Iron 1,10-phenanthroline complex titration indicator

The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

A useful property of a potassium permanganate solution is its intense purple color, which is sufficient to serve as an indicator for most titrations. If you add as little as 0.01 to 0.02 mL of a 0.02 M solution of permanganate to 100 itlL of water, you can perceive the purple color of the resulting solution. If the solution is very dilute, diphenylamine sulfonic acid or the 1,10-phenanthroline complex of iron(II) (see Table 19-2) provides a sharper end point. 

Solutions of cerium(IV) are yellow-orange, but the color is not intense enough to act as an indicator in titrations. Several oxidation/reduction indicators are available for titrations with standard solutions of cerium(IV). The most widely used of these is the iron(II) complex of 1,10-phenanthroline or one of its substituted derivatives (see Table 19-2). 

Some organic ligands, notably 1,10-phenanthroline and 2,2 -bipyridine (Table 6.7), stabilize the lower of two oxidation states of a metal. This is apparent from the values of FF for the appropriate half-reactions in Table 7.1. The observation is associated with the ability of the phen and bpy ligands to accept electrons. Iron(II) complexes of bpy and phen are used as indicators in redox reactions. For example, in a redox titration of Fe with powerful oxidizing... 

According to Buschmann, in very add solution the ionisation of the sulphonic acid group is sufficiently suppressed for sulphobetaines to function as cationics, forming TPB salts which can be extracted by 1,2-dichloroethane. If the TPB salt is titrated in such a two-phase system with a quat, the sulphobetaine is displaced and returns to the aqueous phase. The indicator is the dodecyl sulphate of the iron (II)-1,10-phenanthroline complex, which is initially in the organic layer. The titration is done by the technique of AC voltammetry. The current rises during the titration. At the end-point the quat reacts with the dodecyl sulphate, releasing the iron complex, which passes into the aqueous layer, causing the current to fall. 

The paper [22] leaves some questions unanswered. The author states that sulphobetaines behave like cationics only at acid concentrations exceeding 1.5 M, but the procedure described involves sulphuric acid at less than 0.05 M. It is not clear why the colour transfer is not a satisfactory indication of the end-point the iron (II)-l,10-phenanthroline complex is deep red. Although the author claims to have successfully titrated 3-(dodecyldimethylammonium)propyl sulphonate by this procedure, he does not cite any supporting data. Nevertheless, this is an interesting development and the method is worth investigation. The AC voltammetry apparatus described is no longer available, and an alternative indicator mechanism is required, preferably visual. 

Similar considerations apply to redox titrations, where the indicator dye should possess differently coloured oxidation states and a redox potential which is appropriate for the reaction to be studied. A large number of dyes are known which are suitable for this purpose. Ferroin (Scheme 7), a complex between iron(ll) and phenanthrolin with a deeply red colour, upon oxidation forms a pale blue iron(lll) complex. Another redox indicator is diphenylamine (Scheme 8), which is colourless in the reduced state. It is oxidized in acidic solution irreversibly to diphenylbenzidine (Scheme 9) which can be further oxidized reversibly to the intensely coloured diphenylbenzidine violet (Scheme 10). 

In another version of this procedure the nonionic surfactant was first extracted batch-wise with sodium tetraphenylborate into 1,2-dichloroethane. The tetraphenylborate in the isolated organic phase was then titrated with a cationic surfactant, using Victoria Blue B as indicator (70). This titration can also be performed to an electrochemically detected end point. In this version, an excess of anionic surfactant is added to the cationic complex formed by the ethoxylated nonionic surfactant and potassium ion. The ion pair is extracted into dichloroethane, separated from the initial aqueous phase, then titrated with cationic surfactant in the presence of additional water. The ion pair of the anionic surfactant and Fe(II)(l,10-phenanthroline)3 is added as indicator. The end point of the titration is indicated when the last of the anionic surfactant is complexed by the cationic titrant, causing the iron-phenanthroline cation to migrate to the aqueous phase, where it is detected as a change in potential at a platinum electrode (71). 

Amine oxides have cationic properties at low pH and so can be titrated with anionic reagents according to the procedures developed for cationic surfactants. Titrations with tetraphenylborate (81) and with dodecylsulfate (82) have been demonstrated, using a potentiometric end point. A turbidimetric end point has also been demonstrated (28). An am-perometric end point may be used in a two-phase system with dodecylsulfate titrant, if a suitable cationic indicator is added such as the iron(II) 1,10-phenanthroline complex (83). Long-chain amines, including any unreacted amine from synthesis of the amine oxide, will quantitatively interfere. 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

There are two main types of indicator. One involves a complex of a metal ion, which changes colour when the oxidation state of the metal ion changes. The iron-1,10-phenanthroline system is a typical example, the iron(//) chelate (ferroin) being red, the iron(//7) chelate (ferrion) being essentially colourless (actually very pale blue). The transition potential is about 1,1 V, so the indicator is very suitable for use in titrations with cerium(Ty) in sulphuric acid. The second type of indicator comprises various types of organic compound (aromatic amines, triphenylmethane dyestuffs, for example) which can be oxidised and reduced reversibly, and change colour on doing so. 7V,iV -Diphenylbenzidine is a typical example. 

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