Ascorbic acid measurement

Formation of the free radical of ascorbic acid (measured at 360 nm) accompanied reduction of tocopherol free radicals (38). Bielski et al. (18,46) showed that the ascorbic acid free radical in the 3-position absorbs at 360 nm and that in the 2-position absorbs at 290 nm also, ascorbic acid reacts with superoxide. 

Figure 17.5 Flow rate effect of the mean size of gold nanoparticles prepared in a microflow-through reduction of tetrachloroauric acid with ascorbic acid (measurement of size by centrifugal sedimentation spectroscopy).

Old reports describing decreases in ascorbate with aging in many mammals exist (Kirk, 1962). A clear decrease in serum levels of ascorbic acid (measured by colorimetry) was found in male Wistar rats between 3 and 12 months of age but no changes were observed between 12- and 24-month-old rats (De and Darad, 1991). In the same study no changes in liver ascorbate levels were found among the three... 

On the basis of Khatami s work it seems reasonable to guess that the high levels of ascorbic acid measured in the neural retina are the result of active transport... 

Reduced ascorbic acid has its optical absorbance maximum at 245 to 270 nm depending strongly on pH. At pH 2, the absorption maximum is at 245 nm, and at pH 6.4 it is at 265 nm. Since fixed-wavelength 254-nm mercury lamp detectors are relatively inexpensive, 254-nm UV detectors are commonly used for ascorbic acid measurement. DHA absorbs at 210 to 227 nm, which limits the choice of the sample matrix, solvents, buffers, and other reagents used. Moreover, DHA degradation products may interfere with DHA in the chromatogram (12). Thus the direct UV detection of DHA is complicated, and usually DHA is reduced to ascorbic acid before chromatographic separation and measured as total ascorbic acid (43,66,81,82). 

As noted, this procedure is adapted from a standard spectrophotometric method. The instructions for the spectrophotometric method indicate that the absorbance should be measured 5-10 min after adding the ascorbic acid. Why is this waiting period necessary in the spectrophotometric method, but not necessary in the FIA method ... 

Food process optimi2ation measurements may link a single chemical such as a vitamin, or a physical change such as viscosity, to process conditions and to consumer acceptance. Retention levels of ascorbic acid [50-81-7] C HgO, or thiamine can often be used as an indicator of process conditions (see... 

Another method that determines both ascorbic acid and dehydroascorbic acid first reduced the dehydroascorbic acid to ascorbic acid and then retains the ascorbic acid on an anionic Sephadex column (82). The ascorbic acid is oxidized on the column to dehyroascorbic acid by -benzoquinone, which simultaneously elutes the dehydroascorbic acid. The dehydroascorbic acid is reacted with 4-iiitro-l,2-phenylenediainine and absorbance of the resulting yeUow solution produced is measured at 375 nm. 

Procedure. Use a solution containing 0.15-1.8 mg antimony per 100 mL it should be slightly acidic with sulphuric acid (1.2-1.5M). Transfer a 10 mL aliquot to a 50 mL graduated flask, add 25 mL of the potassium iodide-ascorbic acid reagent, and dilute to the mark with 25 per cent v/v sulphuric acid. Mix thoroughly and measure the absorbance at 425 nm or at 330 nm using a reagent blank as reference solution. 

In order to concentrate the lead extract, remove the lead from the organic solvent by shaking this with three successive 10 mL portions of the dilute hydrochloric acid solution, collecting the aqueous extracts in a 250 mL beaker. To the combined extracts add 5 mL of 20 per cent ascorbic acid solution and adjust to pH 4 by the addition of concentrated ammonia solution. Place the beaker in a fume cupboard, add 3 mL of the 50 per cent potassium cyanide solution and immediately adjust the pH to 9-10 with concentrated ammonia solution. Transfer the solution to a 250 mL separatory funnel with the aid of a little de-ionised water, add 5 mL of the 2 per cent NaDDC reagent, allow to stand for one minute and then add 10 mL of methyl iso butyl ketone. Shake for one minute and then separate and collect the organic phase, filtering it through a fluted filter paper. This solution now contains the lead and is ready for the absorption measurement. 

In order to use any of the results obtained by objective methods as the basis for the acceptance or rejection of a product, there must be available reliable information as to the relationship between the values obtained and organoleptic quality in terms of consumer acceptance and utility. Where standards are based upon measurement of such labile constituents as ascorbic acid or sugar, a knowledge of the normal values for good commercial practice is necessary. Such values have not yet been adequately established. This should constitute a useful field for research of inestimable value to the industry. 

