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1、INTERNATIONAL STANDARD INTERNATIONAL ORGANIZATION FOR STANDARDIZATION l MtXHilYHAPO!ZHAtl OPr.4HM3.4UMtl I-IO CTAHAAPTM3AUMM .ORGANISATION INTERNATIONALE DE NORMALISATION Agricultural food products - General directions for the determination of nitrogen by the Kjeldahl method Produits agricoles alime
2、n taires - Directives g these documents are now in the process of being transformed into International Standards. As part of this process, Technical Committee ISO/TC 34 has reviewed IS0 Recommendation R 1871 and found it technically suitable for transformation. International Standard IS0 1871 theref
3、ore replaces IS0 Recommendation R 1871-1971 to which it is technically identical. IS0 Recommendation R 1871 was approved by the Member Bodies of the following countries : Australia Greece Austria Hungary Brazil India Czechoslovakia Israel Denmark Netherlands Egypt, Arab Rep. of New Zealand Finland P
4、eru France Poland Portugal Romania South Africa, Rep. of Sweden Turkey United Kingdom U.S.S.R. No Member Body expressed disapproval of the Recommendation. The Member Body of the following country disapproved the transformation of lSO/R 1871 into an International Standard : United Kingdom 0 Internati
5、onal Organization for Standardization, 1975 l Printed in Switzerland Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 02:03:04 MDTNo reproduction or networking permitted without l
6、icense from IHS -,-,- INTERNATIONAL STANDARD IS0 1871-1975 (E) Agricultural food products - General directions for the determination of nitrogen by the Kjeldahl method 0 INTRODUCTION 0.1 The analysis of products of animal or vegetable origin, particularly food products, often includes a determinatio
7、n of the so-called total nitrogen by the Kjeldahl method. If therefore seems useful, in order to make the results comparable, to recommend a single procedure for this determination. Experience has shown a) that different procedures are in use, depending on the products and the operators in the vario
8、us countries; b) that these different procedures, when correctly applied, give very similar results. 0.2 As far as can be seen, the spread of the results of the determination of nitrogen, due to the variety of procedures, generally seems less than the spread of the results attributable to the hetero
9、geneity of the products examined. Furthemore, the fact of choosing a special procedure for each type of product to be analysed has the following results : a) it is undoubtedly satisfactory to those analysts who, since they always analyse the same product, have chosen the procedure to which they are
10、accustomed and which suits them; b) it compels laboratories which analyse various types of products, and wish to follow International Standards, to multiply their procedures. This obliges them to have available a variety of types of apparatus and reagents and to train personnel in their use, which i
11、s contrary to the spirit of standardization. In addition, for the analysis of compound products, of which each constituent should in principle be analysed by a special procedure, it is necessary to choose a method which will not necessarily be that adopted for each of the constituents of the product
12、. This can only be damaging to the application of International Standards; c) it means, as often as not, that laboratories which analyse different types of products find themselves unable to use the exact procedures specified because they do not have at their disposal all the necessary types of appa
13、ratus and reagents for these various procedures. They are therefore forced either to use their normal procedure (which may not correspond to all the requirements) or to work out a compromise between their own procedure and that specified, and thus to deviate from the International Standards. 0.3 The
14、 conclusion reached is that the Kjeldahl method can be standardized in principle, but that it is possible to agree that various forms of apparatus or procedures are equivalent if they give equivalent results. It has therefore been thought preferable a) to define the general directions necessary for
15、the correct application of the Kjeldahl method for the determination of nitrogen in agricultural food products; this is the purpose of the present International Standard; b) to leave it to the International Standards which are specific to certain products to describe detailed procedures which are in
16、 accordance with the provisions of the general directions and which can constitute working documents for laboratories. There are products containing nitrogenous compounds in which the nitrogen cannot be determined by the Kjeldahl method; these are special cases not compatible with the general direct
17、ions, and other suitable methods should be the subject of special International Standards for these products. 1 SCOPE This International Standard gives general directions for the apparatus and procedures used for the determination of nitrogen in agricultural food products by the Kjeldahl method. All
