Diphenylamine titration

Titration curve for 50.00 ml of 0.0500 M Fe + with 0.0500 M Ce + showing the range of f and volume of titrant over which the indicators ferroin and diphenylamine sulfonic acid are expected to change color. 

The amount of Fe in a 0.4891-g sample of an ore was determined by a redox titration with K2Cr20y. The sample was dissolved in HCl and the iron brought into the +2 oxidation state using a Jones reductor. Titration to the diphenylamine sulfonic acid end point required 36.92 mL of 0.02153 M K2Cr20y. Report the iron content of the ore as %w/w FeyOy. 

Mention should be made of one of the earliest internal indicators. This is a 1 per cent solution of diphenylamine in concentrated sulphuric acid, and was introduced for the titration of iron(II) with potassium dichromate solution. An intense blue-violet coloration is produced at the end point. The addition of phosphoric(V) acid is desirable, for it lowers the formal potential of the Fe(III)-Fe(II) system so that the equivalence point potential coincides more nearly with that of the indicator. The action of diphenylamine (I) as an indicator depends upon its oxidation first into colourless diphenylbenzidine (II), which is the real indicator and is reversibly further oxidised to diphenylbenzidine violet (III). Diphenylbenzidine violet undergoes further oxidation if it is allowed to stand with excess of dichromate solution this further oxidation is irreversible, and red or yellow products of unknown composition are produced. 

For the titration of colourless or slightly coloured solutions, the use of an indicator is unnecessary, since as little as 0.01 mL of 0.02 M potassium permanganate imparts a pale-pink colour to 100 mL of water. The intensity of the colour in dilute solutions may be enhanced, if desired, by the addition of a redox indicator (such as sodium diphenylamine sulphonate, AT-phenylanthranilic acid, or ferroin) just before the end point of the reaction this is usually not required, but is advantageous if more dilute solutions of permanganate are used. 

The green colour due to the Cr3+ ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include AT-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution) the latter must be used in presence of phosphoric) V) acid. 

Elemental composition K 28.22%, Cl 25.59%, and 0 46.19%. An aqueous solution is analyzed for potassium by AA, ICP, and other methods (see Potassium). Perchlorate ion may be analyzed by ion chromatography or a liquid-membrane electrode. Iodide, bromide, chlorate, and cyanide ions interfere in the electrode measurement. Alternatively, perchlorate ion may be measured by redox titration. Its solution in 0.5M H2SO4 is treated with a measured excess standard ferrous ammonium sulfate. The excess iron(II) solution is immediately titrated with a standard solution of potassium dichromate. Diphenylamine sulfuric acid may be used as an indicator to detect the end point ... 

Cautiously add, with stirring, 15 ml of. sulfuric acid, 5 ml of phosphoric acid, and 6 to 8 drops of Na diphenylamine sulfonate indicator soln (0.2g/100 ml of w). Titrate slowly with 0.05N std K dichromate soln until the pure grn color changes to a gray-green. Then add the dichromate one drop at a time until the first tinge of purple or violet-blue appears... 

Pipet 25 ml of the above soln in a 500ml wide mouth Erlenmeyer flask (F), showh in Fig 1 and Fig 2, add 25 ml of coned HCl and heat nearly to boiling. Add the above stannous chloride soln dropwise and with stirring until the yel color of ferric chloride disappears and then add exactly one drop in excess. Dilute to ca 200 ml with distd w and let stand for 5 min with occasional shaking. Add 15 ml of diphenylamine sulfonate indicator and titrate with std 0.20QQN K dichromate soln until the change at the end point... 

Method S(when acardite II and diphenylamine. are present) a)Extract ca lOg of finely divided propellant with methylene chloride or with chlf, evap the solvent and dry and weigh the extracted residue (P = x + y, where x is the wt of acardite II and y is the wt of DPhA in propellant) b)Dissolve the extracted material in 50cc chlf, add 300cc water and an excess of K bromide-bromate soln of known concn c)After brominating for 4 hrs at RT, add aq soln of KI and titrate the liberated iodine with N/10 Na thiosulfate (lcc of thiosulfate is required for 0.0021 g of DPhA) d)If the ealed wt of DPhA is equal to P, mol wt of DPhA = 1 ( 9 and wt of 2 mols of acardite II is equal to 452, the P1 ... 

Titration with K4Fe(CN)6 to produce K2Zn3[Fe(CN)6]2. End-point detection with diphenylamine. 

Sulfonic acid derivatives of diphenylamine and diphenylbenzidine are suitable indicators in many redox titrations. The structures of these indicators are shown below ... 

Fig. 6.6. Relationship between pKb(water) of N-bases and half - neutralization potential El/2 for potentiometric base titration in glacial acetic acid. 1) 4-aminopyridine (4.83), 2) morpholine (5.30), 3) 2,4,6-trimethylpyridine (6.68), 4) pyridine (8.85), 5) p-toluidine (8.88), 6) o-toluidine (9.61), 7) 3-acetylpyridine (10.82), 8) chloroaniline (11.36), 9) pyrazole (11.47), 10) diphenylamine (13.10) (Pawlak et al., 1985)...

