1、BRITISH STANDARD BS 1742-5.2: 1991 Methods for Chemical analysis of condensed milks Part 5: Determination of nitrogen content Section 5.2 Routine methodBS1742-5.2:1991 This British Standard, having been prepared under the directionof the Agriculture andFood Standards Policy Committee,was published u
2、nderthe authority of the Standards Board and comes intoeffect on 28June 1991 BSI 11-1999 The following BSI references relate to the work on this standard: Committee reference AFC/3 Draft for comment 90/56625 DC ISBN 0 580 19658 5 Committees responsible for this British Standard The preparation of th
3、is 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 Associa
4、tion 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 and Fo
5、od 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 CommentsBS1742-5.2:1991 BSI 11-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 De
6、finition 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 coverBS1742-5.2:1991 ii BSI 11-1999 Foreword This Section of BS1742 has been prepared under the direction of the Agriculture
7、and Food Standards Policy Committee, and introduces into the standard a routine method for the determination of nitrogen content of condensed milks. NOTEThis Section should be read in conjunction with Part 1 “General introduction, including preparation of samples”, published separately. A British St
8、andard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for 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 cove
9、r, an inside front cover, pages i and ii, pages1 to 4, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS1742-5.2:1991 BSI 11-1999 1 1 Scope Th
10、is Section of BS1742 describes a routine method for the determination of the nitrogen content of condensed milks. NOTE 1BS 1742-5.1 describes a reference 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 th
11、is Section of BS1742, the following definition applies. nitrogen content of milk the quantity of nitrogen corresponding to the ammonia produced and determined under the conditions specified in this Section of BS 1742, expressed as grams per100g of sample 3 Principle An aliquot portion of the diluted
12、 test sample is digested rapidly, using a block digestion apparatus, with a mixture of sulphuric acid, hydrogen peroxide and potassium sulphate together with a catalyst such as a mixture of copper(II) sulphate and titanium dioxide, to convert the organic nitrogen present to ammonium sulphate. Excess
13、 sodium hydroxide is added to the cooled digest to liberate ammonia. The ammonia is steam distilled out of the digestion mixture in a manual, semi-automatic or fully automatic steam distillation unit. In the case of manual or semi-automatic steam distillation, the ammonia is distilled into an excess
14、 of boric acid solution and titrated with standard sulphuric acid solution. If a fully automatic distillation unit is employed, the ammonia is automatically titrated and end point detection is by means of a photometric or pH system. The nitrogen content of the sample is calculated from the amount of
15、 ammonia produced. 4 Reagents NOTEThe reagents given in 4.1 to 4.12 are usually required for use in the digestion procedure and for manual titration of the ammonia content of the distillate. Additional and/or different reagents, as specified by the manufacturer, may be required for automatic titrati
16、on. 4.1 Kjeldahl catalyst tablets NOTE 1The use of commercially available tablets is recommended. NOTE 2Tablets comprising3.5g of potassium sulphate,0.105g of copper (II) sulphate pentahydrate and0.105g of titanium dioxide per tablet are suitable. Information on sources of supply can be obtained by
17、writing toCustomer Information, BSI, Linford Wood, Milton Keynes, MK14 6LE. Other types of tablet may be used provided that: a) they contain a quantity of potassium sulphate such that7g of potassium sulphate can be dispensed using an integral number of tablets; and b) they do not contain salts of to
18、xic metals such as selenium or mercury. 4.2 Sulphuric acid, at least 98.0% (m/m), density at20 C approximately 1.84g/mL. 4.3 Hydrogen peroxide solution, containing about300g of hydrogen peroxide (H 2 O 2 ) per litre. 4.4 Antifoaming agent NOTEA silicone preparation is recommended, e.g. a 30% (m/m) a
19、queous emulsion. 4.5 Sodium hydroxide solution, low in nitrogen, containing approximately40g of sodium hydroxide (NaOH) per100mL of solution. 4.6 Dilute sodium hydroxide solution, low in nitrogen, containing approximately2g of sodium hydroxide (NaOH) per litre of solution. 4.7 Boric acid solution. D
20、issolve40g of boric acid in1L of hot water, allow to cool and store in a borosilicate glass bottle. Neutralize before use withthe dilute sodium hydroxide solution (4.6), using 1mL of the indicator solution (4.8). 4.8 Indicator solution. Dissolve0.01g of methyl red and0.15g of bromocresol green in100
21、mL 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% (m/m). 4.11 Ammonium sulphate. Minimum assay99.9% on dried material. The ammonium sulphate shall be dried at102 2 C for not less than2h, immediatel
22、y before use. 4.12 Phenacetin NOTEMaterial supplied as being an organic analytical standard with an assay (ex N) of 100 0.5% (m/m) should be used. 5 Apparatus 5.1 Ordinary laboratory apparatus 5.2 Balance, capable of weighing to an accuracy of0.0001g. 5.3 Temperature controlled digestion block 5.4 D
23、igestion tubes, suitable for use with the digestion block (5.3). 5.5 Exhaust manifold, suitable for use with the digestion tubes (5.4).BS1742-5.2:1991 2 BSI 11-1999 5.6 Centrifugal scrubber apparatus or filter pump, constructed of glass and for use with mains water supply. 5.7 Automatic pipettes, to
24、 deliver 10mL portions of sulphuric acid and5mL portions of hydrogen peroxide, respectively. 5.8 Distillation unit, manual, semi-automatic or automatic. 5.9 Conical flasks, of capacity 250mL. 5.10 Burette, of capacity 25 mL, class B, complying with BS 846. 6 Sampling Take a representative sample of
