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New approaches to moisture determination in complex matrices based on the Karl Fischer Reaction in methanolic and non-alcoholic media
Umeå University, Faculty of Science and Technology, Chemistry.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Vattenhaltsbestämning är av stor vikt i många sammanhang. T.ex. kan vattenhalten påverka utbytet av en kemisk syntes, eller ha negativ inverkan på hållbarheten av läkemedel och livsmedel.

Standardmetoden för vattenhaltsbestämning är Karl Fischer-titrering, baserad på antingen volymetri eller coulometri. I den här avhandlingen presenteras nya infallsvinklar för bestämning av mycket låga halter vatten i komplexa provmatriser, som t.ex. tekniska oljor och substanser som interfererar med alkoholbaserade Karl Fischer-reagens.

Vattnet avskiljs ofta från oljematrisen före titrering genom förångning. I samband med framtagningen av nya referensmaterial för vatten i olja ifrågasattes förångningsteknikernas effektivitet av National Institute of Standards and Technology (NIST). NIST menade att en fraktion av vattnet bands hårt i oljefasen och att det inte kunde frigöras och detekteras annat än med en modifierad volymetrisk metod där reagenset innehöll minst 65% kloroform. I den här avhandlingen presenteras en alternativ metod som uppfyller det ställda kravet för en fullständig upplösning av oljefasen. Med denna metod visas att det inte finns någon anledning att ifrågasätta förångningsteknikernas effektivitet och att den modifierade metoden som NIST använder ger systematiskt för höga resultat.

Fördelar som enklare handhavande, kortare konditioneringstider och att endast ett reagens behövs har gjort att diafragmafri coulometri har blivit allt mer populär. Spårhaltsbestämning med denna teknik ställer dock speciellt höga krav på reagensen eftersom strömtätheten vid katoden är låg. Med anledning av detta testades olika typer av kommersiella reagensblandningar för bestämning av små vattenmängder och kritiska parametrar identifierades. Dekanol visade sig ha en gynnsam effekt på katodreaktionen i reagens modifierade med xylen enligt standardmetodbeskrivningen för bestämning av vatten i oljor.

För provtyper som inte går att analysera med alkoholbaserade reagenser presenteras en ny typ baserad på N-metylformamid. Med ett sådant reagens bestämdes vattenhalten i ett reaktivt salt som används i litiumjonbatterier. Liknande alkoholfria reagens undersöktes mer utförligt i en djupare studie som även inkluderade formamid och dimetylformamid. För- och nackdelar med dessa alternativa lösningsmedel diskuteras och möjliga reaktionsförlopp föreslås. Det visade sig att läget på jämvikten mellan svaveldioxid och vätesulfit är en avgörande faktor för att förklara den stora skillnaden i reaktionshastighet i dessa lösningsmedel.

Abstract [en]

Moisture determination is of great importance in the production and use of many substances. For example, the moisture content can affect the efficiency of a chemical reaction or determine the shelf life of pharmaceuticals or foods. The standard method for moisture determination is Karl Fischer (KF) titration, based on either volumetry or coulometry. This thesis concerns new approaches to trace determination in complex sample matrices and is focused on oils and substances that interfere with alcoholic KF reagents.

Moisture is frequently separated from oil matrices before titration by means of evaporation techniques. In connection with the preparation of new reference materials for moisture in oil, the National Institute of Standards and Technology (NIST) questioned the efficiency of such evaporation techniques. NIST claimed that some of the moisture was sequestered in the oil phase and that it could only be released and detected by using a modified volumetric KF method with a reagent containing at least 65% chloroform. In this thesis, an alternative KF method that meets the proposed requirement for a complete dissolution of the oil sample is presented. With this method it is shown that there is no reason to question the efficiency of the evaporation techniques and that the criticized volumetric method used by NIST is biased high.

Ever since its introduction diaphragm-free coulometry has gained popularity due to its ease of use, with a single reagent and short conditioning times. Trace determination with this technique sets great demands on the reagent due to the resulting low current densities at the generator cathode. The performance of several commercial reagents is evaluated under such unfavorable conditions and critical titration parameters are identified. It is also shown that decanol has a favorable effect on the cathode process when using reagents modified with xylene according to standard methods for moisture determination in oils.

