow to pink each equivalent of NaOH used corresponds to an equivalent of NH3 or an equivalent of N in original sample. The weight in mg of N is given by X milliequivalents of base 14 (the equivalent weight of N).ANALYSIS AND RESULTSAssessing the distillate with HCl until the color shift. Given that Mol of HCl = Mol of NH3 = Mol of N in the sampleMol of H2SO4 = NH3 = 2Mol, 2Mols of N in the sample.

Table No. 1. data

Prueba volumen

Vol1 0,2 ml

Vol20,1 ml

Vol30,2 ml

X0,166 ml

Note: the acid titrant dilution was performed, so that the final concentration is 0.037 N.Calculated by the following equation nitrogen content:mg N = N x V x 14where:N = normality of the acid titration(1N HCl dilution carried dilution factor triplicate 0.33 0.037 N)V = volume of acid consumed (ml)14 = Atomic weight of nitrogen.0.037 Nx0, values approaching 166 mlx14mg N = 0.1036 mg

Assessing the sample may be performed with hydrochloric acid or sodium hydroxide as the reaction that occurs is related to the titration of the excess boric acid as set forth the following equations:H3BO3 =H+ + H2BO3 (1)H2BO3=H+ + HBO3 (2)HBO3-= H+ + BO3 (3)Hydroxide holder is shifting the equilibrium equation 2, where as if hydrochloric acid is titrated with the equilibrium shifts according to equation 1, it is advisable to perform acid titration as the dissociation constant for this equilibrium is greater than in Equation 2 and incur fewer errors.Tabla 4.9 Criteria for assessing soil based on its total nitrogen content(Moreno, 1978).

El porcentaje de nitrgeno contenido en la muestra de suelo se calcula a partir de la siguiente expresin:

Where:T = ml of spent clorhidric acid in the sample.B = ml of clorhidric acid in the white worn.N = exact normality of clorhidric acid.S = weight of the soil sample.Nitrogen=1.162%CONCLUSIONS According to the table is reported that the nitrogen content is extremely rich as it is greater than 0.221%, however it is not possible to determine a possible use for this is unknown ground as the carbon content and the pH so that it is recommended for soils which contain a high content of nitrogen that would accelerate the metabolism of many organisms of great importance in the life cycle of the system.It is important to note that in this case the evaluation was performed without the determination of the target, which is why the percentage of nitrogen is quite high, although this result the nitrogen content may be taken as a reference value to establish useful use of it, such as in extremely poor soils in nitrogen is recommended to subject the sample to new studies, mainly carbon.