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IS0
I NT E RN AT I ON A L O R G A N IZATl O N FOR STA N DA RD IZATl O N
IS0 RECOMMENDATION
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R 760
DETERMINATION OF WATER BY THE KARL FISCHER METHOD
1st EDITION
June 1968
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The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 760, Determination of water by the Karl Fischer method, was
drawn up by Technical Committee ISO/TC 47, Chemistïy, the Secretariat of which is held by the
Ente Nazionale Italian0 di Unificazione (UNI).
Work on this question led, in 1962, to the adoption of a Draft IS0 Recommendation.
In November 1963, this Draft IS0 Recommendation (No. 660) was circulated to all the IS0
Member Bodies for enquiry. It was approved, subject to a few modifications of an editorial nature, by
the following Member Bodies :
Australia Hungary Portugal
Austria India Romania
Belgium Israel Spain
Chile Italy U.A.R.
Colombia Japan United Kingdom
Czechoslovakia Korea, Rep. of U.S.S.R.
France Netherlands Yugoslavia
Germany
Poland
Two Member Bodies opposed the approval of the Draft
New Zealand
U.S.A. .~
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council
which decided, in June 1968, to accept it as an IS0 RECOMMENDATION.
CONTENTS
Page
1. Scope .
3
2. Principle .
3
3. Reactions .
3
4. Reagents and auxiliary materials .
3
5. Apparatus .
5
6. Visual method .
6
7. Direct electrometric titration .
8
8. Electrometric back titration .
9
9. Test report .
11
ANNEX A .
13
ANNEX B .
14
ANNEX C . 15
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ISO/R 760-1968 (E
IS0 Recommendation R 760 June 1968
DETERMINATION OF WATER BY THE KARL FISCHER METHOD*
1. SCOPE
This IS0 Recommendation describes a method suitable for the determination of free water or water
of crystallisation in most solid or liquid chemical products, both organic and inorganic.
Precautions are necessary in certain cases and these are specified in the appropriate IS0 Recommen-
dations.
There are two methods of titration, depending on whether the end point is detected visually or
electrometrically. The visual method can be used only with colourless solutions and when no electro-
metric apparatus is available; it is always a direct titration. The electrometric method on the other
hand may involve either a direct titration or a back titration. The electrometric method, whether by
direct titration or back titration, is the more accurate and for this reason it is recommended.
2. PRINCIPLE
Reaction of any water present with a solution of iodine and sulphur dioxide in a pyridine methanol
mixture (Karl Fischer reagent), previously standardized by titration with an exactly known mass of
water (see clauses 6.1, 7.1 and 8.1).
NOTE. ~ Methanol may be replaced by 2-methoxyethanol (ethylene glycol monomethyl ether). With this solvent a
more constant titration volume is obtained and the reagent can be used with aldehydes and ketones, without using
any special technique.**
3. REACTIONS***
H, O + I, + SO, + 3C, H, N----t2C, H, N.HI + C, H, N.SO,
C,H,N.SO, + ROH -C,H,NH.OSO,OR
4. REAGENTS AND AUXILIARY MATERIALS
4.1 Methanol, analytical reagent grade not containing more than 0.05 of water. If the reagent
contains more than this quantity of water, dry it by distillation from magnesium turnings
activated with iodine. Collect the distillate in a receiver protected from atmospheric moisture
by means of a guard tube filled with anhydrous aluminium sodium silicate or activated silica
gel (4.9).
4.2
2-Methoxyethanol (ethylene glycol rnonornethyl ether), not containing more than 0.05 of
water. If the reagent contains more than this quantity of water, dry it by distillation, rejecting
the first portion of distillate which will contain the water.
*
Fischer, Karl. Angewandre Chemie 1935,48, page 394.
**
Peters, E.D. and Jungnikel, I.L. Anal. Chern. 1955,27, 450-53.
*** Smith, D.M. Bryant, W.N.D. and Mitchell Jr., J. Journal of the American Chemical Society 1939, 61,
page 2407.
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ISO/R 760-1968 (E
Pyridine, not containing more than 0.05 of water. If the reagent contains more than this
4.3
quantity of water, dry it by distillation, rejecting the first portion of distillate which will
contain the water.
4.4 Sample solvent. Either methanol (4.1), a mixture containing 4 parts of methanol (4.1) and 1
part of pyridine (4.3) (by volume) or (preferably for determinations with compounds containing
carbonyl groups) a mixture containing 4 parts of 2-methoxyethanol(4.2) and 1 part of pyridine
(4.3) (by volume). In special cases, other solvents may be recommended, for example, acetic
acid, pyridine or a mixture containing 1 part of methanol (4.1) and 3 parts of chloroform (by
volume).
