1、BRITISH STANDARD BS 1742-5.1: 1991 Methods for Chemical analysis of condensed milks Part 5: Determination of nitrogen content Section 5.1 Reference methodBS1742-5.1:1991 This British Standard, having been prepared under the directionof the Agriculture and Food Standards Policy Committee,was publishe
2、d underthe authority of the Standards Board and comes intoeffect on 31 July 1991 BSI 11-1999 The following BSI references relate to the work on this standard: Committee reference AFC/3 Draft for comment 90/56626 DC ISBN 0 580 19656 9 Committees responsible for this British Standard The preparation o
3、f this British Standard was entrusted by the Agriculture and Food Standards Policy Committee (AFC/-) to Technical Committee AFC/3, upon which the following bodies were represented: Association of British Preserved Milk Manufacturers Association of Public Analysts of Scotland Creamery Proprietors Ass
4、ociation Department of Trade and Industry (Laboratory of the Government Chemist) Intervention Board for Agricultural Produce Joint Committee of the Milk Marketing Board and the Dairy Trade Federation Milk Marketing Board Milk Marketing Board for Northern Ireland Ministry of Agriculture, Fisheries an
5、d Food Royal Association of British Dairy Farmers Royal Society of Chemistry Society of Chemical Industry Society of Dairy Technology Amendments issued since publication Amd. No. Date CommentsBS1742-5.1:1991 BSI 11-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1
6、2 Definition 1 3 Principle 1 4 Reagents 1 5 Apparatus 1 6 Sampling 2 7 Procedure 2 8 Expression of results 3 9 Precision 3 10 Test report 3 Publication(s) referred to Inside back coverBS1742-5.1:1991 ii BSI 11-1999 Foreword This Section of BS 1742 has been prepared under the direction of the Agricul
7、ture and Food Standards Policy Committee. It is a revision of the methods described in clause 7 of BS 1742:1951, which is deleted by amendment. This edition of the standard specifies only copper (II) sulphate as the catalyst unlike the previous edition which permitted the use also of selenium or sui
8、table selenium compounds. NOTEThis Section should be read in conjunction with Part 1 “General introduction, including preparation of samples”, published separately. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible f
9、or their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 4, an inside back cover and a back cover. This standard has been update
10、d (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS1742-5.1:1991 BSI 11-1999 1 1 Scope This Section of BS 1742 describes a reference method for the determination of nitrogen content of condensed milks. NOTE 1BS 1
11、742-5.2 describes a routine method. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition For the purposes of this Section of BS 1742, the following definition applies. nitrogen content of condensed milk the quantity of nitrogen correspon
12、ding to the ammonia produced and determined under the conditions specified in this Section of BS1742 expressed as grams per100g of sample 3 Principle A test portion is digested with a mixture of concentrated sulphuric acid and potassium sulphate, using copper (II) sulphate as the catalyst, to conver
13、t organic nitrogen present to ammonium sulphate. Excess sodium hydroxide is added to the cooled digest to liberate ammonia from the ammonium sulphate. The ammonia is distilled into an excess of boric acid solution, followed by titration with standard volumetric sulphuric acid solution. The nitrogen
14、content of the sample is calculated from the amount of ammonia produced. 4 Reagents 4.1 Potassium sulphate 4.2 Copper (II) sulphate solution. Dissolve 5.0g of copper (II) sulphate pentahydrate (CuSO 4 .5H 2 O) in water and dilute to the mark in a100mL one-mark volumetric flask. 4.3 Sulphuric acid, a
15、t least 98.0% (m/m), density at20 C approximately1.84g/mL. 4.4 Antifoaming agent NOTEA silicone preparation is recommended, e.g. a 30% (m/m) aqueous emulsion. 4.5 Sodium hydroxide solution, low in nitrogen, containing approximately40g of sodium hydroxide (NaOH) per100mL of solution. 4.6 Dilute sodiu
16、m hydroxide solution, low in nitrogen, containing approximately2g of sodium hydroxide (NaOH) per litre of solution. 4.7 Boric acid solution. Dissolve40g of boric acid in1 L of hot water, allow to cool and store in a borosilicate glass bottle. Neutralize before use withthe dilute sodium hydroxide sol
