Patent Application
20050287677
For quantitative determination of substances in compounds, frequently a suitable form of titration is applied. Here, usually the compound to be analyzed is dissolved in a suitable solvent (test solution) and mixed with a specific reagent (titration solution) likewise dissolved in a suitable solvent. In special forms of titration, the reagent can also be generated, e.g., electrochemically, in the solution. The recognition of the end or neutral point of the titration can be realized in various ways, e.g., by a change in color of an indicator, electrochemically (by a change in potential on an electrode), thermometrically by a change in temperature, optically (by absorption of radiation), or by other suitable means.
Other preparations are realized using the example of Karl-Fischer titration (KF); however, it is clear to someone skilled in the art that the described problem, especially with regards to the solvent for the test substances, can be transferred freely to other titration methods.
The state of the art for the determination of water content by means of KF is the volumetric or coulometric titration of the water present in the sample with the help of so-called Karl-Fischer reagents. For this purpose, the sample can be dissolved in a suitable non-aqueous solvent and this solution can be titrated. Aside from with the water portion, if the components of the sample do not react with the titration system, then reliable measurement values are obtained in this way.
If no suitable solvent, which dissolves the sample homogeneously, can be found for certain samples, usually the suspension or emulsion of the sample in titrated in a solvent. This leads to changed kinetics of the titration reaction due to the necessary transition of water between phases. This impairs the recognition of the end point. In addition, deposits of the undissolved components on the electrodes of a coulometric cell can lead to non-reproducible results. As problem cases, for example, tests on cheese, cooking oils, and machine oils can be named.
Another method for testing hard-to-dissolve substances is the indirect determination of water content, in which the water contained in the sample is evaporated by heating and led into the measurement cell with a dry carrier gas flow and titrated there. Here, finding the temperature, at which the free water is stripped from the sample as much as possible, but without causing loss of water from the organic matrix, is problematic. This problem becomes especially clear in the testing of, e.g., biopolymers, which dissolve poorly in most solvents and for which water is lost from the molecule even at relatively low temperatures. In addition, this determination method also requires a comparatively high expense in terms of apparatus.
Ionic liquids have been known since the end of the 1940s. They concern molten salts, which are a liquid at room temperature and below and which present a novel class of solvents with non-molecular, ionic character. A common definition of ionic liquids in distinguishing them from known molten salts is a melting point below 80° C. Other references here name a melting point below room temperature. In this scope of this patent, ionic liquids are understood to be salts, which have a melting point below 80° C., preferably below room temperature, in the pure state.
Typical cation/anion combinations, which lead to ionic liquids, are, e.g., dialkylimidazolium, pyridinium, ammonium, and phosphonium with halogenide, tetrafluoroborate, methylsulfate. In addition, many other combinations of cations and anions are conceivable, which lead to such low-melting point salts.
In view of their use in technical processes, the basic properties of this material class are of interest:
Overviews on ionic liquids, their production, properties, and use can be found, e.g., in: “Ionic Liquids in Synthesis,” P. Wasserscheid, T. Welton (editors), Wiley; “Green Industrial Applications of Ionic Liquids” (NATO Science Series, Ii, Mathematics, Physics, and Chemistry, 92); “Ionic Liquids: Industrial Applications for Green Chemistry” (ACS Symposium Series, 818) by Robin D. Rogers (editor).
Ionic liquids can dissolve a wide range of substances homogeneously. The dissolving properties can be customized within a wide scope. Thus, cellulose, which is extremely difficult to dissolve, can be dissolved homogeneously without changing the chemical structure in a chloride-containing ionic fluid. Similar examples can be found from sectors of enzymatic analysis and biocatalysis, petrochemistry, and many others.
Particularly interesting is the property of ionic liquids to function simultaneously as a polar solvent and also as a homopolar solvent. In this way, within certain limits, polar and homopolar substances can be dissolved simultaneously. For example, it is possible to dissolve in the ionic liquid tetraalkylphosphoniumtosylate in addition to 10 mass % decant as a homopolar substance and also 5 mass % water as a polar substance. Another advantage of the ionic liquids is their great chemical stability. They can dissolve a large number of substances viewed as very reactive, without reacting with these substances. In this way, very reactive titration reagents (e.g., strong oxidants, such as H2O2) can also be dissolved in ionic liquids, without attacking the solvent itself.
