1、BRITISH STANDARD BS2511:1970 Methods for the Determination of water (Karl fischer method)BS2511:1970 This British Standard, having been approved by theChemical Industry StandardsCommittee, waspublished under the authority of the Executive Boardon 30 April1970 BSI07-1999 First published October1954 F
2、irst revised April1970 The following BSI references relate to the work on this standard: Committee references CIC/4 and CIC/4/-/1 Draft for comment68/728 ISBN580 05638 4 Co-operating organizations The Chemicals Industry Standards Committee, under whose supervision this British Standard was prepared,
3、 consists of representatives from the following Government departments and industrial organizations: Board of Trade British Steel Industry Chemical Industries Association* Department of Health and Social Security Fertiliser Manufacturers Association Gas Council Institution of Gas Engineers Ministry
4、of Agriculture, Fisheries and Food Ministry of Technology Laboratory of the Government Chemist National Sulphuric Acid Association Royal Institute of Public Health and Hygiene The industrial organization marked with an asterisk in the above list, together with the following, was directly represented
5、 on the committee entrusted with the preparation of this British Standard: British Laboratory Ware Association British Plastics Federation Ministry of Defence, Navy Department Oil and Colour Chemists Association Pharmaceutical Society of Great Britain Research Association of British Paint, Colour an
6、d Varnish Manufacturers Royal Institute of Chemistry Society of Motor Manufacturers and Traders Ltd. Amendments issued since publication Amd. No. Date CommentsBS2511:1970 BSI 07-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii 1 Scope 1 2 Double burette method using an
7、electrometric end point 1 3 Single burette method using an electrometric end point 7 4 Single burette method using a visual end point 8 5 Determination of water in ketones 11 6 Determination of microgramme quantities of water 13 Appendix A Notes on sampling for the determination of low water content
8、s 22 Figure 1 Suggested alternative forms of reaction vessel 2 Figure 2 Series circuit for double burette apparatus 3 Figure 3 Mains operated shunt circuit for double burette apparatus 3 Figure 4 Apparatus for single burette system using an electrometric end point 9 Figure 5 Apparatus for single bur
9、ette system using a visual end point 12 Figure 6 General assembly (showing microburette) 15 Figure 7 Detail of titration vessel (alternative design) 16 Figure 8 Electrical circuit 17 Figure 9 Dry-box assembly 18 Figure 10 Automated assembly 19 Figure 11 Sampling apparatus 19BS2511:1970 ii BSI 07-199
10、9 Foreword This standard makes reference to the following British Standards: BS506, Methanol. BS846, Burettes and bulb burettes. BS1428, Microchemical apparatus Part D1: Burettes with pressure-filling device and automatic zero. BS1583, One-mark pipettes. BS1792, One-mark volumetric flasks. Small qua
11、ntities of water in various substances may be determined chemically by the Karl Fischer method, this method being of particular value where the common physical methods are inapplicable, especially for the determination of small percentages of water in certain liquids and in certain volatile solids.
12、The method is based on the reaction of water with iodine and sulphur dioxide in pyridine/methanol solution (hereinafter referred to as Fischer reagent) 1) . The pyridine serves to prevent loss of sulphur dioxide from the reagent by uniting with it to form an additive compound, and also facilitates c
13、ompletion of the reaction with water by combining with the reaction products. Ethanol may be used as the solvent, but methanol is generally preferred. The course of the reaction in its simplest form may be expressed as follows 2) : Although, according to this formula, one molecule of iodine should b
14、e equivalent to one molecule of water, in practice this stoichiometric ratio is not attained and Fischer reagent must be standardized against a known mass of water. The end point of the titration may be determined visually by the appearance of a brown colour when excess of iodine is present. Alterna
15、tively, an electrometric procedure may be employed where the colour of the material under test renders the visual end point difficult to determine accurately. For very low water contents the electrometric procedure utilizing the “dead stop” end point technique 3)also has the advantage of greater acc
16、uracy, even in colourless solutions. On account of the extreme hygroscopicity of Fischer reagent, precautions must be taken to exclude all atmospheric moisture during the preparation, storage and use of the reagent. 1) Karl Fischer, Angew. Chem., 1935,48,394. 2) D. M. Smith, W. M. D. Bryant and J. M