17、ution (4.6), using 1 mL of the indicator solution (4.8). 4.8 Indicator solution. Dissolve 0.01 g of methyl red and 0.15g of bromocresol green in100mL of ethanol (70% V/V). 4.9 Sulphuric acid, standard volumetric solution, c(H 2 SO 4 ) = 0.05 mol/L. 4.10 Sucrose, nitrogen content not more than0.002%
18、(m/m). 4.11 Ammonium sulphate. Minimum assay 99.9% on dried material. The ammonium sulphate shall bedried at102 2 C for not less than2h, immediately before use. 4.12 Phenacetin NOTEMaterial supplied as being an organic analytical standard with an assay (ex N) of100 0.5% (m/m) should be used. 5 Appar
19、atus Ordinary laboratory apparatus together with the following. 5.1 Balance, capable of weighing to an accuracy of0.0001g. 5.2 Kjeldahl flasks, of capacity 500 mL. 5.3 Glass balls, of diameter approximately 5mm. 5.4 Burette or automatic pipette, for delivering4.0mL portions of the copper sulphate so
20、lution (4.2). 5.5 Graduated measuring cylinders, complying with BS604, of capacities 50mL, 100mL and 250mL respectively. 5.6 Digestion apparatus, to hold the Kjeldahl flasks(5.2) in an inclined position (approximately45 ), with electric heaters or gas burners that do not heat the flasks above the le
21、vel of their contents, and with a fume extraction system. 5.7 Distillation apparatus, borosilicate glass, to which a Kjeldahl flask (5.2) can be fitted, consisting of a splash head connected to an efficient condenser with a straight inner tube and an outlet tube attached to its lower end; the connec
22、ting tubing and stopper(s) shall be close fitting and preferably made of neoprene. 5.8 Anti-bumping granules, e.g. of fused alumina. 5.9 Pipette or automatic pipette, for delivering0.10mL portions of the indicator solution(4.8). 5.10 Conical flasks, of capacity 500mL, graduated at 200mL. 5.11 Burett
23、e of capacity50mL, class A, complying with BS846.BS1742-5.1:1991 2 BSI 11-1999 6 Sampling Take a representative sample of the condensed milk to be tested using, where appropriate, the procedures described in BS 1742-1. 7 Procedure 7.1 Carry out duplicate determinations using two test portions taken
24、from the same test sample. 7.2 To the Kjeldahl flask (5.2) add three glassballs(5.3), 10g of the potassium sulphate(4.1),4.0mL of the copper sulphate solution (4.2), approximately 3g of prepared sample (clause 6), weighed to the nearest 0.001g and 25mL of the sulphuric acid (4.3), using the acid to
25、wash down any copper sulphate solution, potassium sulphate or sample left on the neck of the flask. Gently mix the contents of the flask. 7.3 Heat each Kjeldahl flask on the digestion apparatus (5.6), very gently at first, taking care to prevent the black froth entering the neck of the flask. When t
26、he initial frothing has ceased and copious white vapour appears, boil vigorously (acid vapour condensing halfway up the neck of the flask) until no black particles remain and until the digest becomes a clear pale blue-green in colour. On reaching this stage, adjust the heating to give gentle boiling
27、 and continue the heating for 2h. If black particles enter the neck of the flask and these are not all washed down into the bulb by the refluxing acid during the initial stages of the vigorous boiling period (this is facilitated, for example, by rotating the flask) allow the flask to cool sufficient
28、ly and wash the particles into the bulb with the minimum of water. Then continue the digestion as described above. The timing of the 2h period of gentle boiling shall not be started until a pale blue-green tint becomes visible in the digest. 7.4 When the Kjeldahl flasks are cool, add 210mL of water
29、plus three or four drops of the antifoaming agent (4.4), using the water to wash down the neck of the flask. Mix the contents thoroughly and ensure that any crystals which separate out are dissolved. Add some anti-bumping granules (5.8) and then add100 mL of the sodium hydroxide solution (4.5) by ge
30、ntly pouring the solution down the inclined neck of the flask to form a layer at the bottom of the bulk of the flask. NOTEIf the digest solidifies on cooling, the test should be repeated using a new test portion. 7.5 For each Kjeldahl flask, immediately after the addition of the sodium hydroxide sol
31、ution, connect the flask to a distillation apparatus (5.7), which hasthe tip of its condenser outlet tube immersed in50 mL of the boric acid solution (4.7) plus 0.10mL of the indicator solution (4.8), contained in a conical flask (5.10). Swirl the contents of each Kjeldahl flask to mix thoroughly an
