Ascorbic acid estimation

The methods for the determination of ascorbic acid have been authoritatively reviewed from time to time (R21, A9, 02, R16, C4). It is intended here only to survey the variety of methods available and their individual advantages. 

The curative method involves the use of a scorbutigenic diet for ten days. On the eleventh day when weight loss would be expected to begin, groups of animals are placed on graded doses of the test sample and the ascorbic acid, and the biological value of the sample is calculated from the resultant dose response curves (H6). 

The bio-assay methods were remarkably accurate in the hands of skilled 

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. 

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Ascorbic acid, H2C6H606, also known as vitamin C, is present in many citrus fruits. It is a diprotic acid with the following values = 7.9 X 105 K = 1-6 X 10 12. What is the pH of a 0.63 M solution of ascorbic acid Estimate [HC6H606-] and [C4H6062-]. 

Mobilization and Metabolism. The total ascorbic acid body pool in healthy adults has been estimated to be approximately 1.5 g, which increases to 2.3—2.8 g with intakes of 200 mg/d (151—158). Depletion of the body pool to 600 mg initiates physiological changes, and signs of clinical scurvy are reported when the body pool falls below 300 mg (149). Approximately 3—4% of the body pool turns over daily, representing 40—60 mg/d of metabolized, or consumed, vitamin C. Smokers have a higher metaboHc turnover rate of vitamin C (approximately 100 mg/d) and a lower body pool than nonsmokers, unless compensated through increased daily intakes of vitamin C (159). The metaboHsm of ascorbic acid varies among different species. 

Approaching the matter from an entirely different angle, a semiquantitative estimation of ascorbic acid or total iodine-reducing substances might provide a suitable basis. Delays in handling involve rather marked losses of ascorbic acid. Kramer and Mahoney (20) have observed a relationship between quality and the amount of iodine-reducible substances remaining in lima beans. 

A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

The main value of ACU measurement is that it makes it possible to give a fast estimate of vitamin C deficiency because in healthy subjects ascorbic acid is about 90% of total ACU [33], A strong deficit of ACU, possibly due to vitamin C deficiency, was found in patients with breast tumors (Fig. 11). 

An extractive spectrophotometric procedure based on the complexation of reduced Iron(II) with 5-Chloro-7-iodo-8-hydroxyquinoline (CIHQ) for the estimation of micro amounts of vitamin C. The resulting brown colored complex was extracted into chloroform to give a reddish brown extract which shows an absorption band at 485 nm. This chelate was formed immediately and the apparent molar absorptivity and Sandell s sensitivity for vitamin C was found to be 8.5 x 105 dm3 mol"1 cm 1 and 2.072xl0 4g cm 2. Linear relationship between absorbance and concentration of ascorbic acid is observed up to 0.8 pg ml"1. Interference studies of different substances including sugars, vitamins and amino acids, metal ions and organic acids were carried out. The utility of the method was tested by analysing some of the marketed products of vitamin C... 

There is a very simple and quick method that can be used to detect the reducing power of sulphur dioxide, developed in the last century and often called the Ripper titration (Ough, 1988). In this method, sulphur dioxide is titrated against iodine or potassium iodate/potassium iodide solution in the presence of starch. When all the sulphur dioxide has been oxidised, a blue colour is produced by the reaction of free iodine with the starch. This is a very quick method but will give only an estimate of the level of sulphur dioxide as other reducing substances, such as ascorbic acid, will interfere consequently, this method is not particularly appropriate for juices with high ascorbate levels. 

Many vegetables and fruits, other than citrus, contain ascorbic acid. It was estimated that citrus fruits and tomatoes provided only 18% of the total vitamin C intake in the American diet during the decade of 1910. These two fruits supplied 41% of vitamin C in 1956-58 (6j. Today orange juice alone provides nearly 60% of the U.S. Recommended Daily Allowance (U.S. RDA) of vitamin C in the American diet (7). 

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. 