25、the condensed milk to be tested using, where appropriate, the procedures described in BS 1742-1. 7 Procedure NOTEIn the normal course of events, samples are analysed in batches, according to the procedure described. 7.1 Gravimetrically prepare an approximately10%(m/m) dilution of the test sample (cl
26、ause 6), using at least10g of test sample and weigh to an accuracy of0.001g. Record the exact strength of the dilution. 7.2 Add the number of Kjeldahl catalyst tablets (4.1) containing7g of potassium sulphate to a clean, dry digestion tube (5.4). Take an aliquot portion of diluted sample (7.1) conta
27、ining approximately0.7g of the test sample (clause 6) into the tube Record the mass (m) of the test sample in the aliquot portion to the nearest0.001g (see 8.1). Carefully add 10mL of the sulphuric acid (4.2) by means of an automatic pipette (5.7), add three or four drops of the antifoaming agent (4
28、.4) and gently swirl the tube to mix the contents. Allow the tube to stand for5min. Carefully add5mL of the hydrogen peroxide solution (4.3) by means of an automatic pipette, allowing the solution to run down the inside of the tube. Again swirl the tube gently in order to mix the contents and then l
29、eave to stand for10min to15min. Transfer the tube to the digestion block (5.3), set at the temperature specified by the manufacturer and immediately place the exhaust manifold (5.5), which is itself connected to a centrifugal scrubber or similar device (5.6), in the top of the tube. Digest the sampl
30、e for the period specified by the manufacturer of the block, usually 40min, and then remove the tube from the block with the exhaust manifold in place and allow to cool for at least15min. Once the tubes are sufficiently cool to handle, remove the exhaust manifold and carefully add50mL of water to ea
31、ch tube. 7.3 Transfer the digestion tube to the distillation unit (5.8). If titration of the ammonia content of the distillate is performed manually, use the following procedure. Place a conical flask (5.9) containing 50 mL of the neutralized boric acid solution (4.7) and two or three drops of indic
32、ator solution (4.8) under the outlet of the condenser, in such a way that the delivery tube is below the surface of the boric acid solution. Operate the distillation unit in accordance with the manufacturers instructions and distil the ammonia liberated by addition of excess sodium hydroxide solutio
33、n (4.5), collecting the distillate in the boric acid solution. NOTEIn a semi-automatic distillation unit, the addition of excess sodium hydroxide and the steam distillation are performed automatically. Titrate the distillate with the standard sulphuric acid solution (4.9) until the colour matches th
34、at of a freshly prepared solution consisting of a volume of water corresponding to the amount of distillate produced,50mL of the boric acid solution (4.7), and two to three drops of indicator solution (4.8), contained in a conical flask. Estimate each burette reading to0.01mL. If the distillation un
35、it is fully automated to include titration of the ammonia content of the distillate, follow the manufacturers instructions for operation of the distillation unit. 7.4 Carry out a blank test following the procedure described in 7.2 and 7.3 using 2mL of water with about 0.25g of the sucrose (4.10) ins
36、tead of the aliquot portion. It is advisable to add an additional three or four drops of the antifoaming agent (4.4) to the digested blank prior to distillation, to reduce frothing. NOTE 1Titration of the blank distillate will require only a very small volume of the standard volumetric solution, nor
37、mally not more than0.15mL. NOTE 2The accuracy of the procedure should be checked periodically by means of the following recovery trials. a) Check that the digestion and distillation procedures are efficient by using a test portion of0.10g to 0.15g of the phenacetin (4.12) weighed to the nearest 0.00
38、1g. Determine the nitrogen content according to the procedure described in7.2 and 7.3. The recovery of nitrogen from the phenacetin should be not less than 98.5%. b) If the digestion and distillation procedures are found to give low recoveries then the distillation procedure alone may be checked as
39、follows. Prepare a solution of the ammonium sulphate (4.11) of concentration exactly0.05mol/L. Pipette a10.0 mL aliquot portion of the ammonium sulphate solution into a digestion tube and add 50mL of water. Determine the nitrogen content of the solution according to the procedure described in 7.3. T
40、he recovery of nitrogen from the ammonium sulphate should be not less than98.5%. NOTE 3It is recommended that analysts should periodically undertake determinations to verify that results complying with the repeatability (see clause 9) are being obtained.BS1742-5.2:1991 BSI 11-1999 3 8 Expression of
41、results 8.1 Method of calculation Calculate the value of the nitrogen content, in grams per100 grams of sample, by means of the formula: where V is the volume of the standard volumetric solution of acid used in the determination (inmL); V ois the volume of the standard volumetric solution of acid us
42、ed in the blank test (in mL); m is the mass of the sample in the aliquot of test portion (in g). 8.2 Result Express the result to two decimal places. 9 Precision NOTEThe precision data were obtained from a collaborative study undertaken in 1991 involving 14 laboratories and two sweetened and two uns
43、weetened 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 should not exceed0.04 g/100 g of test portion. 9.2 Reproducibility The difference between the
44、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 0.10g/100g of test portion. 10 Test report The test report shall be in accordance wit
45、h BS 1742-1. 0.14 VV o () m -4 blankBS1742-5.2:1991 BSI 11-1999 Publication(s) referred to BS 846, Specification for burettes. BS 1742, Methods for chemical analysis of condensed milks. BS 1742-1, General introduction, including preparation of test samples. BS 1742-5, Determination of nitrogen conte
46、nt. BS 1742-5.1, Reference method. BS 1742-5.2: 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 Europe and at the international level. It is incorpora
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