For samples that are incompatible with the alcohol component in ordinary KF reagent a new reagent based on N-methylformamide is presented. It is shown that is works well for determinations of moisture in a conductive salt used in lithium-ion batteries. The concept of alcohol-free KF reagents is taken a step further in a systematic investigation, also including formamide and dimethylformamide. Advantages and disadvantages with these solvents are discussed and possible reaction paths are surveyed. It is shown that the position of the sulfur dioxide/hydrogen sulfite equilibrium is the main explanation for the large differences in the KF reaction rates in these solvents.

Place, publisher, year, edition, pages
Umeå: Kemi , 2008. , 48 p.
Keyword [en]
Karl Fischer titration, non-alcoholic reagent, trace moisture determination.
National Category
Analytical Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-1918ISBN: 978-91-7264-687-2 (print)OAI: oai:DiVA.org:umu-1918DiVA: diva2:142419
Public defence
2008-12-10, KB3A9, KBC, Umeå Universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2008-11-14 Created: 2008-11-14 Last updated: 2009-06-24Bibliographically approved
List of papers
1. Efficiency of methods for Karl Fischer determination of water in oils based on oven evaporation and azeotropic distillation
Open this publication in new window or tab >>Efficiency of methods for Karl Fischer determination of water in oils based on oven evaporation and azeotropic distillation
2003 (English)In: Analytical Chemistry, ISSN 0003-2700, Vol. 75, no 6, 1227-32 p.Article in journal (Refereed) Published
Abstract [en]

The efficiency of azeotropic distillation and oven evaporation techniques for trace determination of water in oils has recently been questioned by the National Institute of Standards and Technology (NIST), on the basis of measurements of the residual water found after the extraction step. The results were obtained by volumetric Karl Fischer (KF) titration in a medium containing a large excess of chloroform (> or = 65%), a proposed prerequisite to ensure complete release of water from the oil matrix. In this work, the extent of this residual water was studied by means of a direct zero-current potentiometric technique using a KF medium containing more than 80% chloroform, which is well above the concentration recommended by NIST. A procedure is described that makes it possible to correct the results for dilution errors as well as for chemical interference effects caused by the oil matrix. The corrected values were found to be in the range of 0.6-1.5 ppm, which should be compared with the 12-34 ppm (uncorrected values) reported by NIST for the same oils. From this, it is concluded that the volumetric KF method used by NIST gives results that are much too high.

Identifiers
urn:nbn:se:umu:diva-8346 (URN)10.1021/ac026229+ (DOI)12659179 (PubMedID)
Available from: 2008-06-11 Created: 2008-06-11Bibliographically approved
2. Coulometric Karl Fischer titration of trace water in diaphragm-free cells
Open this publication in new window or tab >>Coulometric Karl Fischer titration of trace water in diaphragm-free cells
2005 (English)In: Talanta: The International Journal of Pure and Applied Analytical Chemistry, ISSN 0039-9140, Vol. 65, no 5, 1349-54 p.Article in journal (Refereed) Published
Abstract [en]

Factors influencing the accuracy and precision for diaphragm-free Karl Fischer coulometric determinations of low mu g-amounts of water have been studied using the Metrohm 756 (pulsed current) coulometer and eight different types of commercial coulometric reagents and some modifications of these. As in the case of diaphragm-free coulometric titration of large amounts of water, the positive errors, due to the formation of oxidizable reduction products (of sulfur dioxide) in the cathode reaction (besides hydrogen), were found to be minimized by the use of highest possible pulse current (in the range 100-400 mA) in combination with the fastest possible titration rate. Most accurate (102-103%) and precise results (typical relative standard deviation 1.8%) were obtained for reagents containing very large concentrations of imidazole in combination with the presence of modifiers like hexanol, chloroform and propylene glycol (i.e. the HYDRA-POINT reagents). Similar results were obtained when this type of reagent was mixed 60/40 with xylene according to the ASTM recommendation for water determinations in petroleum products like crude and lubricating oils. Addition of decanol to this type of reagent mixture was found to reduce the influence from the oxidative reduction products significantly. A reduction of the error from 3.6% relative to 1.6% was achieved by addition of 9% (v/v) of decanol to a 60/40 reagent mixture of HYDRA-POINT Coulometric Gen (containing hexanol as modifier) and xylene. For larger concentrations of decanol the pulse current had to be lowered to 100 mA and this might explain why no further improvement was observed. An additional attempt to minimize the interference by lowering the concentration of sulfur dioxide in the reagents gave no significant effect. However. by means of a home-built computer-controlled coulometric instrumentation based on continuous instead of pulsed current (including a large cathodic current density) it was possible to achieve recovery rates close to 100% for the best reagents investigated. The reason for this improvement is discussed.