18、 variants of the procedure should be in accordance with these directions in order to obtain equivalent results. 2 FIELD OF APPLICATION This International Standard applies to products containing only nitrogenous compounds which are directly determinable by the Kjeldahl method. Products containing, fo
19、r example, nitrites or nitrates in major amounts require special treatment, although this International Standard can be applied to cured meat products in view of their low residual nitrite and nitrate content. 1 Copyright International Organization for Standardization Provided by IHS under license w
20、ith ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/27/2007 02:03:04 MDTNo reproduction or networking permitted without license from IHS -,-,- lSO1871-1975(E) 3 PRINCIPLE Destruction of organic matter by sulphuric acid in the presence of a catalyst, rendering of the reaction pr
21、oduct alkaline, distillation and titration of the liberated ammonia. 4 TEST PORTION Since many samples of products of animal or vegetable origin (particularly food products) cannot be obtained in a state of perfect homogeneity after preparation in the laboratory, it is advisable to adopt macro-metho
22、ds. The test portion, which varies in amount according to the assumed nitrogen content determinable by the Kjeldahl method, shall be representative of the sample and shall contain between 0,005 and 02 g of nitrogen and, preferably, more than 0,OZ g. 1 n the case of insufficiently homogeneous product
23、s, however, the test portion shall be large (exceeding I g) and, if the nitrogen content is high, the determination shall be carried out on an aliquot portion of the liquid resulting from the destruction of organic matter. The test portion shal I be weighed precision at least equal to 0, 1 %. or mea
24、sured with a Particularly in the case of a viscous liquid or a product in paste form, the test portion can be taken in a small glass container which is placed in the flask, or in a sheet of aluminium, paper or plastics material, which does not yield additional nitrogen or of which the nitrogen conte
25、nt is known. 5 DESTRUCTION OF ORGANIC MATTER 5.1 Sulphuric acid The acid used shall be practically free from nitrogenous compounds (see 8.2). If acid of density p20 = I,83 to I,84 g/ml is used, at least 12 ml of acid should be taken for a test portion containing at most I g of dry substance and 6 to
26、 12 ml per gram of additional dry substance. This information is given only as an indication and should be adjusted for each type of product considered. Avoid unnecessary excess. 5.2 Catalysts A distinction should be drawn between substances intended to raise the boiling point of the liquid during t
27、he destruction of organic matter, and catalysts proper which facilitate such destruction. The former substances are generally sodium sulphate or preferably potassium sulphate; they are introduced in sufficient quantity to raise the boiling point to approximately 360 to 380 “C at the end of the diges
28、tion. These substances are frequently mixed in advance with the appropriate catalyst, and then a reagent known as a “compound catalyst” is obtained. Agreement on the choice of catalyst has proved particularly difficult. Various formulae can be shown in the individual International Standards. All cat
29、alysts which are 1 effective and the check tests are acceptabl and satisfy e. blank When the sample is in powder form, it is often advisable to mix the test portion with the catalyst dry, in the Kjeldahl flask, before adding the sulphuric acid. 5.3 Heating The start of heating is a critical moment i
30、n the Kjeldahl method : in many cases foam appears which may rise into the neck of the flask or even escape from it. Particular attention should be paid to this point in the manipulation; moderate heating should be applied at the beginning of the operation. It is sometimes advisable to add an anti-f
31、oaming agent (paraffin or various substances which alter the surface tension). It is then essential to ensure that these substances do not yield any additional nitrogen. If the heat source gives off an intense infra-red radiation, it is often observed that substances (for example, carbohydrates) whi
32、ch generally have a tendency to cause foam, form instead masses of carbonaceous material which then take longer to dissolve but do so without foaming strongly. It may also be advantageous in certain cases to defer the heating, for example overnight. It is difficult to give a general description of s
33、uitable heating sources using gas or electricity. The intensity of heating may be specified to some extent by indicating the time necessary to raise the temperature of a given volume of water, in a flask similar to that used for the test, from 20 “C to boiling point. In practice, heating is adequate
