Ascorbic acid estimation chemical methods

Ohtsuka et al. used UTEVA-Resin to set up a Pu analysis method, taking advantage of the ability to retain Pu in the Pu(IV) state, wash away other trivalents, and then selectively reduce and elute the Pu as Pu(III), thus separating it from still-retained U.58 59 Samples were loaded and washed in 3 M HN03, and Pu(III) was released with 0.01 M ascorbic acid in 3 M HN03. Ascorbic acid provides rapid reduction and is destroyed in the ICP-MS torch. A decontamination factor of 6 to 7 orders of magnitude was estimated for the Pu-U separation. After Pu elution, U could be removed in dilute nitric acid. Chemical recovery of Pu was 70% in the analysis of several sediment reference samples. These authors used the PrepLab system set up as shown in Figure 9.18. 

Colorimetric Methods. The most frequently used colorimetric methods have been recently reviewed by Omaye et al. (5). Several methods of analyses are based upon the fact that ascorbic acid and dehydroascorbic acid possess certain chemical properties characteristic of sugars such as formation of osazones and conversion to furfural. Colorimetric determination of furfural, an aniline derivative, has been used to a limited extent for the estimation of ascorbic acid in certain materials. These methods have generally been found to be unsatisfactory... 

The simplicity of the chemical methods for estimating ascorbic acid has diverted attention from the several physical methods available for measuring ascorbic acid. For certain applications these methods can be useful. 

Polarography has had limited use in the estimation of ascorbic acid, since early investigations disclosed no advantage over the simpler methods of chemical titration, even as regards specificity. A recent application of this method to vegetables and fruits was reported by Krauze and Boxyk (K16). No dehydroascorbic acid was found in the samples after reduction by H2S. 

Certain microorganisms, such as Escherichia coli and Staphylococcus albus strains will reduce dehydroascorbic acid. This reduction has been used for the estimation of ascorbic acid after its oxidation in extracts by ascorbic acid oxidase (S27) and also in the chemical reduction methods as a way of reducing dehydroascorbic acid to ascorbic acid. The reduction is not specific for L-ascorbic acid, but interfering compounds are unlikely in natural products. The use of this reducing action of bacteria has been improved by Mapson and Ingram (M8). 

Figure 13.8 shows the first separation of small molecules by FFF. Ascorbic acid was separated from toluene through a secondary chemical equilibrium with field-retained microemulsiom droplets. Once again, the exchange between the aqueous phase and the swollen micelles is low, i. e., the efficiency is low and broad peaks are obtained (Figure 13.8). There are so many powerful techniques for small molecule separation that micellar FFF was not used for this purpose. Its interest could be in the physicochemical study of the micellar or microemulsion structure. For example, in the case of the Figure 13.8 experiment, the separation allowed the estimation of the average mass of the mobile phase microemulsion droplets (1.4x 10 %) and consequently, its radius (35 nm) [38], These values can be obtained by heavy methods such as small angle neutron scattering or high resolution NMR [38]. Micellar FFF can be an easy alternative in such studies.

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