1、 Reference number ISO 20938:2008(E) ISO 2008INTERNATIONAL STANDARD ISO 20938 First edition 2008-12-01 Instant coffee Determination of moisture content Karl Fischer method (Reference method) Caf instantan Dtermination de la teneur en eau Mthode de Karl Fischer (Mthode de rfrence) ISO 20938:2008(E) PD
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11、 identifying any or all such patent rights. ISO 20938 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 15, Coffee. INTERNATIONAL STANDARD ISO 20938:2008(E) ISO 2008 All rights reserved 1 Instant coffee Determination of moisture content Karl Fischer method (Reference meth
12、od) 1 Scope This International Standard specifies a method for the determination of moisture content in instant coffee by the Karl Fischer titration method, suitable for use as a reference method. 2 Normative references The following referenced documents are indispensable for the application of this
13、 document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 4788, Laboratory glassware Graduated measuring cylinders 3 Terms and definitions For the purposes of this document, the follo
14、wing terms and definitions apply. 3.1 moisture content content of water, determined according to the procedure specified in this International Standard NOTE The moisture content is expressed as a percentage mass fraction. 4 Principle 4.1 General In a Karl Fischer apparatus, a test portion of instant
15、 coffee is dissolved in a water-free solution of formamide, methanol, and salicylic acid (FMS). Subsequent titration with a Karl Fischer reagent is applied under continuous stirring until the endpoint of the titration is reached and electrometrically detected. The volume of Karl Fischer reagent cons
16、umed is used to calculate the moisture content of the test portion. 4.2 Reaction During the Karl Fischer titration, water present in the sample reacts with iodine and sulfur dioxide in the presence of an amine and an alcohol (e.g. methanol): 222 3 3 3 4 H O I SO ROH 3R NH 2R NH HI R NH HSO R nnnnnn
17、+ + + + where R is an alkyl or alkoxyl group. The endpoint of the reaction is detectable by a small, surplus amount of iodine and quantified electrometrically (either amperometrically or voltametrically). The procedure described in this International Standard is performed only with pyridine-free Kar
18、l Fischer (one-component) reagent (KFR). ISO 20938:2008(E) 2 ISO 2008 All rights reserved5 Reagents and materials Unless otherwise specified, use only reagents of recognized analytical grade, and only distilled or deionized water or water of equivalent purity. 5.1 Karl Fischer reagent (KFR, one-comp
19、onent system) 1) , titre approximately 5 mg/ml, water mass concentration, pyridine-free. 5.2 Methanol, with low water mass concentration, preferably 0,5 mg/ml. 5.3 Formamide, with low water mass concentration. WARNING The use of formamide is of outstanding importance with regard to the time of titra
20、tion and the clear determination of the endpoint. Operators should be carefully instructed and adequately trained so as to avoid direct contact with the reagent (use splash guards and protective clothing), and to wash immediately and thoroughly both in cases of exposure to formamide, as well as at t
21、he end of the workshift. 5.4 Salicylic (2-hydroxybenzoic) acid. NOTE Salicylic acid is applied for reasons of stabilizing the pH of the solution. Under these conditions, uncontrolled side reactions do not take place. 5.5 Disodium tartrate (disodium 2,3-dihydroxybutanedioate) dihydrate. 6 Apparatus U
22、sual laboratory equipment and in particular, the following. All devices used shall be as dry as possible. 6.1 Analytical balance, capable of weighing to an accuracy of 0,1 mg. 6.2 Titration apparatus, for the determination of the moisture content according to the Karl Fischer method. Complete sets o
23、f titration apparatus are commercially available, consisting of the individual components given in 6.2.1 to 6.2.5. 6.2.1 Karl Fischer titrator, for titrations according to the amperometric or the voltametric procedure. 6.2.2 Platinum double electrode. 6.2.3 Magnetic stirrer equipment, fitted with a
24、polytetrafluoroethylene-coated stirring bar. 6.2.4 Titration vessel, of approximately 100 ml capacity, with at least four basic glass sockets. One of these is connected to the burette (6.2.5), the second to the platinum double electrode (6.2.2). The third serves for adding the FMS solution (8.1.1) a
25、nd the fourth for the introduction of the sample. The latter shall be tightly closed during titration by a drying tube or receiver (6.2.5). NOTE Attaching thermostatic mantling to the titration vessel enables an option of slightly raising the temperature, resulting in reaction acceleration. 6.2.5 Bu
26、rettes, reservoirs and drying tubes. A piston burette is used for accurately measuring the volume of KFR (5.1). The reagent bottle, burette and connecting tubes for transferring KFR should be protected from light. All ventilation sockets of the system shall be closed by either drying tubes that cont
27、ain effective drying agents (e.g. silica gel with moisture indicator or molecular sieves), or with drying receivers filled with KFR in order to exclude any ambient moisture. 1) Suitable products are commercially available. ISO 20938:2008(E) ISO 2008 All rights reserved 3 6.3 Syringe or weighing pipe
28、tte, suitable for measuring 25 mg to 30 mg water in the determination of the KFR titre according to 8.2.2. If determining water content of liquid coffee extract, a syringe can also be used to measure the liquid coffee sample and to transfer it into the titration vessel. 6.4 Weighing spoon, made of g
29、lass with a protective cover, purchased from the titration apparatus supplier. 6.5 Graduated measuring cylinder, in accordance with ISO 4788, class A, of capacity 500 ml. 7 Sampling 7.1 General A representative sample should have been sent to the laboratory. It should not have been damaged or change