4.5 Karl Fischer reagent. Place 670 ml of methanol (4.1) or 2-methoxyethanol(4.2) in a previously
dried flask, coloured brown or painted black on the outside, fitted with a ground glass stopper
and having a capacity slightly greater than 1 litre.
Add about 85 g of iodine. Stopper the flask and shake it occasionally until the iodine is
completely dissolved. Then add approximately 270 ml of pyridine (4.3), stopper the flask again
and mix thoroughly. Dissolve 65 g of sulphur dioxide in this solution, cooling to ensure that
the temperature of the liquid does not exceed 20 OC.
NOTE. - The reaction being exothermix, it is necessary to cool the flask from the beginning and to maintain
it at about O OC, for example, by immersing in an ice bath or in crushed solid carbon dioxide.
Replace the ground glass stopper by an attachment for introducing sulphur dioxide. This con-
sists of a cork with a thermometer and an inlet glass tube 6 mm X 8 mm, reaching to within
10 mm of the bottom of the flask, and a small capillary tube for connecting to the atmosphere.
Place the whole assembly with the ice bath on a balance and weigh to the nearest 1 g. Connect
the inlet tube to a siphon of sulphur dioxide by means of a flexible connection and a drying tube
filled with anhydrous aluminium sodium silicate (4.9) as desiccant and gently open the tap on
the siphon.
Adjust the rate of flow of sulphur dioxide so that all the gas is absorbed without the liquid
showing any sign of rising in the inlet tube.
Maintain the equilibrium of the balance by gradually increasing the tare and ensure that the
temperature of the liquid does not rise above 20 OC. Close the tap on the siphon as soon as the
increase in mass reaches 65 g.
Immediately remove the flexible connection and reweigh the flask and its inlet attachment. The
mass of dissolved sulphur dioxide should be between 60 and 70 g. A slight excess is not
harmful.
Stopper the flask, mix the solution and leave for at least 24 hours before using it. In fact, as
the result of imperfectly understood reactions which occur in the fresh reagent, the water
equivalent of the reagent decreases rapidly to begin with and then much more slowly.
This water equivalent is between 3.5 and 4.5 mg/ml. It should be determined daily if methanol
has been used, but may be done less frequently if 2-methoxyethanol has been used. '
It is possible to prepare the Karl Fischer reagent with a lower water content by diluting the
solution prepared as described above with the sample solvent (4.4).
Store the reagent out of the light and protected from atmospheric moisture. It should pre-
ferably be stored in a reagent bottle of brown or black-painted glass.
4.6 Sodium rarzrate, crystalline, (Na,C,H,O, .2H,O) or water.
4.1 Waterlmethanol, 1 O mg/ml standard solution.
Using a microburette or a pipette, place 1 ml of water in a dry 100 ml one-mark volumetric
flask, containing approximately 50 ml of methanol (4.1). Dilute to the mark with methanol
(4.1) and mix (for standardization with this solution see Annex A, clause A.l or A.2 depending
on whether the visual method or the direct electrometric method is used).
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ISO/R 760-1968 (E
4.8 Waterlmethanol solution, approximately 2 g/l.
Using a microburette or a pipette, place 1 ml of water into a perfectly dry 500 ml one-mark
volumetric flask containing approximately 100 ml of methanol (4.1). Dilute to the mark with
methanol (4.1) and mix (see the correspondence by volume of this solution with the Karl
Fischer reagent in clause 8.2.2).
4.9 Aluminium sodium silicate, anhydrous, granules of diameter of 1.7 mm for use as a desiccant.
These granules may be regenerated by washing with water and drying at 350 “C for at least
48 hours.
Or activated silica gel.
4.10 Silicone base grease, for lubricating the ground glass joints.
5. APPARATUS
5.1 For direct titration (by visual method or electrometric method).
5.1.1 A suitable form of apparatus for this method is given in Annex B, should no commerical
apparatus be available. This standard apparatus comprises the parts described below.
5.1.1.1 Automatic burette, 25 ml capacity with a fine point, graduated in 0.05 ml and pro-
tected from atmospheric moisture by a guard tube filled with the desiccant (4.9).
5.1.1.2 Titration vessel, effective capacity of 100 ml, connected to the tap of the burette
(5.1.1 .l) by a ground glass joint and having two side tubes, one permitting the intro-
duction of platinum electrodes if the electrometric method is used and the other
fitted with a “vaccine cap” to permit the introduction of liquid test samples by means
of a syringe without opening the vessel.
