1、BRITISH STANDARD BS 1741-5.2: 1990 Methods for Chemical analysis ofliquid milk and cream Part 5: Determination of the nitrogen content of liquid milk Section 5.2 Routine methodBS1741-5.2:1990 This British Standard, having been prepared under the directionof the Dairying Standards Policy Committee, w
2、aspublished under the authorityof the Board of BSI andcomes into effect on 28September 1990 BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment 89/51961 DC ISBN 0 580 18787 X Committees responsible for this British Standard The pr
3、eparation of this British Standard was entrusted by the Dairying Standards Policy Committee (DAC/-) to Technical Committee DAC/3, upon which the following bodies were represented: Association of British Preserved Milk Manufacturers Association of Public Analysts of Scotland Creamery Proprietors Asso
4、ciation 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
5、 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 CommentsBS1741-5.2:1990 BSI 08-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2
6、 Definition 1 3 Principle 1 4 Reagents 1 5 Apparatus 1 6 Sampling 2 7 Procedure 2 8 Expression of results 2 9 Precision 3 10 Test report 3 Publication(s) referred to Inside back coverBS1741-5.2:1990 ii BSI 08-1999 Foreword This Section of BS 1741 has been prepared under the direction of the Dairying
7、 Standards Policy Committee, and introduces into the standard a routine method for the determination of nitrogen content of liquid milk. NOTEThis Section should be read in conjunction with Part 1: “General introduction including preparation of samples”, published separately. A British Standard does
8、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 cover, an inside
9、 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.BS1741-5.2:1990 BSI 08-1999 1 1 Scope This Section o
10、f BS 1741 describes a routine method for the determination of the nitrogen content of liquid milk. NOTE 1BS 1741-5.1 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 this Section of B
11、S 1741, 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 BS1741, expressed as grams per 100g of sample 3 Principle A test portion of the sample is digested rapidl
12、y, 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 sodium hydroxide is then added
13、 to the cooled digest to liberate ammonia. The ammonia is then steam distilled out of the digestion mixture using 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 of boric acid sol
14、ution and then 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 then calculated from the amount of ammonia
15、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 titration. 4.1 K
16、jeldahl catalyst tablets NOTE 1The use of commercially prepared tablets is recommended. NOTE 2Tablets comprising 3.5g 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 writing to
17、 BSI Enquiry Section, Linford Wood, Milton Keynes, MK14 6LE, enclosing a stamped addressed envelope for reply. Other types of tablet may be used provided that they comply with the following. a) They contain a quantity of potassium sulphate such that7g of potassium sulphate can be dispensed using an
18、integral number of tablets. b) They do not contain salts of toxic 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 agen
19、t 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) per 100mL of solution. 4.6 Dilute sodium hydroxide solution, low in nitrogen, containing approximately2g of sodium hy
20、droxide (NaOH) per litre of solution. 4.7 Boric acid solution. Dissolve 40 g of boric acid in1L of hot water, allow to cool, and store in a borosilicate glass bottle. Neutralize before use with the sodium hydroxide solution(4.6), using1mL of the indicator solution(4.8). 4.8 Indicator solution. Disso
21、lve 0.01g of methyl red and0.15g of bromocresol green in100mL of ethanol(70% V/V). 4.9 Sulphuric acid, standard volumetric solution, c(H 2 SO 4 ) = 0.05mol/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
22、 shall be dried at102 2 C for not less than2h, immediately before use. 4.12 Phenacetin NOTEMaterial supplied as being an organic analytical standard with an assay (exN) of100 0.5% should be used. 5 Apparatus 5.1 Ordinary laboratory apparatus 5.2 Balance, capable of weighing to an accuracy of 0.0001g
23、. 5.3 Temperature controlled digestion block 5.4 Digestion tubes, suitable for use with the digestion block (5.3). 5.5 Exhaust manifold, suitable for use with the digestion tubes (5.4). 5.6 Centrifugal scrubber apparatus or filter pump, constructed of glass and for use with mains water supply. 5.7 A