Keyword
titration, trace water, diaphragm-free cells
Identifiers
urn:nbn:se:umu:diva-13571 (URN)doi:10.1016/j.talanta.2004.09.005 (DOI)
Available from: 2007-05-11 Created: 2007-05-11Bibliographically approved
3. Interference-free coulometric titration of water in lithium bis(oxalato)borate using Karl Fischer reagents based on N-methylformamide
Open this publication in new window or tab >>Interference-free coulometric titration of water in lithium bis(oxalato)borate using Karl Fischer reagents based on N-methylformamide
2006 (English)In: Talanta: The International Journal of Pure and Applied Analytical Chemistry, ISSN 0039-9140, Vol. 69, no 1, 276-280 p.Article in journal (Refereed) Published
Abstract [en]

A non-alcoholic coulometric reagent based on N-methylformamide (NMF) was shown to eliminate the severe interference effect caused by the alcohol component of the conventional Karl Fischer (KF) reagent on the battery electrolyte lithium bis(oxalato)borate (LiBOB). For sample amounts up to 240 μg of water, the stoichiometry of the KF reaction deviated only slightly from the ideal 1:1 ratio for the best reagent composition. Both solid and dissolved (in acetonitrile, tetrahydrofuran (THF), and ethylene carbonate/ethyl methyl carbonate) LiBOB were titrated successfully using a Metrohm 756 KF Coulometer with a diaphragm cell. The detection limit was estimated to be 0.5–1 μg of water using 100 ml of reagent in this system.

Keyword
N-Methylformamide; Non-alcoholic KF reagent; Lithium bis(oxalato)borate; Water determination
Identifiers
urn:nbn:se:umu:diva-12418 (URN)doi:10.1016/j.talanta.2005.10.004 (DOI)
Available from: 2007-04-04 Created: 2007-04-04Bibliographically approved
4. Investigation of Amine-Buffered Amide Reagents for Coulometric Karl Fischer Titration
Open this publication in new window or tab >>Investigation of Amine-Buffered Amide Reagents for Coulometric Karl Fischer Titration
2009 (English)In: Talanta: The International Journal of Pure and Applied Analytical Chemistry, ISSN 0039-9140, Vol. 78, no 4-5, 1452-1457 p.Article in journal (Refereed) Published
Abstract [en]

Formamide (FA), N-methylformamide (NMF), and dimethylformamide (DMF),were evaluated as solvents for coulometric Karl Fischer (KF) reagents in combination with several amine bases. Except for the effect of the iodine species (iodine or triiodide), the pH of the reagent and the position of the sulfur dioxide/ hydrogen sulfite equilibrium were found to be the main factors explaining the large difference in the observed reaction rates betweenwater and the KF reagent in these solvents. Acid–base titrations showed that hydrogen sulfite is the main sulfur reactant in these media. The results will be of great importance in finding suitable combinations of base and solvent with respect to stoichiometry, side reactions caused by active carbonyl compounds, and reagent stability.

Keyword
non-alcoholic KF reagent, water determination, amide, amine
Identifiers
urn:nbn:se:umu:diva-3628 (URN)10.1016/j.talanta.2009.02.047 (DOI)
Available from: 2008-11-14 Created: 2008-11-14Bibliographically approved

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