Karl Fischer titration: difference between revisions
Diff·revision 6 → 7·01:19, 21 Jul 2024
Difference between revision 6 and revision 7 of Karl Fischer titration. 9 lines changed; the page grew by 761 bytes.
| Revision 6 — 07:03, 16 Jul 2024 NewLabNell (talk) add citation for the incidence figure 1,651 bytes ±0 | Revision 7 — 01:19, 21 Jul 2024 LOD_Lorcan (talk) promote subsection to section 2,412 bytes +761 | ||
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| 10 | The method matters to readers of a [[Certificate of analysis|certificate of analysis]] for a specific and frequently misunderstood reason: a chromatographic purity figure is a statement about the ''relative'' composition of the material that dissolved, not about ''how much'' peptide the vial contains. Water, counterions and residual salts occupy mass. A vial of material reported at 99% purity by [[High-performance liquid chromatography|HPLC]] may nevertheless be 6% water and 8% [[Trifluoroacetate counterion|trifluoroacetate]] by mass, in which case its [[Peptide content|peptide content]] is closer to 85%.{{r|usp1503}} | 10 | The method matters to readers of a [[Certificate of analysis|certificate of analysis]] for a specific and frequently misunderstood reason: a chromatographic purity figure is a statement about the ''relative'' composition of the material that dissolved, not about ''how much'' peptide the vial contains. Water, counterions and residual salts occupy mass. A vial of material reported at 99% purity by [[High-performance liquid chromatography|HPLC]] may nevertheless be 6% water and 8% [[Trifluoroacetate counterion|trifluoroacetate]] by mass, in which case its [[Peptide content|peptide content]] is closer to 85%.{{r|usp1503}} |
| 11 | 11 | ||
| + | 12 | == Chemical basis == | |
| + | 13 | The underlying reaction is the oxidation of sulfur dioxide by iodine, which consumes water stoichiometrically: | |
| + | 14 | ||
| + | 15 | {{math|H_{2}O + I_{2} + SO_{2} + CH_{3}OH + 2\,RN → 2\,RNH^{+}I^{-} + RNH^{+}CH_{3}SO_{4}^{-}}} | |
| + | 16 | ||
| + | 17 | Methanol participates directly, forming a methyl sulfite intermediate that is the species actually oxidised; the base (historically pyridine, now typically imidazole) buffers the liberated acid and keeps the reaction in the pH window where the stoichiometry holds at 1 mol iodine per mol water.{{r|scholz1984}} | |
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| 12 | == References == | 19 | == References == |
| 13 | {{reflist}} | 20 | {{reflist}} |
| 14 | <ref name="usp921">United States Pharmacopeia, General Chapter <921>, "Water Determination". USP–NF, current revision.</ref> | 21 | <ref name="usp921">United States Pharmacopeia, General Chapter <921>, "Water Determination". USP–NF, current revision.</ref> |
| 15 | <ref name="usp1503">United States Pharmacopeia, General Chapter <1503>, "Quality Attributes of Synthetic Peptide Drug Substances" (informational). USP–NF, current revision.</ref> | 22 | <ref name="usp1503">United States Pharmacopeia, General Chapter <1503>, "Quality Attributes of Synthetic Peptide Drug Substances" (informational). USP–NF, current revision.</ref> |
| + | 23 | <ref name="scholz1984">Scholz E. ''Karl Fischer Titration: Determination of Water''. Springer, Berlin (1984). The standard monograph on reagent chemistry and interferences.</ref> | |
| 16 | 24 | ||
| 17 | {{DEFAULTSORT:Karl Fischer titration}} | 25 | {{DEFAULTSORT:Karl Fischer titration}} |
| 18 | [[Category:Analytical methods]] | 26 | [[Category:Analytical methods]] |
| 19 | [[Category:Compendial testing]] | 27 | [[Category:Compendial testing]] |
| + | 28 | [[Category:Certificates of analysis]] | |
| 20 | 29 |