Reagents, methods and kits for water determination

An essentially iodine-free and pyridine-free single component reagent for volumetric water analysis using the Karl Fischer reaction, a process for making the reagent, and the use of the reagent to determine the water content of a sample. The reagent contains triiodide ions as an oxidizing agent, a buffer such as an amine, a reducing agent such as SO.sub.2, and a solvent. The presence of iodine, if any, is in an amount less than 1% of the amount of the triiodide ions.

DESCRIPTION 
1. Field of the Invention 
This invention relates to improved one-component reagents for water 
determination using the Karl Fischer reaction, and more particularly to 
volumetric one-component, pyridine-free reagents which are essentially 
iodine-free, containing triiodide, a reducing agent such as SO.sub.2, a 
buffer such as an amide, and a solvent. 
2. Background Art 
The determination of moisture in materials such as liquids and solids by 
the Karl Fischer reaction is well known and widely used since it was first 
described by Karl Fischer in Angewandte Chemie 48, pages 394-396 (1935). 
Numerous publications have also described this technique for water 
determination, and reference is made to a general text by J. Mitchell, Jr. 
and D. M. Smith, entitled "Aquametry", published by John Wiley and Sons, 
1980. Reference is also made to a publication by E. Scholz entitled "Karl 
Fischer Titration", published by Springer Verlag in 1984. 
In a Karl Fischer reaction, the water to be determined reacts with iodine 
on a quantitative basis and consequently, the amount of reacted iodine is 
a measure of the amount of water present in the sample. The reaction 
proceeds according to the following expression: 
EQU (1) H.sub.2 O+SO.sub.2 +I.sub.2 =2H.sup.+ +2I.sup.- +SO.sub.3 
KF reagents are used in several types of analysis. A volumetric analysis 
using a volumetric reagent determines moisture by measuring the volume of 
the Karl Fischer reagent consumed during the analysis. A coulometric 
analysis using a coulometric reagent generates iodine by passing a current 
through the reagent and determines the moisture from the amount of 
current. 
Karl Fischer reagents are divided into two groups, single-component and 
two-component systems. In the single-component systems, all ingredients 
(iodine, buffer, SO.sub.2, and solvent) are in one solution. In the 
two-component systems, the "vessel" solution contains the buffer, 
SO.sub.2, and a solvent, while the "titrant" solution contains iodine in a 
suitable solvent. 
Both types of systems, one-component and two-component, have their 
advantages. The one-component reagents are more economical for users 
because they have to buy only one solution. However, there are 
disadvantages, particularly with respect to stability and shelf life. As 
soon as iodine, SO.sub.2 and amine buffers are combined in the same 
solution they slowly react with each other. This reaction decreases the 
iodine level and therefore reduces the titer strength. This in turn limits 
the stability and shelf life of the reagent. This complication (which does 
not exist in two-component reagents) requires that the type of amine and 
the ratio of amine to SO.sub.2 have to be carefully controlled to furnish 
good one-component reagents. 
This stability problem has been recognized by Blomgren et al in U.S. Pat. 
Nos. 2,780,601 and 2,967,155, both of which describe pyridine based 
reagents. In the former, a suitable concentration of iodide ions was added 
as a stabilizing agent to reduce the speed of the spontaneous titer 
decrease so that the titer of the reagent will be less affected by the 
aforementioned spontaneous side reactions. However, it was found that the 
problem persisted even in reagents containing iodide ions as stabilizing 
agents. Therefore, the invention of U.S. Pat. No. 2,967,155 was directed 
to the use of a stabilizing base in the reagent, where the base strength 
of the stabilizing additive was chosen to be greater than that of the 
accelerating base (pyridine) used in these reagents. Generally, the use of 
pyridine is to be avoided due to odor and health problems, as well as its 
inferior performance characteristics. 
U.S. Pat. No. 5,102,804 describes a modified Karl Fischer reagent for the 
determination of water which contains another iodine source instead of 
iodine. This source is an iodine halide or a mixture of this halide and a 
slat of an aromatic nitrogen containing heterocyclic compound. Advantages 
are stated to be that of increased stability and quicker reaction times. 
While the prior art has provided some solutions to the problem of shelf 
life in one-component Karl Fischer reactions, it is desirable to provide 
one-component volumetric reagents which exhibit improved accuracy in 
addition to enhanced shelf life. This has been accomplished in the present 
invention wherein essentially iodine-free one-component reagents are 
described where the oxidizing reagent is triiodide that is present in the 
species as the triiodide ion I.sub.3. 
Accordingly, it is an object of this invention to provide an essentially 
iodine-free one-component volumetric reagent for the Karl Fischer 
determination of water content, where the oxidizing species is triiodide. 