5.1.1.3 Platinum electrodes (Fig. 1 and 2, Annex B) welded to a glass tube enabling them to be
introduced into the bottom of the titration vessel (5.1.1.2) and joined to two copper
wires which in turn connect to the device for the electrometric detections of the end
point (5.1.1.7) (these are omitted in the case of the visual method).
5.1.1.4
Electromagnetic stirrer, operating at 150 to 300 revolutions per minute with a mild
steel bar coated with glass or PTFE, fixed on a base of adjustable height.
5.1.1.5 Reagent bottle for the Karl Fischer reagent (capacity approximately 3 litres) of brown
or black-painted glass in which the filling tube of the automatic burette is immersed
through the ground glass stopper.
5.1.1.6 Rubber bulb connected to an air dryer filled with the desiccant (4.9) for the admission
of dry air under pressure into the reagent bottle (5.1.1.5) in order to fil the burette
(5.1.1.1).
5.1.1.7 Device for the electrometric detection of the end point shown diagramatically .in
Figure 3, Annex B (to be omitted in the case of the visual method).
5.1.2 Medical syringes, of suitable capacity and calibrated.
5.1.3 A small glass tube, closed at one end and fitted at the other with a rubber stopper, used for
weighing and introducing into the titration vessel, for example, the crystalline sodium
tartrate (approximately 250 mg) used to standardize the Karl Fischer reagent, or possibly
test samples of solid products.
NOTE. - All glassware used should previously be dried at 130 OC in an oven, then placed in a desiccator
containing the desiccant (4.9).
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ISO/R 760-1968 (I
For the electrometric back titration method
5.2
5.2.1 A suitable form of apparatus for this method is given in Annex C, should no commercial
apparatus be available.
This standard apparatus comprises the parts described below.
5.2.1.1 Two automatic burettes, 25 ml capacity with fine points, connected directly to their
filling containers, one of brown or black-painted glass for the Karl Fischer reagent
(4.5) and the other for the standard water/methanol solution (4.8).
5.2.1.2 Titration vessel, effective capacity of 100 ml, connected to the burette by means of
ground glass joints and having two side tubes, one permitting the introduction of
platinum electrodes and the other fitted with a “vaccine cap” to permit introduction
of liquid test samples by means of a syringe without opening the vessel.
5.2.1.3 Drying tube, connecting the fdling containers of the burettes and the stopper of the
titration vessel by a closed circuit.
5.2.1.4 Platinum electrodes (Fig. 4 and 5, Annex C) welded to a glass tube enabling them to
be introduced into the bottom of the titration vessel (5.1.1.2) and joined to two
copper wires which in turn connect to the device for the electrometric detection of
the end point (5.2.1.6).
5.2.1.5
Electromagnetic stirrer, operating at 150 to 300 revolutions per minute, with a mild
steel bar coated with glass or PTFE, fixed on a base of adjustable height.
5.2.1.6 Device for the electrometric detection of the end point shown diagrammatically in
Figure 3, Annex B.
5.2.2 Medical syringes, of suitable capacity,of which the volume is calibrated.
5.2.3 A small glass tube, closed at one end and fitted at the other with a rubber stopper, used for
weighing and introducing into the titration vessel, for example, the mass of crystalline
sodium tartrate (approximately 250 mg) used to standardize the Karl Fischer reagent or
possibly test samples of solid products.
NOTE. - All glassware used should previously be dried at 130 OC in an oven and then placed in a
desiccator containing the desiccant (4.9).
6. VISUAL METHOD
6.1
Principle for the detection of the end point
Development of colour in the test portion by addition of the first drop of excess Karl Fischer
reagent, coloured with iodine which gradually becomes colourless on addition to the test por-
tion containing the water to be determined.
6.2 Procedure
6.2.1 Standardization of Karl Fischer reagent
6.2.1.1 With the apparatus assembled as shown in Annex B and with the joints lubricated
with the grease (4.10), introduce by means of a syringe (5.1.2) 25 ml of methanol (4.1)
into the titration vessel (5.1.1.2) through the “vaccine cap”.
Switch on the electromagnetic stirrer (5.1.1.4). Add the Karl Fischer reagent (4.5)
from the automatic burette (5.1.1.1) until a brown colour is obtained.
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ISO/R 760-1968 (E
6.2.1.2
In the small tube (5.1.3) weigh 250 mg of sodium tartrate (4.6) to the nearest 0.1 mg.
Place this in the titration vessel very quickly,removing the “vaccine cap” for a few
seconds. Weigh the small tube (5.1.3
...