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1、BRITISH STANDARD CONFIRMED FEBRUARY 1991 BS 684-2.11: 1976 Methods of analysis of Fats and fatty oils Part 2: Other methods Section 2.11: Determination of volatile acids (Reichert, Polenske and Kirschner values) IMPORTANT NOTE. It is essential that this Section be read in conjunction with the inform
2、ation in the “General introduction” to BS 684 which is published separately. UDC 665.1.014:543:543.852.8 Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 This British Standard, having been prepared under the direction of the
3、Oils and Fats Standards Committee, was published under the authority of the Executive Board on 29 October 1976 BSI 12-1999 The following BSI references relate to the work on this standard: Committee reference OFC/24 Draft for comment 74/54405 ISBN 0 580 09145 7 A British Standard does not purport to
4、 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 front cover,
5、pages i and ii, pages 1 to 5 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. Amendments issued since publication Amd. No.DateComments Licensed Copy: sheffieldun sh
6、effieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 BSI 12-1999i Contents Page 1Scope1 2References1 3Definitions1 4Principle1 5Reagents1 6Apparatus1 7Sampling and preparation of sample for analysis4 8Procedure4 9Expression of results5 10Test report5 Figure
7、1 Polenske distillation apparatus2 Figure 2 Still-head for Polenske distillation apparatus3 Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii blank Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled
8、 Copy, (c) BSI BS 684-2.11:1976 BSI 12-19991 1 Scope This Section describes a method for the determination of volatile acids, which is applicable to all fats, but is especially useful for fats and blends containing short and medium carbon chain length fatty acids. The method described is technically
9、 similar to the standard method of the Society for Analytical Chemistry (Analyst. 1936, 61, 404 408). The method for the determination of volatile acids is empirical; only a part of the volatile acids in the sample is actually determined. Consequently in order to obtain accurately reproducible resul
10、ts it is essential that the dimensions of the apparatus and details of the procedure are strictly adhered to. 2 References The following standards publications are referred to in this Section: BS 410, Test sieves. BS 604, Graduated measuring cylinders. BS 675, Sugar flasks. BS 1583, One-mark pipette
11、s. BS 1796, Methods for the use of British Standard fine-mesh test sieves. BS 2021, Separating and dropping funnels. 3 Definitions For the purposes of this Section of BS 684 the following definitions apply. Reichert value the number of millilitres of 0.1N aqueous alkali solution required to neutrali
12、ze the water-soluble volatile fatty acids distilled from 5 g of the fat under the precise conditions specified in the method Polenske value the number of millilitres of 0.1N aqueous alkali solution required to neutralize the water-insoluble volatile fatty acids distilled from 5 g of the fat under th
13、e precise conditions specified in the method Kirschner value the number of millilitres of 0.1N aqueous alkali solution required to neutralize the water-soluble volatile fatty acids which form water-soluble silver salts and which are distilled from 5 g of the fat under the precise conditions specifie
14、d in the method 4 Principle The method consists of distilling the steam-volatile fatty acids that are liberated after acidifying the soaps formed by saponification of the fat. The distillate is separated by filtration into water-insoluble and water-soluble fractions. The water-soluble fraction is ti
15、trated with alkali to determine the Reichert value. The water-insoluble fraction is dissolved in ethanol and titrated to determine the Polenske value. Silver sulphate is added to the titrated water-soluble fatty acids, the insoluble silver salts formed are removed by filtration, and the filtrate (co
16、ntaining the fatty acids that form soluble silver salts) is acidified and the liberated fatty acids distilled. The filtered distillate is titrated to determine the Kirschner value. 5 Reagents The following reagents are required (see also BS 684: General introduction). 5.1 Glycerol 5.2 Sodium hydroxi
17、de, 50 % (m/m) solution. Dissolve sodium hydroxide in an equal mass of water and store the solution in a bottle protected from carbon dioxide. Use the clear portion which is free from deposit. 5.3 Sulphuric acid, reagent solution, N. 5.4 Phenolphthalein indicator, 5 g/l solution in ethanol 95 % (v/v
18、). 5.5 Ethanol1), 95 % (v/v), neutralized to phenolphthalein immediately before use. 5.6 Sodium hydroxide, 0.1N standard volumetric solution. 5.7 Barium hydroxide, 0.1N standard volumetric solution. 5.8 Silver sulphate, powdered. 5.9 Boiling aid, ground pumice powder passing through a 300 4m test si
