Non-Spectrophotometric Methods

Non-spectral methods are generally less useful although in specific cases, they may represent the only monitoring method. 

In a clever variation of the use of a pH change to monitor rate, the pH change is minimized hy the controlled and registered addition of acid or base to maintain a constant pH during 

The types of reaction for which pH monitoring is useful are as follows. The mode of monitoring, (a) or (b) above, will be determined by the rate. 

Reduction of D-proline by D-amino acid oxidase at pH 8 shows two steps when monitored at 640 nm. These are interpreted as the build-up and breakdown of a reduced enzyme-imino acid charge transfer complex. If the reaction is monitored using phenol red the same two rates are observed but additionally the release of = 1 proton for each step can be assessed and interpreted. The indicator changes are followed at 505 nm and 385 nm, which are isosbestic wavelengths for the two steps (without indicator), 

The acid-base properties of a transient can be assessed directly by placing a glass electrode in a streaming fluid that is generating that transient a few ms after the mixer. The pH is measured after the electrode has equilibrated. A series of measurements of the extent of protonation vs pH yields a value in the usual way. The pJifa s of H3VO4 (and lower pro-tonated species) and Ni(H20)g could be determined, without interference by slightly slower polymerization. 

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Arya, S. P., M. Mahajan, and P. Jain. 2000. Non-spectrophotometric methods for the determination of Vitamin C. Anal. Chim. Acta 417 1-14. 

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