17、itchell, Jr., J.Amer. Chem. Soc.,1939,61,2407. 3) C. W. Foulk and A. T. Bawden, J. Amer. Chem. Soc.,1926,48,2045.BS2511:1970 BSI 07-1999 iii In this British Standard instructions are given for the preparation of Fischer reagent, its standardization and its application to the determination of water i
18、n a variety of substances. In this revision the opportunity has been taken to amend the text of the existing clauses editorially to bring the wording into line with current conventions, but the only new material is Clause6. In Clause2 details are given for the determination of water of the order of0
19、.020.5%, in liquids and in solids capable of solution in methanol or in other suitable solvents, using the electrometric “dead stop” procedure. In Clause3 details are given for the determination of water of up to0.5% by a method using more simple apparatus (single burette and electrometric end point
20、) and giving a less accurate result. In Clause4 details are given of the visual method of end point detection which can be used for colourless materials where the highest degree of accuracy is not required. Clause5 gives details of modifications necessary for the determination of water in ketones. T
21、wo methods are given, the first involving the use of a modified reagent and the second requiring the addition of a methanol-pyridine mixture to the reaction vessel. The modified reagent is intended to give enhanced precision at very low water contents. Clause6 gives a method for the determination of
22、 microgramme quantities of water. The method is an extension of the Karl Fischer procedure;1-ethylpiperidine is used as a catalyst and constant current potentiometry is used in end point detection. The methods described in Clauses2, 3 and4 are technically similar to the methods in ISO Recommendation
23、 R760, “Method for the determination of water by the Karl Fischer method”. It must be appreciated that the Karl Fischer method as here described cannot be employed directly with any material which would react with any of the constituents of Fischer reagent, e.g.alkalis, metallic oxides, oxidizing ag
24、ents, ketones, quinones, amines, etc. When testing materials other than those for which this method for the determination of water is quoted in British Standards, a preliminary investigation must be carried out to ascertain whether the material in question, or any normally occurring impurities there
25、in, themselves react with the components of Fischer reagent, thereby invalidating this method for the determination of water content. It is sometimes possible, however, to obviate any such difficulties by modifications of the procedure such as: 1) extracting the water by means of some inert solvent,
26、 2) distillation with a solvent with which water forms an azeotrope, 3) varying the proportions of the components of the reagent, or 4) preparation of derivatives which do not react with Fischer reagent. Experience has shown that when determining the water content of some materials, erroneously low
27、results can be obtained owing to the very slow rate of reaction between the water and the Fischer reagent. In these cases the addition of1-ethylpiperidine (N-ethylpiperidine) catalyses the reaction and accurate results can then be obtained. Addition of the catalyst in solution has been found unsatis
28、factory and it should be added to the reaction vessel as the pure liquid. As received, it normally contains some water and needs drying. The amount to be added varies with the material under test and should be established in each case; usually amounts of up to1ml are sufficient. Examples of material
29、s for which this addition has been found beneficial are olefine oxides and aliphatic polyhydroxy compounds with water contents less than100p.p.m.BS2511:1970 iv BSI 07-1999 In many cases the addition of the catalyst is unnecessary and it can be omitted after tests have confirmed this. A British Stand
30、ard 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 pages This document comprises a front cover,
31、an inside front cover, pagesi toiv, pages1 to22, 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.BS2511:1970 BSI 07-1999 1 1 Scope This British
32、 Standard specifies methods for the preparation and standardization of Fischer reagent and its application to the determination of water in a variety of substances. 2 Double burette method using an electrometric end point 2.1 General. This method is intended for use where the highest degree of accur
33、acy is required and is suitable for the determination of water in organic solvents and similar materials having water contents of the order of0.1%. At this level the maximum deviation from the mean of results obtained in different laboratories will not normally exceed0.01% of water. 2.2 Apparatus. C
34、omplete sets of apparatus for determination of water by the Karl Fischer method are available from the trade. These are of different types designed to meet different requirements as to sensitivity and accuracy. In order to avoid unnecessary restriction the apparatus is described in general terms, ex
