Ascorbic Acid titration

There is good evidence for the presence of a five-membered ring in D-araboascorbic acid (XXXIX). This analog of L-ascorbic acid, prepared by method 2 (page 87) from methyl 2-keto-D-gluconate shows the same chemical properties and the same absorption characteristics as L-ascorbic acid. Titration of XXXIX with diazomethane affords, as in the case of L-ascorbic acid, a 3-methyl derivative LXXXII which upon further action with diazomethane gives rise to the 2,3-dimethyl-D-arabo-ascorbic acid (LXXXIII)3 ... 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

If the weak acid is monoprotic, then the FW must be 58.78 g/mol, eliminating ascorbic acid as a possibility. If the weak acid is diprotic, then the FW may be either 58.78 g/mol or 117.6 g/mol, depending on whether the titration was to the first or second equivalence point. Succinic acid, with a formula weight of 118.1 g/mol is a possibility, but malonic acid is not. If the analyte is a triprotic weak acid, then its FW must be 58.78 g/mol, 117.6 g/mol, or 176.3 g/mol. None of these values is close to the formula weight for citric acid, eliminating it as a possibility. Only succinic acid provides a possible match. 

The content of ascorbic acid, in milligrams per 100 mL, in orange juice is determined by a redox titration using either 2,6-dichlorophenolindephenol or N-bromosuccinimide as the titrant. 

The titration of ascorbic acid using coulometrically generated I2 and Br2 is described in this experiment. Details are also given for the polarographic analysis of ascorbic acid. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Sometimes the metal may be transformed into a different oxidation state thus copper(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, nickel or cobalt can be titrated using, for example, murexide as indicator without interference from the copper, which is now present as Cu(I). Iron(III) can often be similarly masked by reduction with ascorbic acid. 

C04-0144. Vitamin C (also called ascorbic acid) is an acid whose formula is HCg H7 Og. When treated with strong base, it undergoes the following reaction HCg H7 0(5 + OH H7 Og + H2 O A pharmacist suspects that the vitamin C tablets received in a recent shipment are not pure. When a single 500.0-mg tablet is dissolved in 200.0 mL of water and titrated with a standard base that is 0.1045 M, it takes 24.45 mL to reach the stoichiometric point. Are the tablets pure If not, what is the mass percentage of impurities (Assume no impurity is either an acid or a base.)... 

Trap H2S in an aqueous NaOH and ascorbic acid in a midget impinger titrate resulting sulfide ion with CdS04 solution. Potentiometry ppb levels NR Ehman 1976... 

It will oxidise substances with lower reduction potentials, e.g. the titration of ascorbic acid is carried out as shown in Figure 3.12. 

Direct iodometric titration is used in pharmacopoeial assays of ascorbic acid, sodium stilbigluconate, dimercaprol injection and acetarsol. 

A 300 mg sample of chlorpromazine hydrochloride can be determined by titration with O.IN HCIO4 in anhydrous acetic acid medium, in the presence of 5% Hg(II) acetate (10 mL) and ascorbic acid (1 g) using crystal violet as the indicator [73]. Ascorbic acid prevents the formation of a red color which would otherwise interfere. 

When I- is titrated with Cu(Cl04)2 in AN, it is oxidized in two steps, I ->If and If ->I2. The formal potentials of the two steps are +0.396V and -0.248V vs Ag/Ag+, respectively. Many organic compounds, such as hydroquinone, ascorbic acid, ferrocene and its derivatives, allylamine, hydroxylamine, phenylhydrazine, thiourea and SH compounds, can also be titrated with Cu(II) in AN. Figure 4.9 shows the titration curves of tetramethylbenzidine (TMB) in AN [12]. In dry AN, TMB is oxidized in two steps as follows ... 

To measure hardness, the sample is treated with ascorbic acid (or hydroxylamine) to reduce Fe3+ to Fe2+ and with cyanide to mask Fe2+, Cu+, and several other minor metal ions. Titration with EDTA at pH 10 in NH3 buffer then gives the total concentrations of Ca2+ and Mg2+. Ca2+ can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. Interference by many metal ions can be reduced by the right choice of indicators.21... 