It is another object of this invention to provide an improved process for 
the volumetric determination of water in a sample using the Karl Fischer 
reaction, in which the one-component reagent that is employed contains 
triiodide, a buffer, SO.sub.2, and a solvent. 
It is another object of this invention to provide an improved 
pyridine-free, essentially iodine-free, one-component volumetric reagent, 
a method for making this reagent, and a method for using this reagent to 
determine water content, wherein improved accuracy results. 
It is another object of this invention to provide an improved one-component 
Karl Fischer reagent containing triiodide, SO.sub.2, a buffer, and a 
solvent wherein any iodine present in said reagent is present in an amount 
less than 1% of the amount of triiodide. 
It is an object of this invention to provide an essentially iodine-free, 
pyridine-free volumetric Karl Fischer reagent containing triiodide as an 
oxidizing agent, a reducing agent, a buffer and a solvent. 
It is another object of this invention to provide a process for water 
determination using the reagents described in the preceding objects.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The reagents of this invention are pyridine-free single component 
volumetric reagents that are essentially iodine-free, and contain 
triiodide as an oxidizing agent. The reagents also contain a reducing 
agent such as SO.sub.2, a buffer such as an amine, and a solvent. 
In addition to the basic Karl Fischer reaction described above as equation 
1, it is known that iodide ions can be added to a Karl Fischer reagent 
containing SO.sub.2, a buffer such as an amine, and a solvent by adding 
water to the Karl Fischer reagent. The iodide ions combine with iodine 
according to the following expression: 
EQU (22) I.sub.2 +I=I.sub.3 
The species I.sub.3 is the triiodide ion. Reaction 2 occurs in aqueous and 
in methanolic solutions, as well as in other solutions. 
The reaction given by Equation 2 can be used to make the improved reagents 
of this invention since the reaction is driven predominantly to the right 
hand side of the equation, i.e., to I.sub.3. As an example, in a first 
step a buffer (amine) and water are dissolved in a suitable solvent to 
produce a solution. Sulphur dioxide is then dissolved in the solution. 
Following this, iodine is added and dissolved by stirring the solution. 
The presence of water creates the iodide ions which then combine with 
iodine in accordance with Equation 2 to make the triiodide ions. By 
driving the equilibrium of this reaction toward I.sub.3, triiodide will be 
formed and the solution will be essentially iodine-free. This is in 
contrast with reagents of the prior art where SO.sub.2, an amine and 
iodine are dissolved in a suitable solvent and only very small amounts, if 
any, of I.sub.3 are produced. In the present invention, sufficient water 
is present so that (essentially) all of the iodine is converted to 
triiodide in accordance with Equations 1 and 2. This will be explained in 
more detail by reference to the following examples which are merely 
illustrative and not limitative of the invention. 
EXAMPLE 1 
In the preparation of a pyridine-free, essentially iodine-free single 
component reagent, 136 grams (2 moles) of imidazole and 3.4 grams (0.19 
moles) of water were dissolved in one liter of ethylene glycol monomethyl 
ether. After this, 96 grams (1.5 moles) of sulphur dioxide SO.sub.2 were 
dissolved in the solution. Then 113 grams (0.445 moles) of iodine were 
dissolved in the solution by stirring. The 0.19 moles of water will react 
with 0.19 moles of iodine in accordance with Equation 1 and will form 0.38 
moles of iodide (I.sup.-), leaving 0.255 moles of iodine. This 0.255 moles 
of iodine will react with the 0.38 moles of iodide according to Equation 2 
to produce a final reagent solution containing 0.255 moles of triiodide, 
and essentially no iodine. 
EXAMPLE 2 
This one-component reagent was made by dissolving 200 grams (1.9 moles) 
diethanolamine and 96 grams (1.5 moles) of sulphur dioxide and 3.6 grams 
(0.2 moles) of water in one liter of methanol. After this 140 grams (0.55 
moles) of iodine are added and dissolved in the solution by stirring it. 
As with Example 1, and in accordance with the reactions given by Equations 
1 and2, triiodide is produced and the resultant reagent is essentially 
iodine-free. 
EXAMPLE 3 
This one-component reagent was prepared by dissolving 188 grams (2 moles) 
of n-propylamine, 102 grams (1.6 moles) sulphur dioxide, and 4.0 grams 
(0.22 moles) of water in one liter of ethyleneglycolmonomethyl ether. 
After this, 152 grams (0.6 moles) of iodine were added and dissolved into 
solution by stirring. As in Example 1, the reactions described in 
Equations 1 and 2 then occur to produce an essentially iodine-free reagent 
wherein the oxidizing agent is triiodide. 
