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ABSTRACT.
The thermometric titration of hygroscopic moisture with acid catalysed 2,2-dimethoxypropane (DMP) is a superior alternative to the traditional Karl Fischer (KF) method. In contrast to KF reagent, DMP is indefinitely stable, is of low toxicity, and may be applied to a much larger range of samples.
1. INTRODUCTION.
When catalysed by the addition of acid, 2,2-dimethoxypropane (DMP) reacts instantly with water to produce a strong endothermic response1. The reaction:
has proved to be the basis of a very versatile titrimetric procedure for the determination of hygroscopic moisture in a wide variety of substances2,3. Compared to the traditional Karl Fischer (KF) method which has been in use since 1935, the thermometric DMP method offers advantages in the following areas:
(a). Toxicity. DMP is of low toxicity. KF reagent contains pyridine (or other nitrogen-containing organic chemicals), iodine, and SO2.
(b). Stability. DMP is indefinitely stable without special precautions. Calibration before each set of titrations is eliminated. KF reagent is inherently unstable, requires frequent recalibration, and special precautions for storage must be undertaken.
(c). Versatility. Thermometric DMP titration may be applied to a range of samples which are either excluded from the repertoire of KF, or subject to interference. These samples include those which are strongly acidic, contain amines, sulphur compounds, or redox agents.
(d). Sensor. Thermometric titrations employ a sensitive but robust thermistor, which is not subject to clogging or fouling.
(e). Disposal
of waste. Generally subject to less stringent regulations.
2. METHODOLOGY AND APPARATUS.
2.1. Apparatus. All determinations were performed with a FACTSTM (Fully Automated Computerised Titration System) manufactured by SANDA Corporation, Philadelphia USA. The titrator was fitted with a thermistor sensor and a thermometric titration board, essentially comprising a Wheatstone bridge. Titrations were performed in a magnetically stirred insulated vessel. In contrast to dedicated Karl Fischer titrators, the SANDA FACTSTM system may be used for hundreds of different thermometric, potentiometric, conductometric and chemiluminescence titration applications.
2.2. Method Outline. Measured amounts of samples were dissolved in a suitable polar, substantially anhydrous solvent such as acetonitrile (a solvent blank is determined during the calibration process). Non-acidic samples were acidified with a small but defined amount of a suitable acid, such as methane sulphonic acid. Titration endpoints are determined from the second derivative of the direct temperature curve of the titration. Time of determination may range from about 30 seconds to 2 minutes.
2.3. Calibration. The titrant was calibrated against a commercial standard, #34813 Hydranal (Riedel de Haën), normally used for standardising Karl Fischer reagent. The reagent contains 5.00 +/- 0.02 mg H2O/ml. Ten titrations of Hydranal resulted in a determination of 5.015 +/- 0.005 (1s) mg H2O/ml. A typical titration curve for moisture with DMP as titrant is illustrated in Figure 1.
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2.4. Applications.
The applications outlined here have been chosen to illustrate how the thermometric DMP method may be clearly differentiated from the Karl Fischer method. Samples were either commercially available reagents, or submitted by interested companies.
2.4.1. Water Contents of Concentrated Acids.
2.4.1.1.
Phosphoric Acid, 85% (Aldrich).
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H3PO4 by Difference, % |
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2.5. Water in Mercaptans.
Three different
types of mercaptans were analysed. Mercaptans which are not miscible in
acetonitrile may be dispersed in isopropanol.
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Nitrogenous compounds such as alkaloids interfere with the KF chemistry and may give misleading results.
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Dimethyl sulphoxide
(DMSO) is quite hygroscopic, and readily absorbs water on standing. Samples
from a freshly opened bottle were compared with those from a bottle which
had been in use for some time. Determination of water in DMSO by KF has
been reported to be problematic.
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The thermometric DMP titration has also been successfully demonstrated for a wide variety of materials including the following:
Foodstuffs: granulated sugar, chocolate, orange juice, apple cider, cheese, margarine.
Polymers and polymer precursors: Nylon, melamine, caprolactam, polyols.
Organic Chemicals: Aldehydes and ketones, aliphatic amines, chlorinated hydrocarbons, phenolic compounds, detergents.
Pharmaceuticals: Aspirin, ascorbic acid.
Oxidizing Agents: Hydrogen peroxide, organic peroxides.
Automotive Fluids: Motor oil (new and used), brake fluid, automatic transmission fluid, white oil.
Minerals and catalysts: Alumina, carbon black.
4. REFERENCES.
1. Wiberg,
K. B., Martin, E. J., Squires, R. R., J. Org. Chem.50, 4717,1985
2. Jordan,
J., Grime, J. K., Waugh, D. H., Miller, C. D., Cullis, H. M., Lohr, D.,
Anal. Chem. 48, 427, 1976;
3. Sadtler,
P., and Sadtler, T., Am. Lab., 69, 85, 1982
For more information contact:
Tom Smith
Antom Technologies
Pty Ltd
15 Tarpon
Street, The Gap
Qld 4061
Australia
Telephone & Facsimile: +61 7 3300 2614
e-mail:
antom@globec.com.au