24、utomatic pipettes, to 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 250 mL.BS1741-5.2:1990 2 BSI 08-1999 5.10 Burette, of capacity 25mL, classB, complying with BS
25、846. 6 Sampling Take a representative sample of the milk to be tested using, where appropriate, the procedures described in BS 1741-1. 7 Procedure NOTEIn the normal course of events, samples are analysed in batches, according to the procedure described. 7.1 Add the number of Kjeldahl catalyst tablet
26、s (4.1) containing7g of potassium sulphate to a clean, dry digestion tube (5.4) and weigh approximately2g of milk into the tube. Record the mass of the milk to the nearest0.001g. Carefully add10mL of the sulphuric acid (4.2) by means of an automatic pipette (5.7), add three or four drops of the anti
27、foaming agent (4.4) and then 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 (5.7), allowing the solution to run down the inside of the tube. Again swirl the tube gently in order to m
28、ix the contents and then leave to stand for10 min 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
29、 the tube. Digest the sample 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 least 15 min. Once the tubes are sufficiently cool to handle, remove the exhaust manifold and car
30、efully add50mL of water to each tube. 7.2 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) containing50mL of the neutralized boric acid solution (4.7) and t
31、wo or three drops of indicator 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 the s
32、odium hydroxide solution (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) unti
33、l the colour matches that 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. Take each burette reading to 0.01mL. If
34、 the distillation unit is fully automated to include titration of the ammonia content of the distillate, follow the manufacturers instructions for operation of the distillation unit. 7.3 Carry out a blank test following the procedure described in 7.1 and 7.2 using2mL of water with about0.25g of the
35、sucrose (4.10) instead of the prepared sample. It is advisable to add a further 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
36、 solution, normally 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 to0.15g of the phenacetin (4.12) weighed to the
37、 nearest 0.001g. Determine the nitrogen content according to the procedure described in7.1 and 7.2. The recovery of nitrogen from the phenacetin should be not less than98.5%. b) If the digestion and distillation procedures are found to give low recoveries then the distillation procedure alone may be
38、 checked as follows. Prepare solution of the ammonium sulphate (4.11) of concentration exactly 0.05mol/L. Pipette a10.0mL aliquot portion of the ammonium sulphate solution into a digestion tube and add50mL of water. Determine the nitrogen content of the solution according to the procedure described
39、in7.2. The recovery of nitrogen from the ammonium sulphate should be not less than98.5%. NOTE 3It is recommended that analysts periodically undertake determinations to verify that results complying with the repeatability recommendation (see 9.1) are being obtained. 8 Expression of results 8.1 Method
40、 of calculation Calculate the value of the nitrogen content, in grams per100g 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 0 is the volume of the standard volumetric solution of acid used in the blank test (
41、inmL); m is the mass of the test portion (ing). 0.14VV 0 () m -BS1741-5.2:1990 BSI 08-1999 3 8.2 Result Express the result to two decimal places. 9 Precision NOTEThe precision data were obtained from a collaborative study undertaken in1990, involving11 laboratories and two skimmed and two whole milk
42、s. 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 exceed0.02g/100g of test portion. 9.2 Reproducibility The difference between the results obtained on sampl
43、es of the same test material by two different analysts in different laboratories (where each resultis the mean of two values that comply with the repeatability requirement) should not exceed0.03g/100g of test portion. 10 Test report The test report shall be in accordance with BS 1741-1.4 blankBS1741
44、-5.2:1990 BSI 08-1999 Publication(s) referred to BS 846, Specification for burettes. BS 1741, Methods for chemical analysis of liquid milk and cream. BS 1741-1, General introduction including preparation of samples. BS 1741-5.1, Reference method. BS 1741-5.2: 1990 BSI 389 Chiswick High Road London W
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