The nurse measures the fluid intake and output, especially when the primary health care provider orders an increase in fluid intake or when a kidney infection is being treated. The primary health care provider may also order daily urinary pH levels when methenamine or nitrofurantoin is administered. These drugs work best in acid urine failure of the urine to remain acidic may require administration of a urinary acidifier, such as ascorbic acid. 

BENZIE i FFand STRAIN J J (1999) Ferric reducing/antioxidant power assay Direct measure of total antioxidant activity of biological fluids and modified version for simultaneous measurement of total antioxidant power and ascorbic acid concentration, Meth Enzymol, 299, 15-27. 

Glassy carbon electrodes polished with alumina and sonicated under clean conditions show activation for the ferrl-/ ferro-cyanlde couple and the oxidation of ascorbic acid. Heterogeneous rate constants for the ferrl-/ ferro-cyanlde couple are dependent on the quality of the water used to prepare the electrolyte solutions. For the highest purity solutions, the rate constants approach those measured on platinum. The linear scan voltammetrlc peak potential for ascorbic acid shifts 390 mV when electrodes are activated. 

Apples (Red Belle de Boskoop, Jonagold or Mutzu) were cut and milled (1.5 mm) and 5% (w/w) of a 2% ascorbic acid solution was added immediately. Enzyme preparations (25 mg enzyme protein / kg mash) were added and the mash was incubated for 2 hours at 20°C whereafter it was pressed. The resulting apple juice was pasteurised at 85°C to discontinue further enzyme degradation. The cloud was measured as turbidity in EF/F units [15]. The cloud stability was determined by a centrifugation test as the amount of turbidity remaining after centrifugation at 4,200 x g for 15 minutes [15]. 

A more recent study, which measured three established markers of free-radical activity in addition to serum ascorbic acid in two groups of elderly diabetic patients (with and without retinopathy), found no significant differences in any of the markers between patients and age-matched controls despite significant depletion of ascorbic acid in patients with diabetes, especially those with retinopathy (Sinclair et al., 1992). These rather paradoxical findings suggest the existence of a complex interrelationship between the levels of individual antioxidant molecules in cells and tissues. 

Ohnishi (Sakamoto etal., 1991) has described an oligomeric derivative of prostaglandin Bi (PGB2) and ascorbic acid. In a rat bilateral carotid occlusion-reperfiision injury complicated by haemorrhagic hypotension, this compound reduced a-phenyl-r-butyl nitrone (PBN) spin-trapped radicals and thiobarbituric acid-reactive products (TBARs) (a measure of lipid peroxidation) in isolated... 

Tetrazolium salts can be reduced nonselectively by many endogenous reductants such as thiol-containing proteins, as well as exogenous ones such as ascorbic acid.521 -523 This can lead to serious interferences and several measures have been described to reduce or eliminate their effect.524,525,650... 

Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized by Sato and co-workers [32]. These compounds seem to be promising fluorescent chemo-sensors for the measurement of reducing species such as Fe2+, ascorbic acid, and hydroxyl radicals. 

Under stress conditions, such as cutting or light exposure, ascorbate oxidase has been described as promoting the transformation of ascorbic acid to dehydroascorbic acid (Wright and Kader 1997b). However, because ascorbic acid can be easily converted into dehydroascorbic acid, it is necessary to measure both ascorbic and dehydroascorbic acids to observe that the content of vitamin C was well preserved in fresh-cut fruit. 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Eberlein and Kattner [194] described an automated method for the determination of orthophosphate and total dissolved phosphorus in the marine environment. Separate aliquots of filtered seawater samples were used for the determination orthophosphate and total dissolved phosphorus in the concentration range 0.01-5 xg/l phosphorus. The digestion mixture for total dissolved phosphorus consisted of sodium hydroxide (1.5 g), potassium peroxidisulfate (5 g) and boric acid (3 g) dissolved in doubly distilled water (100 ml). Seawater samples (50 ml) were mixed with the digestion reagent, heated under pressure at 115-120 °C for 2 h, cooled, and stored before determination in the autoanalyser system. For total phosphorus, extra ascorbic acid was added to the aerosol water of the autoanalyser manifold before the reagents used for the molybdenum blue reaction were added. For measurement of orthophosphate, a phosphate working reagent composed of sulfuric acid, ammonium molyb-... 

In similar work, Sturgeon et al. [125] compared direct furnace methods with extraction methods for cadmium in coastal seawater samples. They could measure cadmium down to 0.1 pg/1. They used 10 pg/1 ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont [116] for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/1 in a seawater sample containing 0.07 pg/1. Generally, he charred at 300 °C and atomised at 1500 °C. This method required compromise between char and atomisation temperatures, sensitivity, heating rates, and so on, but the analytical results seemed precise and accurate. Nitrate added as sodium nitrate delayed the cadmium peak and suppressed the cadmium signal. 

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