EXAMPLE 4 
This one-component volumetric reagent was prepared by dissolving 112 grams 
(0.25 mole) of imidazole triiodide, 83 grams (1.3 moles) of SO.sub.2 and 
119 grams (1.75 moles) imidazole in 1 liter of ethyleneglycolmonomethyl 
ether. 
Application of Reagents 
The reagents prepared in examples 1-4 were used as volumetric reagents in a 
commercial titration apparatus (Ericsen Instruments Corp. Cat. No. AQ100). 
Each moisture analysis used 50 ml of methanol in the titration vessel. The 
methanol was pretitrated to a first endpoint. Then 50 mgs. of water were 
added to the vessel and a second titration undertaken to an identical 
endpoint. Accurate reproducible results were obtained. 
In all of these examples the amount of water is more than one third the 
amount of iodine on a molar basis. This means that the Karl Fischer 
reaction given by Equation 1 and the reaction given by Equation 2 will 
occur while a solution is being prepared. This will convert iodine into 
the triiodide ion so that the finished solutions contain no or essentially 
no iodine, the oxidizing agent being triiodide ions (there being a 
possible excess of I.sup.-). 
In the preparation of these improved reagents, it is preferable that the 
buffer sulphur dioxide, and water are present before adding the iodine. If 
the iodine were added before the water, the iodine may enter side 
reactions leading to an inferior reagent. 
As soon as the ratio of iodine/iodide is equal to 1, practically all of the 
oxidizing species becomes I.sub.3 (triiodide) because of the reaction in 
Equation 2. The best results are obtained as soon as the iodide amount is 
at least equal to the iodine amount, i.e., I.sup.- .vertline.I.sub.2 
.gtoreq.1. If this is so, the resulting solution will be essentially 
iodine-free. 
As the iodide concentration is further increased, at a constant iodine 
concentration, it has been found that the best results are obtained when 
the iodide/iodine ratio is in the range of about 1-2.5. However, superior 
reagents are still obtained when this ratio is above 2.5. 
It is recognized that presently used Karl Fischer reagents of the single 
component type may contain limited amounts of triiodide in addition to 
some iodide. This occurs because all solvents contain small amounts of 
water. Due to this, the reactions given in Equations 1 and 2 occur to a 
limited extent and therefore presently used reagents may contain some 
triiodide in addition to iodine. This relative amounts of triiodide and 
iodine in those prior art reagents are significantly different that those 
in the improved reagents of the present invention, however, as will be 
explained in more detail. 
Several electrochemical experiments have been conducted to show that the KF 
reagents of this invention contain essentially only triiodide and no 
iodine. These experiments involved the measurement of the 
oxidation-reduction potential of the reagents. A platinum wire was 
inserted into the solutions and its voltage was measured against a 
reference cell. The solution contained a known amount of iodine but no 
iodide. Then iodide was added in known amounts while the voltage was 
measured. At the point where the amount of iodide added was equal to the 
amount of iodine (I.sub.2 +I.sup.- =I.sup.-.sub.3) the redox potential of 
the platinum wire dropped suddenly by about 200 millivolt. These results 
prove to an electrochemist skilled in the art that triiodide was formed 
because the voltage drop occurred at an iodine to iodide ratio of 1. The 
large size of the drop (200 millivolts) shows that the amount of iodine 
left after the reaction is much less than 1% of the original amount of 
iodine. 
As noted, very small amounts of iodine may be present in the reagents of 
this invention, as is apparent of a review of Equation 2. The amount of 
iodine that is present will, however, be so small that the iodine is 
immaterial as a titration agent. The equilibrium constant K of Equation 2 
is given by the following expression: 
##EQU1## 
Typically, K is in the order of 10.sup.4 -10.sup.6 (moles.sup.-1 liter) in 
these new KF reagents and at a minimum is at least 10.sup.3 (moles.sup.-1 
liter). It can be easily shown that under these conditions there is very 
little iodine present as shown as more iodide than iodine is added to the 
reagent. For example, if 1 mole iodine per liter is combined with 1.01 
moles of iodide per liter, 1 mole of triiodide is formed and 0.01 mole of 
iodide is left over. Even if a low K of 10.sup.4 is assumed, when these 
numbers are put into Equation 3 it yields I.sub.2 =10.sup.-2 moles per 
liter. In this example, worst case assumptions were made i.e., K=10.sup.4 
and I.sup.- =0.01. In the more realistic case where K=10.sup.5, even for 
I.sup.- =0.01, I.sub.2 is present in an amount 10.sup.-3 moles per liter. 