Assay Transfer about 170 mg of sample, previously ground to a fine powder and accurately weighed, into a 250-mL wide-mouth Erlenmeyer flask, and dissolve in 10 mL of methanol. Add 150 mL of water, 1 mL of 1 N sulfuric acid, and 4 drops of diphenylamine indicator (3 mg of / -di phenyl ami ncsu I Ionic acid sodium salt per milliliter of 0.1 N sulfuric acid), and titrate with 0.1 N ceric sulfate to the first complete color change from yellow to red-violet. Record the volume, in milliliters, of 0.1 N ceric sulfate required as V. Calculate the percentage of C10H14O2 in the sample, uncorrected for hydroquinone and 2,5-di-tm-butylhydroquinone, by the formula... 

Ceric Sulfate, 0.01 N [3.322 g Ce(S04)2 per 1000 mL] Dissolve 4.2 g of ceric sulfate [Ce(S04)2-4H20] or 5.5 g of the acid sulfate [Ce(HS04)4] in about 500 mL of water containing 28 mL of sulfuric acid, and dilute to 1000 mL. Allow the solution to stand overnight, and filter. Standardize this solution daily as follows Weigh accurately about 275 mg of hydroquinone (CgHg02), dissolve it in sufficient 0.5 N Alcoholic Sulfuric Acid to make 500.0 mL, and mix. To 25.0 mL of this solution add 75 mL of 0.5 N sulfuric acid, 20 mL of water, and 2 drops of Diphenylamine TS. Titrate with the ceric sulfate solution at a rate of about 25 drops per 10 s until an endpoint is reached that persists for 10 s. Perform a blank determination using 100 mL of 0.5 N Alcoholic Sulfuric Acid, 20 mL of water, and 2 drops of Diphenylamine TS, and make any necessary correction. Calculate the normality of the ceric sulfate solution by the formula... 

NaC103 volumetric by reaction of the clilorate with FeS04 in acid solution and titrating the excess FeS04 with potassium dichromate using diphenylamine sulfonic acid indicator... 

The interest in the application of indicators in oxidation-reduction titrations has followed on the discovery that the familiar color change undergone by diphenylamine on oxidation could be used to determine the end-point of the titration of ferrous ion by dichromate in acid solution. Diphenylamine, preferably in the form of its soluble sulfonic acid, at first undergoes irreversible oxidation to diphenylbenzidine, and it is this substance, with its oxidation product diphenylamine violet, that constitutes the real indicator. ... 

The standard potential of the indicator system is not known exactly, but experiments have shown that in not too strongly acid solutions the sharp color change from colorless to violet, with green as a possible intermediate, occurs at a potential of about — 0.75 volt. The standard potential of the ferrous-ferric system is 0.78 whereas that of the di-chromate-chromic ion system in an acid medium is approximately — 1.2 volt hence a suitable oxidation-reduction indicator might be expected to have a standard potential of about — 0.95 volt. It would thus appear that diphenylamine would not be satisfactory for the titration of ferrous ions by acid dichromate, and this is actually true if a simple ferrous salt is employed. In actual practice, for titration purposes, phosphoric acid or a fluoride is added to the solution these substances form complex ions with the ferric ions with the result that the effective standard potential of the ferrous-ferric system is lowered (numerically) to about — 0.5 volt. The change of potential at the end-point of the titration is thus from about — 0.6 to — 1.1 volt, and hence diphenylamine, changing color in the vicinity of — 0.75 volt, is a satisfactory indicator. 

Diphenylamine This compound was introduced by Knop as an indicator for the Fe(II)-dichromate titration. Since then the chemistry of this indicator has been studied... 

In the foregoing discussion the indicator has tacitly been assumed to come rapidly to equilibrium at each point of the titration curve. That this is an over-simplihcation is evident from a number of experimental observations. Kolthoflf and Sarver found that the oxidation of diphenylamine with dichromate is induced by the Fe(II)-dichromate reaction. The direct oxidation is so slow that the indicator blank is best determined by comparison of the visual with the potentiometric end point. With ferroin. Smith and Brandt and Stockdale foimd that the reverse titration, dichromate with iron, gave satisfactory results at sufficiently high acidities, whereas the direct titration failed because the indicator could not be oxidized. Here the oxidation seems to be slow and the reduction rapid because of the irreversible nature of the oxidant and the reversible nature of the reductant. 

Because the coefficients of expansion for most nonaqueous solvents are high compared with water, the technique of gravimetric titration is attractive. Precise titrations can be performed in this way with small volumes of titrant and sample. End points can be obtained potentiometrically or with visual indicators such as ferroin and diphenylamine. 

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