1、 DEUTSCHE NORM May 2005DIN 10772-2 ICS 67.140.20 Supersedes DIN 10772-2:1986-01 Analysis of coffee and coffee products Karl Fischer method for the determination of water content Part 2: Reference method for soluble coffee Untersuchung von Kaffee und Kaffee-Erzeugnissen Bestimmung des Wassergehaltes
2、nach Karl Fischer Teil 2: Referenzverfahren fr Kaffee-Extrakt Document comprises 12 pagesTranslation by DIN-Sprachendienst. In case of doubt, the German-language original should be consulted as the authoritative text. No part of this translation may be reproduced without prior permission of DIN Deut
3、sches Institut fr Normung e. V., Berlin. Beuth Verlag GmbH, 10772 Berlin, Germany, has the exclusive right of sale for German Standards (DIN-Normen). English price group 9 www.din.de www.beuth.de !,imz“05.06 9707487DIN 10772-2:2005-05 2 Contents Page Foreword3 1 Scope 4 2 Terms and definitions .4 3
4、Principle4 4 Background to the method .4 5 Reagents.4 6 Apparatus .5 7 Sampling.6 8 Preparation.6 9 Procedure .8 10 Evaluation.8 11 Precision.8 12 Test report 9 Annex A (informative) Results of interlaboratory testing 10 Annex B (informative) Examples of titration curves. 11 Bibliography. 12 Figures
5、 Figure B.1 Titration curves of individual samples. 11 Tables Table 1 Validation data 9 Table A.1 Results of interlaboratory testing 10 DIN 10772-2:2005-05 3 Foreword This standard has been prepared by Technical Committee Kaffee of the Normenausschuss Lebensmittel und landwirtschaftliche Produkte (F
6、oodstuffs and Agricultural Products Standards Committee). Newly acquired knowledge relating to the importance of a stable pH during the reaction as well as the availability of new reagents and advances in the design of titrating apparatus have made it necessary to revise the January 1986 edition. DI
7、N 10772 consists of the following parts: Part 1: Method for the analysis of roasted coffee Part 2: Method for the analysis of coffee extract Amendments This standard differs from the January 1986 edition as follows. a) A commercially available pyridine-free single-component Karl Fischer reagent is n
8、ow used for the titration. b) The pH of the solution to be titrated is now stabilized by adding salicylic acid. c) Determination of the titre of the Karl Fischer reagent with disodium tartrate dihydrate has been included as an alternative. d) The figure showing the titration vessels has been deleted
9、 and the description of the titration apparatus has been updated. e) Examples of titration curves have been included. f) Precision data have been given. g) The interlaboratory test has been performed by an expanded group of laboratories based throughout the EU. h) The standard has been editorially r
10、evised. Previous edition DIN 10772-2: 1986-01 DIN 10772-2:2005-05 4 1 Scope This standard specifies a method of determining the water content of coffee extract by titrating directly with Karl Fischer reagent. 2 Terms and definitions For the purposes of this standard, the following terms and definiti
11、ons apply. 2.1 water content of solid and liquid coffee extracts the proportion by mass of water in g/100 g of sample as determined by the method specified in this standard 3 Principle A sample of coffee extract is dissolved in a titrated mixture of formamide, methanol and salicylic acid (FMS) and i
12、mmediately titrated with the single-component Karl Fischer reagent (KFR) while stirring constantly until the electrometric end point is reached. The water content of the sample is calculated from the volume of KFR used. 4 Background to the method In the Karl Fischer determination, the following reac
13、tion occurs in the presence of an amine and an alcohol (here: methanol) between the water in the sample, on the one hand, and iodine and sulfur dioxide, on the other: H2O + I2+ SO2+ ROH + 3 RnNH3-n 2 RnNH3-n HI + RnNH3-n HSO4R, where R is an alkyl or alkoxy radical. The end point, which is detected
14、by the excess iodine used in the reaction, is determined electrometrically, i.e. amperometrically or voltametrically. The method described in this standard employs only pyridine-free single-component Karl Fischer reagents (KFR). 5 Reagents 5.1 General Only analytical grade reagents shall be used, an
15、d the water used shall be distilled water or water of equivalent purity. The following reagents shall be used. 5.2 Single-component Karl Fischer reagent (KFR)1)having a titre of about 5 mg H2O/ml. 5.3 Methanol having a water content of less than 0,5 mg H2O/ml. 1) Suitable products are commercially a
16、vailable. DIN 10772-2:2005-05 5 5.4 Formamide having a low water content. CAUTION The advantages of using formamide are that it reduces the titration time and renders the end point clearly detectable. However, it has recently been found that it may be toxic to human reproduction and it is therefore
17、important to give laboratory staff detailed instructions on handling it and on the need to avoid direct contact with the skin through wearing gloves and taking steps to prevent splashes. 5.5 Salicylic acid. NOTE Salicylic acid stabilizes the pH of the solution being titrated and keeps it in a range