30、d during transport or storage. Sampling is not part of the method specified in this International Standard. The sampling procedure shall be subject to agreement by the interested parties. 7.2 Sample preparation The sample should be truly representative and well homogenized, without damage or change
31、during transportation and storage. To protect the laboratory sample from ambient moisture, it shall be transferred to an airtight glass jar immediately after opening the original packaging. NOTE Instant coffee is highly hygroscopic. 8 Preparative steps 8.1 Preparation of solvent mixture and receiver
32、 for titration 8.1.1 Preparation of the formamide-methanol-salicylic acid-mixture (FMS) For better solubility, first dissolve salicylic acid (5.4) in methanol (5.2), after which add formamide (5.3). In the FMS mixture, the ratio of formamide to methanol shall be 1 + 2 (parts by volume). EXAMPLE 1 A
33、mass of 60 g salicylic acid (5.4) is dissolved in 200 ml methanol (5.2) in a stoppered cylindrical flask under continuous stirring. A volume of 100 ml formamide (5.3) is subsequently added, the flask is then closed and the FMS completely homogenized by further stirring. Direct preparation of the mix
34、ture in the titration vessel is also feasible. EXAMPLE 2 A volume of 10 ml formamide (5.3) is placed in the titration vessel, then mixed with 20 ml methanol (5.2) in which 6 g salicylic acid (5.4) have been previously dissolved. Prepare fresh solution daily. NOTE FMS stabilizes the pH value during t
35、he Karl Fischer titration. 8.1.2 Conditioning of the titration system Transfer 30 ml FMS into the titration vessel, optionally using a second piston burette (6.2.5). Allow the system to condition in a so-called stand-by titration with KFR for 20 min, during which time adherent, residual water is com
36、pletely removed. ISO 20938:2008(E) 4 ISO 2008 All rights reservedThe electrometric default parameters are the polarization current, I pol , of 50 A, the titration stop at about 60 % to 80 %, with a waiting time of 30 s. Manufacturers instructions for use, both for the titration apparatus and reagent
37、s, should be adhered to. The buffer capacity of the Karl Fischer one-component reagent, in combination with the FMS receiver, varies according to the reagent used, and the type of coffee extract analysed. It is feasible that depending on the water content more than one titration may be performed in
38、the same receiver. The course of titration can be used for reaction control (see Figure B.1). The titration time may prove a suitable indicator. 8.2 Determination of KFR titre 8.2.1 Determination with disodium tartrate dihydrate Weigh approximately 0,12 g disodium tartrate dihydrate (5.5) (Karl Fisc
39、her primary titre substance) to the nearest 0,1 mg in a closed weighing spoon (6.4) to determine the gross mass, and transfer it via the appropriate socket into the titration vessel containing the conditioned, water-free FMS. Close the titration vessel again and immediately initiate the titration un
40、der continuous stirring. Weigh the weighing spoon back to the nearest 0,1 mg to give the tare. At the end of titration, note the volume of KFR used to the nearest 0,01 ml. 8.2.2 Determination with water (alternative method) Fill the syringe (6.3) with water, drain to about 0,2 ml, and, after the sur
41、face of the injection needle has been dried, weigh the syringe on the analytical balance (6.1) to the nearest 0,1 mg (gross mass). Insert the injection needle of the syringe through the appropriate socket into the titration vessel. Once the needle has almost reached the solution surface, inject appr
42、oximately 25 mg to 30 mg water into the FMS receiver. The water shall be added quickly, trying to reduce the influence of ambient air humidity; in order to accelerate water addition, the number of water drops equivalent to the mass of water needed should be determined in a pre-trial with the syringe
43、. Close the titration vessel again and immediately start titration under continuous stirring. Weigh the syringe back to the nearest 0,1 mg to give the tare. At the end of titration (same default values applied), note the volume of KFR consumed to the nearest 0,01 ml. Using a weighing pipette (6.3),
44、the introduction of water may be performed in an analogous manner. NOTE Other water standards can be used alternatively. 8.2.3 Calculation of titre Calculate the titre, , according to Equation (1); see also Clause 10: 2 HO m V = (1) where m H 2 Ois the mass, in milligrams, of water used as titre (gr
45、oss mass minus tare); V is the volume, in millilitres, of KFR used. ISO 20938:2008(E) ISO 2008 All rights reserved 5 When determining the titre with disodium tartrate dihydrate, the mass of the tartrate used corresponds to the mass of water as follows: 2 HO T 0,156 6 mm = (2) where m Tis the mass, i
46、n milligrams, of tartrate used as titre (gross mass minus tare). A mass of 1 g disodium tartrate dihydrate corresponds to 0,156 6 g water. Carry out the determination in duplicate, and use the mean as the resulting titre. Although pyridine-free one-component reagents generally show high stability, c
47、heck the titre from time to time. 9 Procedure with instant coffee 9.1 Instant coffee Place 30 ml FMS (8.1.1) in the titration vessel and condition via a pre-titration (8.1.2). Weigh a test portion of instant coffee (between 300 mg and 500 mg) to the nearest 0,1 mg and transfer it into the titration
48、vessel (8.2.1). Close the titration vessel and start the titration immediately, without changing any titration parameters up to the endpoint. In order to exclude ambient air humidity, the protective cover of the weighing spoon shall be used in all cases during the weighing procedure. 9.2 Liquid coff
49、ee extract Perform the moisture content determination in liquid coffee extract concentrates according to the procedure given in 8.2.2. Use a microlitre syringe (6.3) to measure the volume of the extract sample. 10 Expression of results Calculate the moisture content in the coffee extract, w H 2 O , expressed as a percentage mass fraction, according to Equation (3) 2 KFR 1 HO E 10 V w m = (3) where KFRis the KFR titre, in milligrams of water per millilitre KFR, as determined in 8.2; V 1is the volume, in millilitr