The amount of this radical precursor in the reaction mixture of glucose with t-butylamine was estimated from intensity of the ESR signal that appeared 30 min after ascorbic acid was added to the mixture. The increase in ESR signal during the reaction is shown in Fig. 20. The radical precursor was observed at a very early stage of the reaction and increased rapidly for about 25 min, then decreased along with the development of the free radical. This pattern is very similar to that observed in the... 

When several temperature-dependent rate constants have been determined or at least estimated, the adherence of the decay in the system to Arrhenius behavior can be easily determined. If a plot of these rate constants vs. reciprocal temperature (1/7) produces a linear correlation, the system is adhering to the well-studied Arrhenius kinetic model and some prediction of the rate of decay at any temperature can be made. As detailed in Figure 17, Carstensen s adaptation of data, originally described by Tardif (99), demonstrates the pseudo-first-order decay behavior of the decomposition of ascorbic acid in solid dosage forms at temperatures of 50° C, 60°C, and 70°C (100). Further analysis of the data confirmed that the system adhered closely to Arrhenius behavior as the plot of the rate constants with respect to reciprocal temperature (1/7) showed linearity (Fig. 18). Carsten-sen suggests that it is not always necessary to determine the mechanism of decay if some relevant property of the degradation can be explained as a function of time, and therefore logically quantified and rationally predicted. 

While many aspects of nonlinear pharmacokinetic behavior may impact on the above equation, the more relevant pharmacokinetic processes for ASOs are absorption and distribution at or below therapeutic or nontoxic plasma concentrations. Nonlinear absorption or distribution processes can affect AUC terms in a nonproportional manner when different doses are compared, thereby resulting in an inaccurate determination of bioavailability. This has been shown to occur on numerous occasions for compounds such as ascorbic acid or naproxen [59-62]. Such cases require an understanding of the capacity-limiting cause of the nonlinearity and the pharmacokinetic processes upon which this impacts, in case of ASOs absorption and intercompartmental distribution processes from the central compartment into the peripheral tissues. With this understanding, various methods may then be applied to best approximate the rate of change of the plasma concentrations from one sampling time to the next allowing for an estimate of the absolute BAV. 

Another important field of application concerns food and beverages, especially wine, juices, and tea (A2, A11, A17, B4, K12, V7, Yl). The antioxidant components of food include vitamin E (a-tocopherol), vitamin A (retinoids), vitamin C (ascorbic acid), and also fi-carotene (provitamin A), other carotenoids (of which more than 600 compounds have been identified), flavonoids, simple phenols, and glucobrasicins (H3). Unfortunately, the TAC value of a food is not informative on the bioavailability of its antioxidants. It has been estimated that polyphenols are normally present in blood plasma at concentrations of 0.2-2 //M (PI). However, it has been demonstrated that feeding rats a quercetin-augmented diet can increase their plasma levels of quercetin and its metabolites up to 10-100 //M (M27), and transient increases in the concentration of plant-derived phenolic compounds can take place after ingestion of food and beverages, which may affect blood plasma TAC (see later). 

Indicine JV-oxide (53) is an anti-cancer drug that is now undergoing clinical trials. Methods for its estimation include oxidation to the pyrrole derivative and treatment with Ehrlich s reagent,53 g.l.c. analysis of its trimethylsilyl derivatives,54 and differential pulse radiography.55 The aqueous degradation of indicine iV-oxide (53) to retronecine /V-oxide and (—)-trachelanthic acid has been studied.56 Indicine jV-oxide is reduced to indicine by Fem, by reduced cytochrome c, by ascorbic acid, and by denatured haemoglobin.57... 

Reactions of hydroquinone, catechol, and L-ascorbic acid with dicyanobis(l,10-phenan-thn>line)iion(III) were studied in dimethyl sulfoxide (DMSO). Application of the Marcus theory to the reactions of catechol and hydroquinone provided the electron exchange rate constant for the Fe(III/II) couple in DMSO. The self-exchange rate constant for the ascorbic acidAadical couple was estimated for the first time in DMSO. The one electron-oxidation process of L-ascorbic acid in an aprotic solvents such as DMSO may be completely different from that in aqueous solutions. 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

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