Boric acid Kjeldahl method

Method 2.13 Determination of 1 to 90% Organic Nitrogen in Polymers. Kjeldahl Digestion - Boric Acid Titration Method... 

METHOD 83 - DETERMINATION OF DOWN TO 50 PPM ORGANIC NITROGEN IN POLYOLEFINS. KJELDAHL DIGESTION - BORIC ACID TITRATION METHOD. 

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 ... 

If, improbably, trapping capacity were in question, the boric acid could be set at 2%, not 1%. It is striking that air lacking NaOH can be passed through the trap contents for hours without producing any change in color. In common with the NIOSH method, the specificity is merely for alkali as distinct from NaOH the customary use of boric acid has in fact hitherto been for estimating ammonia in Kjeldahl distillates. 

Nitrogen is determined by the Kjeldahl method using Devarda s alloy. The complex and the alloy are placed in a standard Kjeldahl apparatus and the ammonia distilled off from a 7.5 M sodium hydroxide solution into 2% boric acid. The ammonia is titrated with standard hydrochloric acid using bromocresol green-methyl red as indicator. 

The boric acid method (a direct method) is simpler and is usually more accurate since it requires the standardization and accurate measurement of only one solution. However, the end point break is not so sharp, and the indirect method requiring back-titration is usually preferred for micro-Kjeldahl analysis. A macro-Kjeldahl analysis of blood requires about 5 mL blood, while a micro-Kjeldahl analysis requires about 0.1 mL. 

A 2.00-mL serum sample is analyzed for protein by the modified Kjeldahl method. The sample is digested, the ammonia is distilled into boric acid solution, and 15.0 mL of standard HCl is required for the titration of thb ammonium borate. The HCl is standardized by treating 0.330 g pure ( [4)2804 in the same manner. If 33.3 mL acid is required in the standardization titration, what is the concentration of protein in the serum in g% (wt/vol) ... 

The nitrogen contained in organic substances in water is converted to ammonium ions by KjeldahEs decomposition method. The ammonium ions are distilled from alkaline solution as ammonia, collected in boric acid solution and determined acidimetrically or photometrically in the receiver. Ammonium ions originally contained in the sample are separated, identified and deducted, or separated off by distillation before carrying out the Kjeldahl decomposition. Nitrite and nitrate are volatilized by H2SO4 in the... 

In the classical Kjeldahl method, the proteins are digested (wet oxidized) in sulfuric acid with a catalyst (mercury and selenium tablets now succeed by the much safer potassium and copper sulfate tablets). An acid solution of ammonium sulfate is formed which is then diluted in water. The solution is made alkaline with sodium hydroxide and heated to distil off ammonia into excess standard acid sulfuric acid. The excess acid is back-titrated with standard sodium hydroxide to determine the amount of ammonia. It is more usual now to use boric acid in which to collect the ammonia and titrate with standard hydrochloric acid. The Kjeldahl procedure has been partially automated in systems such as the Kjeltec Analyzer. Total protein can be calculated as nitrogen content x 6.38. 

Scales FM, Harrison AP (1920) Boric acid modification of the Kjeldahl methods for crop and soil analysis. J Ind Eng Chem 12 350-352... 

Some workers prefer to distil the ammonia into a 4 per cent solution of boric acid, in which it may then be titrated directly with standard acid using methyl red as indicator. This is the method used in the B,P, for Kjeldahl determinations and has the obvious advantage that only one standard solution is used. However, in our experience this method sometimes leads to unsatisfactory end-points and we prefer to recommend the reliable back-titration procedure. 

The Kjeldahl method for nitrogen determination is a good example of a back titration. The sample (for example, a food product) is oxidized by concentrated sulfuric acid to remove carbonaceous matter. Excess sodium hydroxide solution is then added, and the ammonia released is carefully distilled off into a known volume of standard acid, such as 0.1 M boric acid. The excess acid is then titrated with standard alkali. 

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