32、d boil, gently at first to prevent excessive frothing. When 100 mL to 125mL of distillate have been collected, lower each conical flask until the tip of the condenser outlet tube is approximately 40mm above the 200mL mark. Continue each distillation until irregular boiling (bumping) starts and then
33、immediately stop the heating. Disconnect each Kjeldahl flask and rinse the tip of each condenser outlet tube with a little water, collecting the rinsings in the conical flask. The distillation rate shall be such that approximately150mL of distillate are collected when irregular boiling (bumping) sta
34、rts; the volume of the contents of each conical flask will be approximately 200mL,i.e. approximately 150mL of the distillate will have been collected as required. The efficiency of each condenser shall be such that the temperature of the contents of each conical flask does not exceed25 C during the
35、distillation. 7.6 Titrate each distillate with the standard volumetric sulphuric acid solution (4.9) until the colour matches that of a freshly prepared solution consisting of150mL of water,50mL of the boric acid solution and0.10mL of the indicator solution, contained in a conical flask (5.10). Esti
36、mate each burette reading to 0.01 mL. 7.7 Carry out a blank test following the procedure described in 7.2 to 7.6, taking 3mL of distilled water with about0.1g of sucrose (4.10) instead of the prepared sample. NOTE 1The titration of the blank distillate will require only a very small volume of the st
37、andard volumetric solution, normally not more than 0.15mL. NOTE 2The accuracy of the procedure should be checked regularly by means of the following recovery trials, carried out in accordance with 7.2 to 7.6. a) Check that the digestion procedure is efficient by using a test portion of approximately
38、 0.35g of phenacetin (4.12) weighed to the nearest0.001g. The recovery of nitrogen from the phenacetin should be not less than 99.0%. b) If the digestion and distillation procedures are found to give low recoveries then the distillation procedure alone may be checked as follows. Prepare solution of
39、the ammonium sulphate (4.11) of concentration exactly 0.05mol/L. Pipette a 10.0mL aliquot portion of the ammonium sulphate into a digestion flask and add 50 mL of water. Determine the nitrogen content of the solution according to the procedure described in 7.4 to 7.6. The recovery of nitrogen from t
40、he ammonium sulphate should be not less than 98.5%.BS1742-5.1:1991 BSI 11-1999 3 8 Expression of results 8.1 Method of calculation Calculate each duplicate value of the nitrogen content, in grams of nitrogen per100g of sample, by means of the formula: where Round the results to the nearest0.001 g pe
41、r 100g. 8.2 Result Take as the result the arithmetic mean of the results of two determinations, provided that the requirement for repeatability (see 9.1) is satisfied. Express the result to three decimal places. 9 Precision NOTEThe precision data were obtained from a collaborative study undertaken i
42、n 1991, involving nine laboratories and two sweetened and two unsweetened condensed milks. 9.1 Repeatability The difference between the values obtained from two determinations carried out simultaneously or in rapid succession by the same analyst using the same apparatus shall not exceed0.05g/100g of
43、 test portion. 9.2 Reproducibility The difference between the results obtained on samples of the same test material by two different analysts in different laboratories (where each resultis the mean of two values that comply withthe repeatability requirement) should not exceed 0.06g/100g of test port
44、ion. 10 Test report The test report shall be in accordance with BS 1742-1. V is the volume of the standard volumetric solution of acid used in the determination (inmL); V o is the volume of the standard volumetric solution of acid used in the blank test (inmL); m is the mass of the test portion (in
45、g). 0.14 VV o () m -4 blankBS1742-5.1:1991 BSI 11-1999 Publication(s) referred to BS 604, Specification for graduated glass measuring cylinders. BS 846, Specification for burettes. BS 1742, Methods for chemical analysis of condensed milks. BS 1742-1, General introduction, including preparation of te
46、st samples. BS 1742-5, Determination of nitrogen content. BS 1742-5.2, Routine method. BS 1742-5.1: 1991 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for preparing BritishStandards. It presents the UK view on standards in Eu
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