BS-770-8-1987.pdf
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1、BRITISH STANDARD CONFIRMED OCTOBER 1993 BS 770-8:1987 Methods for Chemical analysis of cheese Part 8: Determination of nitrogen content (reference method) NOTEThis Part should be read in conjunction with Part 1 “General Introduction”, published separately. UDC 637.3.07:543.846 Licensed Copy: London
2、South Bank University, London South Bank University, Fri Dec 08 13:50:01 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 770-8:1987 This British Standard, having been prepared under the direction of the Dairying Standards Committee, was published under the authority of the Board of BSI and comes into
3、effect on 30 January 1987 BSI 11-1999 The Committees responsible for this British Standard are shown in Part 1. The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment 85/54445 DC ISBN 0 580 15683 4 Foreword This Part of BS 770 has been prepared
4、under the direction of the Dairying Standards Committee. It supersedes the method for determination of nitrogen given in clause 7 of BS 770:1963. Other new Parts of BS 770 supersede the remaining methods given in BS 770:1963 and that standard is therefore withdrawn. The method described in this Part
5、 differs from the previous method chiefly in the use of copper sulphate rather than mercuric oxide or mercuric sulphate solution as the catalyst and in the use of boric acid as the collecting agent. A collaborative study organized on behalf of the committee established the precision values for the m
6、ethod as given in clause 9. A British Standard 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 pa
7、ges This document comprises a front cover, an inside front cover, pages i and ii, pages 1 and 2, 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 cove
8、r. Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 13:50:01 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 770-8:1987 BSI 11-1999i Contents Page ForewordInside front cover 1Scope1 2Definition1 3Pri
9、nciple1 4Reagents1 5Apparatus1 6Sampling1 7Procedure1 8Expression of results2 9Precision2 10Test report2 Publications referred toInside back cover Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 13:50:01 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii blank Licens
10、ed Copy: London South Bank University, London South Bank University, Fri Dec 08 13:50:01 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 770-8:1987 BSI 11-19991 1 Scope This Part of BS 770 describes a reference method for the determination of nitrogen in cheese and processed cheese of all types. NOTET
11、he titles of the publications referred to in this standard are listed on inside back cover. 2 Definition For the purposes of this Part of BS 770, the following definition applies. 2.1 nitrogen content the quantity of nitrogen corresponding to the ammonia produced and determined under the conditions
12、specified in this British Standard and expressed as a percentage by mass 3 Principle A test portion is digested with a mixture of concentrated sulphuric acid and potassium sulphate using copper (II) sulphate as a catalyst, to convert organic nitrogen present to ammonium sulphate. Excess sodium hydro
13、xide is added to the cooled digest to liberate ammonia from the ammonium sulphate. The ammonia is distilled into an excess of boric acid solution, then titrated with standard sulphuric acid solution. The nitrogen content of the sample is calculated from the amount of ammonia produced. 4 Reagents Use
14、 the following reagents, which shall be of recognized analytical grade and low in nitrogen. Water used shall comply with the requirements of BS 3978. 4.1 Copper (II) sulphate solution. Dissolve 25 g of copper sulphate pentahydrate (CuSO4.5H2O) in 10 % (V/V) sulphuric acid solution and dilute to the
15、mark with the acid in a 500 mL one-mark volumetric flask. 4.2 Potassium sulphate (K2SO4), anhydrous. NOTECommercially available potassium sulphate tablets may be used. 4.3 Sulphuric acid, density 1.84 g/mL at 20 C. 4.4 Sodium hydroxide solution, low in nitrogen, containing approximately 40 g of sodi
16、um hydroxide (NaOH) per 100 mL of solution. 4.5 Orthoboric acid solution. Dissolve 40 g of orthoboric acid (H3BO3) in warm water and dilute to 1 L. 4.6 Sulphuric acid, standard volumetric solution, c(“ H2SO4) = 0.10 mol/L. 4.7 Mixed indicator solution. Dissolve 0.01 g of methyl red, 0.02 g of bromot
17、hymol blue and 0.06 g of bromocresol green in 100 mL of ethanol 70 % (V/V). 4.8 Boiling regulators 4.8.1 Glass balls, undrilled, of diameter approximately 5 mm (for the digestion). 4.8.2 Anti-foaming emulsion (for the distillation). 5 Apparatus Ordinary laboratory apparatus and, in particular, the f
18、ollowing. 5.1 Balance, capable of weighing to an accuracy of 1 mg. 5.2 Kjeldahl flasks, of capacity 500 mL. 5.3 Digestion apparatus, with heat supplied by a gas bunsen or electric element adjusted such that 250 mL of water in a 500 mL Kjeldahl flask plus two glass balls will come to a rolling boil i
19、n approximately 5 min. 5.4 Effective suction device or efficient water scrubbing system, for attachment to the digestion range to allow for safe ventilation and extraction of acid fumes evolved during digestion. 5.5 Distillation units, for use with a Kjeldahl flask fitted with a rubber stopper throu
20、gh which passes the lower end of a bulb-trap to prevent splash-over of the digest mixture during distillation, with the upper end of the bulb-trap connected to a condenser by rubber tubing. 5.6 Graduated dispenser or automatic pipettes, of capacity 0.2 mL, 1 mL and 25 mL, used to deliver the volumes
21、 of indicator solution, copper (II) sulphate solution and concentrated sulphuric acid. 5.7 Greaseproof paper, low in nitrogen. 5.8 Conical flasks, of capacity 250 mL. 6 Sampling Carry out the sampling in accordance with the methods described for cheese in BS 809. 7 Procedure 7.1 Prepare a test sampl
22、e in accordance with BS 770-1. 7.2 Carry out duplicate determinations on two test portions taken from the same test sample. 7.3 Carry out two blank tests using the procedure described in 7.4 but omitting the test portions. Licensed Copy: London South Bank University, London South Bank University, Fr
23、i Dec 08 13:50:01 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 770-8:1987 2 BSI 11-1999 7.4 Weigh to the nearest 1 mg, 1 g 20 mg of the prepared sample onto a piece of the greaseproof paper (5.7) of size approximately 100 mm 60 mm. Transfer the greaseproof paper and the test portion to a Kjeldahl f
24、lask (5.2). NOTE 1The quantity of sample can be weighed by difference. This procedure is particularly useful in the case of soft cheeses. Place 1 mL of the copper (II) sulphate solution (4.1), two glass balls (4.8.1), 15 g of the potassium sulphate (4.2) and 25 mL of the concentrated sulphuric acid
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