Finally, through their immeasurable vapor pressure, ionic liquids offer the possibility of storing titration or sample solutions over longer periods of time, without causing changes in concentration due to evaporation of the solvent, e.g., in the storage containers of automatic titration systems. This can be important especially for applications, which take place at a permanently elevated ambient temperature.
In addition, the extremely low vapor pressure and the high thermal stability of the ionic liquids permits titration to be performed at an elevated temperature, without having to fear changes in concentration due to evaporation of the solvent. This can be advantageous in cases, in which, e.g., the reaction rate of a selected titration reaction is not sufficiently high and is to be increased by raising the temperature, e.g., in the saponification of fats with KOH. In other cases, elevated temperatures are advantageous in order to stop undesired reactions, for example, the polymerization of the indicator eriochrome black T at low pH values and high concentrations of alkali metals. Ionic fluids here present the possibility of opening up a significantly larger temperature range than with conventional solvents without building up pressure.
The use of ionic liquids as a solvent for titration can be realized in various ways:
In one embodiment, the otherwise difficult-to-dissolve sample substance is dissolved in an ionic liquid and titrated with a conventional titration solution. Here, the ionic liquid is advantageously miscible with the titration solution and the volume ratios are selected, so that after reaching the titration end point, a homogeneous solution is present, except for conditional precipitates produced by the type of titration.
In another embodiment, the sample solution is produced in a conventional solvent and the titration reagents are dissolved in an ionic liquid. This embodiment is preferred in the application of titration reagents that are more reactive or more difficult to dissolve, for which a conventional solvent is to be found only with difficulty. Here, a homogeneous mixture should also be present over the entire mixing range of sample and titration solutions.
In an especially preferred embodiment, both the sample solution and also the titration solution are produced with ionic liquids as the solvent, so that the ionic liquids used in both solutions are miscible with each other and total solubility of the sample substance and the titration reagents exists over the entire mixing range.
Other preparations are performed with the example of the Karl-Fischer titration; however, it is clear to someone skilled in the art that the described problems, especially with regards to the solvent for the sample substances, can be transferred freely to other titration methods.
For the common Karl-Fischer reagents, it is to be emphasized that all components have good solubility and are stable in ionic liquids. In this way, it is also possible to produce a titration solution based on ionic liquids as the solvent. These solutions produced in this way distinguish themselves in that for the typical storage of Karl-Fischer titration solutions in bottles with dry caps, there is no change in concentration due to evaporation of the solvent.
For sample substances viewed as problematic with regards to their solubility, in the ionic liquids suitable for these substances, freely soluble, inhomogeneous multi-component systems, such as, e.g., cheese or tar-like compounds can advantageously be dissolved homogeneously in ionic liquids. In particular, hydrophobic substances, such as machine oil and transformer oils are dissolved homogeneously by ionic liquids.
In the claims, the SMILES nomenclature (Simplified Molecular Input Line Entry System) is used for chemical formulas, see Weininger et al., Chem. Des. Autom. News 1986, 1, 2-15.
This is shown in the examples 1 and 2, as well as 3, which illustrate the invention, without limiting its scope.
Thus, ionic liquids are very well suited in view of their dissolving properties for preparing solvents for otherwise difficult-to-dissolve substances for the Karl-Fischer titration.
In 20 ml of dry ionic liquid, trihexyltetradecylphosphonium-chloride, approximately 2 g of the butter oil is weighed in exactly and dissolved by stirring. This solution is titrated with the help of a titrino with the use of a hydranal composite 5 solution. The triple determination produces a water content of 852 ppm.
A titration according to ISO 5536 produces a water content of 848 ppm.
As the solvent, EMIM-EtSO4 (ethylmethylimidaxolium-ethylsulfate) is used. In an amount of 10 mL EMIM-EtSO4, the following substances, which are difficult to dissolve in methanol, are dissolved:
None of these substances is soluble in methanol, which is typically used for sample preparation.
In a Thunder Scientific moisture generator, different ionic liquids are exposed to a relative air humidity of 45% at 25° C. For comparison, methanol, which is the solvent typically used for the Karl-Fischer titration, is exposed to these same conditions. The ionic liquids are stirred for better mixing. To determine the hygroscopicity, at intervals of approx. 15 minutes, samples were removed and the moisture content was determined by means of the Karl-Fischer titration. Table 1 shows the resulting measurement values (moisture in mass %).
Where:
This shows that the ionic liquids are comparable to methanol in terms of their hygroscopicity.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 025 756.6 | May 2004 | DE | national |