18、in which no uncontrolled secondary reactions occur. 5.6 Disodium tartrate dehydrate. 6 Apparatus 6.1 General In addition to standard laboratory equipment, the following shall be used. Only completely dry glassware shall be used. 6.2 Analytical balance. 6.3 Titration apparatus consisting of the follo
19、wing components for determining water content by the Karl Fischer method. Complete titration equipment are available commercially. 6.3.1 Karl Fischer titrator, for amperometric or voltametric titration. 6.3.2 Platinum double electrode. 6.3.3 Magnetic stirrer, equipped with a polytetrafluoroethylene
20、(PTFE) coated magnetic follower. 6.3.4 Titration vessel, of nominal capacity 100 ml, having at least four ground glass joints: one for connecting the burette (as in 6.3.5), one for the platinum double electrode (as in 6.3.2), one for introducing the FMS mixture and one for adding the sample, respect
21、ively, the last being sealed with a drying tube or drying receptacle during titration. NOTE Sheathing the titration vessel results in a gentle heating, thus accelerating the reaction. 6.3.5 Burettes, storage jars and drying tubes. A piston-operated burette shall be used to measure the volume of KFR
22、(as in 5.2) used. The storage jar, burette and KFR feed tube shall be protected from light. To protect the whole system from atmospheric moisture, all the aeration and venting tubes of the titration apparatus shall be sealed with drying tubes filled with effective drying agents, such as silica gel w
23、ith indicator or a molecular sieve, or with a drying receptacle filled with KFR. 6.4 Syringe, weighing pipette or precision pipette (e.g. micropipette or microsyringe) for measuring out 25 to 30 mg of water to determine the titre of the KFR. NOTE The syringe can also be used to measure out liquid co
24、ffee extract to determine its water content. 6.5 Glass weighing scoop, with protective covering, obtainable e.g. from the titration apparatus manufacturer. 6.6 Measuring cylinder, of nominal capacity 500 ml. DIN 10772-2:2005-05 6 7 Sampling 7.1 General Until a DIN standard is published, the sampling
25、 method shall be agreed between the parties. It shall be based on a representative, well-mixed sample that has not been affected by transportation or storage. 7.2 Sample preparation To protect it from atmospheric moisture, the sample shall be stored in a dry, gastight glass bottle immediately after
26、opening the package. NOTE Coffee extract is highly hygroscopic. 8 Preparation 8.1 Titration medium 8.1.1 Preparation of formamide-methanol-salicylic-acid mixture (FMS) The FMS mixture shall contain formamide and methanol in a ratio by volume of 1:2 and 60 g of salicylic acid for every 200 ml of meth
27、anol. First dissolve the salicylic acid in the methanol and then add 100 ml of formamide for every 200 ml of methanol. EXAMPLE 1 60 g of salicylic acid are dissolved in 200 ml of methanol in a conical shoulder reagent bottle (the storage jar of a second piston-operated burette may be used) while sti
28、rring. After adding 100 ml of formamide, the bottle is closed and the FMS mixture is thoroughly mixed again. EXAMPLE 2 10 ml of formamide are introduced into the titration vessel and mixed with 20 ml of methanol in which 6 g of salicylic acid are dissolved. As the FMS mixture is stable for only one
29、day, prepare a fresh solution every day. NOTE FMS stabilizes the pH during the Karl Fischer titration of the coffee extract. It is recommended that a second piston-operated burette be used to meter out the FMS mixture into the titration cell. 8.1.2 Dehydration of titration medium Introduce 30 ml of
30、FMS into the titration cell and dehydrate it by titration with Karl Fischer reagent. Taking account of the information provided by the manufacturers of the titration apparatus and reagents, it is recommended by way of example that the polarization current, Ipol, be set to 50 A, Istopto 60 % to 80 %
31、and the waiting time to 30 seconds. NOTE The buffer capacity of the single-component Karl Fischer reagent in combination with the FMS titration medium varies with the reagent supplier and the coffee extract sample. Depending on the water content of the sample, several determinations may be performed
32、 with the same FMS titration medium. Taking the titration time as a guide, the titration curve (see figure in Annex B) can be used to check whether this is possible. DIN 10772-2:2005-05 7 8.2 Titre of Karl Fischer reagent (KFR) 8.2.1 Determination with disodium tartrate dihydrate Using the analytica
33、l balance, weigh about 0,12 g of disodium tartrate dehydrate (as in 5.6), to an accuracy of 0,1 mg, into the weighing scoop (total mass). Add the disodium tartrate dihydrate to the dehydrated FMS in the titration cell through the upper ground joint. Then seal the cell. Start the stirrer and titrate