35、cept where strict adherence to a particular type or size is considered essential. 1) Reaction vessel. The reaction vessel (see Figure 1) has a working capacity of not less than60ml. The wide neck has an airtight detachable head fitted to accommodate two burette jets (extended to suitable length) and
36、 tubes for entry and exit of nitrogen. An additional hole, protected by a suitable seal, accommodates the spindle of a stirrer (unless magnetic stirring is provided). Two platinum wire electrodes extending nearly to the bottom of the vessel are either fused in through the side of the vessel or inser
37、ted through the head, if preferred. A drain cock is provided. A side tube, closed by a translucent latex vaccine cap, provides for insertion of liquid samples without exposing the contents of the vessel to atmospheric moisture during the operation. NOTECare should be taken in the construction of the
38、 vessel to avoid excessive “dead space” in the region of the drain cock. 2) Two burettes. The two burettes are of25ml capacity (or other capacity as preferred). When solid samples are to be examined there is a third burette of100ml capacity. All are Class A, complying with BS846 4) . 3) Two reagent
39、reservoirs. These are suitably connected to the burettes for gravity, suction or pressure feed. Provision is made for drying the air used for blowing or displacing the reagents. 4) Glass stirrer. The stirrer is either mechanically driven from above or magnetically driven from below. 5) Electrical eq
40、uipment. This is to provide for the application of not more than20mV to the electrodes when there is an excess of0.2ml of Fischer reagent. Suitable circuits are shown diagrammatically in Figure 2 and Figure 3. 6) Galvanometer. This has a sensitivity such that full scale deflection is obtained by a c
41、urrent of not more than1004A. The galvanometer may be in series with the reaction vessel or in a shunt position. In the former case the galvanometer will give a “null” reading at the end point and in the latter case a positive reading. For use in the series position the galvanometer should have a re
42、sistance not exceeding1007. 7) Tapping key or switch 8) Pipettes 5) . Pipettes are of10ml and20ml capacity, fitted with hypodermic needles of about1mm bore(1.5mm for use with the more viscous liquids such as the higher alkyl phthalates) and provided with suitable protection against atmospheric moist
43、ure. 9) Three additional hypodermic needles. These are of about1mm bore. 10) Weighing pipette. This is fitted with a hypodermic needle, of about1mm bore, in place of the usual jet. 11) Three drying, bottles. The bottles are for drying the nitrogen supply, one containing concentrated sulphuric acid a
44、nd two containing freshly activated silica gel, in that order. 12) One-mark volumetric flasks 6) . These are of100ml capacity, closed with translucent latex vaccine caps. NOTE 1All possible points of entry of atmospheric moisture into the apparatus (including the suction end of the pipettes) are pro
45、tected by guard tubes filled with freshly activated silica gel or other efficient desiccant. On safety grounds the use of granular magnesium perchlorate is not recommended. NOTE 2Silicon grease is used for lubrication of stopcocks. Petroleum jelly is not suitable, owing to its reactivity with Fische
46、r reagent. 4) BS846, “Burettes and bulb burettes”. 5) Pipettes complying with BS1583, “One-mark pipettes”, Class A, are suitable. 6) Flasks complying with BS1792, “One-mark volumetric flasks”, are suitable.BS 2511:1970 2 BSI 07-1999 Figure 1 Suggested alternative forms of reaction vesselBS2511:1970
47、BSI 07-1999 3 * Values shown for resistances will depend to some extent on the size and distance apart of the electrodes. Figure 2 Series circuit for double burette apparatus Total resistance of microammeter and resistor R in series to be approximately 4600 . Figure 3 Mains operated shunt circuit fo
48、r double burette apparatusBS2511:1970 4 BSI 07-1999 2.3 Reagents. The following reagents are required: 1) Methanol, complying with BS506 7)and dried as described in2.4, or anhydrous methanol denatured with pyridine. 2) Nitrogen, from a cylinder of compressed gas. 3) Fischer reagent, having a water e
49、quivalent of3.04.0mg/ml. This may be prepared as described in2.4 from the methanol and: i) pyridine, laboratory reagent quality; ii) iodine, resublimed; iii) sulphur dioxide, liquid, refrigeration grade, from a glass siphon. NOTEThis strength of reagent has been found generally convenient for determining percentages of water of the order of0.1%, using the apparatus and procedure detailed, but any other strength of reagent between about2.5mg/ml anti5.0mg/ml may be adopted if preferred. 4) 1-ethylpiperidine (N-ethylpiperidine), dried ove
