isosbesticadj.

/ʌɪsəʊsˈbɛstɪk/

Forms: Also erron. isobestic.

Etymology: < German isosbestisch (A. Thiel et al. 1924, in Fortschr. d. Chem. XVIII. 116), < Greek ἴσος equal + σβεστός extinguished ( < σβεννύναι to quench, extinguish): see -ic suffix.

Physical Chemistry.

1. isosbestic point n. a wave-length at which the absorption of light by a liquid remains constant as the acidity varies or, more generally, as the state of equilibrium between two interconvertible substances or states shifts.

ΘΚΠ

the world > matter > chemistry > physical chemistry > phases > [noun] > phase transitions > points

triple point1872

isosbestic point1925

1925 Chem. Abstr. 19 2180 Photometric measurements of the absorption spectra were plotted for equal degrees of acidity and these curves intersected in general in the ‘isobetic’ point [sic: rendering Ger. isosbestischer Punkt.].

1943 W. R. Brode Chem. Spectrosc. (ed. 2) ix. 249 (caption) Spectrophotometric determination of hydrogen-ion concentration; mixed indicator (methyl red + bromothymol blue). (Note isosbestic points at 467 and 500 mμ.)

1954 Trans. Faraday Soc. 50 802 The ionized form exhibits absorption at a longer wave-length (251 mμ) than the non-ionized (239 mμ) and there is an isobestic point at 245 mμ.

1958 L. Meites & H. C. Thomas Advanced Analyt. Chem. viii. 277 The spectra of a number of solutions having equal formal concentrations of bromthymol blue but different pH values will all intersect at 501 mμ. Such a point is known as an isosbestic point, and the appearance of such a point on a family of spectra is a necessary (but not quite sufficient) criterion of the presence of two and only two forms of the absorbing substance in equilibrium with each other.

1960 Jrnl. Biol. Chem. 235 1026/2 Each of the absorption maxima and isosbestic points is essentially the same for normal as well as for sickle cells and homozygous C hemoglobins.

1962 R. E. Dodd Chem. Spectrosc. v. 299 The absence of an isobestic point indicates a more complex system.

1971 F. A. Bettelheim Exper. Physical Chem. xiii. 144 The absence of an isobestic point is definite proof of the presence of more than two absorbing species.

1972 Nature 24 Mar. 140/2 Control measurements are made at an isosbestic point for the rhodopsin and the product.

2. Other anomalous forms (with quot. 1939 cf. quot. 1943 for isosbestic point n. at sense 1 above).

ΚΠ

1939 W. R. Brode Chem. Spectrosc. ix. 206 There will be a point..where the extinction curves should have a common value at any pH concentration. Such a point is known as an isobastique point.

1949 A. C. Candler Pract. Spectrosc. iv. 90 As one absorption curve fades, another develops, while the absorption at an intermediate wavelength, called by Brode the isobastic point, remains unaltered.

1949 A. C. Candler Pract. Spectrosc. iv. 91 The indicator should be of standard strength, but slight variations may be corrected if the absorption at both the centre of the absorption band and at the isobastic point are measured.

This entry has not yet been fully updated (first published 1976; most recently modified version published online December 2020).

单词	isosbestic
释义	isosbesticadj. /ʌɪsəʊsˈbɛstɪk/ Forms: Also erron. isobestic. Etymology: < German isosbestisch (A. Thiel et al. 1924, in Fortschr. d. Chem. XVIII. 116), < Greek ἴσος equal + σβεστός extinguished ( < σβεννύναι to quench, extinguish): see -ic suffix. Physical Chemistry. 1. isosbestic point n. a wave-length at which the absorption of light by a liquid remains constant as the acidity varies or, more generally, as the state of equilibrium between two interconvertible substances or states shifts. ΘΚΠ the world > matter > chemistry > physical chemistry > phases > [noun] > phase transitions > points triple point1872 isosbestic point1925 1925 Chem. Abstr. 19 2180 Photometric measurements of the absorption spectra were plotted for equal degrees of acidity and these curves intersected in general in the ‘isobetic’ point [sic: rendering Ger. isosbestischer Punkt.]. 1943 W. R. Brode Chem. Spectrosc. (ed. 2) ix. 249 (caption) Spectrophotometric determination of hydrogen-ion concentration; mixed indicator (methyl red + bromothymol blue). (Note isosbestic points at 467 and 500 mμ.) 1954 Trans. Faraday Soc. 50 802 The ionized form exhibits absorption at a longer wave-length (251 mμ) than the non-ionized (239 mμ) and there is an isobestic point at 245 mμ. 1958 L. Meites & H. C. Thomas Advanced Analyt. Chem. viii. 277 The spectra of a number of solutions having equal formal concentrations of bromthymol blue but different pH values will all intersect at 501 mμ. Such a point is known as an isosbestic point, and the appearance of such a point on a family of spectra is a necessary (but not quite sufficient) criterion of the presence of two and only two forms of the absorbing substance in equilibrium with each other. 1960 Jrnl. Biol. Chem. 235 1026/2 Each of the absorption maxima and isosbestic points is essentially the same for normal as well as for sickle cells and homozygous C hemoglobins. 1962 R. E. Dodd Chem. Spectrosc. v. 299 The absence of an isobestic point indicates a more complex system. 1971 F. A. Bettelheim Exper. Physical Chem. xiii. 144 The absence of an isobestic point is definite proof of the presence of more than two absorbing species. 1972 Nature 24 Mar. 140/2 Control measurements are made at an isosbestic point for the rhodopsin and the product. 2. Other anomalous forms (with quot. 1939 cf. quot. 1943 for isosbestic point n. at sense 1 above). ΚΠ 1939 W. R. Brode Chem. Spectrosc. ix. 206 There will be a point..where the extinction curves should have a common value at any pH concentration. Such a point is known as an isobastique point. 1949 A. C. Candler Pract. Spectrosc. iv. 90 As one absorption curve fades, another develops, while the absorption at an intermediate wavelength, called by Brode the isobastic point, remains unaltered. 1949 A. C. Candler Pract. Spectrosc. iv. 91 The indicator should be of standard strength, but slight variations may be corrected if the absorption at both the centre of the absorption band and at the isobastic point are measured. This entry has not yet been fully updated (first published 1976; most recently modified version published online December 2020). < adj.1925
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