19、eve and remaining on a 180 4m test sieve complying with the requirements of BS 410, when tested using the procedure described in BS 1796. 6 Apparatus The following items of apparatus are required (see also BS 684: General introduction). 6.1 Graduated cylinder, 100 ml, complying with the requirements
20、 of BS 604. 1) Ethanol may be replaced by industrial methylated spirits (ethanol containing 5 % by volume of approved wood naphtha) of equivalent strength. It should be noted that the use of industrial methylated spirit is governed by The Methylated Spirits Regulations 1952 (SI 1952 no. 2230). Licen
21、sed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 2 BSI 12-1999 6.2 Pipettes, 100 ml and 50 ml, complying with the requirements of BS 1583, class B. 6.3 Distillation apparatus, as shown in Figure 12). The constituent parts are as l
22、isted in 6.3.1 to 6.3.6. 2) The apparatus is supported on a retort stand, not shown in the diagram. Figure 1 Polenske distillation apparatus Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 BSI 12-19993 6.3.1 Flat bottomed bo
23、iling flask (Polenske), of borosilicate glass, with a capacity to the base of the neck of 310 10 ml. 6.3.2 Still-head, of glass tubing of wall thickness 1.25 0.25 mm and conforming to the shape and dimensions shown in Figure 2. A rubber stopper fitted below the bulb of the longer arm of the still-he
24、ad, and used for connecting it to the flask, shall have its lower surface 10 mm above the centre of the side hole of the still-head. 6.3.3 Condenser, of glass tubing. The wall thickness of the inner tube shall be 1.0 0.2 mm and that of the outer jacket and of the wider part at the top 1.25 0.25 mm.
25、6.3.4 Receiver, consisting of a flask complying with the requirements of BS 675 with two graduation marks on the neck, one at 100 ml and the other at 110 ml. 6.3.5 Asbestos board, 120 mm in diameter and 6 mm in thickness, with a circular hole about 65 mm in diameter, used to support the flask over t
26、he burner. During distillation the Polenske flask shall fit snugly into the hole in the board so as to prevent the flame from impinging on the surface of the flask above the hole. NOTEA new asbestos board may be conveniently prepared by bevelling the edge of the hole, soaking in water, moulding the
27、edge with a flask and drying. Appropriate safety procedures should be observed when shaping asbestos board. 6.3.6 Bunsen burner, fitted with a flame guard. The burner should be sufficiently large to allow the distillation to be completed in the time specified below. 6.4 Separating funnel, 250 ml, co
28、mplying with the requirements of BS 2021. 6.5 Burette, 25 ml, with a coarse jet and fitted with a soda-lime guard tube. 6.6 Filter paper3), with a diameter of 9 cm. Figure 2 Still-head for Polenske distillation apparatus 3) Whatman no. 4 or no. 41, or Barcham Green no. 801 have been found suitable.
29、Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 4 BSI 12-1999 7 Sampling and preparation of sample for analysis See BS 684: General introduction. 8 Procedure 8.1 Test portion. Weigh, to the nearest 10 mg, 5.00 g of the fat i
30、nto the Polenske flask (6.3.1). 8.2 Determination 8.2.1 Saponification. To the test portion (8.1) add 20 g of glycerol (5.1) and then from the burette (6.5) add 2 ml of sodium hydroxide solution (5.2). NOTEThe jet of the burette should be wiped clean from carbonate deposit, and the first few drops o
31、f sodium hydroxide withdrawn from the burette should be rejected. Heat the flask over a naked flame with continuous mixing until the test portion, including any drops adhering to the top of the flask, is saponified and the liquid becomes clear. Avoid overheating during the saponification. Cover the
32、flask with a watch glass. Carry out a blank test omitting the test portion but using the same quantities of reagents and following the same procedure, again avoiding overheating during the heating with sodium hydroxide (such overheating will be indicated by darkening of the solution). Allow the soap
33、 to cool to about 90 C. Meanwhile fill the 100 ml cylinder (6.1) with boiling water that has been boiling vigorously for 15 min, empty the cylinder and refill it to the 93 ml mark with similar boiling water. Immediately pour the water from the cylinder into the flask draining the cylinder for 5 s an
34、d dissolve the soap by gentle shaking of the flask. If the solution is not clear (indicating incomplete saponification) or is darker than light yellow (indicating overheating), repeat the saponification with a new test portion. 8.2.2 Distillation of volatile acids. Add 0.1 g of boiling aid (5.9), fo
