Isobestic wavelengths

Figure 6.4 Plotting absorbance versus wavelength at several levels of pH for a BCG optical sensor shows that the alkaline form of the indicator absorbs at about 620 nm and the acidic form absorbs at about 450 nm. At about 550 nm both forms have the same molar absorptivity (isobestic wavelength). (Reproduced from LASERFOCUSWORLD.COM.)...

Sims and Horn (S13) did not consider that the addition of urea to the sodium benzoate solution was of any advantage and observed speedy coupling at pH 6.0-7.0 without it. The addition of nitrite to the diazo blank was found to eliminate errors due to hemolysis, and colorimetric readings were made at 525 mp since this is the isobestic wavelength for oxyhemoglobin and methemoglobin. Urea-ethanol solution has also been used to facilitate rapid coupling of bilirubin with diazotized sulfanilic acid. Patterson et al. (PI) have been able to eliminate the cloudiness that tends to develop with this method by the addition of concentrated phenol. 

Table 5 presents the extremes of the e values found for the light absorption maxima and minima in Table 6 some of the isobestic wavelengths and corresponding e values are given. Benesch el al. (Bl) foxmd = 132, based on 6 n = H-5. 

Isobestic Points (IBP) op HbCO with Eto, HsOt, and Hi c Values op HbCO at THE Isobestic Wavelengths... 

The total concentration of light-absorbing substances c is introduced, and c — C2 is substituted for Ci. If, furthermore, X2 is an isobestic wavelength, then These conditions being fulfilled, Eq. (8) may be... 

Although Refsum (Rl) has demonstrated that very good results can be achieved using X = 576 and 560 nm (nonisobestic), the methods described (Section 5.1.1) have been chosen because the use of an isobestic wavelength offers distinct advantages (Section 4). 

Blood (0.1 ml) is mixed with 20 ml 0.1% ammonia solution, and 20 mg sodium dithionite (NaiS204) is added to convert HbOa to Hb. The optical density is measured at X = 538 and 578 nm (isobestic wavelength) I = 1.000 cm slit width = 0.02 mm. The measurements should be carried out within 10 minutes of the addition of Naa8204. The CO saturation is calculated from the equation... 

The optical density is measured at X = 562 and 540 nm (isobestic wavelength). The CO saturation is calculated from the equation... 

Figure 9.21 Isobesticpoint. Alkaline hydrolysis of methyl salicylate at 25°C. Superimposition of the successive spectra recorded between 280 and 350 nm at 10 min intervals. At isobestic wavelength, the absorbance is invariant.

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