Ferroin indicator

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. 

Low-spin, octahedral complexes are formed by ligands such as bipy, phen and CN , and their stability is presumably enhanced by the symmetrical configuration. [Fe(bipy)3] + and [Fe(phen)3] + are stable, intensely red complexes, the latter being employed as the redox indicator, ferroin , due to the sharp colour change which occurs when strong oxidizing agents are added to it ... 

A redox indicator is a compound that changes color when it goes from its oxidized to its reduced state. The indicator ferroin changes from pale blue (almost colorless) to red. 

Ce4+ is yellow and Ce3+ is colorless, but the color change is not distinct enough for cerium to be its own indicator. Ferroin and other substituted phenanthroline redox indicators (Table 16-2) are well suited to titrations with Ce4+. 

Ferroin With the introduction of Ce(IV) as an oxidant and the evaluation of the formal potential of the Ce(rV)-Ce(III) couple, the need for indicators with higher electrode potentials became evident. The indicator ferroin, tris(l,10-phenanthroline)-iron(II), was discovered by Walden, Hammett, and Chapman, and its standard potential was evaluated at 1.14 V. Hume and KolthofiF found that the formal potential was 1.06 V in 1 M hydrochloric or sulfuric acid. The color change, however, occurs at about 1.12 V, because the color of the reduced form (orange-red) is so much more intense than that of the oxidized form (pale blue). From Figure 15-1 it can be seen that ferroin should be ideally suited to titrations of Fe(II) and other reductants with Ce(lV), particularly when sulfuric acid is the titration medium. It has the further advantages of undergoing a reversible oxidation-reduction reaction and of being relatively stable even in the presence of oxidant. 

The addition of the redox indicator ferroin causes the classical oscillation system, which was first described in detail hy A. M. Zhabotinsky, to go through the various differently colored stationary states, which undergo interconversion at regular intervals. 

Of all the oxidation/reduction indicators, ferroin approaches most closely the ideal substance. It reacts rapidly and reversibly, its color change is pronounced, and its solutions are stable and easily prepared. In contrast to many indicators, the oxidized form of ferroin is remarkably inert toward strong oxidizing agents. At temperatures above 60°C, ferroin decomposes. 

The excess of K2Cr207 is determined by back titration with a ferrous salt with a redox indicator, ferroin ... 

The reaction oscillates equally well if one replaces cerium by iron, or bromine by iodine. If the redox indicator ferroin is used, the solution switches continuously between red (Fe " ) and blue (Fe ), with an oscillation period between seconds and minutes, depending on the concentrations. In addition to the temporal oscillations shown in Figure 2.11, the reaction also exhibits spatial oscillations. Provided the reaction is carried out in a thin layer, there will be beautiful patterns of concentric rings travelling through the solution. 

Spirals are frequently arranged in pairs due to the procedure of their generation. Note that the collision of two fronts leads to their mutual annihilation, as shown in the three-dimensional perspective representation of Figure 4. This presentation allows to recognize in detail the height of the wave crest and its modulation in the collision area between both fronts, where the two waves start to annihilate each other. In terms of chemistry the dip in the area of interaction indicates a more reduced state of the catalyst and indicator ferroin produced by the collision. It also visualizes clearly the sharp cusps formed in the collision area. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

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. 

Peroxymonosulphuric acid (PMSA, H SO ) proved to be a promising oxidizer in reactions with chemiluminescent substances (luminol) with participation of such ions as Mn(II), Cu(II), Ni(II), Cr(IV), V(V). The literature data show the possibility of utilization PMSA in indicating reaction with ferroin ([Fe(l,10-phenanthrolyne) ] ) which is accelerated by Mn(II) compounds. 

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. 

M cerium(IV) solution, and the equivalence point is at 1.10 volts. Ferroin changes from deep red to pale blue at a redox potential of 1.12 volts the indicator will therefore be present in the red form. After the addition of, say, a 0.1 per cent excess of cerium(IV) sulphate solution the potential rises to 1.27 volts, and the indicator is oxidised to the pale blue form. It is evident that the titration error is negligibly small. 

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. 

Internal indicators suitable for use with cerium(IV) sulphate solutions include AT-phenylanthranilic acid, ferroin [1,10-phenanthroline iron(II)], and 5,6-dimethylferroin. 

Method A Standardisation with arsenic (III) oxide. Discussion. The most trustworthy method for standardising cerium(IV) sulphate solutions is with pure arsenic(III) oxide. The reaction between cerium(IV) sulphate solution and arsenic(III) oxide is very slow at the ambient temperature it is necessary to add a trace of osmium tetroxide as catalyst. The arsenic(III) oxide is dissolved in sodium hydroxide solution, the solution acidified with dilute sulphuric acid, and after adding 2 drops of an osmic acid solution prepared by dissolving 0.1 g osmium tetroxide in 40mL of 0.05M sulphuric acid, and the indicator (1-2 drops ferroin or 0.5 mL /V-phenylanthranilic acid), it is titrated with the cerium(IV) sulphate solution to the first sharp colour change orange-red to very pale blue or yellowish-green to purple respectively. 

Procedure. Weigh out accurately about 0.2 g of arsenic(III) oxide, previously dried at 105-110 °C for 1-2 hours, and transfer to a 500 mL beaker or to a 500 mL conical flask. Add 20 mL of approx. 2M sodium hydroxide solution, and warm the mixture gently until the arsenic(III) oxide has completely dissolved. Cool to room temperature, and add 100 mL water, followed by 25 mL 2.5M sulphuric acid. Then add 3 drops 0.01 M osmium tetroxide solution (0.25 g osmium tetroxide (CARE FUME CUPBOARD) dissolved in 100 mL 0.05M sulphuric acid) and 0.5 mL AT-phenylanthranilic acid indicator (or 1-2 drops of ferroin). Titrate with the 0.1 M cerium(IV) sulphate solution until the first sharp colour change occurs (see Discussion above). Repeat with two other samples of approximately equal weight of arsenic(III) oxide. 