35、llowed by 50 ml of sulphuric acid (5.3) and connect the flask at once with the distillation apparatus. Heat the flask without boiling its contents until the insoluble acids are completely melted, then increase the rate of heating and distil over 110 ml in 19 min to 21 min. Keep the water flowing in
36、the condenser at a sufficient rate to maintain the temperature of the issuing distillate between 18 C and 21 C. When the distillate reaches the 110 ml mark, remove the flame and replace the receiver (6.3.4) with a cylinder of about 25 ml capacity to catch drainings. Close the receiver (6.3.4) with i
37、ts stopper and, without mixing the contents, place it in water at 15 C for 10 min so as to immerse the 110 ml mark. Remove the flask from the water, dry the outside and invert the flask carefully, avoiding wetting the stopper with the liquid insoluble acids. Mix the distillate by four or five double
38、 inversions, without violent shaking. Filter the entire distillate, through a dry filter paper (6.6) fitting snugly into a funnel, into a dry 250 ml conical flask, but reject the first runnings. Stopper the flask and retain the filtrate for titration as described in 8.2.1. The filtrate should be fre
39、e from insoluble fatty acids, but this is difficult to achieve with coconut oil. NOTEIn such cases where liquid-insoluble fatty acids pass through the filter paper, collect the filtrate in a separating funnel (6.4). Allow the insoluble acids to rise to the surface and carefully draw off the lower (a
40、queous) layer into a dry 250 ml conical flask, ensuring that any insoluble acids are retained in the separating funnel. 8.2.3 Filtration. Detach the still-head, wash the condenser with a 15 ml portion of cold water and pass the washings through the cylinder, the 110 ml flask, the filter and the sepa
41、rating funnel, if used, so that the washings nearly fill the filter paper. Repeat this washing twice more, allowing the filter to drain completely before adding the next portion of washings. Discard the washings. Dissolve the insoluble acids by passing a 15 ml portion of neutralized ethanol (5.5) th
42、rough the condenser, the cylinder, the filter and the separating funnel, if used, so that the ethanol nearly fills the filter paper. Repeat this procedure twice more, allowing the filter to drain completely before adding the next portion of neutralized ethanol. Collect the filtrate in a 250 ml conic
43、al flask. Stopper the flask and retain the solution for titration as described in 8.2.2. 8.2.4 Determination of Reichert value. Using a pipette (6.2), transfer 100 ml of the filtrate containing the soluble volatile acids from the receiver (6.3.4) to a dry titration flask, add 0.2 ml of phenolphthale
44、in indicator (5.4) and titrate with the barium hydroxide (5.7) until the liquid becomes pink. Note the volume of barium hydroxide solution used. NOTE0.1N sodium hydroxide solution (5.6) may be used for the titration if the Kirschner value is not required. If the Kirschner value is to be determined,
45、it is essential that the titration flask is dry before use. Close the titration flask with a cork and continue as in 8.2.3. Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:16:31 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.11:1976 BSI 12-19995 8.2.5 Determination of Polenske value.
46、Titrate the ethanolic solution of the insoluble volatile acids, after addition of 0.25 ml of phenolphthalein indicator (5.4), with the 0.1N barium or sodium hydroxide solution (5.6 or 5.7) until the solution becomes pink. 8.2.6 Determination of Kirschner value. Add 0.5 g of silver sulphate (5.8) to
47、the neutralized solution from 8.2.1. Allow the flask to stand in the dark for 1 h with occasional shaking and filter the contents in the dark through a dry filter. Transfer 100 ml of the filtrate to a dry Polenske flask (6.3.1), add 35 ml of cold water recently boiled for 15 min, 10 ml of sulphuric
48、acid (5.3) and a loosely wound 5 mm coil of 300 mm of aluminium wire about 1 mm thick, or 0.1 g of boiling aid (5.9). Connect the flask with the standard apparatus and repeat the process as described above, i.e. the distillation of 110 ml in 19 min to 21 min, the mixing (but without the cooling for
49、10 min), the filtration and the titration of 100 ml of the filtrate with the barium hydroxide (5.7). 9 Expression of results 9.1 Method of calculation and formulae. Calculate the results using the following formulae. NOTE 1Polenske values, and to a much slighter extent Reichert values, have been found to be low when determined at low barometric pressures such as may occur at high altitudes. The following factors may be applied to values determined at a barometric pressure of mmHg to convert them to the values determined at normal pressure (Kirkham, An
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