The figure shows the results of a bipotentiometric titration of ascorbic acid with If. Ascorbic acid (146 mg) was dissolved in 200 mL of water in a 400-mL beaker. Two Pt electrodes were attached to the K-F outlets of the pH meter and spaced about 4 cm apart in the magnetically stirred solution. The solution was titrated with 0.04 M If (prepared by dissolving 2.4 g of K1 plus 1.2 g of I2 in 100 mL of water), and the voltage was recorded after each addition. Prior to the equivalence point, all the If is reduced to I- by the excess ascorbic acid. Reaction B can occur, but Reaction A cannot. A voltage of about 300 mV is required to support a constant current of 10 pA. (The ascorbate dehydroascorbate couple does not react at a Pt electrode and cannot carry current.) After the equivalence point, excess If is present, so Reactions A and B both occur, and the voltage drops precipitously. 

The direct Ripper iodometric titration is still used, but it is subject to error. In its place, direct iodate-iodide titration is used (44). This is followed by fixing the sulfur dioxide with glyoxal in a second sample and retitrating. The difference represents the free sulfur dioxide. The second titration roughly represents the amount of reduction and the amount of ascorbic acid present. Formulas for calculating the amount of sulfur dioxide to add in order to produce a predetermined level of free sulfur dioxide have been given by Stanescu (45). 

Because of the clinical significance of vitamin C, it is essential to In-able to detect and quantify its presence in various biological materials. Ana lytical methods have been developed to determine the amount of ascorbic acid in foods and in biological fluids such as blood and urine. Ascorbic acid may be assayed by titration with iodine, reaction with 2,4-dinitrophenylhy-drazine, or titration with a redox indicator, 2,6-dichlorophenolindophenol (DCIP) in acid solution. The latter method will be used in this experiment because it is reasonably accurate, rapid, and convenient and can be applied to many different types of samples. 

The reaction of DCIP with ascorbic acid is shown in Figure El 1.3. Ascorbic acid reduces the indicator dye from an oxidized form (red in acid) to a reduced form (colorless in acid). The procedure is simple, beginning with dissolution of the sample to be tested in metaphosphoric acid. An aliquot of the sample is then titrated directly with a solution of DCIP. Although the original DCIP solution is blue, it becomes light red in the acid solution. Upon reaction with ascorbic acid in the sample, the dye becomes colorless. Titration is continued until there is a very slight excess of dye added (faint pink color remains in the acid solution). 

Samples for analysis often contain traces of other compounds, in addition to ascorbic acid, that reduce DCIP. One way to minimize the interference of other substances is to analyze two identical aliquots of the sample. One aliquot is titrated directly and the total content of all reducing substances present is determined. The second aliquot is treated with ascorbic acid oxidase to destroy ascorbic acid and then titrated with DCIP. The... 

Obtain a sample containing an unknown amount of ascorbic acid from your instructor. Place 1.0 mL of the unknown in a 50-mL Erlenmeyer flask. Add 5.0 mL of metaphosphoric acid solution to the flask and titrate as before with DCIP. Repeat with two more samples of the unknown. 

To test for the presence of interfering substances in the juice, pipet a 10.0-mL sample of the fresh, undiluted juice into a 50-mL Erlenmeyer. Add a few crystals of ascorbic acid oxidase to destroy the ascorbic acid. Let stand, after gentle mixing, for 10 minutes. Add 40 mL of metaphosphoric acid solution. Titrate three 10.0-mL portions of this diluted juice with DCIP. 

Calculate the amount of ascorbic acid in the juices and report in terms of mg/100 mL of pure juice. Again, calculate confidence limits at the 95°/o confidence level. Remember that you started with 10 mL of pure juice, diluted it with 40 mL of metaphosphoric acid solution, and titrated 10 mL of the diluted juice. Are there reducing substances in addition to ascorbic acid in the juice ... 

B 5. A 10-mL sample of pure orange juice was diluted with 40 mL of metaphosphoric acid solution. Then 10 mL of the diluted juice was titrated to an end point with 9.27 mL of DCIP. A blank required 0.25 mL of DCIP. A 1-mg sample of pure ascorbic acid required 6.52 mL (after blank correction) of DCIP. What is the concentration of ascorbic acid in the orange juice in mg/100 mL ... 

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