These iodine concentrations of 0.01 and 0.001 moles per liter, 
respectively, are so low that they are far outside the range of present 
conventional volumetric reagents in which iodine is in the range of 
0.05-0.33 moles per liter. 
Practical present volumetric reagents contain 0.05-0.33 moles per liter of 
iodine. Since the new reagents have to contain an equal amount of 
triiodide that means that the most preferred range of triiodide is 
0.05-0.33 moles of triiodide per liter. Since the new reagents can also be 
prepared somewhat stronger, the preferred triiodide range is 0.05-0.6 
moles of triiodide per liter. 
The buffers used in these new reagents are non-pyridine buffers and 
preferably are numerous types of amines. It has been found that imidazole 
and its derivatives give the best results. These derivatives are compounds 
that contain the imidazole ring and wherein the hydrogen of the imidazole 
is substituted by one or more aliphatic or aromatic groups. Good results 
have also been obtained with diethanol amines or other aliphatic amines 
such as triethylamine--in general, aliphatic amines can be used. 
Suitable amine buffers, besides the most preferred imidazole, include 
aliphatic amines, primary, secondary, or tertiary amines optionally 
containing zero to three oxygen atoms. Examples include diethanolamine, 
ethanolamine, triethanolamine, diethylamine, triethylamine, 
diisopropylamine, tri-n-butylamine, ethylenediamine and the like. Mixtures 
of such amines can also be used. In addition to the above-mentioned types 
of amines, other suitable amines include dimethylaniline, diphenylamine 
and other equivalent amines. Diethanolamine is a preferred amine. 
The suitable range of amine:SO.sub.2 ratio depends on the kind of amine 
used. For weak amines such as imidazole, the ratio is 10:1-0.5:1. For 
strong amines such as diethanolamine, it is only 2:5:1-0.5:1. 
The solvents used for these improved reagents can be chosen from those 
customarily used. For example, an anhydrous low molecular weight alcohol 
can be used, such as ethylene glycol-monomethyl ether. Another suitable 
solvent is methanol. 
By far the most preferred reducing agent is SO.sub.2. Other reducing agents 
are described by Delmonte in U.S. Pat. No. 3,656,907. An example is 
dimethylsulfoxide. SO.sub.2 may alternatively be used in an admixture with 
an acid such as a carboxylic acid. Suitable acids include formic, oxalic, 
sulfuric, hydriodic, and acetic acid. The molar ratio of sulfur dioxide to 
acid is typically in the range from about 20:1 to 1:5, with the preferable 
range being about 2:1 to 1:2. 
In practice, these improved pyridine-free, iodine-free one-component 
volumetric reagents are used in the same manner as are other one-component 
reagents. That is, the reagent is added in measured amounts to titrate to 
an endpoint identical to the beginning endpoint. 
The improved reagents of this invention have surprisingly shown increased 
accuracy in comparison to reagents wherein the titration agent is iodine. 
A comparison experiment was carried out to compare the performance of the 
titration reagent of Example 1 with that of a conventional reagent. The 
composition of the conventional reagent was identical to that given in 
Example 1, except that no water was used in its formulation. Consequently, 
the comparison was between the new triiodide reagent and the conventional 
iodine reagent. Both solutions were used as titrants in a manual 
conventional titration apparatus to titrate a known amount of 50 
milligrams of water in methanol. Ten titrations were performed with each 
solution, the accuracy being found as follows: 
New reagent: 50 mg.+-.0.9 mg. Accuracy 1.8% 
Old reagent: 50 mg.+-.2.1 mg. Accuracy 4.2%. 
Other experiments relating to accuracy were carried out, yielding the same 
result: namely, that the new reagents containing triiodide and essentially 
no iodine were more accurate. This is a very desirable feature which can 
for example, provide increased efficiency in a manufacturing process where 
the degree of accuracy is critical. 
In these improved, essentially iodine-free reagents, a range of triiodide 
of 0.03-1 mole per liter is useful. However, the preferred range of 
triiodide is 0.05-0.6 moles per liter. In order to convert substantially 
all of the iodine into triiodide during formulation of the reagent in 
accordance with equation (2), the water that is present prior to adding 
iodine should be present in an amount at least 1/3 of the amount of iodine 
on a molar basis. 
These new reagents can be employed in kits that are sold to users for the 
determination of water content. An example is a sealed vial containing 
these new reagents, where the unknown sample can be introduced into the 
vial (as by breaking a seal). 
While the invention has been described with respect to particular 
embodiments thereof, it will be apparent to those of skill in the art that 
variations can be made therein without departing from the spirit and scope 
of the invention. The scope of the invention is intended to be limited 
only by the issued claims thereof.