34、 if the boiling acid condenses towards the middle of the neck of the usual type of Kjeldahl flask, for example of capacity 300 ml. In all cases, it is essential to avoid overheating of the walls of the flask where they are not in contact with liquid; this can be arranged, for example, by placing the
35、 flask on a sheet of asbestos with a hole of diameter slightly less than that of the free surface of the liquid in the flask. Throughout the heating, it is advisable to place the flask on a support so that its axis is inclined at an angle of 30 to 45” to the vertical. Many techniques specify agitati
36、on of the flask from time to time during destruction of the organic matter. This procedure can often be avoided by placing in the flask a glass ball, for example 5 to 7 mm in diameter. When the liquid has become clear, the absence of further change of colour does not necessarily indicate complete de
37、struction of the organic matter; the nitrogen in certain resistant compounds, such as lysine, tryptophan or tyrosine, is rendered inorganic only by prolonging the heating for 30 to 90 min after the liquid has become clear. An additional heating of 30 to 40 min is generally sufficient. For any one pr
38、oduct, and with a test portion of the same mass, the duration of heating depends both on the source of heat available and on the catalyst selected. 2 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for
39、 Resale, 04/27/2007 02:03:04 MDTNo reproduction or networking permitted without license from IHS -,-,- ISOl871-1975 (E) The optimum heating conditions are those which enable the highest results for the determination of nitrogen to be obtained, after having eliminated all sources of error. During the
40、 heating, the mouth of the flask can be partially blocked, for example with a glass bulb with a short stem, or connected to a device for absorbing or aspirating fumes. Such a device should not involve any risk of retaining acid liquid which has been splashed up, or of contamination by residues from
41、an earlier operation. In all cases, it is advisable during the cooling to take the necessary steps to protect the contents of the flask from any ammonia fumes which may be present in the laboratory. To summarize, use an electric or gas appliance which does not cause overheating of the walls of the f
42、lask not in contact with the liquid, and which is capable of ensuring sufficient boiling for the acid to condense towards the middle of the neck of the usual type of Kjeldahl flask; continue heating for at least 30 min after the liquid has become clear and no longer changes colour. 5.4 Precipitation
43、 of mercury If the catalyst used contains mercury, the mercury shall be precipitated before the distillation of the ammonia is carried out. Sodium hypophosphite or potassium hypophosphite is the best reagent for precipitating mercury. This reagent shall be introduced in the dry state, after diluting
44、 the medium and before making it alkaline. In practice, 1 g of sodium hypophosphite or of potassium hypophosphite suffices for the precipitation of up to 1 g of mercury. NOTE -The precipitation of mercury by alkaline sulphides or thiosulphates involves a risk of the release of hydrogen sulphide or s
45、ulphur dioxide into the atmosphere in the apparatus if, during mixing, these reagents are temporarily in contact with an acid area of the medium. These acid gases, if passing directly into the distillate, then neutralize part of the ammonia, and this causes an error leading to a low result. 6 DISTIL
46、LATION OF AMlVlONlA 6.1 Apparatus 6.1.1 The various procedures known at the present time describe a great variety of distillation apparatus. A distinction can be made between a) apparatus which allows distillation to be carried out without transfer of the sulphuric acid solution contained in the dig
47、estion flask; b) apparatus which requires transfer of the acid solution, either as a whole or by the removal of an aliquot portion. 6.1.2 The distillation of ammonia can be carried out by various methods : a) by simple distillation after dilution with water; b) by steam distillation, possibly with t
48、hermal insulation or auxiliary heating of the vessel containing the solution to be distilled; c) by distillation with superheated steam, which not require auxiliary heating. does 6.1.3 Every apparatus shall include a device for condensing steam and collecting the ammonia vapour. All these various ty
49、pes of apparatus are admissible, if they satisfy the check tests described in 8.3. The apparatus shall have the following characteristics : a) it shall prevent any loss of ammonia, either by volatilization in the atmosphere at the moment of adding alkali, or by leakage during distillation; b) it shall ensure complete distillation of the ammonia; c) it shall prevent any accidental carry-over of the sodium hydroxide solution, droplets of which shall be retained by means of an e