34、immediately with KFR. Weigh the empty scoop to an accuracy of 0,1 mg (mass of weighing scoop). When the end point is reached, record the volume of KFR used to an accuracy of 0,01 ml. 8.2.2 Determination with water (alternative method) Fill a microsyringe with water, if necessary empty it except for
35、about 0,2 ml and, after drying the injection needle, weigh it on the analytical balance. Now insert the injection needle of the syringe into the opening provided for the purpose in the titration vessel until it is just above the liquid level of the FMS mixture in the vessel and add the required amou
36、nt of water (25 to 30 mg) dropwise, but quickly to minimize any effect due to atmospheric moisture. Then pull the syringe piston back slightly to prevent further droplets being added. To speed up the operation, determine the number of drops equal to 25 to 30 mg in a preliminary trial with the syring
37、e. After adding the water, close the titration vessel immediately and determine the amount of water added by weighing the syringe on the analytical balance. An amount of 25 to 30 mg of water can also be introduced in a similar way using a weighing pipette. Titrate the water added to the titration ve
38、ssel with KFR without altering the titration parameters (see sub-clause 8.1.2) until the end point is reached. NOTE Other water standards can also be used. 8.2.3 Titre calculation Calculate the titre, T, using the following equation: VmTw= (1) where mwis the mass of water added, in mg; V is the volu
39、me of KFR used, in ml. In the case of standardization with disodium tartrate dihydrate, 1 g of disodium tartrate dihydrate is equivalent to 0,156 6 g of water and the equivalent mass of water is given by: mw= 0,156 6 mT, where mTis the mass of disodium tartrate dihydrate without weighing scoop. Repe
40、at this determination and report the mean of the results. DIN 10772-2:2005-05 8 NOTE Although pyridine-free single-component reagents are generally very stable, it is advisable to check the titre from time to time. 9 Procedure 9.1 Solid coffee extract Introduce 30 ml of FMS mixture prepared as descr
41、ibed in subclause 8.1 into the titration vessel and dehydrate it by pretitration. Weigh 300 mg to 500 mg of solid coffee extract sample, to an accuracy of 0,1 mg, and transfer it to the titration vessel as described in subclause 8.2.1. To exclude atmospheric moisture, always use the protective cover
42、 of the weighing scoop for this and other weighings. Quickly seal the titration vessel and immediately titrate to the end point with KFR without altering the titration parameters (see subclause 8.1.2). 9.2 Liquid coffee extract To determine the water content of liquid coffee extracts, use the titre
43、determined as in subclause 8.2 and meter out the sample using a microsyringe, taking account of the expected water content. 10 Evaluation 10.1 Calculation Calculate the water content, ww, of the coffee extract, in g/100 g of sample, using equation (2): EwmVTw=101(2) where T is the titre of the KFR,
44、in mg of water/ml of KFR, determined as in clause 9; V1is the volume of KFR used to titrate the sample solution, in ml; mEis the initial sample mass, in g. Provided the repeatability conditions as specified in subclause 11.2 are fulfilled, report the result as the arithmetic mean of two determinatio
45、ns. 11 Precision 11.1 General Details of an interlaboratory test designed to determine the precision of the method will be found in Annex A. The values found in that test cannot be applied to other types of sample and concentration ranges. DIN 10772-2:2005-05 9 11.2 Repeatability limit (same operato
46、r, same equipment) The absolute difference between two successive results obtained under repeatability conditions will not exceed the repeatability limit, r, reported in table 1 in more than 5 % of cases. 11.3 Reproducibility limit (different operators, different equipment) The absolute difference b
47、etween two individual results obtained under reproducibility conditions will not exceed the reproducibility limit, R, reported in table 1 in more than 5 % of cases. Table 1 Validation data Sample x g/100 g r g/100 g R g/100 g Spray dried coffee extract (sample A) 4,442 0,234 0,336 Agglomerated coffe
48、e extract (sample B) 4,598 0,193 0,329 Freeze dried coffee extract (sample C) 1,722 0,129 0,289 12 Test report The test report shall refer to this standard and include the following details as a minimum: a) all the data required to identify the sample unambiguously; b) the test method used; c) the t
49、est results and the units in which they are reported; d) date on which the analysis was completed; e) whether the required repeatability limit was fulfilled; f) whether all the operations not specified in this standard were performed by choice; g) any circumstances that may have affected the result(s). DIN 10772-2:2005-05 10 Annex A (informative) Results of interlaboratory testing The precision was determined by Technical Committee Kaffee of the Normenausschuss Lebensmittel und landwirtschaftliche Produkte (Foodstuffs and Agricultural Products Standards Commi