Procedure. Prepare an approximately 0.1 M solution of ammonium iron(II) sulphate in dilute sulphuric acid and titrate with the cerium(IV) sulphate solution using ferroin indicator. 

Weigh out accurately about 0.2 g sodium oxalate into a 250 mL conical flask and add 25-30 mL 1M sulphuric add. Heat the solution to about 60 °C and then add about 30 mL of the cerium(IV) solution to be standardised dropwise, adding the solution as rapidly as possible consistent with drop formation. Re-heat the solution to 60 °C, and then add a further 10 mL of the cerium(IV) solution. Allow to stand for three minutes, then cool and back-titrate the excess cerium(IV) with the iron(II) solution using ferroin as indicator. 

Discussion. Copper(II) ions are quantitatively reduced in 2M hydrochloric acid solution by means of the silver reductor (Section 10.140) to the copper(I) state. The solution, after reduction, is collected in a solution of ammonium iron(III) sulphate, and the Fe2+ ion formed is titrated with standard cerium(IV) sulphate solution using ferroin or AT-phenylanthranilic acid as indicator. 

Procedure (copper in crystallised copper sulphate). Weigh out accurately about 3.1 g of copper sulphate crystals, dissolve in water, and make up to 250 mL in a graduated flask. Shake well. Pipette 50 mL of this solution into a small beaker, add an equal volume of ca AM hydrochloric acid. Pass this solution through a silver reductor at the rate of 25 mL min i, and collect the filtrate in a 500 mL conical flask charged with 20 mL 0.5M iron(III) ammonium sulphate solution (prepared by dissolving the appropriate quantity of the analytical grade iron(III) salt in 0.5M sulphuric acid). Wash the reductor column with six 25 mL portions of 2M hydrochloric acid. Add 1 drop of ferroin indicator or 0.5 mL N-phenylanthranilic acid, and titrate with 0.1 M cerium(IV) sulphate solution. The end point is sharp, and the colour imparted by the Cu2+ ions does not interfere with the detection of the equivalence point. 

Procedure (copper in copper(I) chloride). Prepare an ammonium iron(III) sulphate solution by dissolving 10.0 g of the salt in about 80 mL of 3 M sulphuric acid and dilute to 100 mL with acid of the same strength. Weigh out accurately about 0.3 g of the sample of copper(I) chloride into a dry 250 mL conical flask and add 25.0 mL of the iron(III) solution. Swirl the contents of the flask until the copper(I) chloride dissolves, add a drop or two of ferroin indicator, and titrate with standard 0.1 M cerium(IV) sulphate. 

Hydrogen peroxide. The diluted solution, which may contain nitric or hydrochloric acid in any concentration between 0.5 and 3M or sulphuric add in the concentration range 0.25 to 1.5M, is titrated directly with standard cerium(IV) sulphate solution, using ferroin or /V-phenylanthranilic acid as indicator. The reaction is ... 

To 25.0 mL of 0.01-0.015 M persulphate solution in a 150 mL conical flask, add 7 mL of 5 M sodium bromide solution and 2 mL of 3 M sulphuric acid. Stopper the flask. Swirl the contents, then add excess of 0.05M ammonium iron(II) sulphate (15.0mL), and allow to stand for 20 minutes. Add 1 mL of 0.001 M ferroin indicator, and titrate the excess of Fe2+ ion with 0.02 M cerium(IV) sulphate in 0.5 M sulphuric acid to the first colour change from orange to yellow. 

Ferroin 175, 365, 381 modification by substituents, 366 prepn. of indicator solution, 175, 365 Ferromanganese analysis of, (ti) 336 Ferrous ammonium sulphate see Ammonium iron(II) sulphate Ferrous iron see Iron(II)... 

Ferrocyanides stability, 6, 830 Ferrocytochrome c oxidation, 6, 621 Ferroin, 4,1203 redox indicator. 1,558 Ferrokinetics... 

For demonstrations add to a 1 -L beaker, 600 mL water, 60 mL cone, sulfuric acid, 20 g malonic acid, 7.8 g potassium bromate, 0.7-0.8 g (NHj ),Ce(NCL), and about 1 mL 0.025 M [Fe(phen)i]S04 ( ferroin indicator) to give a visible color. Stir magnetically. A short but variable length of time can be expected before oscillations begin their frequency depends on the temperature. 

The effects of substituents upon the ferroin reduction have also been recorded (Table 25) . A marked correlation between E and log A is found, indicating a single type of cation-anion interaction. 

One important group of colour indicators is derived from 1 10 phenantholine ortho-phenanthroline) which forms a 3 1 complex with iron(II). The complex known as ferroin undergoes a reversible redox reaction accompanied by a distinct colour change... 

Where the reduction potentials of two analytes are sufficiently different a mixture may be analysed. Titanium(III), = 0-lOV may be titrated with cerium(IV) in the presence of iron(II), =0.77 V usjng methylene blue as indicator. Subsequently the total, iron plus titanium, may be determined using ferroin as indicator. The determination of iron is illustrative of some practical problems which are encountered in direct titration procedures. 

Procedure Weigh accurately about 0.3 g of ferrous fumarate and dissolve in 15 ml of dilute sulphuric acid by the help of gentle heating. Cool, add 50 ml of water and titrate immediately with 0.1 N ammonium ceric sulphate, employing ferroin sulphate solution as indicator. Each ml of 0.1 N ammonium ceric sulphate is equivalent to 0.01699 g of C4H2Fe04. 

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