Titrations with Ascorbic Acid

Hydrogen peroxide is an efficient oxidizing agent (Section 16-3), particularly in alkaline solution. The excess peroxide is usually decomposed by boiling the alkaline solution the process is hastened by a number of catalysts, including nickel salts, iodide, and platinum black. Schulek and Szakacs removed the excess with chlorine water and then added potassium cyanide to destroy the excess chlorine. Examples of oxidation reactions are the oxidation of Cr(III) to chromate in 2Af sodium hydroxide, Co(II) to Co(III) in bicarbonate solution, Mn(II) to Mn(IV) in the presence of tellurate, and Fe(II) to Fe(III) followed by titration with ascorbic acid. Sodium peroxide, an even more vigorous oxidant, is applied in alkaline fusions. The fusion of chromite ore to form chromate has been critically studied, ... 

Svehla, G., L. Koltai, and L. Erdey. 1963. The use of 2,6-dichlorophenol-indo-phenol as indicator in iodometric titrations with ascorbic acid. Anal. Chim. Acta 29 442-447. 

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. 

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

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 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... 

R. A. Verdini and C. M. Lagier, Voltametric iodometric titration of ascorbic acid with dead-stop end-point detection in fresh vegetables and fruit samples, J. Agric. Food. Chem., 48 (2000) 2812-2817. 

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu " can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa"] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

Mapson (14) found that in solutions of pH 0.6 there is no significant condensation of formaldehyde with ascorbic acid. However, at this pH sulfides, sulfites, and thiol compounds combine readily with formaldehyde. He, therefore, proposed that the indophenol titration be carried out at pH 0.6 after the addition of formaldehyde to estimate ascorbic acid in the presence of the interfering sulfur compounds. 

Another aliquot of the metaphosphoric acid extract is brought to pH 2.0 by adding 40% sodium citrate solution dropwise. A quantity of formaldehyde solution is then added that will yield a concentration of 8% formaldehyde the mixture is made up to a convenient volume and set aside at room temperature. Samples of this solution are removed at 10 minute intervals and titrated promptly with standardized indophenol reagent. As 30 to 40 minutes are required for completion of the condensation with ascorbic acid, it is desirable to cany out the titrations at 10 minute intervals for not less than 70 minutes. 

For standardization of iron (III) solution [17,18], a 5 ml aliquot of iron (III) solution was treated either with 5 ml of O.Olmol 1-1 ascorbic acid or 10ml of 0.01 mol 1-1 mercaptoacetic acid and swirled for min. About 0.1 g of potassium iodide and ml of 1% starch were added, and the residual amount of ascorbic acid or mercaptoacetic acid from the reaction was evaluated by back titration with 0.01 mol 1 chloramine T, a blue colour was obtained at the end-point. Iron (III) reacts with ascorbic acid or mercaptoacetic acid in a molar ratio of 2 1 and 1 1, respectively. The strength of iron (III) solution determined by two methods agreed within 1 %. 

Determination of the solubility of ascorbic acid in the above solvents Since Mn(HI) sulphate solutions react rapidly with ascorbic acid solution (Sec. 12.4.1), the solubility of ascorbic acid can be determined by titrating a known volume of the saturated solution at 298 K. 

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. 

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). 

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... 

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 ... 

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 ... 

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). 

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